research paper on oled technology

Materials Chemistry Frontiers

Recent advances in organic light-emitting diodes: toward smart lighting and displays.

* Corresponding authors

a Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou, Jiangsu, P. R. China E-mail: [email protected] , [email protected]

b School of Physics and Electronics Science, Ministry of Education Nanophotonics & Advanced Instrument Engineering Research Center, East China Normal University, Shanghai, China

c Institute of Organic Optoelectronics (IOO), JITRI, Wujiang, Suzhou 215215, China

Organic light-emitting diodes (OLEDs) have rapidly grown as one of the leading technologies for full-color display panels and eco-friendly lighting sources due to their outstanding features including superior color quality, wide viewing angle, mercury-free manufacture, fascinating flexibility, etc. A variety of materials, device architectures, as well as processing techniques have been investigated for optimizing device performance in order to fulfill the requirements of lighting and display applications. In this review, we first summarize the light emission mechanisms of electroluminescent materials. Then, the designed device architectures aiming at the realization of various light emission mechanisms are reviewed. An overview of recent advances in light extraction strategies is presented since all efficient OLEDs have a multi-thin-film structure, which leads to severe light trapping in devices. In addition, the progress of flexible OLEDs is reviewed from the aspect of flexible transparent electrodes because of their great potential in flexible displays. Most recent breakthroughs of solid-state lighting and displays are briefly addressed as well. A brief perspective on future research is also proposed for pursuing the commercialization of OLEDs.

• This article is part of the themed collection: 2020 Materials Chemistry Frontiers Review-type Articles

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S. Zou, Y. Shen, F. Xie, J. Chen, Y. Li and J. Tang, Mater. Chem. Front. , 2020,  4 , 788 DOI: 10.1039/C9QM00716D

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Recent advances in efficient emissive materials-based OLED applications: a review

• Published: 13 September 2021
• Volume 56 , pages 18837–18866, ( 2021 )

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• Jayanta Bauri 1 ,
• Ram Bilash Choudhary   ORCID: orcid.org/0000-0002-1464-5649 1 &
• Gobind Mandal 1  

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In the present time, organic light-emitting diode (OLED) is a very promising participant over light-emitting diodes (LEDs), liquid crystal display (LCD), and also another solid-state lighting device due to its low cost, ease of fabrication, brightness, speed, wide viewing angle, low power consumption, and high contrast ratio. The most prominent layer of OLED is the emissive layer because the device emission color, contrast ratio, and external efficiency depend of this layer’s materials. This review ruminates on the basics of OLEDs, different light emission mechanisms, OLEDs achievements, and different types of challenges revealed in the field of OLEDs. This review’s primary intention is to broadly discuss the synthesizing methods, physicochemical properties of conducting polymer polymethyl methacrylate (PMMA), and its polymeric nanocomposite-based emissive layer materials for OLEDs application. It also discusses the most extensively used OLED fabrication techniques. PMMA-based polymeric nanocomposites revealed good transparency properties, good thermal stability, and high electrical conductivity, making suitable materials as an emissive layer for OLED applications.

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Acknowledgements

The authors are thankful to the Director IIT(ISM), Dhanbad, for providing the research opportunities and continuously supporting them by providing the institute infrastructure and fellowship.

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Bauri, J., Choudhary, R.B. & Mandal, G. Recent advances in efficient emissive materials-based OLED applications: a review. J Mater Sci 56 , 18837–18866 (2021). https://doi.org/10.1007/s10853-021-06503-y

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DOI : https://doi.org/10.1007/s10853-021-06503-y

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Approaches for Long Lifetime Organic Light Emitting Diodes

Sujith sudheendran swayamprabha.

1 Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013 Taiwan, Republic of China

Deepak Kumar Dubey

Rohit ashok kumar yadav, mangey ram nagar, aayushi sharma.

2 Birla Institute of Technology & Science‐Pilani, Shamirpet‐Keesara Road, Jawahar Nagar, Shameerpet, Hyderabad Telangana, 500078 India

Fu‐Ching Tung

3 Department of Solid State Lighting Technology, Mechanical and Mechatronics Systems Research Labs., Industrial Technology and Research Institute, Hsinchu 31057 Taiwan, Republic of China

Jwo‐Huei Jou

Organic light emitting diodes (OLEDs) have been well known for their potential usage in the lighting and display industry. The device efficiency and lifetime have improved considerably in the last three decades. However, for commercial applications, operational lifetime still lies as one of the looming challenges. In this review paper, an in‐depth description of the various factors which affect OLED lifetime, and the related solutions is attempted to be consolidated. Notably, all the known intrinsic and extrinsic degradation phenomena and failure mechanisms, which include the presence of dark spot, high heat during device operation, substrate fracture, downgrading luminance, moisture attack, oxidation, corrosion, electron induced migrations, photochemical degradation, electrochemical degradation, electric breakdown, thermomechanical failures, thermal breakdown/degradation, and presence of impurities within the materials and evaporator chamber are reviewed. Light is also shed on the materials and device structures which are developed in order to obtain along with developed materials and device structures to obtain stable devices. It is believed that the theme of this report, summarizing the knowledge of mechanisms allied with OLED degradation, would be contributory in developing better‐quality OLED materials and, accordingly, longer lifespan devices.

This work provides an in‐depth overview of the state of art of stability of organic light emitting diodes and covers important degradation issues involved in this technology. Several intrinsic and extrinsic degradation mechanisms within the materials and evaporator chambers are thoroughly analyzed and discussed.

1. Introduction

1.1. world revenue of display and lighting.

Organic light emitting diodes (OLEDs) are at the cusp of becoming the dominant technology for high‐quality flat panel display as well as for solid‐state lighting owing to its unique disruptive features such as energy‐saving, wide view‐angle, fast response, high contrast, and high color purity. OLEDs are emitting flat, plan, diffused and soft light and producing eye‐catching images that have the class of natural light color, making them an ideal candidate for high‐quality flat panel display. [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 ] On the other hand, LEDs have to undergo multilevel “processing” in order to get close to the light quality offered by the OLED based counterparts. It is also difficult to obtain even dispersal into a near‐plane light source in LEDs. [ 15 ] Similarly for general lighting purposes in offices or reading areas, which are usually larger than an average‐sized room, uniformly illuminated lighting sources are required. This requirement can be fulfilled by using almost similar size OLED lighting panels. However, this proportionately increases the manufacturing costs of the components which make this technology less feasible for general lighting purpose.

OLED technology also offers new approaches to be fabricated on flexible and stretchable substrates and its thinness, flexibility and extraordinary durability during worst mechanical conditions such as bending and twisting making it suitable for wearable electronics, biomedical appliances, electronic skins, and robotics. OLED components are capable of emitting stable and transformable light which can be tuned by the device architecture. The color and color temperature of OLEDs can be adjusted along with a wide range from 1500 to 20000 K with almost no restrictions, which generate the possibility of mimicking natural light style artificial light. [ 16 ] Both OLED researchers and market analysts are strongly believed that the OLED lighting market will, sooner or later, take up a major portion of revenue and eventually disrupt the conventional technologies.

According to Global OLED Display Market reported the global OLED Display market is valued at 42 490 million US$ in 2020 and is expected to reach 185 830 million US$ by the end of 2026, growing at a compound annual growth rate (CAGR) of 23.2% during 2021–2026 ( Figure   1 a ). [ 17 ] Similarly, the global OLED Lighting Panels market is valued at 45 million US$ in 2020 and is expected to reach 65 million US$ by the end of 2026, growing at a CAGR of 5.5% during 2021–2026 (Figure  1b ). [ 18 ]

Overview of OLED global market revenue by a) display and b) lighting application. c) Contribution of OLED technology by application in next‐generation electronic devices.

The informative display is a key tool for information dissemination and human–machine communication and become an indispensable part of our lifestyle and gradually shaping it. The machine offers data to the user via the display, to which the user responds by providing feedback to the machine. The market of OLED displays has grown rapidly and has started to challenge other existing major technology owing to its potential applications in smartphones, tablets, computer monitors, televisions (TVs), automobile, head‐up‐display, smart watches, and so on. As per IDTechEx report, OLEDs for cellphones and TVs dominate the OLED sector, comprising 78% and 17% of the OLED market revenue in 2020, respectively (Figure  1c ). Despite being 17% of the revenue, OLED TVs are 43% of the OLED market by area of the display. The third largest OLED application is wearables, which is 2% of the total OLED display market value and 0.3% by area in 2020. [ 19 ] OLED display can be divided into two categories on the basis of driving mode, i.e., passive matrix OLED (PMOLED) and active matrix OLED (AMOLED). Among, AMOLED is superior because of its low weight, thickness, wide color gamut, and fast response time. It is also the most widely used large‐sized display in OLED sector with potential applications in cellphones, tablets and TVs. [ 20 , 21 ] However, it is criticized for a poor lifetime when compared with other counterparts, i.e., PMOLED. [ 20 , 21 ]

Nowadays, the human being is spending their maximum time in an environment that is created by artificial lighting. It is used everywhere, including our home, school, office, shopping malls, factories as well as also for other outdoor purposes. According to the International Energy Agency's 2006 report, lighting consumes about 20% of total generated electric energy and 30–40% of total consumption of this energy in residential buildings and offices. [ 22 ] Moreover, energy‐inefficient light sources such as incandescent bulbs are still lighting majors in many developing countries. In order to solve the energy crisis, both academics and industries have made a considerable effort to devise energy saving and long‐lasting lighting sources are in demand to solve the energy crisis. Amongst, all the existing lighting majors still LED is more dominating technology, but in the last few years OLED based lighting sources also show a great potential owing to its energy saving and other physiological features. Although a notable development has been made in OLED lighting, but still there are several challenges to realize such as high cost and performance at high brightness. [ 23 , 24 , 25 , 26 ] In lighting, several companies have launched trial products such as: Osram in Germany, Philips in the Netherlands, Visionox in the mainland, Lumiotec in Japan, and GE in the United States. Kaneka, LG, Samsung, Konica Minolta, etc., are also some noted companies.

OLED are used today to make efficient and beautiful lighting panels. Moreover, it is the only technology that can create large “area” flexible and transparent lighting panels. In 2016, Audi launched its first automobile with OLED lighting with the help of OSRAM. Furthermore, in 2018, Audi A8 uses four small vertical OLED taillight modules on each side, that are provided by Hella. [ 27 ] In 2018, Acuity Brands launched 4′, 6′, and 8′ lamp based on OLED technology and named it the “Peerless OLE4 Olessence.” [ 28 ] In 2019, they further launched the slimmer version of Olessence with the help of OLEDWorks. [ 28 ] Despite being efficient and beautiful, OLED lighting also offers many physiological features. In this regard, in December 2018, First‐o‐lite, together with NTHU, launched the first candlelight OLED desk lamp that is blue hazard free as well as also free from Hg, glare and flicker, and providing a sensational pleasant environment. [ 29 , 30 , 31 ] A complete chart of OLED development history has been demonstrated in the Figure   2 .

A brief history of OLED technology evolution.

1.2. Challenges of OLED

OLEDs are conceptually different from most of the current lighting and display majors and offer arrange of attractive features such as high efficiency, low power consumption, fast switching, wide viewing angle, light weight, and flexibility. [ 32 , 33 ] Because of sustainable development in material chemistry, device physics and manufacturing technology, OLEDs are continuously improving in terms of performance, durability, and manufacturability, and many products based on it had already come into our daily life. But still this technology is suffering from some serious drawbacks, like the reduced life expectation given the extreme sensitiveness to oxygen and moisture as well as difficulties into encapsulate devices which cause an aggravation in the degradation that making the devices very sensitive and not efficient. [ 33 ] This sensitiveness demands highly controlled production environment, which requires complex methods of fabrication and an increased production cost. Furthermore, roughness of the metal electrodes, poor interfacial bonding between organic and inorganic layer and the migration of the metal ions into the organic layers from the electrodes are also highly responsible for device efficiency and lifetime.

Besides the aforementioned issue, OLED's device physicists also have to answer some other questions related to the thermodynamics of OLED material like which parameters are responsible for their degradation, solubility, and proper dispersion in the phases that they're in. OLED's material chemists are also spending a lot of times on the molecular design of emitters.

1.3. Growth of OLED Paper and Patent

Since the unveiling of the first OLED in 1987, numerous strategies have been applied to improve device efficiency and lifetime. In this section ( Figures   3 – 10 ), we are trying to provide clear statistics about the number of papers and patents growth curve per year for the particular color of OLEDs and the highest efficiencies reported per year for a specific color of OLED.

Number of research articles on WOLED per year (Web of Science).

Illustrates the number of patents on WOLEDs filed per year (Web of Science).

Figure  3 shows the number of publications on white OLEDs (WOLEDs) from 1995 to 2018. After the invention of first WOLED by Kido et al. in 1994, many research groups are focusing to improve the OLEDs efficiency and lifetime. [ 34 ] Moreover, several studies dealing with the tuning of Commission Internationale de L'Eclairage (CIE) coordinates, color rendering index (CRI), spectrumresemblance index (SRI), and color temperature (CT) in WOLEDs. [ 23 ] Till 2005, there is a moderate growth in WOLED development but later it gains huge attention from the academia and industry due to the successive advancement in RGB emitters. Hence, there is a tremendous increment in the number of WOLED publications after 2005. In 2018, 53 papers were published, describing the materials and methods for the production of white OLEDs. It is very crucial to reduce the fabrication and material cost for WOLEDs to become a disruptive technology in the lighting market.

Figure  4 shows the highest power efficiency (PE) of WOLEDs per year. The PE of OLEDs depends on a number of critical factors like material property, fabrication technique, and device architecture, etc. In 1994, Kido et al. demonstrated the first multilayer WOLED, which exhibited a PE of 0.83 lm W −1 . [ 34 ] They have used three constructive emissive layers namely blue, green, and red with different carrier transporting properties. Till 2008, the overall device power efficiencies were considerably lower than 60–70 lm W −1 but in the year 2009, Reineke et al. demonstrated a WOLED device achieving PE of 90 lm W −1 via implementing a periodic out‐coupling structure. [ 35 ] Later, in 2014, Liu et al. fabricated a flexible WOLED which exhibited a maximum PE of 101.3 lm W −1 . [ 36 ] In the same year, Ou et al. demonstrated a WOLED with the highest PE of 123.4 lm W −1 by utilizing multilayered energy cascading structure. [ 37 ] In the year 2016, Xu et. al. demonstrated WOLED with maximum PE of 112.4 lm W −1 using quasi‐random out‐coupling. [ 38 ] In 2017, Tong and group members reported an efficient light extraction of OLEDs on a fully solution‐processed flexible substrate and reported a PE of 107 lm W −1 . [ 39 ] In the same year, Wu et al. displayed a WOLED possessing PE of 105 lm W −1 without light out‐coupling enhancement technique. [ 40 ] Recently, Ying et al. revealed a high‐efficiency hybrid WOLED by introducing ultrathin nondoped phosphorescent emitters in a blue exciplex host exhibited PE of 97.1 lm W −1 . [ 41 ] Ying et al. reported an efficient WOLED through managing triplet excitons in the emission layer and achieved a 95.3 lm W −1 efficiency. [ 42 ]

Highest power efficiency WOLED per year (Web of Science).

Figure  5 demonstrated the number of papers published on blue OLEDs in each year, the average number of papers per year is significantly increasing after 2009. Lighting and display industries facing critical issues concerning in the short operational lifetime of blue OLEDs. Very recently, Lee et al. reported a review article regarding the current status, encounters, and future viewpoint of blue OLEDs. The short lifetime of blue OLEDs mainly attributed to wide bandgap and long triplet exciton lifetime. [ 43 ] It is very crucial to develop deep blue emitters with high efficiency and lifetime, which should follow the National Television System Committee (NTSC) standard color coordinates of (0.14, 0.08). [ 44 ] After 2014, there is drastic progress in the development of blue OLEDs, which may be influenced by the 2014 Nobel prize for bright blue light LED. [ 45 ] The OLED industries are still trying to develop efficient and long lifetime blue OLEDs from phosphorescent and thermally activated delayed flourescence (TADF) emitters.

Number of research articles on blue OLED per year (Web of Science).

Figure  6 shows the increment in the highest PE of blue OLEDs per year. Until 2005, the reported blue OLEDs PE was less than 15 lm W −1 . In 2006, Padmaperuma et al. demonstrated a blue OLED with a maximum PE of 25.1 lm W −1 by utilizing a novel charge‐transporting host material. [ 46 ] In 2008, Su's research group demonstrated blue electrophosphorescent OLED with a PE of 55 lm W −1 at 100 cd cm −2 . [ 47 ] In 2009, Bhansali et al. revealed a high efficient blue electrophosphorescence OLED exhibited PE of 61.2 lm W −1 via utilizing Pt(II)‐pyridyltriazolate complex as an emitter molecule. [ 48 ] Through employing a pyridine containing electron transport layer, Ye et al. demonstrated blue OLED with maximum a PE of 65.8 lm W −1 . [ 49 ] In the year 2017, Sasabe et al. reported a blue OLED with a power efficiency of 66 lm W −1 through an isonicotinonitrile‐based novel TADF emitter. [ 50 ]

Highest power efficiency of blue OLED per year (Web of Science).

Figure  7 shows the number of papers based on red OLEDs per year. Red emission is one of the key components for displays, WOLEDs, and low CT OLEDs. [ 31 , 51 , 52 ] Numerous studies were going on to improve the efficiency and lifetime of deep red, red, and orange‐red OLEDs. In 2018, 27 papers were discussed about red OLEDs. Red OLEDs are showing higher lifetime and efficiency compared to blue OLEDs. Fluorescent, phosphorescent, and TADF materials are utilizing to fabricate high efficiency and long lifetime red OLEDs. The color purity, efficiency, and long lifetime of red OLEDs strong influencing the durability of OLED displays. [ 51 ]

Number of research articles on red OLED per year (Web of Science).

Figure  8 shows the highest PE of red OLED per year. In 2005, Wellmann et al. reported a highly efficient PIN red OLED, which exhibited a PE of 10.0 lm W −1 at 100 cd m −12 with CIE coordinates of (0.69, 0.31). [ 53 ] In 2008, Meerheim et al. demonstrated a red OLED with a PE of 81 lm W −1 with an out‐coupling device mechanism through a microcavity amplification between the cathode and Ag layer. [ 54 ] In 2017, Cui et al. displayed a red OLED with maximum PE of 69.11 lm W −1 based on terbium and gadolinium complexes as sensitizers. [ 55 ] In 2018, Wang et al. demonstrated a high efficiency red phosphorescent OLED (PhOLED) with a PE of 63.6 lm W −1 by employing a novel TADF host material. [ 56 ] In the same year, Wang et al. reported a double emissive layer based high efficiency OLED device with a PE 53.5 lm W −1 . [ 57 ]

Highest power efficiency of red OLED per year (Web of Science).

Li et al. reported organic shell‐molecule to avoid the transition problem of triplet in OLEDs. [ 58 ] They fabricated OLEDs using stable neutral π radicals and achieved an EQE of 2.4%. [ 58 ] They reported a new stable room‐temperature luminescent radical, ( N ‐carbazolyl)bis(2,4,6‐tirchlorophenyl)‐methyl radical (CzBTM), which exhibited deep red to near infrared emission. [ 59 ] In another report, they demonstrated open‐shell, doublet red dopants that emit light after donor–radical charge transfer. [ 60 ] In 2019, they reported a deep‐red/near‐infrared OLED with a maximum quantum efficiency of 5.3% with donor–acceptor neutral radicals not following the Aufbau principle. [ 61 ] Tris(2,4,6‐trichlorophenyl)methyl–pyridoindolyl derivatives showed a high photoluminescence quantum yield of >90% and the device showed pure red emission with an EQE of 12%. [ 62 ]

Figure  9 illustrates the number of published papers focused on the OLEDs lifetime perspective. However, the OLEDs lifetime still remains the ongoing challenge in front of the researcher communities that repress it from present lighting and display technologies. In 2015, Scholz et al. published an informative review regarding the external and internal factors influencing the device lifetime. [ 33 ] It's always crucial to solve these challenges associated with the device lifetime. Mainly, blue OLEDs facing unacceptably short lifetimes issues compared with red and green OLEDs. [ 43 ]

Number of papers of lifetime per year (Web of Science).

Every year a huge number of patents were filling based on the novel material, methods or technology perspective for improving the performance and lifetime of WOLEDs. Figure  10 illustrates the number of WOLEDs filed per year. Vast varieties of approaches were adopted to improve the efficiency of OLEDs compared to lifetime. Most of the publications and patents in OLED field are discussing the efficiency improvement. Despite the great achievements with device efficiency, an effective way to improve device lifetime is needed for the wider acceptance of OLED in display and lighting market. We need to focus on the development of devices with high efficiency as well as a long lifetime.

1.4. Ongoing Challenges

1.4.1. longer lifetime at higher brightness.

It is always crucial to obtain extended operational lifetime at higher brightness. The brightness of OLED devices increases with the applied current. With the increase in applied current, the lifetime of device decreasing. That means the lifetime of OLEDs is inversely proportional to the brightness, which is calculated as

where the L 0 is the initial brightness, t 1/2 is the time required to decay 50% of initial brightness (LT 50 ), and n is the acceleration factor.

Achieving high brightness at low voltage can prevent the device degradation associated with higher current. Lee et al. reported an OLED exhibits high brightness at low voltage, 10 000 cd m −2 at 4 V. They achieved the same by applying high mobility electron transporting layer (ETL) material and mixed hosts. [ 63 ] Recently, Kido's group reported a sterically bulky hole transporting material to attain a higher lifetime at high luminance. At 1000 cd m −2 , the TADF device showed and EQE > 20%, 50 lm W −1 PE and LT 50 of 10 000 h . [ 64 ] It is necessary to develop OLEDs with a longer lifetime at higher brightness and lower voltage to save energy and money. Table   1 is showing the Universal Display Corporation (UDC) OLED material performance, which is adopted from Prof. S. R. Forrest presentation at American Physical Society (APS) March meeting 2018. [ 65 ]

Universal Display Corporation (UDC) OLED material performance [ 65 ]

1.4.2. Longer Lifetime at Elevated Temperatures

Both external and internal temperature influence the OLED lifetime and performance. Displays and mobile phone screens are exposed to different temperature ranges. Therefore, temperature management is crucial for the performance of OLEDs at high brightness. The device operating temperature influences both lifetime and efficiency. [ 66 ] High heat generation may lead to the device meltdown or fracture. Thermal degradation is one of the major barriers to the production of longer lifetime OLED. Nowadays many techniques are employing to dissipate the heat energy associated with OLEDs. [ 67 , 68 , 69 , 70 ] We discussed the high heat caused device meltdown or substrate fracture and Joule heat in Sections  2.1.2 and  2.2.9 .

1.4.3. Longer Lifetime at Harsh Environments

Harsh environments associated with device fabrication and operating strong affecting the long lifetime operation. In 2016, Kaur et al. published a review paper discussing the influence of environmental factors on organic light emitting diode (OLED) displays. [ 71 ] Presence of moisture, oxygen, and impurities strongly affecting the OLED lifetime, we are discussing the same issues in the Sections  2.2.1 , 2.2.2 , and  2.2.10 . In 2016, Fujimoto et al. reported the influence of vacuum chamber impurities on the lifetime of OLEDs. [ 72 ] The vacuum chamber impurities significantly influenced the lifetime of the experiment and reproducibility of the results. In order to achieve higher efficiency and longer lifetime, need to eliminate moisture, oxygen, and impurities mainly from the fabrication environment.

2. Degradation Phenomena and Failure Mechanisms

2.1. degradation phenomena, 2.1.1. dark spot.

“Dark spots” or “black spots” are the nonemissive regions, which formed in the active area of OLEDs under the operating or storage conditions according to the studies by Scholz et al., [ 33 ] Turak et al., [ 73 ] Ke et al., [ 74 ] and Zardareh et al. [ 75 ] . Liew et al. and Fujihira et al. found the dark spots to form at the interfaces between the organic and conducting layers. [ 76 , 77 ] Güney et al. reported the growth of such dark spots under electrical stress. [ 78 ] According to McElvain et al. and Aziz et al. efficiency, brightness, and lifetime of OLED decrease with the increase of dark spots. [ 79 , 80 ]

The main external factor influencing the dark spot formation is the presence of humidity, dust particles, pinholes, spikes on ITO, short circuit, etc. In 2018, Azrain et al. reported a review article, which reviewed the mechanisms responsible for the formation of dark spots in OLEDs. [ 81 ] The report summarized that the main reasons for dark spot formation are pinholes and high electrical current density. Weijer et al. reported that the penetration of water through pinholes in the cathode causes local oxidation of the cathode and which leads to the formation of nonemissive area in the device. [ 82 ] Ohzu et al. calculated the dark spot formation in flexible OLEDs with the amount of water penetrated into the device. [ 83 ] Ding et al. described the nature of catastrophic OLED lighting panel failure and the dark spot formation. [ 84 ]

In the 1990s, the process of encapsulation was proposed as a solution for dark spot formation, thereby increasing the lifetime of the device. The different encapsulation techniques are discussed in Section 2.2.2. According to Ke et al. the roughening of the polymer/electrode interface because of metal migration increases the local current thereby leading to dark spot formation. It was further suggested to smoothen the polymer/electrode interface in order to prevent the dark spot formation. [ 74 ] Chan et al. and Phatak et al. reported that elevated temperature deposition of cathode layer reduced the black dot formation because of the increase in the cathode/organic layer adhesion. [ 85 , 86 ] Liu et al. observed that the base etching process, without changing the ITO thickness and sheet resistance smoothens the anode (ITO) surface, which hence enhances the OLED lifetime by preventing dark spot formation. [ 76 ] Liew et al. removed the cathode by scotch tape and the new cathode deposited immediately in a vacuum chamber, in order to prevent cathode delamination and dark spots. [ 87 ] The invention of encapsulation technique was an important milestone in OLED technology, which effectively protects the device from oxygen and moisture. Minimization of dark spots will improve the device performance with effective brightness, efficiency and lifetime. The mechanism of dark spot formation shows in Figure   11 .

Electrolysis of water molecules and formation of dark spots inside an OLED device.

2.1.2. High Heat Caused Device Melt Down or Substrate Fracture

Device meltdown or substrate fracture occurring mainly due to the high heat created inside the device. According to Chen et al. one of the major challenges facing by flexible OLEDs is fracture of particularly thin brittle conducting transparent oxide films, it may depend on the deposition temperature also. [ 88 ] Production of Joule heat originates thermal expansion, interlayer diffusion of materials and crystallization or melting of organic materials, which reduce the lifetime of OLEDs. Regarding generation of Joule heat, after effects and reduction methods of Joule heating described in Section  2.2.8 . Yan et al. introduced a polycarbonate high‐temperature capable substrate for organic electronics. [ 89 ] Many studies are going on to defend the high heat caused problems in OLEDs.

2.1.3. Downgrading Luminance and/or Color

Generally, the lifetime of an OLED is defined as the time required for the luminance to become 50% of the initial luminance. [ 12 ] According to Van Slyke et al., luminance decay is coulombic, which is directly proportional to the current density applied. [ 90 ] The luminance efficiency may be reduced by elevated temperature, accumulation of OLED degradation products, accumulation of immobile charge carriers, etc. Parker et al. reported that the luminance falloff increases with increase in temperature. [ 91 ] According to him, higher temperature will change the morphology of polymeric materials and reducing the luminance efficacy. So et al. reported that accumulation of OLED degradation products affects luminance efficiency and operating voltage. [ 92 ] He observed that formation of traps producing nonradiative recombination centers and reduce the luminance efficiency. Kondakov et al. reported, the accumulation of immobile charge carriers also affects the luminance efficiency. [ 93 ] Wang et al. conducted the constant‐brightness driving mode experiment to found the decay behavior, he divided the decay stages as i) linear increase, ii) exponential increase, and iii) vertical increase of current density over time. [ 94 ]

According to Aziz et al. luminance degradation occurs mainly through three degradation modes i) dark‐spot degradation, ii) catastrophic failure, and iii) intrinsic degradation. [ 95 ] Ishii et al. described that the luminance decay happening via two steps; Initially the exponential luminance decay occurring due to chemical degradation and in the next step, rapid decline because of internal electric field. [ 96 ] Young et al. reported that NPB + (Radical ion of hole transporting material N , N ′‐Bis(naphthalen‐1‐yl)‐ N , N ′‐bis(phenyl)benzidine, NPB) is an active quencher of blue luminescence via Förster energy transfer. [ 97 ] OLEDs with tandem device structure are showing better efficiency and brightness. [ 98 ] Lee et al. increased the luminance efficiency by introducing hole‐transporting interlayers between HTL and emissive layer (EML). [ 99 ] Kitamura et al. improved the luminance efficiency by introducing SiO 2 /SiNx photonic crystals on ITO substrate. [ 100 ] Li et al. introduced CNT templates as external electron source, which compensate the electron deficiency in the device and enhanced the luminance efficiency. [ 101 ] Microlens array is an excellent method to improve luminance efficiency of OLED device. [ 102 ]

2.2. Failure Mechanisms and Plausible Solutions

2.2.1. moisture attack.

The presence of moisture in OLEDs will drastically reduce the device lifetime mainly via the formation and/or growth of dark spot formation, the decrease of electroluminescence (EL) intensity, the change in the electronic structure of organic layers, and the corrosion of cathode. [ 103 , 104 ] The penetration of moisture through materials can be conducted by the damp heat (DH) test as revealed in the “61215 Test” defined by the International Electrotechnical Commission (IEC). [ 105 ] Peike et al. described that grid corrosion or reduced conductivity between the emitter and grid is the most likely cause of DH‐induced degradation. [ 106 ] Laronde et al. employed the same DH testing to investigate the degradation of photovoltaic modules subjected to corrosion. [ 107 ] According to Burrows et al., water and oxygen are the main factors influencing premature device failure. [ 108 ] Liao et al. examined the effect of moisture on the electroluminescence intensity of OLEDs. [ 103 ] He found that both the HTL and ETL were negatively affected by moisture, leading to dark spot formation and operational instability and hence device degradation. The same group reported the tris‐(8‐hydroxyquinoline) aluminum (Alq 3 ) layer to undergo electronic structure change under the exposure of moisture. [ 109 ]

Papadimitrakopoulos et al. explained the chemistry behind Alq 3 degradation ( Figure   12 ) in the presence of moisture and oxygen. [ 110 ] Aziz et al. reported the electrolysis reaction (Equation ( 1 )) to occur in the Mg/Ag cathode due to the moisture adsorbed on the electrode surface. [ 111 ] Lim et al. examined the effect of moisture on the dark spot formation, and they reported that the presence of pinholes enhanced the moisture attack by providing the undesirable penetration pathway. [ 104 ] The moisture and oxygen penetration through pinholes is demonstrated in Figure   13 . Nevertheless, the most dependable effective approaches to reduce moisture attack is the employment of encapsulation, introduction of moisture seal, usage of desiccants, etc. Different types of encapsulation techniques are discussed in Section 2.2.2. Chun et al. reported a moisture seal containing alternating organic and inorganic layers, such as of epoxy and SiNH, to prevent moisture attack. [ 112 ] Torres et al. studied different type desiccants for avoiding moisture attack. [ 113 ] As observed, effective prevention of infiltration of moisture into the OLED device will increase both the device performance and lifetime

The reaction of Alq 3 with moisture and oxygen [ 110 ]

Pinholes in metal layer causing device failure.

2.2.2. Oxidation

From the earlier days of organic electroluminescent device invention, researchers were trying to resolve the problems like oxidation and corrosion. [ 114 ] Oxidation in OLED devices occurs mainly due to electrochemical reactions, moisture attack, and creates dark, nonemissive spots in device and limits the device lifetime. [ 79 ] According to Juharia et al., carrier mobility of OLED materials could be destroyed by oxidation. [ 115 ] According to Burrows et al., variations in the degree of wetting of organic surface by electrodes and defects formed during fabrication promotes oxidation. [ 116 ] Do et al. reported that chemical oxidation of organic layers leads to evolution of heat and gases, which enhances device degradation by detachment of the electrode thin layer from organic/polymer layer. [ 117 ] According to Schaer et al. the thermal diffusion of oxygen leads to the oxidation of both electrodes and organic layers. [ 118 ] In order to prevent oxidation, Shen et al. introduced a metallic capping layer after the electrode. [ 119 ] Nevertheless, avoidance of moisture will reduce the oxidation process in OLEDs. The encapsulation of OLED devices is a proficient method to prevent oxidation .

Various encapsulation techniques have been developed to reduce the influence of impurities on the overall performance of OLEDs. Some important encapsulation techniques adopted in OLED industry include Al 2 O 3 /ZrO 2 nanolamination by Meyer et al. [ 120 ] and Seo et al., [ 121 ] silica nanoparticle incorporated organic/inorganic nanocomposites by Jin et al., [ 122 ] graphene oxide nanocomposites by Jeon et al., [ 123 ] atomic layer deposition (ALD) of AlOx films by Park et al., [ 124 ] chemical vapor deposition of polymer films by Yamashita et al., [ 125 ] and siloxane or siloxane derivatives by Biebuyck et a1., [ 126 ] etc.

2.2.3. Corrosion

The chemical corrosion is mainly happening due to the exposure of materials/electrodes to moisture, the mechanism described in Section  2.2.1 . The corrosion processes in OLED affect electrodes [ 127 ] or organic/polymer layers, [ 128 ] which affects the OLED lifetime. According to Aziz et al. galvanic corrosion leads to bubble formation in OLED devices. [ 128 ] Lin et al. reported that under electrical stress conditions, the presence of bases enhances both corrosion of polymers and bubble formation. [ 129 ] According to Sierros et al., in flexible optoelectronics the ITO undergoes stress corrosion cracking, the rate of which would be higher in the presence of acids. [ 130 ] Effective encapsulation methods can prevent the corrosion rate. According to Paetzold et al. in order to slow down the corrosion rate of different layers in OLED, needs to apply substrates with low permeation rate for air and water. [ 131 ] Calcium corrosion test is extensively used for the determination of moisture permeation extent. [ 132 ] Arai et al. introduced a gold doped Mg cathode, which prevents the nonemissive area formation due to corrosion. [ 133 ]

2.2.4. Electron Induced Migrations

In OLEDs, due to the continuous flow, electrons collide each other, which leads to molecular migration, resulting in the loss of device efficiency and a decrease in lifetime. [ 134 , 135 ] According to Shen et al. mobile ions induced voltage changes, leading to device degradation. [ 136 ] According to Probst et al., if the metal/organic layer interaction is feeble in the interface, it will enhance the metal migration. [ 137 ] Lee et al. reported that the presence of Indium in the organic layer increased the driving voltage and reduced luminance efficacy. [ 138 ] Lin et al. reported that the small organic molecules like TPBi can start migration even at a low voltage bias. [ 139 ] Png et al. observed the PEDOT + accumulation at the interface between poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and hole injection layer (HIL) even low electric field 1 V cm −1 . [ 140 ] Ke et al. and Chua et al. introduced a parylene layer at the electrode–organic interface, in order to prevent atomic migration, the device showed better luminance efficacy and lifetime. [ 141 ] The molecular and atomic migration failure shown in Figures   14 and  15 .

Molecular migration causing OLED device failure.

Atomic migration causing OLED device failure.

2.2.5. Photochemical Degradation

Organic and polymeric materials are susceptible to various photo‐ and electrochemical reactions in the device. [ 142 , 143 , 144 ] Scholz et al. propose that the photochemical reactions mainly occur at ITO/organic interface in three key steps; light absorption by the polymer layer, the exciton movement toward ITO, and the formation and release of oxygen from the ITO surface. [ 142 ] He also studied the effect of light‐induced dimerization of ETL materials, namely BPhen, BCP, and Alq 3 . [ 142 ] In 1996, Rothberg et al. reported the significant role of excitons in OLED device degradation. [ 145 ] In 2017, Bell et al. describe the exciton induced intramolecular cyclization of arylamine HTL moieties. [ 146 ] Wang et al. reported that the interaction between HTM positive polarons and singlet excitons increases the hole transport material/electron transport material (HTM/ETM) interface degradation. [ 147 ] The UV irradiation enhances the device degradation by the formation of singlet excitons (Section  2.2.3 ). According to Meerheim et al., incorporation of a hole blocking layer plays a significant role in controlling exciton distribution. [ 148 ] Laser‐desorption/ionization time‐of‐flight mass spectrometry (LDI‐TOF‐MS) is a highly efficient technique to understand the mechanism of photochemical reaction in the device. [ 149 ]

Photochemical degradations are happening due to the irradiation of UV light also. According to Patel et al., continuous exposure of UV light on OLED devices would lead to the formation of dim or dark spots. [ 150 ] Kondakov et al. found, under UV irradiation, CBP ( N , N ′‐dicarbazolyl‐4,4′‐biphenyl) underwent an exocyclic C—N bond cleavage. [ 151 ] In 2017, Yu et al. reported that the PL quantum yield of CBP decreases 80% after irradiation of UV for 72 h. They attributed that to the morphological changes associated with UV exposure. [ 152 ] Quirino et al. reported that the decrease in PL intensity was directly proportional to the UV irradiation time, based on an Eu‐ β ‐diketonate complex (Eu(btfa)3bipy) containing device. [ 153 ] According to Seifert et al., UV irradiation and electrical aging showed the same undesirable impact on device lifetime and the singlet excitons formed by UV exposure played an important role in device degradation. [ 154 ] By reducing the undesirable UV irradiation, one can control the formation of singlet excitons and the dissociation of weak bonds.

Besides, photochemical degradation, exciton induced degradation mechanism also play a significant role in the device degradation. In 2018, Kim et al. explained the charge neutral generation of polaron pairs by electron transfer from dopant to host excitons. [ 155 ] Aziz et al. reported exciton diffusion the generation of quenchers in HTL and diffusion of excitons from HTL to EML play a crucial role in the device degradation. [ 156 ] Hany Aziz et al. is one of the pioneers in the field of OLED degradation studies. Recently, his team investigated the exciton‐induced degradation of HTLs and the influence on the efficiency and lifetime of phosphorescent OLEDs. [ 158 ] Exciton stress on HTLs may reduce the EQE and lifetime of PhOLEDs. Besides, they examined the influence of excitons and electrons in PEDOT:PSS (HIL) degradation. [ 157 ] They conclude that the incorporation of HTL with PEDOT:PSS HIL can reduce the electron leakage to HIL and improves device stability. His team compared the morphological properties of spin‐coated and blade coated organic semiconductor films. [ 158 ] The device with the blade coated HTLs and EMLs were less aggregated and showed better efficiency and lifetime. They studied the exciton induced molecular aggregation in organic small‐molecule electroluminescent materials, which showed solution‐processed films are more susceptible to exciton induced aggregation compared to vacuum deposited thin film. [ 159 ] They reported the influence of the deposition rate on the thin film morphology and PhOLED EL performance. [ 160 ] Lower deposition rate can reduce the exciton induced degradation and thereby improving the morphological order and stability.

2.2.6. Electrochemical Degradation

Irreversible chemical/electrochemical reactions show some profound impact on the degradation of OLEDs. [ 149 ] According to Xia et al., the design of electrochemically stable EML is critical for long lifetime OLEDs because the EML contains both electron and hole charge carriers. [ 161 ] Aziz et al. reported the electrochemical reactions between the electrodes lead to both microstructural change and corrosion. In addition, the presence of short circuit point accelerates the electrochemical degradation process. [ 129 ] The same group also reported that the anodic oxidation of tris(8‐hydroxyquinoline)aluminum (Alq 3 ) and instability of Alq 3 cations play an important role in device degradation. [ 162 ] According to Ke et al., [ 74 ] Franky So et al., [ 92 ] and Gardonio et al. [ 163 ] due to the presence of moisture at metal/organic interface the electrochemical processes in OLEDs lead to the formation of “bubbles” probably filled with H 2 gas and hence form dark spot. Savvateev et al. reported that the local current densities formed around the bubble's perimeter would cause to the formation of local luminescence and heating, preventing uniform current injection to the entire device. [ 164 ] The merging of bubbles also reduces the available injection area. Avoidance of moisture by different encapsulation techniques (Section  2.2.2 ) is a most effective approach to prevent the electrochemical degradations. Rudmann et al. suggested that more electrochemically stable metals like Ag be used as the cathode to increase device lifetime. [ 165 ]

2.2.7. Electric Breakdown

The term electric breakdown is mainly associated with the dielectric breakdown of OLED materials. Very few reports are available on the dielectric properties of OLED materials. According to Wang et al. the device efficiency and brightness can improve with an electron injection layer with high dielectric strength. [ 166 ] Ohta et al. examined the effect of dielectric strength of materials in active matrix OLEDs. [ 167 ] In 2018, Jou et al. reported that the dielectric breakdown of organic materials is directly related to the roughness of the substrate using. [ 168 ] Even though the driving voltage of OLED devices are 3–5 V, for higher brightness the device requires a higher voltage. After reaching 10 V or above, luminance starts to decrease. This failure may have attributed to the dielectric breakdown of organic materials.

2.2.8. Thermomechanical Failures

In OLED devices, each layer of different materials has its own thermal expansion coefficient, which generates an intrinsic stress. [ 132 , 169 ] The stress formed in the layers is released through delamination. [ 132 ] According to Lee et al., OLED devices are exposed to mechanical loading like bend, torsion, and folds which leads to delamination. [ 132 ] The thermomechanical properties of materials will strongly influence the flexible OLEDs lifetime and performance. [ 170 ] Brand et al. suggested that it is necessary to match the coefficient of thermal expansion, elastic moduli and adhesion strength of the secondary material in the devices. [ 171 ] In 2017, Hasegawa et al. introduced a super heat resistant polymer poly(benzoxazoleimide)s (PBOI) substrate with a very low coefficient of thermal expansion and adequate ductility for flexible OLED devices. [ 172 ] Ryu et al. fabricated glass fiber reinforced transparent composite films with good thermal conductivity, low coefficient of thermal expansion and high mechanical properties. [ 173 ] Oh et al. reported that due to the low coefficient of thermal expansion (13 ppm °C −1 ) polyethylenenaphthalate (PEN) showed better performance in flexible OLEDs. [ 174 ] According to Behrendt et al. the Al 2 O 3 /TiO 2 nanolaminates grown by atomic layer deposition (ALD) generated a low intrinsic tensile strength. In order to achieve long lifetime OLEDs, it is essential to select the materials with excellent thermomechanical properties.

2.2.9. Thermal Breakdown/Degradation

Thermal instability of the materials used in OLEDs might cause irreversible device degradation. [ 93 ] Nenna et al. reported that electrical failure mechanism in archetypal OLEDs is mainly related to the lower glass transition temperature of the material, which also restricted the device operating voltage. [ 175 ] Using X‐ray specular reflectivity, Fenter et al. reported that large thermal expansion behavior of ( N , N ′‐diphenyl‐ N , N ′‐bis(3‐methylphenyl)‐1,1′‐biphenyl)‐4,4′‐diamine (TPD) to be attributable to its low glass transition temperature, suggesting a strain‐driven failure mechanism. [ 176 ] Kwak et al. reported that the real‐time temperature of the device with the structure of ITO/ α ‐NPD/Alq 3 /TPBi/LiF/Al is much lower than that of the device without the ETL and the electron injection layer (EIL). The insertion of the suitable ETL and EIL lowers the junction potential barrier and thus advances the efficiency of the electron injection. [ 177 ] By using transmission matrix model, Bergemann et al. found that the internal air gap between the package lid and substrate provides a high impedance to heat transfer, and eliminating the gap facilitates heat transfer and allows the device to operate at near ambient temperature even at high brightness. [ 178 ] According to Zhou et al. electricfield induced decomposition of ITO and electro‐migration of indium are responsible for the catastrophic failure or thermal breakdown of OLEDs. [ 179 ] Morphological changes in the NPB ( N , N ‐di(naphthalene‐1‐yl)‐ N , N ‐diphthalbenzidine) and Alq 3 layers at elevated temperature leading to deterioration in the current–voltage characteristics were reported by Xu et al. [ 180 ] The materials degradation above T g is shown in Figure   16 . Above T g , the material is showing degradation, which depicted by black portions in Figure  16 .

General schematic illustration of the influence of glass transition temperature ( T g ) in the thermal degradation of OLED materials.

Joule heat causes thermal expansion, inter‐layer diffusion and crystallization or melting of organic materials, which limits the lifespan of devices. [ 69 , 181 , 182 , 183 ] Gärditz et al. reported that the rise in device temperature increases local current flow, leading to brightness inhomogeneity in OLEDs. [ 181 ] Tyagi et al. attributed the Joule heat to the interfacial resistance existing between the organic/organic and metal/organic interfaces. [ 184 ] Gong et al. used scanning tunneling microscopy (STM) and photoluminescence (PL) to investigate the phase separation, interfacial structural change, and aggregation of BT (1,4‐bis(benzothiazole‐vinyl) benzene) and TPBi (2,2′,2″‐(1,3,5‐Benzinetriyl)‐tris(1‐phenyl‐1‐ H ‐benzimidazole)) caused by joule heat. [ 183 ] Fujihira et al. reported that high localized current in devices would increase the joule heat. [ 77 ] Liao et al. observed bubbles to form within OLED devices that may be due to the release of gases caused by high joule heat generated at localized electrical shorts. [ 185 ]

Several methods had been recommended to reduce the effect of joule heat and hence improve the device performance. Prevention methods for Joule heat are the utilization of substrates or anode with high thermal conductivity, employing graded mixed layer of ETL and HTL, reduction of driving voltage, etc. According to Chung et al. , substrates with high thermal conductivity could enhance the device performance by conducting the joule heat into the substrate. [ 186 ] Kim et al. reported the use of a highly conductive anode, G‐PEDOT (an aqueous dispersion of PEDOT:PSS with glycerol), to prevent the negative impact of joule heat. [ 187 ] Tyagi et al. reported the insertion of F 4 ‐TCNQ (2,3,5,6‐tetrafluoro‐7,7′,8,8′‐tetra cyano quino dimethane) between the anode/hole transport layers to reduce the interface resistance and joule heat. [ 188 ] Chwang et al. introduced the graded mixed layer (uniformly mixed ETL and HTL) to diminish the joule heat by reducing the electricfield across the layers. [ 189 , 190 ] Matsushima et al. reported that the reduction of driving voltage can enhance the lifetime of OLEDs by preventing the joule heat. [ 189 ] In 2014, Park et al. introduced films of heat sink in order to dissipate the heat generated by the organic layers in OLEDs. [ 191 ] Joule heat generation is demonstrated in Figure   17 .

High injection barrier generating high joule heat.

Recently, Reineke et al. reported an experimental proof of Joule heating‐induced switched‐back regions in OLED devices. [ 192 ] Zojer et al. explained how the thermal conductivities of organic layers determine the temperature inside the device. [ 193 ] Schwamb et al. investigated the passive cooling of lagre area OLEDs, mainly focused on convective cooling. [ 194 ] Zakhidov et al. introduced hydroflouroethers for heat dissipation in OLEDs. [ 195 ] Fischer et al. demonstrated electric bistability produced by self‐heating onto the thermally activated conductivity. [ 196 ] In another report, Fischer et al. described temperature‐activated transport in organic semiconductors and the catastrophic failure associated with that. [ 197 ]

2.2.10. Presence of Impurities

The efficiency and lifetime of OLEDs are significantly influenced by the presence of impurities and surface roughness. Vardeny et al. and Zou et al. reported the effect of material purity on device performance. [ 198 , 199 ] According to their finding, high material purity enhances the luminous efficacy and carrier injection conditions by reducing both ionic diffusion and internal electric field formation. Yamawaki et al. reported the halogenated impurities in OLEDs to trigger radical formation, which can react with the organic materials either by excitation or reduction and cause damage to ETL, EIL, and EML. [ 200 ] Yamawaki et al. and Becker et al. suggested that the presence of halogenated impurities could be estimated using high performance liquid chromatography–mass spectroscopy (HPLC‐MS) and combustion ion chromatography (CIC). [ 200 , 201 ] Presence of dust particles increase the formation of pinholes in the cathode, which leads to infiltration of moisture and air into the device. [ 81 ] The dust particle penetration is shown is Figure   18 .

External dust particles causing formation of dark spots.

Adachi et al reported some studies related to the influence of impurities in device performance. According to him, if the vacuum chamber cleaned by plasma cleaning and the partial pressure of the water reduces by a cryotrap, device lifetime will be stable. [ 202 ] In another study, his team reported that impurities floating inside the vacuum chamber seriously influencing the lifetime value and reproducibility of the device. [ 72 ] They reported that the device fabricated in a new chamber can attain a lifetime approximately twice than that of a pre‐existing chamber. [ 203 ]

Several methods are attributed to identify and reduce the impurity content associated with OLED fabrication . Tominetti et al. reported an efficient mass spectrometric technique for detecting internal impurities in OLED displays. [ 204 ] Tsugita et al. fabricated high purity organic films via gas flow deposition method (GFD). [ 205 ] According to Jain et al., [ 206 ] Salati et al., [ 207 ] and Pardo et al. [ 208 ] recrystallization and vacuum deposition are efficient methods to purify both organometallic and organic compounds used in OLEDs.

Various surface treatments were introduced to tune the surface roughness of ITO. [ 209 ] According to Li et al. and Jung et al. the surface roughness can be decreased by etching (acid/alkali), annealing, mechanical polishing, etc. [ 209 , 210 ] Choi et al. reported boron doping affects the surface properties of ITO but increases the OLED performance because it influences surface energy, transmittance, sheet resistance, work function and mobility. [ 211 ] Zhou et al. examined the sand blasting technique for creating surface roughness on OLED substrate, in order to reduce the wave nature of internally generated photons, which could increase the external quantum efficiency from 9% to 11.6%. [ 212 ] Hatton et al. reported that Silane modified ITO shows better power efficiency as compared to normal ITO surface due to tune the anode work function to the HOMO of HTL. [ 213 ] Kim et al. investigated the peak‐to‐valley roughness ( R pv ) of ITO surface directly proportional to the leakage current of OLED. [ 214 ] Park et al. [ 215 ] and Lu et al. [ 215 ] observed plasma treatment reduces both surface roughness and contaminations. Helander et al. reported the chlorination of transparent ITO improved the electrode work function (>6.1 eV). [ 216 ]

3. Materials for Long Lifetime OLED

The performance and lifetime of OLEDs primarily depend upon the characteristics of the hole transporting materials (HTM), electron transporting materials (ETM), emissive materials (EM), and host materials. [ 1 , 3 , 7 ] Fluorescence, phosphorescence, and thermally activated delayed fluorescence (TADF) are the main mechanisms associated with the emission phenomenon in OLEDs. [ 7 ] Fluorescent materials emit light only due to the consumption of singlet excitons, while phosphorescent materials can utilize both singlet and triplet excitons. [ 217 ] TADF materials utilize both singlet and triplet excitons by converting triplet excitons to singlet excitons through reverse intersystem crossing. [ 218 ] Aggregation‐induced emission (AIE) is an interesting phenomenon where molecules which are otherwise nonemissive in the solution state are found to emit strongly in the aggregate form or solid state. [ 219 ] A high energy transfers between the host and guest results in yielding a longer lifetime for the OLED device. [ 220 ] Numerous research studies and key publications proposed that through reduction of host singlet‐triplet splitting, precise adjustment of host–guest energy gap and doping concentration, and enhancement of host emission‐guest absorption overlapping a long operational lifetime TADF and TADF‐sensitized OLED device can be fabricating, even higher than PhOLEDs. [ 221 ]

Many research groups are working on the research and development of blue fluorescent materials. Naphthalimide, pyrene, phosphine oxide, oxidazoles, benzo‐fluoranthene, and phenanthroimidazole materials are extensively using as blue emitters. [ 222 ] In 2018, Jung et al. reported a pyre based blue emitter, which showed the LT95 lifetime of 471 h and extrapolated LT 50 lifetime of 30000 h. [ 223 ] A wide range of phosphorescent emitters have been reported in the past few years, most of them being Iridium based, as shown in Figure   19 . Host materials play a major role in host–guest energy transfer and exciton confinement. [ 224 ] Figure   20 . shows the range of host materials. Figure   21 depicts the range of ETL materials commonly using in OLEDs.

Molecular structures of heavy metal complexes based phosphorescent emitters.

Molecular structures of host materials using in both fluorescent and phosphorescent OLEDs.

Molecular structures of ETL materials using in OLEDs.

Organic small molecules, polymers and dendrimers are widely employed as TADF materials. [ 225 ] Recently, Lee et al. reported an in‐depth review of TADF materials which find use in the fabrication of long lifetime OLEDs. [ 226 ] They proposed an efficient device lifetime expansion strategy for the TADF OLEDs in terms of the rational chemical bond stabilizing molecule design and fast reversed intersystem crossing rate model. According to Adachi et al. n‐type hosts are appropriate for stable electroluminescent devices because of their capability to balance charge fluxes and suppress high‐energy exciton formation. [ 227 ] Liao et al., pure hydrocarbon hosts which possess high glass transition temperature making them suitable for being used as OLED materials. [ 228 ] Green TADF OLEDs with SF3‐TRZ (2‐(9,9′‐spirobi[fluoren]‐3‐yl)‐4,6‐diphenyl‐1,3,5‐triazine (SF3‐TRZ)) as the host were seen to achieve a maximum EQE of 20.6% and T 50 of 10934 h for an initial brightness of 1000 cd m −2 . More importantly, SF3‐TRZ can also function as a host for sky‐blue TADF OLEDs because of its high T 1 . A sky‐blue TADF OLED with a high EQE of 8.8% and a lifetime of 454 h for an initial brightness of 1000 cd m −2 was produced. [ 229 ] Some frequently using TADF emitters and host materials are shown in Figures   22 and  23 .

Molecular structures of TADF emitters using in OLEDs.

Molecular structures of TADF host materials using in OLEDs.

TADF and hyperfluorescent based OLED devices demonstrating better quantum efficiency compared to the phosphorescent and fluorescent counterparts. [ 230 , 231 , 232 , 233 ] The operational lifetime of TADF OLEDs is much lower compared to those of phosphorescent OLEDs. [ 234 ] Many research groups are presently investigating avenues to improve device performance through effective utilization and crucial device engineering to improve the potential ability of TADF materials. Sasabe et al. proposed a lifetime extending strategy of thermally activated delayed fluorescent OLEDs from a molecular design approach that significantly improved device efficiency and lifetime. [ 235 ] A novel sterically bulky hexaphenylbenzene based HTMs (TATT, 4DBTHPB, 4DBFHPB), 4CzIPN based device showed an EQE of 19.4% and LT 50 of 24 000 h.

According to Lee et al., molecules with chemical bond stabilizing design and high reverse intersystem crossing effect can improve the lifetime of TADF OLEDs. [ 234 ] Yu et al. improved the efficiency and lifetime of TADF OLED by the utilization of novel emitters (BCzTrzDBF, TCzTrzDBF, IDCzTrzDBF) via replacing common phenyl linker with dibenzofuran linker. [ 236 ] Recently, Jeon et al. published a very detailed progress report about TADF OLED lifetime improvement for the last seven years. [ 230 ] Jang et al. enhanced the TADF device by the utilization of electrostatic potential dispersing pyrimidine‐5‐carbonitrile acceptor. [ 237 ] Kido et al. improved the TADF OLED lifetime by developing novel hexaphenylbenzene‐based sterically bulky hole transporters. [ 238 ]

According to Yang et al., utilization of both charge‐transfer exciton (CT) and local exciton (LE) is an efficient pathway for obtaining high efficiency devices. [ 239 ] They reported hybridized local and charge‐transfer excited state (HLCT) mechanism can utilize for the fabrication of highly efficient OLEDs. [ 240 ] Ma et al. reported a “hot exciton” reverse intersystem crossing (RISC) process from T2 to S1, which evade the accumulation of long‐lived triplet excitons and the nondoped device showed a maximum EQE of 10.5%. [ 241 ] In another work, they reported a high radiative exciton ratio of 48% (exceeds limit of 25% in conventional fluorescent OLEDs), which may due to the result of efficient RISC through the hot‐exciton process. [ 242 ]

Still, the OLED industry is facing challenges related to the stability of blue OLED. Various approaches are adopting to improve the efficiency and lifetime of blue OLEDs. Recently, Lin et al. published an informative review paper on current status, challenges, and future outlook of blue organic light emitting diodes. [ 43 ] According to them, blue phosphorescent and TADF OLEDs are satisfactory with the efficiency and emission color but not achieving a reliable lifetime. Even though the limited efficiency, blue fluorescent, and triplet‐triplet florescent (TTF) OLEDs are widely using for display applications.

Recently, Yang et al. examined the key factors affecting the lifetime of blue phosphorescent OLED using CN modified blue host materials (O‐39BCzCN, M‐39BCzCN, P‐39BCzCN). [ 243 ] He concluded that bond dissociation energy and Forster energy transfer rate are crucial to the lifetime of blue phosphorescent OLEDs. Park et al. reported a long lifetime red PhOLED by the utilization of benzocarbazole and diphenyltriazine based bipolar host materials (BCTrz1, BCTrz2). The device lifetime increased 30 times compared to the bipolar host material, CBP. [ 244 ] Kido's group improved the red PhOLED lifetime by the introduction of a novel dibenzofuran‐based n‐type exciplex host (4DBF46PM, 4DBF26PM, 4DBF46TRZ). [ 245 ] The device showed an LT 80 of 3300 h, which is six times longer than the previously reported lifetime for deep‐red OLED. [ 246 ] Yamazaki et al. introduced a novel host–guest system that improves the lifetime of a deep‐red phosphorescent OLED ≈5.4 compared to the conventional system. [ 247 ]

The effective utilization of suitable materials in the exciplex forming cohost system improving the lifetime of OLED devices. Recently, Lee et al. reported a p‐type host ((3,3′‐bis(5‐pheny‐ lindolo[3,2‐ a ]carbazol‐12(5 H )‐yl)‐1,1′‐biphenyl (IDCzBP)) for an exciplex host, which improved the device lifetime of green PhOLED by 11 times compared to the conventional p‐type host system. [ 248 ] Yamaguchi et al. reported a high temperature (85 °C) withstanding deep red PhOLED with exciplex forming host and guest material. [ 249 ] Kim et al. synthesized high triplet energy exciplex hosts triphenylsilyl blocking groups, which showed device lifetime of LT 80 of 1900 h at 100 cd m −2 and 21.6% EQE. [ 250 ] Recently Lee et al. published an informative review paper describing exciplex hosts for blue phosphorescent OLEDs. [ 251 ] Wong et al. reported an exciplex forming cohost with biscarbazole donor and a triazine‐based acceptor to improve the efficiency and lifetime of fluorescent and phosphorescent OLEDs. [ 252 ] Song et al. examined the performance RGBY OLEDs by employing an electroplex host system. The devices showed a longer lifetime compared to a single host, mixed host, and exciplex host devices. [ 253 ]

Charge transporting materials have the capability to accept the charge from the electrode and thereafter inject the carriers into the emissive zone. Molecular architecture, thermal stability, morphology, highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) values, ability to block the carriers of opposite charge and charge mobility of charge transporting materials are essential for efficient and long lifetime OLEDs. Hole transporting materials (HTM) possess low ionization potential and low electron affinity, whilst electron transporting materials (ETM) possess high ionization potential and high electron affinity. [ 1 , 254 ] The glass transition temperature ( T g ) of both HTM and ETM strongly influences the lifetime of OLEDs. [ 255 ] Triarylamine based hole transporting materials have low glass transition temperature compared to carbazole‐based materials. [ 256 ] Thermal breakdown of OLEDs has been discussed in Section 2.2.9. Silane moieties also function as hole transporting materials. Some other widely using hole transporting materials have been shown in Figure   24 . Oxidazole molecules, dendrimers, metal chelates, azole‐based materials, etc., are commonly using as electron transport materials. [ 254 ] Thermal evaporated devices showed a better lifetime than solution‐processed devices. Small molecules and polymeric materials are generally used as carrier transporting materials in OLEDs.

Molecular structures of HTL materials using in OLEDs.

4. Device Architecture for Long Lifetime OLED

Device degradation which is quantified by the lifetime of the devices is an efficiency loss process that happens in the OLEDs overtime during the electrical driving process, which originated from the inherently poor stability of organic photochromic materials inserted in the OLEDs. Numerous degradation processes of OLEDs have their origin in the improper management of device structures, which aggravate material degradation. Therefore, both material and device‐related degradation routes should be managed at the same time. Device architecture is considered to be an imperative factor that influences the device efficiency, performance and lifetime. [ 164 , 257 , 260 ] Various OLED devices designed approach for long‐lifetime is discussed below in detail.

4.1. Stepwise Devices

A high operating voltage and short operation lifetime are the major concerns of the current OLED devices in applications as compared to the other competitive display technologies. In order to break through these limitations, organic electroluminescent materials holding high conductivity and good electrical relation with electrodes are desired. Stepwise, double emissive layer, novel carrier/charge transport materials, and metal‐doped complex based structural approach are extensively employed to enhance the device lifetime. Recently, doping techniques, which have been widely utilized in forming inorganic semiconductors, have been pertained for shaping organic p‐ and n‐type materials. [ 258 , 259 ] In 2005, Lee et al. demonstrated high‐performance OLED devices based on the novel metal doped electron transporting and cathode materials result in the drop of 2.59 V in driving voltage, a 47.3% rise in current efficiency and a 3.14 times improvement in operation lifetime. [ 260 ] Because of the high thermal stability, better charge balance and good energy alignment of the electron transport layer with the emissive layer the lifetime of the devices significantly enhanced. In 2006, Tsai et al. fabricated a long lifetime and high efficiency white OLEDs with a mixed host in one of the double emission layers. The device exhibited the half lifetime of ≈100 h at initial 5000 cd m −2 , five times that better than that of the NPB counterpart. [ 261 ]

In 2011, Duan et al. presented a strategy to achieve a white organic light‐emitting diode (WOLED) with an extremely long lifetime through the wise control of the recombination zone. They achieved a record high lifetime of over 150 000 h at an initial brightness of 1000 cd m −2 , ≈40 times longer than the conventional bilayer WOLED. [ 262 ] Device composed of a blue emissive layer of 6,6′‐(1,2‐ethenediyl)bis‐ N ‐2‐naphthalenyl‐ N ‐phenyl‐2‐naphthalenamine (ENPN) doped in 9‐(1‐naphthyl)‐10‐(2‐naphthyl)‐anthracene ( α , β ‐ADN) was deposited on top of the mixed host blue emissive layer to prevent hole penetration inside the electron transport layer and to achieve better confinement of carrier recombination. Therefore, the employment of double emitting layers can stabilize the blue emission, which is the key feature to their device performance. In order to expand the operating lifetime, the uniform or graded mixed host structure has been initiated to eliminate the sharp heterojunction interface. [ 190 , 263 , 264 ] In 2012, Yang et al. demonstrate an excellent phosphorescent green organic light emitting diodes with double emitting layer and lithium fluoride (LiF) doped 2,2′,2″‐(1,3,5‐Benzinetriyl)‐tris(1‐phenyl‐1‐ H ‐benzimidazole) (TPBi) as an electron transporting layer. The device exhibits a current efficiency of 40.5 cd A −1 , power efficiency of 23.7 lm W −1 and operational lifetime of 5300 h, nearly 1.99, 2.95, and 35 times of those of the reference device, respectively. [ 265 ] The reason for the significant improvement is the effective carriers self‐balancing character of double emitting layer OLEDs, less numbers of heterojunction interface and superior electron transport property of TPBi:LiF. In the same year, Nakayama group developed a phosphorescent white OLED device which exhibits a low drive voltage of 3.5 V, a power efficiency of 64 lm W −1 , EQE of 20%, and a lifetime of 10 000 h at an initial luminance of 1000 cd m −2 with a light out‐coupling technique. [ 266 ] Figure   25 . Shows the device architectures of step‐wise energy transfer in OLEDs.

Device architectures of step‐wise energy transfer in OLEDs.

In 2013, Hao and co‐workers demonstrated a highly efficient and long working lifetime phosphorescent organic light emitting diode containing mixed host composed of wide‐bandgap based 4,7‐diphenyl‐1,10‐phenanthroline (Bphen) and 4,4ʹ‐bis(carbazol‐9‐yl)‐biphenyl (CBP). [ 267 ] The mixed host based devices exhibited an operation lifetime of 3530 h at a luminance of 500 cd m −2 intensifying by about 4.1 and 2.4 times relative to that of the counter single host and double emitting layer devices. High‐efficiency and longer working lifetime were credited to the absence of heterojunction and balanced charge carrier transport characteristics in the mixed host based OLED structures. In 2014, Cho et al. investigate a universal host (DCzDCN) material for both green thermally activated delayed fluorescence and phosphorescent OLEDs and achieved a high quantum efficiency of 25% and long lifetime around 200 h. [ 268 ] The suppression of exciton‐polaron annihilation has been recognized as an effective way to increase the operational lifetimes of OLEDs. The operational lifetime of an OLED utilizing a TADF emitter was noticeably improved by employing interlayers of an electron injection material and a ten‐fold rise in the lifetime of a blue PhOLED was attained by employing a graded dopant concentration profile in a broadened emitting layer. [ 269 , 270 ] More recently, Fukagawa et al. proposed that the stability and lifetime of the phosphorescent devices is well‐nigh proportional to the Förster resonance energy transfer rate from host to emitter when thermally activated delayed fluorescence molecules are used as hosts. The 2c host based devices expressed extremely high operational lifetime LT 50 of 20 000 h, Δ E ST 0.29 eV, and an EQE of 21.5%. [ 220 ] For prospect, further improvement of lifetime will be necessary while improving power and current efficiency. If a device of longer lifetime is realized, the foot of the application spreads out greatly.

4.2. Tandem Devices

A tandem organic light‐emitting diode (OLED) has multiple electroluminescence (EL) units attached electrically in series with unique intermediate connectors within the device. Researchers have examined this new OLED architecture with growing interest and have found that the current efficiency of a tandem OLED containing N EL units ( N > 1) should be N times that of a conventional OLED comprising only a single EL unit.

As compared with the conventional organic light emitting diodes, tandem OLED devices have received a broad attention owing to their superior current efficiency, power efficiency, luminescence, and operational lifetime. In tandem OLEDs, two or more individual electroluminescence (EL) units are electrically coupled in series with unique connecting stacks which function as a charge generation layer (CGL), where holes and electrons are generated and injected into the adjacent hole transporting and electron transporting layer, respectively. The key element in fabricating a high performance and long lifetime tandem device is the connecting materials stack, which plays a significant role in the electric field distribution, charge generation and charge injection mechanism. [ 3 , 271 , 272 ]

The interconnecting layers are usually formed via combining the p‐ and n‐type layers between the emission units. The p‐type layer comprises oxides such as indium tin oxide (ITO), vanadium pentoxide (V 2 O 5 ), molybdenum trioxide (MoO 3 ) in addition to p‐doped organic hole‐transporting materials. The n‐type layers composed of an electron transporting materials doped with metals such as Li, Cs and Mg or metal complexes. In 2009, Lee and co‐workers developed an efficient interconnecting layer (TR‐E314:Li/LGC101:NPB) via combining the n‐ and p‐type doped layers. The stacked unit devices expressed a maximum current efficiency, EQE and CIE coordinates of 14.7 cd A −1 , 10.6% and (0.12, 0.22), respectively, under steady current density of 10 mA cm −2 . The 20% decay lifetime ( t 80 ) of the stacked OLED (366 h) is 4.4 times that of the single‐unit OLEDs. [ 273 ] It is apparent that the operating lifetime and current efficiency of the OLEDs is enhanced in the presence of the interconnecting layers. For tandem WOLEDs, the main focus is on how to design an effective charge generation layer, since it plays a significant role in ensuring high efficiency and long lifetime. [ 164 ]

In 2013, Hao et al. demonstrate a group of electrophosphorescence organic light emitting devices with different number of heterojunctions. They proposed that the device performance illustrates a gradual enhancement in efficiency and lifetime with the lessening of heterojunction interfaces, which is credited to eliminating effectively the energy barrier between the heterojunction interfaces and the efficient and balanced carrier injection and transport via consuming an appropriate chemical doping of the wide band gap compounds as the charge/carrier transport layers. A single‐heterojunction OLED device showed a maximum PE of 32.1 lm W −1 , nearly 3.1 times that of the referential multi‐heterojunction PhOLED, and the operational lifetime of 1184 h, beyond 15 times of the reference device. [ 274 ] The two stack tandem device structure is shown in Figure   26 .

Schematic representation of a two stack tandem OLED device.

A tandem device also permits individual subdevices to be controlled independently, such capability may find numerous fascinating applications of these devices such as color tunable light. [ 275 , 276 , 277 ] To make these devices more efficient Tang et al. fabricated a three‐unit tandem device using a p–n junction as the connecting unit between the electroluminescence units and showed excellent light out coupling and carrier injection characteristics with better operational lifetime. [ 99 ] One of the key challenges in tandem OLEDs is to facilitate the effective charge generating interconnection between electroluminescence units. To successfully overcome the above problem, each electroluminescence unit is electrically connected in series via a carrier generation layer. In 2016, Fung et al. reviewed the current research advances in tandem OLEDs architecture with key focus on the material selection and interface studies in the intermediate connectors, as well as the optical design of the tandem OLEDs. [ 272 ] The interface of the intermediate connector is considered as very crucial in determining the driving voltage, current efficiency, power efficiency, and operational lifetime of tandem OLEDs. [ 271 , 278 ] In 2017, Song et al. reviewed the recent result of the degradation mechanism, operational stability and lifetime improvement strategies for blue PhOLEDs via classifying them into device and material based approaches. [ 260 ]

Triplet‐triplet annihilation (TTA) and triplet‐polaron annihilation (TPA) are the foremost degradation mechanisms and these arises due to the long excited state lifetime of the triplet excitons. Both the TTA and TPA processes are required in the operation of OLEDs, but they should be minimized to achieve PhOLEDs with long lifetimes. More recently, Shi et al. fabricated a high‐performance hybrid tandem WOLEDs with a thin layer of Ca incorporated in between Liq and HAT‐CN to act as an intermediate connector and achieved the maximum forward‐viewing current efficiency, power efficiency and EQE of 106.3 cd A −1 , 51.4 lm W −1 and 39.6%, respectively. [ 279 ] In the future, we should still persist to look for other intermediate connectors with negligible voltage drop and high transparency in order to achieve ideal tandem OLEDs with improved voltage stability, current efficiency, power efficiency, power consumption, and operational stability.

4.3. p–i–n Devices

The high performance and long lifetime demands for OLEDs are met by the p–i–n (p‐doped hole transport layer/intrinsically conductive emission layer/n‐doped electron transport layer) approach. The p–i–n OLEDs demonstrated in Figure   27 . The diodes based on p–i–n concept have exponential forward characteristics up to comparatively high current densities. These p–i–n devices enable high luminance and efficiency at extremely low operating voltages, as well as long OLED lifetimes. [ 53 , 258 , 259 , 280 ] In 2005, Wellmann et al. reported very high‐efficiency devices with power efficiencies >70 lm W −1 and lifetimes of more than 220 000 h at a brightness of 150 cd m −2 through integrating double emission layer, comprised of two bipolar layers doped with the emitters, into the p–i–n architecture. [ 53 ]

Device architectures of conventional and p–i–n OLEDs.

Devices with single emission layers lead to a strong decrease in efficiency at elevated brightness. Improvement of the OLED characteristics is possible when p–i–n architecture was combined with the double emission layer concept, where a predominantly hole transport emission layer and a predominantly electron transport emission layer was doped with the phosphorescent emitter dye. Using the p–i–n layer concept, Meerheim et al. fabricated a very high power efficiency of 37.5 lm W −1 and a lifetime of 30 200 h at 100 cd m −2 initial luminance OLEDs. He studied the charge carrier and exciton distribution routes for achieving broad and centered exciton recombination zone to maximize the device performance of OLEDs both in terms of lifetime and efficiency. [ 150 ]

In 2008, Fehse et al. investigate the degradation mechanism of p–i–n and i–n–OLEDs on PEDOT:PSS anodes with respect to different anode pretreatments. The extrapolated lifetime difference between indium tin oxide (ITO) and PEDOT:PSS anodes showed 5000 h longer living OLED on a polymer anode. [ 281 ] Meerheim et al. reviewed the recent advances and improvement strategies concepts in terms of low operating voltages, high power efficiency and long lifetime in the field of p–i–n type organic light emitting diodes (OLEDs). [ 282 ] In 2012, Birnstock et al. reported a new record lifetime value for p–i–n devices 50% lifetimes of more than 1 million h for fluorescent red, 100 000 h for phosphorescent green based on Ir(ppy) 3 and 50 000 h for fluorescent blue of starting brightness of 500 cd m −2 . [ 283 ] In the same year, Birnstock et al. demonstrated lifetimes of phosphorescent bottom emitting p–i–n OLEDs of 30 000 h at a brightness of 500 cd/m². [ 284 ] Further progress can be expected from the newly developed molecular n‐dopant and p‐dopant, hence long‐term and thermal stability of p–i–n OLEDs.

4.4. Inverted Devices

OLEDs have been attracting extensive attention due to their excellent optoelectronic properties and superb energy saving features. However, the poor ambient stability remains a critical hurdle for commercialization of this unique technology. In the last few years, inverted OLEDs (iOLEDs) have been proposed as an ideal structure for realizing air‐stable OLEDs because of its several advantages over convention architecture OLEDs. For example, the environmental stability of OLEDs can be significantly improved because reactive materials such as alkali metals which widely used between organic and metal layers in conventional devices replaced with water‐ and oxygen‐sensitive metal oxides electron injection materials such ZnO, SnO 2 , TiO 2 , and ZnO 2 due to their air stability, nontoxicity, transparency and high electron mobilities. [ 285 , 286 , 287 ] However, iOLED based on these n‐type metal oxides were criticized for lower performance when compared with conventional OLED because there is still a large electron injection barrier from their conduction band to the LUMO of the emitters. [ 288 ] One of the possible solution these issue is use an interlayer between metal oxide ETL and emissive layer. The general device architecture of conventional and inverted OLEDs are shown in Figure   28 .

General device structure of a) conventional and (b) inverted OLED device.

In 2012, Zou et al. introduced a polymers containing simple aliphatic amine groups PEIE and PEI as a “universal” surface modifiers to reduce the energy barrier between ETL and emissive layer that allow low‐cost environmentally fabrication of air‐stable devices. [ 289 ] In 2014, Lee et al. demonstrated iPLEDs with a CE of 61.6 cd A −1 , a PE of 19.4 lm W −1 and an EQE of 17.8% by using a simple and effective method that relies on the nanostructure of ZnO‐R and the 2‐ME + EA polar solvent treatment of the ZnO‐R. [ 290 ] In 2015, they further reported a new interfacial engineering method by introducing a series of amine‐based interfacial molecules that contain 2–6 amine groups (2–6N) for highly efficient iPLED. The best optimized iPLEDs exhibit a maximum luminance of 99 300 cd m −2 , a CE of 23.1 cd A −1 , a PE of 8.83 lm W −1 , and an EQE of 8.40%, which are 30‐, 32‐, 38‐, and 30‐fold higher than that of the reference device without any interlayer. [ 291 ] Fukagawa et al. reported a long‐lived flexible display using efficient and stable iOLEDs, where ZnO and polyethyleneimine (PEI) were used as ETL and interlayer to effectively inject electron into the emissive zone. in which electrons can be effectively injected without the use of alkali metals. The iOLED‐based flexible display emits light over 1 year under the simplified encapsulation, though the cOLED‐based flexible display shows almost no luminosity only after 21 day under the same encapsulation. [ 292 ]

4.5. Exciplex Cohost System Based Devices

OLEDs employing TADF mechanism exciplex cohost systems have gained attractive interest due to the promising low operation voltage, high IQE, low efficiency roll‐off, high light‐out coupling efficiency, and enhanced lifetime. Exciplex‐forming cohost system enables efficient singlet and triplet energy transfers from the host exciplex to the dopant because the singlet and triplet energies of the exciplex are at same level as well as also reduce the probability of direct trapping of charges at the dopant molecules and lower charge‐injection barrier from the charge‐transport layers to the emitting layer. Figure   29 showing the schematic representation of excitons in conventional and exciplex OLEDs.

(Left) Schematic representation of excitons in conventional and exciplex OLEDs. (Right) Essential parameters for donor and acceptor to obtain TADF mechanism enabling exciplex forming cohost systems.

In 2014, Seo et al. reported that the lifetime of OLEDs using exciplex forming cohosts can theoretically reach 1 million h at an initial luminance of 1000 cd m −2 , as long as the exciplex is used as a medium of energy transfer rather than nonradiative relaxation pathways. [ 293 ] In 2018, Shih et al. developed an exciplex forming cohost system consisting of a conventional star‐shaped carbazole TCTA and a triazine 3P‐T2T as a donor and an acceptor molecules, respectively. The resultant device exhibited an operational lifetime ( τ 80 ) of ≈1020 min with the initial brightness of 2000 cd m −2 , which is 56 times longer than the reference device. [ 294 ] In 2019, Liao's group reported a deep‐red OLED with EQE > 22% employing exciplex cohost system as the main matrix and one TADF molecules as a sensitizer. The lifetime of based optimized device based on exciplex cohost: TADF sensitizer:emitter is approximately twentieth time longer than that of single host based device using a conventional host–guest emitting layer. [ 294 ] Recently, Wang et al. demonstrated a PQ2Ir based red phosphorescent OLEDs with a maximum EQE of 19.8% and LT 50 (time to 50% of initial luminance at 1000 cd m −2 ) lifetime up to 10 169 h based on 9,9′‐diphenyl‐9 H ,9′ H ‐3,3′‐bicarbazole (BCzPh):2‐(9,9′‐spirobi[fluoren]‐2‐yl)‐4,6‐diphenyl‐1,3Δ,5‐ triazine (SF2‐TRZ) exciplex cohost system that is ≈16 times longer than the reference group TCTA:2,4,6‐tri [(1,1′‐biphenyl)‐3‐yl]‐1,3,5‐triazine (T2T). [ 295 ] Some of the unique device structures used for both high efficiency and long lifetime tabulated in Table   2 .

Some unique device architectures/materials for both high efficiency and long lifetime

5. Conclusion

OLED will become the disruptive technology in the field of display and lighting in the near future. Primarily, we gave a clear idea about the revenue of current lighting market. We discussed about the number of publications and patents in OLED field per year. From the statistics, it is very clear that many research works are going on to improve the device efficiency compared to device lifetime. It's essential to give more attention to develop long lifetime OLEDs. We covered all the extrinsic and intrinsic factor affecting the device lifetime and pointing out, how we can prevent the same. The selection of materials and designing of device architecture are strong pillars for obtaining higher lifetime and efficiency.

Conflict of Interest

The authors declare no conflict of interest.

Acknowledgements

The authors would like to acknowledge the financial support in part from the Ministry of Science and Technology, Taiwan via a grant MOST 106‐2119‐M‐007‐011.

Biographies

Sujith Sudheendran Swayprabha received his Ph.D. from National Tsing Hua University, Taiwan under the guidance of Prof. Jwo‐Huei Jou in May 2020. After the completion of his B.Sc. (Chemistry) and M.Sc. (Analytical Chemistry) from University of Kerala, India, he worked as a project fellow in National Institute for Interdisciplinary Science and Technology (NIIST‐CSIR), Kerala, India. Currently, he is working as a Research and Development Engineer in Raystar Optronics, Inc., Taichung, Taiwan. His research interests mainly include optoelectronic materials and device engineering.

Deepak Kumar Dubey graduated with a Ph.D. from Department of Materials Science and Engineering, National Tsing Hua University, Taiwan in 2019 under the supervision Prof. Jwo‐Huei Jou. His doctoral research works were focused on the design, fabrication, and characterization of highly efficient OLED devices. Currently, he is as an Alberta Innovates Research Associate at the University of Calgary, Canada under the supervision of Prof. Gregory C. Welch. His current research interests span from fully printed and flexible electronics to advance and enhance the performance of conventional organic electronic devices including OLEDs, solar Cells and OFET by developing new device physics concepts, and charge transporting materials.

Shahnawaz received his Bachelor of Science in Electronics from Zakir Husain Delhi College (Delhi University), India, Master of science in Electronics and Master of Technology in Nanotechnology from Jamia Millia Islamia, India. He was appointed as project fellow in Delhi University, India. He is pursuing Ph.D. degree in Materials Science from National Tsing Hua University, Taiwan. His main research interests are in organic and inorganic materials and devices with a focus on organic electronics, involving synthesis of materials and device fabrication with thin films including organic light emitting diodes, smart healthy lighting panels, and displays.

Rohit Ashok Kumar Yadav received his Ph.D. degree from Department of Materials Science and Engineering, National Tsing Hua University, Taiwan in July 2020 and M.Tech degree in Nanoscience and Technology from University Institute of Chemical Technology, India in June 2014. His research interests mainly surround organic light‐emitting diodes (OLEDs) to design novel device strategies to improve the radiative exciton generations and achieve high‐performance OLEDs.

Jwo‐Huei Jou is a professor of the Department of Materials Science & Engineering in National Tsinghua University, Taiwan. He received his Ph.D. in Macromolecular Science and Engineering Program from the University of Michigan in 1986. Later, he worked as a visiting scientist at IBM Almaden Research Center before joining as an associate professor in 1988 and full professor in 1992. Prof. Jou's research interest includes high‐efficiency and natural light‐style organic light emitting diodes (OLEDs), polymer, thin film stress, and expert system applications.

Sudheendran S. Swayamprabha, Dubey D. K., Shahnawaz, Yadav R. A. K., Nagar M. R., Sharma A., Tung F.‐C., Jou J.‐H., Approaches for Long Lifetime Organic Light Emitting Diodes . Adv. Sci. 2021, 8 , 2002254 10.1002/advs.202002254 [ PMC free article ] [ PubMed ] [ CrossRef ] [ Google Scholar ]

Research on High-Resolution Color OLED Display Technology

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• Review Article
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• Published: 25 October 2021

Augmented reality and virtual reality displays: emerging technologies and future perspectives

• Jianghao Xiong 1 ,
• En-Lin Hsiang 1 ,
• Ziqian He 1 ,
• Tao Zhan   ORCID: orcid.org/0000-0001-5511-6666 1 &
• Shin-Tson Wu   ORCID: orcid.org/0000-0002-0943-0440 1  

Light: Science & Applications volume  10 , Article number:  216 ( 2021 ) Cite this article

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• Liquid crystals

With rapid advances in high-speed communication and computation, augmented reality (AR) and virtual reality (VR) are emerging as next-generation display platforms for deeper human-digital interactions. Nonetheless, to simultaneously match the exceptional performance of human vision and keep the near-eye display module compact and lightweight imposes unprecedented challenges on optical engineering. Fortunately, recent progress in holographic optical elements (HOEs) and lithography-enabled devices provide innovative ways to tackle these obstacles in AR and VR that are otherwise difficult with traditional optics. In this review, we begin with introducing the basic structures of AR and VR headsets, and then describing the operation principles of various HOEs and lithography-enabled devices. Their properties are analyzed in detail, including strong selectivity on wavelength and incident angle, and multiplexing ability of volume HOEs, polarization dependency and active switching of liquid crystal HOEs, device fabrication, and properties of micro-LEDs (light-emitting diodes), and large design freedoms of metasurfaces. Afterwards, we discuss how these devices help enhance the AR and VR performance, with detailed description and analysis of some state-of-the-art architectures. Finally, we cast a perspective on potential developments and research directions of these photonic devices for future AR and VR displays.

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Introduction

Recent advances in high-speed communication and miniature mobile computing platforms have escalated a strong demand for deeper human-digital interactions beyond traditional flat panel displays. Augmented reality (AR) and virtual reality (VR) headsets 1 , 2 are emerging as next-generation interactive displays with the ability to provide vivid three-dimensional (3D) visual experiences. Their useful applications include education, healthcare, engineering, and gaming, just to name a few 3 , 4 , 5 . VR embraces a total immersive experience, while AR promotes the interaction between user, digital contents, and real world, therefore displaying virtual images while remaining see-through capability. In terms of display performance, AR and VR face several common challenges to satisfy demanding human vision requirements, including field of view (FoV), eyebox, angular resolution, dynamic range, and correct depth cue, etc. Another pressing demand, although not directly related to optical performance, is ergonomics. To provide a user-friendly wearing experience, AR and VR should be lightweight and ideally have a compact, glasses-like form factor. The above-mentioned requirements, nonetheless, often entail several tradeoff relations with one another, which makes the design of high-performance AR/VR glasses/headsets particularly challenging.

In the 1990s, AR/VR experienced the first boom, which quickly subsided due to the lack of eligible hardware and digital content 6 . Over the past decade, the concept of immersive displays was revisited and received a new round of excitement. Emerging technologies like holography and lithography have greatly reshaped the AR/VR display systems. In this article, we firstly review the basic requirements of AR/VR displays and their associated challenges. Then, we briefly describe the properties of two emerging technologies: holographic optical elements (HOEs) and lithography-based devices (Fig. 1 ). Next, we separately introduce VR and AR systems because of their different device structures and requirements. For the immersive VR system, the major challenges and how these emerging technologies help mitigate the problems will be discussed. For the see-through AR system, we firstly review the present status of light engines and introduce some architectures for the optical combiners. Performance summaries on microdisplay light engines and optical combiners will be provided, that serve as a comprehensive overview of the current AR display systems.

The left side illustrates HOEs and lithography-based devices. The right side shows the challenges in VR and architectures in AR, and how the emerging technologies can be applied

Key parameters of AR and VR displays

AR and VR displays face several common challenges to satisfy the demanding human vision requirements, such as FoV, eyebox, angular resolution, dynamic range, and correct depth cue, etc. These requirements often exhibit tradeoffs with one another. Before diving into detailed relations, it is beneficial to review the basic definitions of the above-mentioned display parameters.

Definition of parameters

Taking a VR system (Fig. 2a ) as an example. The light emitting from the display module is projected to a FoV, which can be translated to the size of the image perceived by the viewer. For reference, human vision’s horizontal FoV can be as large as 160° for monocular vision and 120° for overlapped binocular vision 6 . The intersection area of ray bundles forms the exit pupil, which is usually correlated with another parameter called eyebox. The eyebox defines the region within which the whole image FoV can be viewed without vignetting. It therefore generally manifests a 3D geometry 7 , whose volume is strongly dependent on the exit pupil size. A larger eyebox offers more tolerance to accommodate the user’s diversified interpupillary distance (IPD) and wiggling of headset when in use. Angular resolution is defined by dividing the total resolution of the display panel by FoV, which measures the sharpness of a perceived image. For reference, a human visual acuity of 20/20 amounts to 1 arcmin angular resolution, or 60 pixels per degree (PPD), which is considered as a common goal for AR and VR displays. Another important feature of a 3D display is depth cue. Depth cue can be induced by displaying two separate images to the left eye and the right eye, which forms the vergence cue. But the fixed depth of the displayed image often mismatches with the actual depth of the intended 3D image, which leads to incorrect accommodation cues. This mismatch causes the so-called vergence-accommodation conflict (VAC), which will be discussed in detail later. One important observation is that the VAC issue may be more serious in AR than VR, because the image in an AR display is directly superimposed onto the real-world with correct depth cues. The image contrast is dependent on the display panel and stray light. To achieve a high dynamic range, the display panel should exhibit high brightness, low dark level, and more than 10-bits of gray levels. Nowadays, the display brightness of a typical VR headset is about 150–200 cd/m 2 (or nits).

a Schematic of a VR display defining FoV, exit pupil, eyebox, angular resolution, and accommodation cue mismatch. b Sketch of an AR display illustrating ACR

Figure 2b depicts a generic structure of an AR display. The definition of above parameters remains the same. One major difference is the influence of ambient light on the image contrast. For a see-through AR display, ambient contrast ratio (ACR) 8 is commonly used to quantify the image contrast:

where L on ( L off ) represents the on (off)-state luminance (unit: nit), L am is the ambient luminance, and T is the see-through transmittance. In general, ambient light is measured in illuminance (lux). For the convenience of comparison, we convert illuminance to luminance by dividing a factor of π, assuming the emission profile is Lambertian. In a normal living room, the illuminance is about 100 lux (i.e., L am  ≈ 30 nits), while in a typical office lighting condition, L am  ≈ 150 nits. For outdoors, on an overcast day, L am  ≈ 300 nits, and L am  ≈ 3000 nits on a sunny day. For AR displays, a minimum ACR should be 3:1 for recognizable images, 5:1 for adequate readability, and ≥10:1 for outstanding readability. To make a simple estimate without considering all the optical losses, to achieve ACR = 10:1 in a sunny day (~3000 nits), the display needs to deliver a brightness of at least 30,000 nits. This imposes big challenges in finding a high brightness microdisplay and designing a low loss optical combiner.

Tradeoffs and potential solutions

Next, let us briefly review the tradeoff relations mentioned earlier. To begin with, a larger FoV leads to a lower angular resolution for a given display resolution. In theory, to overcome this tradeoff only requires a high-resolution-display source, along with high-quality optics to support the corresponding modulation transfer function (MTF). To attain 60 PPD across 100° FoV requires a 6K resolution for each eye. This may be realizable in VR headsets because a large display panel, say 2–3 inches, can still accommodate a high resolution with acceptable manufacture cost. However, for a glasses-like wearable AR display, the conflict between small display size and the high solution becomes obvious as further shrinking the pixel size of a microdisplay is challenging.

To circumvent this issue, the concept of the foveated display is proposed 9 , 10 , 11 , 12 , 13 . The idea is based on that the human eye only has high visual acuity in the central fovea region, which accounts for about 10° FoV. If the high-resolution image is only projected to fovea while the peripheral image remains low resolution, then a microdisplay with 2K resolution can satisfy the need. Regarding the implementation method of foveated display, a straightforward way is to optically combine two display sources 9 , 10 , 11 : one for foveal and one for peripheral FoV. This approach can be regarded as spatial multiplexing of displays. Alternatively, time-multiplexing can also be adopted, by temporally changing the optical path to produce different magnification factors for the corresponding FoV 12 . Finally, another approach without multiplexing is to use a specially designed lens with intended distortion to achieve non-uniform resolution density 13 . Aside from the implementation of foveation, another great challenge is to dynamically steer the foveated region as the viewer’s eye moves. This task is strongly related to pupil steering, which will be discussed in detail later.

A larger eyebox or FoV usually decreases the image brightness, which often lowers the ACR. This is exactly the case for a waveguide AR system with exit pupil expansion (EPE) while operating under a strong ambient light. To improve ACR, one approach is to dynamically adjust the transmittance with a tunable dimmer 14 , 15 . Another solution is to directly boost the image brightness with a high luminance microdisplay and an efficient combiner optics. Details of this topic will be discussed in the light engine section.

Another tradeoff of FoV and eyebox in geometric optical systems results from the conservation of etendue (or optical invariant). To increase the system etendue requires a larger optics, which in turn compromises the form factor. Finally, to address the VAC issue, the display system needs to generate a proper accommodation cue, which often requires the modulation of image depth or wavefront, neither of which can be easily achieved in a traditional geometric optical system. While remarkable progresses have been made to adopt freeform surfaces 16 , 17 , 18 , to further advance AR and VR systems requires additional novel optics with a higher degree of freedom in structure design and light modulation. Moreover, the employed optics should be thin and lightweight. To mitigate the above-mentioned challenges, diffractive optics is a strong contender. Unlike geometric optics relying on curved surfaces to refract or reflect light, diffractive optics only requires a thin layer of several micrometers to establish efficient light diffractions. Two major types of diffractive optics are HOEs based on wavefront recording and manually written devices like surface relief gratings (SRGs) based on lithography. While SRGs have large design freedoms of local grating geometry, a recent publication 19 indicates the combination of HOE and freeform optics can also offer a great potential for arbitrary wavefront generation. Furthermore, the advances in lithography have also enabled optical metasurfaces beyond diffractive and refractive optics, and miniature display panels like micro-LED (light-emitting diode). These devices hold the potential to boost the performance of current AR/VR displays, while keeping a lightweight and compact form factor.

Formation and properties of HOEs

HOE generally refers to a recorded hologram that reproduces the original light wavefront. The concept of holography is proposed by Dennis Gabor 20 , which refers to the process of recording a wavefront in a medium (hologram) and later reconstructing it with a reference beam. Early holography uses intensity-sensitive recording materials like silver halide emulsion, dichromated gelatin, and photopolymer 21 . Among them, photopolymer stands out due to its easy fabrication and ability to capture high-fidelity patterns 22 , 23 . It has therefore found extensive applications like holographic data storage 23 and display 24 , 25 . Photopolymer HOEs (PPHOEs) have a relatively small refractive index modulation and therefore exhibits a strong selectivity on the wavelength and incident angle. Another feature of PPHOE is that several holograms can be recorded into a photopolymer film by consecutive exposures. Later, liquid-crystal holographic optical elements (LCHOEs) based on photoalignment polarization holography have also been developed 25 , 26 . Due to the inherent anisotropic property of liquid crystals, LCHOEs are extremely sensitive to the polarization state of the input light. This feature, combined with the polarization modulation ability of liquid crystal devices, offers a new possibility for dynamic wavefront modulation in display systems.

The formation of PPHOE is illustrated in Fig. 3a . When exposed to an interfering field with high-and-low intensity fringes, monomers tend to move toward bright fringes due to the higher local monomer-consumption rate. As a result, the density and refractive index is slightly larger in bright regions. Note the index modulation δ n here is defined as the difference between the maximum and minimum refractive indices, which may be twice the value in other definitions 27 . The index modulation δ n is typically in the range of 0–0.06. To understand the optical properties of PPHOE, we simulate a transmissive grating and a reflective grating using rigorous coupled-wave analysis (RCWA) 28 , 29 and plot the results in Fig. 3b . Details of grating configuration can be found in Table S1 . Here, the reason for only simulating gratings is that for a general HOE, the local region can be treated as a grating. The observation of gratings can therefore offer a general insight of HOEs. For a transmissive grating, its angular bandwidth (efficiency > 80%) is around 5° ( λ  = 550 nm), while the spectral band is relatively broad, with bandwidth around 175 nm (7° incidence). For a reflective grating, its spectral band is narrow, with bandwidth around 10 nm. The angular bandwidth varies with the wavelength, ranging from 2° to 20°. The strong selectivity of PPHOE on wavelength and incident angle is directly related to its small δ n , which can be adjusted by controlling the exposure dosage.

a Schematic of the formation of PPHOE. Simulated efficiency plots for b1 transmissive and b2 reflective PPHOEs. c Working principle of multiplexed PPHOE. d Formation and molecular configurations of LCHOEs. Simulated efficiency plots for e1 transmissive and e2 reflective LCHOEs. f Illustration of polarization dependency of LCHOEs

A distinctive feature of PPHOE is the ability to multiplex several holograms into one film sample. If the exposure dosage of a recording process is controlled so that the monomers are not completely depleted in the first exposure, the remaining monomers can continue to form another hologram in the following recording process. Because the total amount of monomer is fixed, there is usually an efficiency tradeoff between multiplexed holograms. The final film sample would exhibit the wavefront modulation functions of multiple holograms (Fig. 3c ).

Liquid crystals have also been used to form HOEs. LCHOEs can generally be categorized into volume-recording type and surface-alignment type. Volume-recording type LCHOEs are either based on early polarization holography recordings with azo-polymer 30 , 31 , or holographic polymer-dispersed liquid crystals (HPDLCs) 32 , 33 formed by liquid-crystal-doped photopolymer. Surface-alignment type LCHOEs are based on photoalignment polarization holography (PAPH) 34 . The first step is to record the desired polarization pattern in a thin photoalignment layer, and the second step is to use it to align the bulk liquid crystal 25 , 35 . Due to the simple fabrication process, high efficiency, and low scattering from liquid crystal’s self-assembly nature, surface-alignment type LCHOEs based on PAPH have recently attracted increasing interest in applications like near-eye displays. Here, we shall focus on this type of surface-alignment LCHOE and refer to it as LCHOE thereafter for simplicity.

The formation of LCHOEs is illustrated in Fig. 3d . The information of the wavefront and the local diffraction pattern is recorded in a thin photoalignment layer. The volume liquid crystal deposited on the photoalignment layer, depending on whether it is nematic liquid crystal or cholesteric liquid crystal (CLC), forms a transmissive or a reflective LCHOE. In a transmissive LCHOE, the bulk nematic liquid crystal molecules generally follow the pattern of the bottom alignment layer. The smallest allowable pattern period is governed by the liquid crystal distortion-free energy model, which predicts the pattern period should generally be larger than sample thickness 36 , 37 . This results in a maximum diffraction angle under 20°. On the other hand, in a reflective LCHOE 38 , 39 , the bulk CLC molecules form a stable helical structure, which is tilted to match the k -vector of the bottom pattern. The structure exhibits a very low distorted free energy 40 , 41 and can accommodate a pattern period that is small enough to diffract light into the total internal reflection (TIR) of a glass substrate.

The diffraction property of LCHOEs is shown in Fig. 3e . The maximum refractive index modulation of LCHOE is equal to the liquid crystal birefringence (Δ n ), which may vary from 0.04 to 0.5, depending on the molecular conjugation 42 , 43 . The birefringence used in our simulation is Δ n  = 0.15. Compared to PPHOEs, the angular and spectral bandwidths are significantly larger for both transmissive and reflective LCHOEs. For a transmissive LCHOE, its angular bandwidth is around 20° ( λ  = 550 nm), while the spectral bandwidth is around 300 nm (7° incidence). For a reflective LCHOE, its spectral bandwidth is around 80 nm and angular bandwidth could vary from 15° to 50°, depending on the wavelength.

The anisotropic nature of liquid crystal leads to LCHOE’s unique polarization-dependent response to an incident light. As depicted in Fig. 3f , for a transmissive LCHOE the accumulated phase is opposite for the conjugated left-handed circular polarization (LCP) and right-handed circular polarization (RCP) states, leading to reversed diffraction directions. For a reflective LCHOE, the polarization dependency is similar to that of a normal CLC. For the circular polarization with the same handedness as the helical structure of CLC, the diffraction is strong. For the opposite circular polarization, the diffraction is negligible.

Another distinctive property of liquid crystal is its dynamic response to an external voltage. The LC reorientation can be controlled with a relatively low voltage (<10 V rms ) and the response time is on the order of milliseconds, depending mainly on the LC viscosity and layer thickness. Methods to dynamically control LCHOEs can be categorized as active addressing and passive addressing, which can be achieved by either directly switching the LCHOE or modulating the polarization state with an active waveplate. Detailed addressing methods will be described in the VAC section.

Lithography-enabled devices

Lithography technologies are used to create arbitrary patterns on wafers, which lays the foundation of the modern integrated circuit industry 44 . Photolithography is suitable for mass production while electron/ion beam lithography is usually used to create photomask for photolithography or to write structures with nanometer-scale feature size. Recent advances in lithography have enabled engineered structures like optical metasurfaces 45 , SRGs 46 , as well as micro-LED displays 47 . Metasurfaces exhibit a remarkable design freedom by varying the shape of meta-atoms, which can be utilized to achieve novel functions like achromatic focus 48 and beam steering 49 . Similarly, SRGs also offer a large design freedom by manipulating the geometry of local grating regions to realize desired optical properties. On the other hand, micro-LED exhibits several unique features, such as ultrahigh peak brightness, small aperture ratio, excellent stability, and nanosecond response time, etc. As a result, micro-LED is a promising candidate for AR and VR systems for achieving high ACR and high frame rate for suppressing motion image blurs. In the following section, we will briefly review the fabrication and properties of micro-LEDs and optical modulators like metasurfaces and SRGs.

Fabrication and properties of micro-LEDs

LEDs with a chip size larger than 300 μm have been widely used in solid-state lighting and public information displays. Recently, micro-LEDs with chip sizes <5 μm have been demonstrated 50 . The first micro-LED disc with a diameter of about 12 µm was demonstrated in 2000 51 . After that, a single color (blue or green) LED microdisplay was demonstrated in 2012 52 . The high peak brightness, fast response time, true dark state, and long lifetime of micro-LEDs are attractive for display applications. Therefore, many companies have since released their micro-LED prototypes or products, ranging from large-size TVs to small-size microdisplays for AR/VR applications 53 , 54 . Here, we focus on micro-LEDs for near-eye display applications. Regarding the fabrication of micro-LEDs, through the metal-organic chemical vapor deposition (MOCVD) method, the AlGaInP epitaxial layer is grown on GaAs substrate for red LEDs, and GaN epitaxial layers on sapphire substrate for green and blue LEDs. Next, a photolithography process is applied to define the mesa and deposit electrodes. To drive the LED array, the fabricated micro-LEDs are transferred to a CMOS (complementary metal oxide semiconductor) driver board. For a small size (<2 inches) microdisplay used in AR or VR, the precision of the pick-and-place transfer process is hard to meet the high-resolution-density (>1000 pixel per inch) requirement. Thus, the main approach to assemble LED chips with driving circuits is flip-chip bonding 50 , 55 , 56 , 57 , as Fig. 4a depicts. In flip-chip bonding, the mesa and electrode pads should be defined and deposited before the transfer process, while metal bonding balls should be preprocessed on the CMOS substrate. After that, thermal-compression method is used to bond the two wafers together. However, due to the thermal mismatch of LED chip and driving board, as the pixel size decreases, the misalignment between the LED chip and the metal bonding ball on the CMOS substrate becomes serious. In addition, the common n-GaN layer may cause optical crosstalk between pixels, which degrades the image quality. To overcome these issues, the LED epitaxial layer can be firstly metal-bonded with the silicon driver board, followed by the photolithography process to define the LED mesas and electrodes. Without the need for an alignment process, the pixel size can be reduced to <5 µm 50 .

a Illustration of flip-chip bonding technology. b Simulated IQE-LED size relations for red and blue LEDs based on ABC model. c Comparison of EQE of different LED sizes with and without KOH and ALD side wall treatment. d Angular emission profiles of LEDs with different sizes. Metasurfaces based on e resonance-tuning, f non-resonance tuning and g combination of both. h Replication master and i replicated SRG based on nanoimprint lithography. Reproduced from a ref. 55 with permission from AIP Publishing, b ref. 61 with permission from PNAS, c ref. 66 with permission from IOP Publishing, d ref. 67 with permission from AIP Publishing, e ref. 69 with permission from OSA Publishing f ref. 48 with permission from AAAS g ref. 70 with permission from AAAS and h , i ref. 85 with permission from OSA Publishing

In addition to manufacturing process, the electrical and optical characteristics of LED also depend on the chip size. Generally, due to Shockley-Read-Hall (SRH) non-radiative recombination on the sidewall of active area, a smaller LED chip size results in a lower internal quantum efficiency (IQE), so that the peak IQE driving point will move toward a higher current density due to increased ratio of sidewall surface to active volume 58 , 59 , 60 . In addition, compared to the GaN-based green and blue LEDs, the AlGaInP-based red LEDs with a larger surface recombination and carrier diffusion length suffer a more severe efficiency drop 61 , 62 . Figure 4b shows the simulated result of IQE drop in relation with the LED chip size of blue and red LEDs based on ABC model 63 . To alleviate the efficiency drop caused by sidewall defects, depositing passivation materials by atomic layer deposition (ALD) or plasma enhanced chemical vapor deposition (PECVD) is proven to be helpful for both GaN and AlGaInP based LEDs 64 , 65 . In addition, applying KOH (Potassium hydroxide) treatment after ALD can further reduce the EQE drop of micro-LEDs 66 (Fig. 4c ). Small-size LEDs also exhibit some advantages, such as higher light extraction efficiency (LEE). Compared to an 100-µm LED, the LEE of a 2-µm LED increases from 12.2 to 25.1% 67 . Moreover, the radiation pattern of micro-LED is more directional than that of a large-size LED (Fig. 4d ). This helps to improve the lens collection efficiency in AR/VR display systems.

Metasurfaces and SGs

Thanks to the advances in lithography technology, low-loss dielectric metasurfaces working in the visible band have recently emerged as a platform for wavefront shaping 45 , 48 , 68 . They consist of an array of subwavelength-spaced structures with individually engineered wavelength-dependent polarization/phase/ amplitude response. In general, the light modulation mechanisms can be classified into resonant tuning 69 (Fig. 4e ), non-resonant tuning 48 (Fig. 4f ), and combination of both 70 (Fig. 4g ). In comparison with non-resonant tuning (based on geometric phase and/or dynamic propagation phase), the resonant tuning (such as Fabry–Pérot resonance, Mie resonance, etc.) is usually associated with a narrower operating bandwidth and a smaller out-of-plane aspect ratio (height/width) of nanostructures. As a result, they are easier to fabricate but more sensitive to fabrication tolerances. For both types, materials with a higher refractive index and lower absorption loss are beneficial to reduce the aspect ratio of nanostructure and improve the device efficiency. To this end, titanium dioxide (TiO 2 ) and gallium nitride (GaN) are the major choices for operating in the entire visible band 68 , 71 . While small-sized metasurfaces (diameter <1 mm) are usually fabricated via electron-beam lithography or focused ion beam milling in the labs, the ability of mass production is the key to their practical adoption. The deep ultraviolet (UV) photolithography has proven its feasibility for reproducing centimeter-size metalenses with decent imaging performance, while it requires multiple steps of etching 72 . Interestingly, the recently developed UV nanoimprint lithography based on a high-index nanocomposite only takes a single step and can obtain an aspect ratio larger than 10, which shows great promise for high-volume production 73 .

The arbitrary wavefront shaping capability and the thinness of the metasurfaces have aroused strong research interests in the development of novel AR/VR prototypes with improved performance. Lee et al. employed nanoimprint lithography to fabricate a centimeter-size, geometric-phase metalens eyepiece for full-color AR displays 74 . Through tailoring its polarization conversion efficiency and stacking with a circular polarizer, the virtual image can be superimposed with the surrounding scene. The large numerical aperture (NA~0.5) of the metalens eyepiece enables a wide FoV (>76°) that conventional optics are difficult to obtain. However, the geometric phase metalens is intrinsically a diffractive lens that also suffers from strong chromatic aberrations. To overcome this issue, an achromatic lens can be designed via simultaneously engineering the group delay and the group delay dispersion 75 , 76 , which will be described in detail later. Other novel and/or improved near-eye display architectures include metasurface-based contact lens-type AR 77 , achromatic metalens array enabled integral-imaging light field displays 78 , wide FoV lightguide AR with polarization-dependent metagratings 79 , and off-axis projection-type AR with an aberration-corrected metasurface combiner 80 , 81 , 82 . Nevertheless, from the existing AR/VR prototypes, metasurfaces still face a strong tradeoff between numerical aperture (for metalenses), chromatic aberration, monochromatic aberration, efficiency, aperture size, and fabrication complexity.

On the other hand, SRGs are diffractive gratings that have been researched for decades as input/output couplers of waveguides 83 , 84 . Their surface is composed of corrugated microstructures, and different shapes including binary, blazed, slanted, and even analogue can be designed. The parameters of the corrugated microstructures are determined by the target diffraction order, operation spectral bandwidth, and angular bandwidth. Compared to metasurfaces, SRGs have a much larger feature size and thus can be fabricated via UV photolithography and subsequent etching. They are usually replicated by nanoimprint lithography with appropriate heating and surface treatment. According to a report published a decade ago, SRGs with a height of 300 nm and a slant angle of up to 50° can be faithfully replicated with high yield and reproducibility 85 (Fig. 4g, h ).

Challenges and solutions of VR displays

The fully immersive nature of VR headset leads to a relatively fixed configuration where the display panel is placed in front of the viewer’s eye and an imaging optics is placed in-between. Regarding the system performance, although inadequate angular resolution still exists in some current VR headsets, the improvement of display panel resolution with advanced fabrication process is expected to solve this issue progressively. Therefore, in the following discussion, we will mainly focus on two major challenges: form factor and 3D cue generation.

Form factor

Compact and lightweight near-eye displays are essential for a comfortable user experience and therefore highly desirable in VR headsets. Current mainstream VR headsets usually have a considerably larger volume than eyeglasses, and most of the volume is just empty. This is because a certain distance is required between the display panel and the viewing optics, which is usually close to the focal length of the lens system as illustrated in Fig. 5a . Conventional VR headsets employ a transmissive lens with ~4 cm focal length to offer a large FoV and eyebox. Fresnel lenses are thinner than conventional ones, but the distance required between the lens and the panel does not change significantly. In addition, the diffraction artifacts and stray light caused by the Fresnel grooves can degrade the image quality, or MTF. Although the resolution density, quantified as pixel per inch (PPI), of current VR headsets is still limited, eventually Fresnel lens will not be an ideal solution when a high PPI display is available. The strong chromatic aberration of Fresnel singlet should also be compensated if a high-quality imaging system is preferred.

a Schematic of a basic VR optical configuration. b Achromatic metalens used as VR eyepiece. c VR based on curved display and lenslet array. d Basic working principle of a VR display based on pancake optics. e VR with pancake optics and Fresnel lens array. f VR with pancake optics based on purely HOEs. Reprinted from b ref. 87 under the Creative Commons Attribution 4.0 License. Adapted from c ref. 88 with permission from IEEE, e ref. 91 and f ref. 92 under the Creative Commons Attribution 4.0 License

It is tempting to replace the refractive elements with a single thin diffractive lens like a transmissive LCHOE. However, the diffractive nature of such a lens will result in serious color aberrations. Interestingly, metalenses can fulfil this objective without color issues. To understand how metalenses achieve achromatic focus, let us first take a glance at the general lens phase profile \(\Phi (\omega ,r)\) expanded as a Taylor series 75 :

where \(\varphi _0(\omega )\) is the phase at the lens center, \(F\left( \omega \right)\) is the focal length as a function of frequency ω , r is the radial coordinate, and \(\omega _0\) is the central operation frequency. To realize achromatic focus, \(\partial F{{{\mathrm{/}}}}\partial \omega\) should be zero. With a designed focal length, the group delay \(\partial \Phi (\omega ,r){{{\mathrm{/}}}}\partial \omega\) and the group delay dispersion \(\partial ^2\Phi (\omega ,r){{{\mathrm{/}}}}\partial \omega ^2\) can be determined, and \(\varphi _0(\omega )\) is an auxiliary degree of freedom of the phase profile design. In the design of an achromatic metalens, the group delay is a function of the radial coordinate and monotonically increases with the metalens radius. Many designs have proven that the group delay has a limited variation range 75 , 76 , 78 , 86 . According to Shrestha et al. 86 , there is an inevitable tradeoff between the maximum radius of the metalens, NA, and operation bandwidth. Thus, the reported achromatic metalenses at visible usually have limited lens aperture (e.g., diameter < 250 μm) and NA (e.g., <0.2). Such a tradeoff is undesirable in VR displays, as the eyepiece favors a large clear aperture (inch size) and a reasonably high NA (>0.3) to maintain a wide FoV and a reasonable eye relief 74 .

To overcome this limitation, Li et al. 87 proposed a novel zone lens method. Unlike the traditional phase Fresnel lens where the zones are determined by the phase reset, the new approach divides the zones by the group delay reset. In this way, the lens aperture and NA can be much enlarged, and the group delay limit is bypassed. A notable side effect of this design is the phase discontinuity at zone boundaries that will contribute to higher-order focusing. Therefore, significant efforts have been conducted to find the optimal zone transition locations and to minimize the phase discontinuities. Using this method, they have demonstrated an impressive 2-mm-diameter metalens with NA = 0.7 and nearly diffraction-limited focusing for the designed wavelengths (488, 532, 658 nm) (Fig. 5b ). Such a metalens consists of 681 zones and works for the visible band ranging from 470 to 670 nm, though the focusing efficiency is in the order of 10%. This is a great starting point for the achromatic metalens to be employed as a compact, chromatic-aberration-free eyepiece in near-eye displays. Future challenges are how to further increase the aperture size, correct the off-axis aberrations, and improve the optical efficiency.

Besides replacing the refractive lens with an achromatic metalens, another way to reduce system focal length without decreasing NA is to use a lenslet array 88 . As depicted in Fig. 5c , both the lenslet array and display panel adopt a curved structure. With the latest flexible OLED panel, the display can be easily curved in one dimension. The system exhibits a large diagonal FoV of 180° with an eyebox of 19 by 12 mm. The geometry of each lenslet is optimized separately to achieve an overall performance with high image quality and reduced distortions.

Aside from trying to shorten the system focal length, another way to reduce total track is to fold optical path. Recently, polarization-based folded lenses, also known as pancake optics, are under active development for VR applications 89 , 90 . Figure 5d depicts the structure of an exemplary singlet pancake VR lens system. The pancake lenses can offer better imaging performance with a compact form factor since there are more degrees of freedom in the design and the actual light path is folded thrice. By using a reflective surface with a positive power, the field curvature of positive refractive lenses can be compensated. Also, the reflective surface has no chromatic aberrations and it contributes considerable optical power to the system. Therefore, the optical power of refractive lenses can be smaller, resulting in an even weaker chromatic aberration. Compared to Fresnel lenses, the pancake lenses have smooth surfaces and much fewer diffraction artifacts and stray light. However, such a pancake lens design is not perfect either, whose major shortcoming is low light efficiency. With two incidences of light on the half mirror, the maximum system efficiency is limited to 25% for a polarized input and 12.5% for an unpolarized input light. Moreover, due to the existence of multiple surfaces in the system, stray light caused by surface reflections and polarization leakage may lead to apparent ghost images. As a result, the catadioptric pancake VR headset usually manifests a darker imagery and lower contrast than the corresponding dioptric VR.

Interestingly, the lenslet and pancake optics can be combined to further reduce the system form. Bang et al. 91 demonstrated a compact VR system with a pancake optics and a Fresnel lenslet array. The pancake optics serves to fold the optical path between the display panel and the lenslet array (Fig. 5e ). Another Fresnel lens is used to collect the light from the lenslet array. The system has a decent horizontal FoV of 102° and an eyebox of 8 mm. However, a certain degree of image discontinuity and crosstalk are still present, which can be improved with further optimizations on the Fresnel lens and the lenslet array.

One step further, replacing all conventional optics in catadioptric VR headset with holographic optics can make the whole system even thinner. Maimone and Wang demonstrated such a lightweight, high-resolution, and ultra-compact VR optical system using purely HOEs 92 . This holographic VR optics was made possible by combining several innovative optical components, including a reflective PPHOE, a reflective LCHOE, and a PPHOE-based directional backlight with laser illumination, as shown in Fig. 5f . Since all the optical power is provided by the HOEs with negligible weight and volume, the total physical thickness can be reduced to <10 mm. Also, unlike conventional bulk optics, the optical power of a HOE is independent of its thickness, only subject to the recording process. Another advantage of using holographic optical devices is that they can be engineered to offer distinct phase profiles for different wavelengths and angles of incidence, adding extra degrees of freedom in optical designs for better imaging performance. Although only a single-color backlight has been demonstrated, such a PPHOE has the potential to achieve full-color laser backlight with multiplexing ability. The PPHOE and LCHOE in the pancake optics can also be optimized at different wavelengths for achieving high-quality full-color images.

Vergence-accommodation conflict

Conventional VR displays suffer from VAC, which is a common issue for stereoscopic 3D displays 93 . In current VR display modules, the distance between the display panel and the viewing optics is fixed, which means the VR imagery is displayed at a single depth. However, the image contents are generated by parallax rendering in three dimensions, offering distinct images for two eyes. This approach offers a proper stimulus to vergence but completely ignores the accommodation cue, which leads to the well-known VAC that can cause an uncomfortable user experience. Since the beginning of this century, numerous methods have been proposed to solve this critical issue. Methods to produce accommodation cue include multifocal/varifocal display 94 , holographic display 95 , and integral imaging display 96 . Alternatively, elimination of accommodation cue using a Maxwellian-view display 93 also helps to mitigate the VAC. However, holographic displays and Maxwellian-view displays generally require a totally different optical architecture than current VR systems. They are therefore more suitable for AR displays, which will be discussed later. Integral imaging, on the other hand, has an inherent tradeoff between view number and resolution. For current VR headsets pursuing high resolution to match human visual acuity, it may not be an appealing solution. Therefore, multifocal/varifocal displays that rely on depth modulation is a relatively practical and effective solution for VR headsets. Regarding the working mechanism, multifocal displays present multiple images with different depths to imitate the original 3D scene. Varifocal displays, in contrast, only show one image at each time frame. The image depth matches the viewer’s vergence depth. Nonetheless, the pre-knowledge of the viewer’s vergence depth requires an additional eye-tracking module. Despite different operation principles, a varifocal display can often be converted to a multifocal display as long as the varifocal module has enough modulation bandwidth to support multiple depths in a time frame.

To achieve depth modulation in a VR system, traditional liquid lens 97 , 98 with tunable focus suffers from the small aperture and large aberrations. Alvarez lens 99 is another tunable-focus solution but it requires mechanical adjustment, which adds to system volume and complexity. In comparison, transmissive LCHOEs with polarization dependency can achieve focus adjustment with electronic driving. Its ultra-thinness also satisfies the requirement of small form factors in VR headsets. The diffractive behavior of transmissive LCHOEs is often interpreted by the mechanism of Pancharatnam-Berry phase (also known as geometric phase) 100 . They are therefore often called Pancharatnam-Berry optical elements (PBOEs). The corresponding lens component is referred as Pancharatnam-Berry lens (PBL).

Two main approaches are used to switch the focus of a PBL, active addressing and passive addressing. In active addressing, the PBL itself (made of LC) can be switched by an applied voltage (Fig. 6a ). The optical power of the liquid crystal PBLs can be turned-on and -off by controlling the voltage. Stacking multiple active PBLs can produce 2 N depths, where N is the number of PBLs. The drawback of using active PBLs, however, is the limited spectral bandwidth since their diffraction efficiency is usually optimized at a single wavelength. In passive addressing, the depth modulation is achieved through changing the polarization state of input light by a switchable half-wave plate (HWP) (Fig. 6b ). The focal length can therefore be switched thanks to the polarization sensitivity of PBLs. Although this approach has a slightly more complicated structure, the overall performance can be better than the active one, because the PBLs made of liquid crystal polymer can be designed to manifest high efficiency within the entire visible spectrum 101 , 102 .

Working principles of a depth switching PBL module based on a active addressing and b passive addressing. c A four-depth multifocal display based on time multiplexing. d A two-depth multifocal display based on polarization multiplexing. Reproduced from c ref. 103 with permission from OSA Publishing and d ref. 104 with permission from OSA Publishing

With the PBL module, multifocal displays can be built using time-multiplexing technique. Zhan et al. 103 demonstrated a four-depth multifocal display using two actively switchable liquid crystal PBLs (Fig. 6c ). The display is synchronized with the PBL module, which lowers the frame rate by the number of depths. Alternatively, multifocal displays can also be achieved by polarization-multiplexing, as demonstrated by Tan et al. 104 . The basic principle is to adjust the polarization state of local pixels so the image content on two focal planes of a PBL can be arbitrarily controlled (Fig. 6d ). The advantage of polarization multiplexing is that it does not sacrifice the frame rate, but it can only support two planes because only two orthogonal polarization states are available. Still, it can be combined with time-multiplexing to reduce the frame rate sacrifice by half. Naturally, varifocal displays can also be built with a PBL module. A fast-response 64-depth varifocal module with six PBLs has been demonstrated 105 .

The compact structure of PBL module leads to a natural solution of integrating it with above-mentioned pancake optics. A compact VR headset with dynamic depth modulation to solve VAC is therefore possible in practice. Still, due to the inherent diffractive nature of PBL, the PBL module face the issue of chromatic dispersion of focal length. To compensate for different focal depths for RGB colors may require additional digital corrections in image-rendering.

Architectures of AR displays

Unlike VR displays with a relatively fixed optical configuration, there exist a vast number of architectures in AR displays. Therefore, instead of following the narrative of tackling different challenges, a more appropriate way to review AR displays is to separately introduce each architecture and discuss its associated engineering challenges. An AR display usually consists of a light engine and an optical combiner. The light engine serves as display image source, while the combiner delivers the displayed images to viewer’s eye and in the meantime transmits the environment light. Some performance parameters like frame rate and power consumption are mainly determined by the light engine. Parameters like FoV, eyebox and MTF are primarily dependent on the combiner optics. Moreover, attributes like image brightness, overall efficiency, and form factor are influenced by both light engine and combiner. In this section, we will firstly discuss the light engine, where the latest advances in micro-LED on chip are reviewed and compared with existing microdisplay systems. Then, we will introduce two main types of combiners: free-space combiner and waveguide combiner.

Light engine

The light engine determines several essential properties of the AR system like image brightness, power consumption, frame rate, and basic etendue. Several types of microdisplays have been used in AR, including micro-LED, micro-organic-light-emitting-diodes (micro-OLED), liquid-crystal-on-silicon (LCoS), digital micromirror device (DMD), and laser beam scanning (LBS) based on micro-electromechanical system (MEMS). We will firstly describe the working principles of these devices and then analyze their performance. For those who are more interested in final performance parameters than details, Table 1 provides a comprehensive summary.

Working principles

Micro-LED and micro-OLED are self-emissive display devices. They are usually more compact than LCoS and DMD because no illumination optics is required. The fundamentally different material systems of LED and OLED lead to different approaches to achieve full-color displays. Due to the “green gap” in LEDs, red LEDs are manufactured on a different semiconductor material from green and blue LEDs. Therefore, how to achieve full-color display in high-resolution density microdisplays is quite a challenge for micro-LEDs. Among several solutions under research are two main approaches. The first is to combine three separate red, green and blue (RGB) micro-LED microdisplay panels 106 . Three single-color micro-LED microdisplays are manufactured separately through flip-chip transfer technology. Then, the projected images from three microdisplay panels are integrated by a trichroic prism (Fig. 7a ).

a RGB micro-LED microdisplays combined by a trichroic prism. b QD-based micro-LED microdisplay. c Micro-OLED display with 4032 PPI. Working principles of d LCoS, e DMD, and f MEMS-LBS display modules. Reprinted from a ref. 106 with permission from IEEE, b ref. 108 with permission from Chinese Laser Press, c ref. 121 with permission from Jon Wiley and Sons, d ref. 124 with permission from Spring Nature, e ref. 126 with permission from Springer and f ref. 128 under the Creative Commons Attribution 4.0 License

Another solution is to assemble color-conversion materials like quantum dot (QD) on top of blue or ultraviolet (UV) micro-LEDs 107 , 108 , 109 (Fig. 7b ). The quantum dot color filter (QDCF) on top of the micro-LED array is mainly fabricated by inkjet printing or photolithography 110 , 111 . However, the display performance of color-conversion micro-LED displays is restricted by the low color-conversion efficiency, blue light leakage, and color crosstalk. Extensive efforts have been conducted to improve the QD-micro-LED performance. To boost QD conversion efficiency, structure designs like nanoring 112 and nanohole 113 , 114 have been proposed, which utilize the Förster resonance energy transfer mechanism to transfer excessive excitons in the LED active region to QD. To prevent blue light leakage, methods using color filters or reflectors like distributed Bragg reflector (DBR) 115 and CLC film 116 on top of QDCF are proposed. Compared to color filters that absorb blue light, DBR and CLC film help recycle the leaked blue light to further excite QDs. Other methods to achieve full-color micro-LED display like vertically stacked RGB micro-LED array 61 , 117 , 118 and monolithic wavelength tunable nanowire LED 119 are also under investigation.

Micro-OLED displays can be generally categorized into RGB OLED and white OLED (WOLED). RGB OLED displays have separate sub-pixel structures and optical cavities, which resonate at the desirable wavelength in RGB channels, respectively. To deposit organic materials onto the separated RGB sub-pixels, a fine metal mask (FMM) that defines the deposition area is required. However, high-resolution RGB OLED microdisplays still face challenges due to the shadow effect during the deposition process through FMM. In order to break the limitation, a silicon nitride film with small shadow has been proposed as a mask for high-resolution deposition above 2000 PPI (9.3 µm) 120 .

WOLED displays use color filters to generate color images. Without the process of depositing patterned organic materials, a high-resolution density up to 4000 PPI has been achieved 121 (Fig. 7c ). However, compared to RGB OLED, the color filters in WOLED absorb about 70% of the emitted light, which limits the maximum brightness of the microdisplay. To improve the efficiency and peak brightness of WOLED microdisplays, in 2019 Sony proposed to apply newly designed cathodes (InZnO) and microlens arrays on OLED microdisplays, which increased the peak brightness from 1600 nits to 5000 nits 120 . In addition, OLEDWORKs has proposed a multi-stacked OLED 122 with optimized microcavities whose emission spectra match the transmission bands of the color filters. The multi-stacked OLED shows a higher luminous efficiency (cd/A), but also requires a higher driving voltage. Recently, by using meta-mirrors as bottom reflective anodes, patterned microcavities with more than 10,000 PPI have been obtained 123 . The high-resolution meta-mirrors generate different reflection phases in the RGB sub-pixels to achieve desirable resonant wavelengths. The narrow emission spectra from the microcavity help to reduce the loss from color filters or even eliminate the need of color filters.

LCoS and DMD are light-modulating displays that generate images by controlling the reflection of each pixel. For LCoS, the light modulation is achieved by manipulating the polarization state of output light through independently controlling the liquid crystal reorientation in each pixel 124 , 125 (Fig. 7d ). Both phase-only and amplitude modulators have been employed. DMD is an amplitude modulation device. The modulation is achieved through controlling the tilt angle of bi-stable micromirrors 126 (Fig. 7e ). To generate an image, both LCoS and DMD rely on the light illumination systems, with LED or laser as light source. For LCoS, the generation of color image can be realized either by RGB color filters on LCoS (with white LEDs) or color-sequential addressing (with RGB LEDs or lasers). However, LCoS requires a linearly polarized light source. For an unpolarized LED light source, usually, a polarization recycling system 127 is implemented to improve the optical efficiency. For a single-panel DMD, the color image is mainly obtained through color-sequential addressing. In addition, DMD does not require a polarized light so that it generally exhibits a higher efficiency than LCoS if an unpolarized light source is employed.

MEMS-based LBS 128 , 129 utilizes micromirrors to directly scan RGB laser beams to form two-dimensional (2D) images (Fig. 7f ). Different gray levels are achieved by pulse width modulation (PWM) of the employed laser diodes. In practice, 2D scanning can be achieved either through a 2D scanning mirror or two 1D scanning mirrors with an additional focusing lens after the first mirror. The small size of MEMS mirror offers a very attractive form factor. At the same time, the output image has a large depth-of-focus (DoF), which is ideal for projection displays. One shortcoming, though, is that the small system etendue often hinders its applications in some traditional display systems.

Comparison of light engine performance

There are several important parameters for a light engine, including image resolution, brightness, frame rate, contrast ratio, and form factor. The resolution requirement (>2K) is similar for all types of light engines. The improvement of resolution is usually accomplished through the manufacturing process. Thus, here we shall focus on other three parameters.

Image brightness usually refers to the measured luminance of a light-emitting object. This measurement, however, may not be accurate for a light engine as the light from engine only forms an intermediate image, which is not directly viewed by the user. On the other hand, to solely focus on the brightness of a light engine could be misleading for a wearable display system like AR. Nowadays, data projectors with thousands of lumens are available. But the power consumption is too high for a battery-powered wearable AR display. Therefore, a more appropriate way to evaluate a light engine’s brightness is to use luminous efficacy (lm/W) measured by dividing the final output luminous flux (lm) by the input electric power (W). For a self-emissive device like micro-LED or micro-OLED, the luminous efficacy is directly determined by the device itself. However, for LCoS and DMD, the overall luminous efficacy should take into consideration the light source luminous efficacy, the efficiency of illumination optics, and the efficiency of the employed spatial light modulator (SLM). For a MEMS LBS engine, the efficiency of MEMS mirror can be considered as unity so that the luminous efficacy basically equals to that of the employed laser sources.

As mentioned earlier, each light engine has a different scheme for generating color images. Therefore, we separately list luminous efficacy of each scheme for a more inclusive comparison. For micro-LEDs, the situation is more complicated because the EQE depends on the chip size. Based on previous studies 130 , 131 , 132 , 133 , we separately calculate the luminous efficacy for RGB micro-LEDs with chip size ≈ 20 µm. For the scheme of direct combination of RGB micro-LEDs, the luminous efficacy is around 5 lm/W. For QD-conversion with blue micro-LEDs, the luminous efficacy is around 10 lm/W with the assumption of 100% color conversion efficiency, which has been demonstrated using structure engineering 114 . For micro-OLEDs, the calculated luminous efficacy is about 4–8 lm/W 120 , 122 . However, the lifetime and EQE of blue OLED materials depend on the driving current. To continuously display an image with brightness higher than 10,000 nits may dramatically shorten the device lifetime. The reason we compare the light engine at 10,000 nits is that it is highly desirable to obtain 1000 nits for the displayed image in order to keep ACR>3:1 with a typical AR combiner whose optical efficiency is lower than 10%.

For an LCoS engine using a white LED as light source, the typical optical efficiency of the whole engine is around 10% 127 , 134 . Then the engine luminous efficacy is estimated to be 12 lm/W with a 120 lm/W white LED source. For a color sequential LCoS using RGB LEDs, the absorption loss from color filters is eliminated, but the luminous efficacy of RGB LED source is also decreased to about 30 lm/W due to lower efficiency of red and green LEDs and higher driving current 135 . Therefore, the final luminous efficacy of the color sequential LCoS engine is also around 10 lm/W. If RGB linearly polarized lasers are employed instead of LEDs, then the LCoS engine efficiency can be quite high due to the high degree of collimation. The luminous efficacy of RGB laser source is around 40 lm/W 136 . Therefore, the laser-based LCoS engine is estimated to have a luminous efficacy of 32 lm/W, assuming the engine optical efficiency is 80%. For a DMD engine with RGB LEDs as light source, the optical efficiency is around 50% 137 , 138 , which leads to a luminous efficacy of 15 lm/W. By switching to laser light sources, the situation is similar to LCoS, with the luminous efficacy of about 32 lm/W. Finally, for MEMS-based LBS engine, there is basically no loss from the optics so that the final luminous efficacy is 40 lm/W. Detailed calculations of luminous efficacy can be found in Supplementary Information .

Another aspect of a light engine is the frame rate, which determines the volume of information it can deliver in a unit time. A high volume of information is vital for the construction of a 3D light field to solve the VAC issue. For micro-LEDs, the device response time is around several nanoseconds, which allows for visible light communication with bandwidth up to 1.5 Gbit/s 139 . For an OLED microdisplay, a fast OLED with ~200 MHz bandwidth has been demonstrated 140 . Therefore, the limitation of frame rate is on the driving circuits for both micro-LED and OLED. Another fact concerning driving circuit is the tradeoff between resolution and frame rate as a higher resolution panel means more scanning lines in each frame. So far, an OLED display with 480 Hz frame rate has been demonstrated 141 . For an LCoS, the frame rate is mainly limited by the LC response time. Depending on the LC material used, the response time is around 1 ms for nematic LC or 200 µs for ferroelectric LC (FLC) 125 . Nematic LC allows analog driving, which accommodates gray levels, typically with 8-bit depth. FLC is bistable so that PWM is used to generate gray levels. DMD is also a binary device. The frame rate can reach 30 kHz, which is mainly constrained by the response time of micromirrors. For MEMS-based LBS, the frame rate is limited by the scanning frequency of MEMS mirrors. A frame rate of 60 Hz with around 1 K resolution already requires a resonance frequency of around 50 kHz, with a Q-factor up to 145,000 128 . A higher frame rate or resolution requires a higher Q-factor and larger laser modulation bandwidth, which may be challenging.

Form factor is another crucial aspect for the light engines of near-eye displays. For self-emissive displays, both micro-OLEDs and QD-based micro-LEDs can achieve full color with a single panel. Thus, they are quite compact. A micro-LED display with separate RGB panels naturally have a larger form factor. In applications requiring direct-view full-color panel, the extra combining optics may also increase the volume. It needs to be pointed out, however, that the combing optics may not be necessary for some applications like waveguide displays, because the EPE process results in system’s insensitivity to the spatial positions of input RGB images. Therefore, the form factor of using three RGB micro-LED panels is medium. For LCoS and DMD with RGB LEDs as light source, the form factor would be larger due to the illumination optics. Still, if a lower luminous efficacy can be accepted, then a smaller form factor can be achieved by using a simpler optics 142 . If RGB lasers are used, the collimation optics can be eliminated, which greatly reduces the form factor 143 . For MEMS-LBS, the form factor can be extremely compact due to the tiny size of MEMS mirror and laser module.

Finally, contrast ratio (CR) also plays an important role affecting the observed images 8 . Micro-LEDs and micro-OLEDs are self-emissive so that their CR can be >10 6 :1. For a laser beam scanner, its CR can also achieve 10 6 :1 because the laser can be turned off completely at dark state. On the other hand, LCoS and DMD are reflective displays, and their CR is around 2000:1 to 5000:1 144 , 145 . It is worth pointing out that the CR of a display engine plays a significant role only in the dark ambient. As the ambient brightness increases, the ACR is mainly governed by the display’s peak brightness, as previously discussed.

The performance parameters of different light engines are summarized in Table 1 . Micro-LEDs and micro-OLEDs have similar levels of luminous efficacy. But micro-OLEDs still face the burn-in and lifetime issue when driving at a high current, which hinders its use for a high-brightness image source to some extent. Micro-LEDs are still under active development and the improvement on luminous efficacy from maturing fabrication process could be expected. Both devices have nanosecond response time and can potentially achieve a high frame rate with a well-designed integrated circuit. The frame rate of the driving circuit ultimately determines the motion picture response time 146 . Their self-emissive feature also leads to a small form factor and high contrast ratio. LCoS and DMD engines have similar performance of luminous efficacy, form factor, and contrast ratio. In terms of light modulation, DMD can provide a higher 1-bit frame rate, while LCoS can offer both phase and amplitude modulations. MEMS-based LBS exhibits the highest luminous efficacy so far. It also exhibits an excellent form factor and contrast ratio, but the presently demonstrated 60-Hz frame rate (limited by the MEMS mirrors) could cause image flickering.

Free-space combiners

The term ‘free-space’ generally refers to the case when light is freely propagating in space, as opposed to a waveguide that traps light into TIRs. Regarding the combiner, it can be a partial mirror, as commonly used in AR systems based on traditional geometric optics. Alternatively, the combiner can also be a reflective HOE. The strong chromatic dispersion of HOE necessitates the use of a laser source, which usually leads to a Maxwellian-type system.

Traditional geometric designs

Several systems based on geometric optics are illustrated in Fig. 8 . The simplest design uses a single freeform half-mirror 6 , 147 to directly collimate the displayed images to the viewer’s eye (Fig. 8a ). This design can achieve a large FoV (up to 90°) 147 , but the limited design freedom with a single freeform surface leads to image distortions, also called pupil swim 6 . The placement of half-mirror also results in a relatively bulky form factor. Another design using so-called birdbath optics 6 , 148 is shown in Fig. 8b . Compared to the single-combiner design, birdbath design has an extra optics on the display side, which provides space for aberration correction. The integration of beam splitter provides a folded optical path, which reduces the form factor to some extent. Another way to fold optical path is to use a TIR-prism. Cheng et al. 149 designed a freeform TIR-prism combiner (Fig. 8c ) offering a diagonal FoV of 54° and exit pupil diameter of 8 mm. All the surfaces are freeform, which offer an excellent image quality. To cancel the optical power for the transmitted environmental light, a compensator is added to the TIR prism. The whole system has a well-balanced performance between FoV, eyebox, and form factor. To release the space in front of viewer’s eye, relay optics can be used to form an intermediate image near the combiner 150 , 151 , as illustrated in Fig. 8d . Although the design offers more optical surfaces for aberration correction, the extra lenses also add to system weight and form factor.

a Single freeform surface as the combiner. b Birdbath optics with a beam splitter and a half mirror. c Freeform TIR prism with a compensator. d Relay optics with a half mirror. Adapted from c ref. 149 with permission from OSA Publishing and d ref. 151 with permission from OSA Publishing

Regarding the approaches to solve the VAC issue, the most straightforward way is to integrate a tunable lens into the optical path, like a liquid lens 152 or Alvarez lens 99 , to form a varifocal system. Alternatively, integral imaging 153 , 154 can also be used, by replacing the original display panel with the central depth plane of an integral imaging module. The integral imaging can also be combined with varifocal approach to overcome the tradeoff between resolution and depth of field (DoF) 155 , 156 , 157 . However, the inherent tradeoff between resolution and view number still exists in this case.

Overall, AR displays based on traditional geometric optics have a relatively simple design with a decent FoV (~60°) and eyebox (8 mm) 158 . They also exhibit a reasonable efficiency. To measure the efficiency of an AR combiner, an appropriate measure is to divide the output luminance (unit: nit) by the input luminous flux (unit: lm), which we note as combiner efficiency. For a fixed input luminous flux, the output luminance, or image brightness, is related to the FoV and exit pupil of the combiner system. If we assume no light waste of the combiner system, then the maximum combiner efficiency for a typical diagonal FoV of 60° and exit pupil (10 mm square) is around 17,000 nit/lm (Eq. S2 ). To estimate the combiner efficiency of geometric combiners, we assume 50% of half-mirror transmittance and the efficiency of other optics to be 50%. Then the final combiner efficiency is about 4200 nit/lm, which is a high value in comparison with waveguide combiners. Nonetheless, to further shrink the system size or improve system performance ultimately encounters the etendue conservation issue. In addition, AR systems with traditional geometric optics is hard to achieve a configuration resembling normal flat glasses because the half-mirror has to be tilted to some extent.

Maxwellian-type systems

The Maxwellian view, proposed by James Clerk Maxwell (1860), refers to imaging a point light source in the eye pupil 159 . If the light beam is modulated in the imaging process, a corresponding image can be formed on the retina (Fig. 9a ). Because the point source is much smaller than the eye pupil, the image is always-in-focus on the retina irrespective of the eye lens’ focus. For applications in AR display, the point source is usually a laser with narrow angular and spectral bandwidths. LED light sources can also build a Maxwellian system, by adding an angular filtering module 160 . Regarding the combiner, although in theory a half-mirror can also be used, HOEs are generally preferred because they offer the off-axis configuration that places combiner in a similar position like eyeglasses. In addition, HOEs have a lower reflection of environment light, which provides a more natural appearance of the user behind the display.

a Schematic of the working principle of Maxwellian displays. Maxwellian displays based on b SLM and laser diode light source and c MEMS-LBS with a steering mirror as additional modulation method. Generation of depth cues by d computational digital holography and e scanning of steering mirror to produce multiple views. Adapted from b, d ref. 143 and c, e ref. 167 under the Creative Commons Attribution 4.0 License

To modulate the light, a SLM like LCoS or DMD can be placed in the light path, as shown in Fig. 9b . Alternatively, LBS system can also be used (Fig. 9c ), where the intensity modulation occurs in the laser diode itself. Besides the operation in a normal Maxwellian-view, both implementations offer additional degrees of freedom for light modulation.

For a SLM-based system, there are several options to arrange the SLM pixels 143 , 161 . Maimone et al. 143 demonstrated a Maxwellian AR display with two modes to offer a large-DoF Maxwellian-view, or a holographic view (Fig. 9d ), which is often referred as computer-generated holography (CGH) 162 . To show an always-in-focus image with a large DoF, the image can be directly displayed on an amplitude SLM, or using amplitude encoding for a phase-only SLM 163 . Alternatively, if a 3D scene with correct depth cues is to be presented, then optimization algorithms for CGH can be used to generate a hologram for the SLM. The generated holographic image exhibits the natural focus-and-blur effect like a real 3D object (Fig. 9d ). To better understand this feature, we need to again exploit the concept of etendue. The laser light source can be considered to have a very small etendue due to its excellent collimation. Therefore, the system etendue is provided by the SLM. The micron-sized pixel-pitch of SLM offers a certain maximum diffraction angle, which, multiplied by the SLM size, equals system etendue. By varying the display content on SLM, the final exit pupil size can be changed accordingly. In the case of a large-DoF Maxwellian view, the exit pupil size is small, accompanied by a large FoV. For the holographic display mode, the reduced DoF requires a larger exit pupil with dimension close to the eye pupil. But the FoV is reduced accordingly due to etendue conservation. Another commonly concerned issue with CGH is the computation time. To achieve a real-time CGH rendering flow with an excellent image quality is quite a challenge. Fortunately, with recent advances in algorithm 164 and the introduction of convolutional neural network (CNN) 165 , 166 , this issue is gradually solved with an encouraging pace. Lately, Liang et al. 166 demonstrated a real-time CGH synthesis pipeline with a high image quality. The pipeline comprises an efficient CNN model to generate a complex hologram from a 3D scene and an improved encoding algorithm to convert the complex hologram to a phase-only one. An impressive frame rate of 60 Hz has been achieved on a desktop computing unit.

For LBS-based system, the additional modulation can be achieved by integrating a steering module, as demonstrated by Jang et al. 167 . The steering mirror can shift the focal point (viewpoint) within the eye pupil, therefore effectively expanding the system etendue. When the steering process is fast and the image content is updated simultaneously, correct 3D cues can be generated, as shown in Fig. 9e . However, there exists a tradeoff between the number of viewpoint and the final image frame rate, because the total frames are equally divided into each viewpoint. To boost the frame rate of MEMS-LBS systems by the number of views (e.g., 3 by 3) may be challenging.

Maxwellian-type systems offer several advantages. The system efficiency is usually very high because nearly all the light is delivered into viewer’s eye. The system FoV is determined by the f /# of combiner and a large FoV (~80° in horizontal) can be achieved 143 . The issue of VAC can be mitigated with an infinite-DoF image that deprives accommodation cue, or completely solved by generating a true-3D scene as discussed above. Despite these advantages, one major weakness of Maxwellian-type system is the tiny exit pupil, or eyebox. A small deviation of eye pupil location from the viewpoint results in the complete disappearance of the image. Therefore, to expand eyebox is considered as one of the most important challenges in Maxwellian-type systems.

Pupil duplication and steering

Methods to expand eyebox can be generally categorized into pupil duplication 168 , 169 , 170 , 171 , 172 and pupil steering 9 , 13 , 167 , 173 . Pupil duplication simply generates multiple viewpoints to cover a large area. In contrast, pupil steering dynamically shifts the viewpoint position, depending on the pupil location. Before reviewing detailed implementations of these two methods, it is worth discussing some of their general features. The multiple viewpoints in pupil duplication usually mean to equally divide the total light intensity. In each time frame, however, it is preferable that only one viewpoint enters the user’s eye pupil to avoid ghost image. This requirement, therefore, results in a reduced total light efficiency, while also conditioning the viewpoint separation to be larger than the pupil diameter. In addition, the separation should not be too large to avoid gap between viewpoints. Considering that human pupil diameter changes in response to environment illuminance, the design of viewpoint separation needs special attention. Pupil steering, on the other hand, only produces one viewpoint at each time frame. It is therefore more light-efficient and free from ghost images. But to determine the viewpoint position requires the information of eye pupil location, which demands a real-time eye-tracking module 9 . Another observation is that pupil steering can accommodate multiple viewpoints by its nature. Therefore, a pupil steering system can often be easily converted to a pupil duplication system by simultaneously generating available viewpoints.

To generate multiple viewpoints, one can focus on modulating the incident light or the combiner. Recall that viewpoint is the image of light source. To duplicate or shift light source can achieve pupil duplication or steering accordingly, as illustrated in Fig. 10a . Several schemes of light modulation are depicted in Fig. 10b–e . An array of light sources can be generated with multiple laser diodes (Fig. 10b ). To turn on all or one of the sources achieves pupil duplication or steering. A light source array can also be produced by projecting light on an array-type PPHOE 168 (Fig. 10c ). Apart from direct adjustment of light sources, modulating light on the path can also effectively steer/duplicate the light sources. Using a mechanical steering mirror, the beam can be deflected 167 (Fig. 10d ), which equals to shifting the light source position. Other devices like a grating or beam splitter can also serve as ray deflector/splitter 170 , 171 (Fig. 10e ).

a Schematic of duplicating (or shift) viewpoint by modulation of incident light. Light modulation by b multiple laser diodes, c HOE lens array, d steering mirror and e grating or beam splitters. f Pupil duplication with multiplexed PPHOE. g Pupil steering with LCHOE. Reproduced from c ref. 168 under the Creative Commons Attribution 4.0 License, e ref. 169 with permission from OSA Publishing, f ref. 171 with permission from OSA Publishing and g ref. 173 with permission from OSA Publishing

Nonetheless, one problem of the light source duplication/shifting methods for pupil duplication/steering is that the aberrations in peripheral viewpoints are often serious 168 , 173 . The HOE combiner is usually recorded at one incident angle. For other incident angles with large deviations, considerable aberrations will occur, especially in the scenario of off-axis configuration. To solve this problem, the modulation can be focused on the combiner instead. While the mechanical shifting of combiner 9 can achieve continuous pupil steering, its integration into AR display with a small factor remains a challenge. Alternatively, the versatile functions of HOE offer possible solutions for combiner modulation. Kim and Park 169 demonstrated a pupil duplication system with multiplexed PPHOE (Fig. 10f ). Wavefronts of several viewpoints can be recorded into one PPHOE sample. Three viewpoints with a separation of 3 mm were achieved. However, a slight degree of ghost image and gap can be observed in the viewpoint transition. For a PPHOE to achieve pupil steering, the multiplexed PPHOE needs to record different focal points with different incident angles. If each hologram has no angular crosstalk, then with an additional device to change the light incident angle, the viewpoint can be steered. Alternatively, Xiong et al. 173 demonstrated a pupil steering system with LCHOEs in a simpler configuration (Fig. 10g ). The polarization-sensitive nature of LCHOE enables the controlling of which LCHOE to function with a polarization converter (PC). When the PC is off, the incident RCP light is focused by the right-handed LCHOE. When the PC is turned on, the RCP light is firstly converted to LCP light and passes through the right-handed LCHOE. Then it is focused by the left-handed LCHOE into another viewpoint. To add more viewpoints requires stacking more pairs of PC and LCHOE, which can be achieved in a compact manner with thin glass substrates. In addition, to realize pupil duplication only requires the stacking of multiple low-efficiency LCHOEs. For both PPHOEs and LCHOEs, because the hologram for each viewpoint is recorded independently, the aberrations can be eliminated.

Regarding the system performance, in theory the FoV is not limited and can reach a large value, such as 80° in horizontal direction 143 . The definition of eyebox is different from traditional imaging systems. For a single viewpoint, it has the same size as the eye pupil diameter. But due to the viewpoint steering/duplication capability, the total system eyebox can be expanded accordingly. The combiner efficiency for pupil steering systems can reach 47,000 nit/lm for a FoV of 80° by 80° and pupil diameter of 4 mm (Eq. S2 ). At such a high brightness level, eye safety could be a concern 174 . For a pupil duplication system, the combiner efficiency is decreased by the number of viewpoints. With a 4-by-4 viewpoint array, it can still reach 3000 nit/lm. Despite the potential gain of pupil duplication/steering, when considering the rotation of eyeball, the situation becomes much more complicated 175 . A perfect pupil steering system requires a 5D steering, which proposes a challenge for practical implementation.

Pin-light systems

Recently, another type of display in close relation with Maxwellian view called pin-light display 148 , 176 has been proposed. The general working principle of pin-light display is illustrated in Fig. 11a . Each pin-light source is a Maxwellian view with a large DoF. When the eye pupil is no longer placed near the source point as in Maxwellian view, each image source can only form an elemental view with a small FoV on retina. However, if the image source array is arranged in a proper form, the elemental views can be integrated together to form a large FoV. According to the specific optical architectures, pin-light display can take different forms of implementation. In the initial feasibility demonstration, Maimone et al. 176 used a side-lit waveguide plate as the point light source (Fig. 11b ). The light inside the waveguide plate is extracted by the etched divots, forming a pin-light source array. A transmissive SLM (LCD) is placed behind the waveguide plate to modulate the light intensity and form the image. The display has an impressive FoV of 110° thanks to the large scattering angle range. However, the direct placement of LCD before the eye brings issues of insufficient resolution density and diffraction of background light.

a Schematic drawing of the working principle of pin-light display. b Pin-light display utilizing a pin-light source and a transmissive SLM. c An example of pin-mirror display with a birdbath optics. d SWD system with LBS image source and off-axis lens array. Reprinted from b ref. 176 under the Creative Commons Attribution 4.0 License and d ref. 180 with permission from OSA Publishing

To avoid these issues, architectures using pin-mirrors 177 , 178 , 179 are proposed. In these systems, the final combiner is an array of tiny mirrors 178 , 179 or gratings 177 , in contrast to their counterparts using large-area combiners. An exemplary system with birdbath design is depicted in Fig. 11c . In this case, the pin-mirrors replace the original beam-splitter in the birdbath and can thus shrink the system volume, while at the same time providing large DoF pin-light images. Nonetheless, such a system may still face the etendue conservation issue. Meanwhile, the size of pin-mirror cannot be too small in order to prevent degradation of resolution density due to diffraction. Therefore, its influence on the see-through background should also be considered in the system design.

To overcome the etendue conservation and improve see-through quality, Xiong et al. 180 proposed another type of pin-light system exploiting the etendue expansion property of waveguide, which is also referred as scanning waveguide display (SWD). As illustrated in Fig. 11d , the system uses an LBS as the image source. The collimated scanned laser rays are trapped in the waveguide and encounter an array of off-axis lenses. Upon each encounter, the lens out-couples the laser rays and forms a pin-light source. SWD has the merits of good see-through quality and large etendue. A large FoV of 100° was demonstrated with the help of an ultra-low f /# lens array based on LCHOE. However, some issues like insufficient image resolution density and image non-uniformity remain to be overcome. To further improve the system may require optimization of Gaussian beam profile and additional EPE module 180 .

Overall, pin-light systems inherit the large DoF from Maxwellian view. With adequate number of pin-light sources, the FoV and eyebox can be expanded accordingly. Nonetheless, despite different forms of implementation, a common issue of pin-light system is the image uniformity. The overlapped region of elemental views has a higher light intensity than the non-overlapped region, which becomes even more complicated considering the dynamic change of pupil size. In theory, the displayed image can be pre-processed to compensate for the optical non-uniformity. But that would require knowledge of precise pupil location (and possibly size) and therefore an accurate eye-tracking module 176 . Regarding the system performance, pin-mirror systems modified from other free-space systems generally shares similar FoV and eyebox with original systems. The combiner efficiency may be lower due to the small size of pin-mirrors. SWD, on the other hand, shares the large FoV and DoF with Maxwellian view, and large eyebox with waveguide combiners. The combiner efficiency may also be lower due to the EPE process.

Waveguide combiner

Besides free-space combiners, another common architecture in AR displays is waveguide combiner. The term ‘waveguide’ indicates the light is trapped in a substrate by the TIR process. One distinctive feature of a waveguide combiner is the EPE process that effectively enlarges the system etendue. In the EPE process, a portion of the trapped light is repeatedly coupled out of the waveguide in each TIR. The effective eyebox is therefore enlarged. According to the features of couplers, we divide the waveguide combiners into two types: diffractive and achromatic, as described in the followings.

Diffractive waveguides

As the name implies, diffractive-type waveguides use diffractive elements as couplers. The in-coupler is usually a diffractive grating and the out-coupler in most cases is also a grating with the same period as the in-coupler, but it can also be an off-axis lens with a small curvature to generate image with finite depth. Three major diffractive couplers have been developed: SRGs, photopolymer gratings (PPGs), and liquid crystal gratings (grating-type LCHOE; also known as polarization volume gratings (PVGs)). Some general protocols for coupler design are that the in-coupler should have a relatively high efficiency and the out-coupler should have a uniform light output. A uniform light output usually requires a low-efficiency coupler, with extra degrees of freedom for local modulation of coupling efficiency. Both in-coupler and out-coupler should have an adequate angular bandwidth to accommodate a reasonable FoV. In addition, the out-coupler should also be optimized to avoid undesired diffractions, including the outward diffraction of TIR light and diffraction of environment light into user’s eyes, which are referred as light leakage and rainbow. Suppression of these unwanted diffractions should also be considered in the optimization process of waveguide design, along with performance parameters like efficiency and uniformity.

The basic working principles of diffractive waveguide-based AR systems are illustrated in Fig. 12 . For the SRG-based waveguides 6 , 8 (Fig. 12a ), the in-coupler can be a transmissive-type or a reflective-type 181 , 182 . The grating geometry can be optimized for coupling efficiency with a large degree of freedom 183 . For the out-coupler, a reflective SRG with a large slant angle to suppress the transmission orders is preferred 184 . In addition, a uniform light output usually requires a gradient efficiency distribution in order to compensate for the decreased light intensity in the out-coupling process. This can be achieved by varying the local grating configurations like height and duty cycle 6 . For the PPG-based waveguides 185 (Fig. 12b ), the small angular bandwidth of a high-efficiency transmissive PPG prohibits its use as in-coupler. Therefore, both in-coupler and out-coupler are usually reflective types. The gradient efficiency can be achieved by space-variant exposure to control the local index modulation 186 or local Bragg slant angle variation through freeform exposure 19 . Due to the relatively small angular bandwidth of PPG, to achieve a decent FoV usually requires stacking two 187 or three 188 PPGs together for a single color. The PVG-based waveguides 189 (Fig. 12c ) also prefer reflective PVGs as in-couplers because the transmissive PVGs are much more difficult to fabricate due to the LC alignment issue. In addition, the angular bandwidth of transmissive PVGs in Bragg regime is also not large enough to support a decent FoV 29 . For the out-coupler, the angular bandwidth of a single reflective PVG can usually support a reasonable FoV. To obtain a uniform light output, a polarization management layer 190 consisting of a LC layer with spatially variant orientations can be utilized. It offers an additional degree of freedom to control the polarization state of the TIR light. The diffraction efficiency can therefore be locally controlled due to the strong polarization sensitivity of PVG.

Schematics of waveguide combiners based on a SRGs, b PPGs and c PVGs. Reprinted from a ref. 85 with permission from OSA Publishing, b ref. 185 with permission from John Wiley and Sons and c ref. 189 with permission from OSA Publishing

The above discussion describes the basic working principle of 1D EPE. Nonetheless, for the 1D EPE to produce a large eyebox, the exit pupil in the unexpanded direction of the original image should be large. This proposes design challenges in light engines. Therefore, a 2D EPE is favored for practical applications. To extend EPE in two dimensions, two consecutive 1D EPEs can be used 191 , as depicted in Fig. 13a . The first 1D EPE occurs in the turning grating, where the light is duplicated in y direction and then turned into x direction. Then the light rays encounter the out-coupler and are expanded in x direction. To better understand the 2D EPE process, the k -vector diagram (Fig. 13b ) can be used. For the light propagating in air with wavenumber k 0 , its possible k -values in x and y directions ( k x and k y ) fall within the circle with radius k 0 . When the light is trapped into TIR, k x and k y are outside the circle with radius k 0 and inside the circle with radius nk 0 , where n is the refractive index of the substrate. k x and k y stay unchanged in the TIR process and are only changed in each diffraction process. The central red box in Fig. 13b indicates the possible k values within the system FoV. After the in-coupler, the k values are added by the grating k -vector, shifting the k values into TIR region. The turning grating then applies another k -vector and shifts the k values to near x -axis. Finally, the k values are shifted by the out-coupler and return to the free propagation region in air. One observation is that the size of red box is mostly limited by the width of TIR band. To accommodate a larger FoV, the outer boundary of TIR band needs to be expanded, which amounts to increasing waveguide refractive index. Another important fact is that when k x and k y are near the outer boundary, the uniformity of output light becomes worse. This is because the light propagation angle is near 90° in the waveguide. The spatial distance between two consecutive TIRs becomes so large that the out-coupled beams are spatially separated to an unacceptable degree. The range of possible k values for practical applications is therefore further shrunk due to this fact.

a Schematic of 2D EPE based on two consecutive 1D EPEs. Gray/black arrows indicate light in air/TIR. Black dots denote TIRs. b k-diagram of the two-1D-EPE scheme. c Schematic of 2D EPE with a 2D hexagonal grating d k-diagram of the 2D-grating scheme

Aside from two consecutive 1D EPEs, the 2D EPE can also be directly implemented with a 2D grating 192 . An example using a hexagonal grating is depicted in Fig. 13c . The hexagonal grating can provide k -vectors in six directions. In the k -diagram (Fig. 13d ), after the in-coupling, the k values are distributed into six regions due to multiple diffractions. The out-coupling occurs simultaneously with pupil expansion. Besides a concise out-coupler configuration, the 2D EPE scheme offers more degrees of design freedom than two 1D EPEs because the local grating parameters can be adjusted in a 2D manner. The higher design freedom has the potential to reach a better output light uniformity, but at the cost of a higher computation demand for optimization. Furthermore, the unslanted grating geometry usually leads to a large light leakage and possibly low efficiency. Adding slant to the geometry helps alleviate the issue, but the associated fabrication may be more challenging.

Finally, we discuss the generation of full-color images. One important issue to clarify is that although diffractive gratings are used here, the final image generally has no color dispersion even if we use a broadband light source like LED. This can be easily understood in the 1D EPE scheme. The in-coupler and out-coupler have opposite k -vectors, which cancels the color dispersion for each other. In the 2D EPE schemes, the k -vectors always form a closed loop from in-coupled light to out-coupled light, thus, the color dispersion also vanishes likewise. The issue of using a single waveguide for full-color images actually exists in the consideration of FoV and light uniformity. The breakup of propagation angles for different colors results in varied out-coupling situations for each color. To be more specific, if the red and the blue channels use the same in-coupler, the propagating angle for the red light is larger than that of the blue light. The red light in peripheral FoV is therefore easier to face the mentioned large-angle non-uniformity issue. To acquire a decent FoV and light uniformity, usually two or three layers of waveguides with different grating pitches are adopted.

Regarding the system performance, the eyebox is generally large enough (~10 mm) to accommodate different user’s IPD and alignment shift during operation. A parameter of significant concern for a waveguide combiner is its FoV. From the k -vector analysis, we can conclude the theoretical upper limit is determined by the waveguide refractive index. But the light/color uniformity also influences the effective FoV, over which the degradation of image quality becomes unacceptable. Current diffractive waveguide combiners generally achieve a FoV of about 50°. To further increase FoV, a straightforward method is to use a higher refractive index waveguide. Another is to tile FoV through direct stacking of multiple waveguides or using polarization-sensitive couplers 79 , 193 . As to the optical efficiency, a typical value for the diffractive waveguide combiner is around 50–200 nit/lm 6 , 189 . In addition, waveguide combiners adopting grating out-couplers generate an image with fixed depth at infinity. This leads to the VAC issue. To tackle VAC in waveguide architectures, the most practical way is to generate multiple depths and use the varifocal or multifocal driving scheme, similar to those mentioned in the VR systems. But to add more depths usually means to stack multiple layers of waveguides together 194 . Considering the additional waveguide layers for RGB colors, the final waveguide thickness would undoubtedly increase.

Other parameters special to waveguide includes light leakage, see-through ghost, and rainbow. Light leakage refers to out-coupled light that goes outwards to the environment, as depicted in Fig. 14a . Aside from decreased efficiency, the leakage also brings drawback of unnatural “bright-eye” appearance of the user and privacy issue. Optimization of the grating structure like geometry of SRG may reduce the leakage. See-through ghost is formed by consecutive in-coupling and out-couplings caused by the out-coupler grating, as sketched in Fig. 14b , After the process, a real object with finite depth may produce a ghost image with shift in both FoV and depth. Generally, an out-coupler with higher efficiency suffers more see-through ghost. Rainbow is caused by the diffraction of environment light into user’s eye, as sketched in Fig. 14c . The color dispersion in this case will occur because there is no cancellation of k -vector. Using the k -diagram, we can obtain a deeper insight into the formation of rainbow. Here, we take the EPE structure in Fig. 13a as an example. As depicted in Fig. 14d , after diffractions by the turning grating and the out-coupler grating, the k values are distributed in two circles that shift from the origin by the grating k -vectors. Some diffracted light can enter the see-through FoV and form rainbow. To reduce rainbow, a straightforward way is to use a higher index substrate. With a higher refractive index, the outer boundary of k diagram is expanded, which can accommodate larger grating k -vectors. The enlarged k -vectors would therefore “push” these two circles outwards, leading to a decreased overlapping region with the see-through FoV. Alternatively, an optimized grating structure would also help reduce the rainbow effect by suppressing the unwanted diffraction.

Sketches of formations of a light leakage, b see-through ghost and c rainbow. d Analysis of rainbow formation with k-diagram

Achromatic waveguide

Achromatic waveguide combiners use achromatic elements as couplers. It has the advantage of realizing full-color image with a single waveguide. A typical example of achromatic element is a mirror. The waveguide with partial mirrors as out-coupler is often referred as geometric waveguide 6 , 195 , as depicted in Fig. 15a . The in-coupler in this case is usually a prism to avoid unnecessary color dispersion if using diffractive elements otherwise. The mirrors couple out TIR light consecutively to produce a large eyebox, similarly in a diffractive waveguide. Thanks to the excellent optical property of mirrors, the geometric waveguide usually exhibits a superior image regarding MTF and color uniformity to its diffractive counterparts. Still, the spatially discontinuous configuration of mirrors also results in gaps in eyebox, which may be alleviated by using a dual-layer structure 196 . Wang et al. designed a geometric waveguide display with five partial mirrors (Fig. 15b ). It exhibits a remarkable FoV of 50° by 30° (Fig. 15c ) and an exit pupil of 4 mm with a 1D EPE. To achieve 2D EPE, similar architectures in Fig. 13a can be used by integrating a turning mirror array as the first 1D EPE module 197 . Unfortunately, the k -vector diagrams in Fig. 13b, d cannot be used here because the k values in x-y plane no longer conserve in the in-coupling and out-coupling processes. But some general conclusions remain valid, like a higher refractive index leading to a larger FoV and gradient out-coupling efficiency improving light uniformity.

a Schematic of the system configuration. b Geometric waveguide with five partial mirrors. c Image photos demonstrating system FoV. Adapted from b , c ref. 195 with permission from OSA Publishing

The fabrication process of geometric waveguide involves coating mirrors on cut-apart pieces and integrating them back together, which may result in a high cost, especially for the 2D EPE architecture. Another way to implement an achromatic coupler is to use multiplexed PPHOE 198 , 199 to mimic the behavior of a tilted mirror (Fig. 16a ). To understand the working principle, we can use the diagram in Fig. 16b . The law of reflection states the angle of reflection equals to the angle of incidence. If we translate this behavior to k -vector language, it means the mirror can apply any length of k -vector along its surface normal direction. The k -vector length of the reflected light is always equal to that of the incident light. This puts a condition that the k -vector triangle is isosceles. With a simple geometric deduction, it can be easily observed this leads to the law of reflection. The behavior of a general grating, however, is very different. For simplicity we only consider the main diffraction order. The grating can only apply a k -vector with fixed k x due to the basic diffraction law. For the light with a different incident angle, it needs to apply different k z to produce a diffracted light with equal k -vector length as the incident light. For a grating with a broad angular bandwidth like SRG, the range of k z is wide, forming a lengthy vertical line in Fig. 16b . For a PPG with a narrow angular bandwidth, the line is short and resembles a dot. If multiple of these tiny dots are distributed along the oblique line corresponding to a mirror, then the final multiplexed PPGs can imitate the behavior of a tilted mirror. Such a PPHOE is sometimes referred as a skew-mirror 198 . In theory, to better imitate the mirror, a lot of multiplexed PPGs is preferred, while each PPG has a small index modulation δn . But this proposes a bigger challenge in device fabrication. Recently, Utsugi et al. demonstrated an impressive skew-mirror waveguide based on 54 multiplexed PPGs (Fig. 16c, d ). The display exhibits an effective FoV of 35° by 36°. In the peripheral FoV, there still exists some non-uniformity (Fig. 16e ) due to the out-coupling gap, which is an inherent feature of the flat-type out-couplers.

a System configuration. b Diagram demonstrating how multiplexed PPGs resemble the behavior of a mirror. Photos showing c the system and d image. e Picture demonstrating effective system FoV. Adapted from c – e ref. 199 with permission from ITE

Finally, it is worth mentioning that metasurfaces are also promising to deliver achromatic gratings 200 , 201 for waveguide couplers ascribed to their versatile wavefront shaping capability. The mechanism of the achromatic gratings is similar to that of the achromatic lenses as previously discussed. However, the current development of achromatic metagratings is still in its infancy. Much effort is needed to improve the optical efficiency for in-coupling, control the higher diffraction orders for eliminating ghost images, and enable a large size design for EPE.

Generally, achromatic waveguide combiners exhibit a comparable FoV and eyebox with diffractive combiners, but with a higher efficiency. For a partial-mirror combiner, its combiner efficiency is around 650 nit/lm 197 (2D EPE). For a skew-mirror combiner, although the efficiency of multiplexed PPHOE is relatively low (~1.5%) 199 , the final combiner efficiency of the 1D EPE system is still high (>3000 nit/lm) due to multiple out-couplings.

Table 2 summarizes the performance of different AR combiners. When combing the luminous efficacy in Table 1 and the combiner efficiency in Table 2 , we can have a comprehensive estimate of the total luminance efficiency (nit/W) for different types of systems. Generally, Maxwellian-type combiners with pupil steering have the highest luminance efficiency when partnered with laser-based light engines like laser-backlit LCoS/DMD or MEM-LBS. Geometric optical combiners have well-balanced image performances, but to further shrink the system size remains a challenge. Diffractive waveguides have a relatively low combiner efficiency, which can be remedied by an efficient light engine like MEMS-LBS. Further development of coupler and EPE scheme would also improve the system efficiency and FoV. Achromatic waveguides have a decent combiner efficiency. The single-layer design also enables a smaller form factor. With advances in fabrication process, it may become a strong contender to presently widely used diffractive waveguides.

Conclusions and perspectives

VR and AR are endowed with a high expectation to revolutionize the way we interact with digital world. Accompanied with the expectation are the engineering challenges to squeeze a high-performance display system into a tightly packed module for daily wearing. Although the etendue conservation constitutes a great obstacle on the path, remarkable progresses with innovative optics and photonics continue to take place. Ultra-thin optical elements like PPHOEs and LCHOEs provide alternative solutions to traditional optics. Their unique features of multiplexing capability and polarization dependency further expand the possibility of novel wavefront modulations. At the same time, nanoscale-engineered metasurfaces/SRGs provide large design freedoms to achieve novel functions beyond conventional geometric optical devices. Newly emerged micro-LEDs open an opportunity for compact microdisplays with high peak brightness and good stability. Further advances on device engineering and manufacturing process are expected to boost the performance of metasurfaces/SRGs and micro-LEDs for AR and VR applications.

Data availability

All data needed to evaluate the conclusions in the paper are present in the paper. Additional data related to this paper may be requested from the authors.

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The authors are indebted to Goertek Electronics for the financial support and Guanjun Tan for helpful discussions.

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Jianghao Xiong, En-Lin Hsiang, Ziqian He, Tao Zhan & Shin-Tson Wu

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J.X. conceived the idea and initiated the project. J.X. mainly wrote the manuscript and produced the figures. E.-L.H., Z.H., and T.Z. contributed to parts of the manuscript. S.W. supervised the project and edited the manuscript.

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Xiong, J., Hsiang, EL., He, Z. et al. Augmented reality and virtual reality displays: emerging technologies and future perspectives. Light Sci Appl 10 , 216 (2021). https://doi.org/10.1038/s41377-021-00658-8

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Received : 06 June 2021

Revised : 26 September 2021

Accepted : 04 October 2021

Published : 25 October 2021

DOI : https://doi.org/10.1038/s41377-021-00658-8

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