Organic light‐emitting diodes (OLEDs) have become a mainstream display technology in consumer electronics. Self‐emitting ability, transparency, true dark tone, and capability of being made flexible ...are some of the features of OLED displays, leading to a superior performance compared with liquid crystal displays. In addition to displays, OLEDs are also a strong candidate for lighting applications. Despite great advances in improving the internal quantum efficiency of an OLED to nearly 100%, the external quantum efficiency is still lacking behind due to optical losses. This review reports the latest advances in the optical design of OLEDs that address the external coupling efficiency of OLEDs. Discussed at first are the fundamentals of OLED optics and how the refractive indices of different layers in an OLED stack affect the extraction efficiency. Then, this paper reviews how microlens arrays, scattering layers, and corrugated structures can be used to recover the optical losses and improve the external efficiency, and the general optical designs for different optical structures for light extraction are presented.
A large portion of generated light from organic light‐emitting diodes (OLEDs) is trapped in the device, and developing an outcoupling strategy is important to achieve high‐performance OLEDs. This review presents the optical loss mechanisms and discusses the effects of refractive index and optical structures on light extraction. Finally, the guidelines for light extraction are presented.
Organic light‐emitting diodes (OLEDs) are established as a mainstream light source for display applications and can now be found in a plethora of consumer electronic devices used daily. This success ...can be attributed to the rich luminescent properties of organic materials, but efficiency enhancement made over the last few decades has also played a significant role in making OLEDs a practically viable technology. This report summarizes the efforts made so far to improve the external quantum efficiency (EQE) of OLEDs and discusses what should further be done to push toward the ultimate efficiency that can be offered by OLEDs. The study indicates that EQE close to 58% and 80% can be within reach without and with additional light extraction structures, respectively, with an optimal combination of cavity engineering, low‐index transport layers, and horizontal dipole orientation. In addition, recent endeavors to identify possible applications of OLEDs beyond displays are presented with emphasis on their potential in wearable healthcare, such as OLED‐based pulse oximetry as well as phototherapeutic applications based on body‐attachable flexible OLED patches. OLEDs with fabric‐like form factors and washable encapsulation strategies are also introduced as technologies essential to the success of OLED‐based wearable electronics.
Various approaches to enhance the outcoupling efficiency of organic light‐emitting diodes (OLEDs) are presented along with a discussion as to what can be done further to realize their ultimate potential. Recent efforts to extend the applications of OLEDs beyond displays are also highlighted. As such examples, OLED‐based health‐monitoring sensors, wearable phototherapeutic patches, and fabric‐like OLEDs are introduced.
Perovskite light-emitting diodes (PeLEDs) are strong candidates for next-generation display and lighting technologies due to their high color purity and low-cost solution-processed fabrication. ...However, PeLEDs are not superior to commercial organic light-emitting diodes (OLEDs) in efficiency, as some key parameters affecting their efficiency, such as the charge carrier transport and light outcoupling efficiency, are usually overlooked and not well optimized. Here, ultrahigh-efficiency green PeLEDs are reported with quantum efficiencies surpassing a milestone of 30% by regulating the charge carrier transport and near-field light distribution to reduce electron leakage and achieve a high light outcoupling efficiency of 41.82%. Ni
Mg
O
films are applied with a high refractive index and increased hole carrier mobility as the hole injection layer to balance the charge carrier injection and insert the polyethylene glycol layer between the hole transport layer and the perovskite emissive layer to block the electron leakage and reduce the photon loss. Therefore, with the modified structure, the state-of-the-art green PeLEDs achieve a world record external quantum efficiency of 30.84% (average = 29.05 ± 0.77%) at a luminance of 6514 cd m
. This study provides an interesting idea to construct super high-efficiency PeLEDs by balancing the electron-hole recombination and enhancing the light outcoupling.
Perovskite light‐emitting diodes (PeLEDs) show great application potential in high‐quality flat‐panel displays and solid‐state lighting due to their steadily improved efficiency, tunable colors, ...narrow emission peak, and easy solution‐processing capability. However, because of high optical confinement and nonradiative charge recombination during electron–photon conversion, the highest reported efficiency of PeLEDs remains far behind that of their conventional counterparts, such as inorganic LEDs, organic LEDs, and quantum‐dot LEDs. Here a facile route is demonstrated by adopting bioinspired moth‐eye nanostructures at the front electrode/perovskite interface to enhance the outcoupling efficiency of waveguided light in PeLEDs. As a result, the maximum external quantum efficiency and current efficiency of the modified cesium lead bromide (CsPbBr3) green‐emitting PeLEDs are improved to 20.3% and 61.9 cd A−1, while retaining spectral and angular independence. Further reducing light loss in the substrate mode using a half‐ball lens, efficiencies of 28.2% and 88.7 cd A−1 are achieved, which represent the highest values reported to date for PeLEDs. These results represent a substantial step toward achieving practical applications of PeLEDs.
Highly efficient perovskite light‐emitting diodes are achieved by implementing a simple and cost‐effective method for efficient outcoupling of waveguided light. A record external quantum efficiency of 28.2% is realized for the device based on cesium lead bromide (CsPbBr3), while retaining the same spectral response for broad viewing angles.
Abstract
Organic light‐emitting diodes (OLEDs) are widely used in research and are established in the industry. The building block nature of organic compounds enables a vast variety of materials. On ...top of that, there exist many strategies to improve the light outcoupling of OLEDs making a direct comparison of outcoupling technologies difficult. Here, a novel approach is introduced for the evaluation of light outcoupling structures. The new defined “efficiency of light outcoupling structures” (ELOS) clearly determines the effectiveness of the light outcoupling structure by weighting the experimental efficiency enhancement over the theoretical outcoupling gain. It neither depends on cavity design nor on the chosen organic material. The methodology is illustrated for red phosphorescent OLEDs comprising internal and external light outcoupling structures. Assumptions and further uses are discussed with respect to experimental and theoretical handling. In addition, the ELOS is calculated for various outcoupling techniques from literature to demonstrate the universality. Finally, most suitable reference OLEDs are discussed for application of light outcoupling structures. The presented approach enables new possibilities for studying light outcoupling structures and improves their comparability in a highly material‐driven research field.
Recent advances in highly efficient organic light‐emitting diodes (OLEDs) and their emerging applications, including phototherapeutic patches, biometric sensors, and textile displays are reviewed by ...Kyung Cheol Choi, Seunghyup Yoo, and co‐workers in article number 1907539. For efficiency improvement, the importance of outcoupling structures coupled with horizontally oriented dipole emitters is highlighted.
Top‐Emitting Quantum Dot Light‐Emitting Diodes
In article number 2206133, Jaehoon Kim, Kyung‐Tae Kang, and co‐workers present a novel light extraction structure for top‐emitting quantum dot ...light‐emitting diodes (QLEDs) called RaDiNa. By utilizing molded irregular holes on polydimethylsiloxane (PDMS) made by ZnO nanorods, the authors are able to widen the narrow viewing angle of top‐emitting QLEDs, resulting in a remarkable 60% increase in external quantum efficiency.
Hybrid organic–inorganic perovskite semiconductors have shown potential to develop into a new generation of light‐emitting diode (LED) technology. Herein, an important design principle for perovskite ...LEDs is elucidated regarding optimal perovskite thickness. Adopting a thin perovskite layer in the range of 35–40 nm is shown to be critical for both device efficiency and stability improvements. Maximum external quantum efficiencies (EQEs) of 17.6% for Cs0.2FA0.8PbI2.8Br0.2, 14.3% for CH3NH3PbI3 (MAPbI3), 10.1% for formamidinium lead iodide (FAPbI3), and 11.3% for formamidinium lead bromide (FAPbBr3)‐based LEDs are demonstrated with optimized perovskite layer thickness. Optical simulations show that the improved EQEs source from improved light outcoupling. Furthermore, elevated device temperature caused by Joule heating is shown as an important factor contributing to device degradation, and that thin perovskite emitting layers maintain lower junction temperature during operation and thus demonstrate increased stability.
An important design principle for perovskite light‐emitting diodes is discovered regarding optimal perovskite thickness. Adopting a thinner perovskite layer is beneficial for both device efficiency and stability, with external quantum efficiency (EQE) as high as 17.6% being achieved. The improved EQE is primarily due to better light outcoupling, and the improved stability is correlated with reduced Joule heating.
The unique optical properties of lead halide perovskites have drawn significant attention towards their application in light emitting devices (LEDs) in recent years. While quantum yield, emission ...wavelength and stability are already in the focus of many research groups, the orientation of the emissive transition dipole moments (TDM) has rarely been investigated. As known from other thin film applications such as organic LEDs, this quantity can severely affect the light outcoupling of the device and thereby limit the external quantum efficiency. In this work, we investigate CsPbBr3 nanoplatelets of variable thickness and determine the orientation of their TDMs from thin film radiation pattern analysis. We then apply optical simulations to elucidate the performance limits of perovskite based blue LEDs in prototypical device architectures. We find that with increasingly beneficial horizontal orientation, the maximum efficiency achievable increases to values close to 30%. However, since the photoluminescence quantum efficiency degrades considerably for decreasing thickness, the overall device efficiency does not significantly improve. Thus, for the currently available material sets we can conclude that while for nanocubes the non-ideal orientation limits device performance, devices with nanoplatelets are limited by non-optimal photoluminescence quantum yields.
•Reducing the thickness of CsPbBr3 nanocrystal platelets from 6 to 2 monolayers allows tuning their emission colour from green to blue.•Concomitantly the transition dipole moment is confined towards more in-plane orientation.•This has strong potential for improved light outcoupling from perovskite light-emitting diodes.
Theoretical estimates indicate that graphene thin films can be used as transparent electrodes for thin-film devices such as solar cells and organic light-emitting diodes, with an unmatched ...combination of sheet resistance and transparency. We demonstrate organic light-emitting diodes with solution-processed graphene thin film transparent conductive anodes. The graphene electrodes were deposited on quartz substrates by spin-coating of an aqueous dispersion of functionalized graphene, followed by a vacuum anneal step to reduce the sheet resistance. Small molecular weight organic materials and a metal cathode were directly deposited on the graphene anodes, resulting in devices with a performance comparable to control devices on indium−tin-oxide transparent anodes. The outcoupling efficiency of devices on graphene and indium−tin-oxide is nearly identical, in agreement with model predictions.
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