Query: Perovskite Light-Emitting Diodes， 2020

title: Microcavity top emission perovskite light emitting diodes
abstract: Light emitting diodes (LEDs) based on perovskites show great potential in lighting and display applications. However, although perovskite films with high photoluminescence quantum efficiencies are commonly achieved, the efficiencies of perovskite LEDs are largely limited by the low light out coupling efficiency. Here, we show that high efficiency perovskite LEDs with a high external quantum efficiency of 20.2% and an ultrahigh radiant exitance up to 114.9 mW cm 2 can be achieved by employing the microcavity effect to enhance light extraction. The enhanced microcavity effect and light out coupling efficiency are confirmed by the study of angle dependent emission profiles. Our results show that both the optical and electrical properties of the device need to be optimized to achieve high performance perovskite LEDs.

title: Recent Advancements and Challenges for Low Toxic Perovskite Materials.
abstract: Lead based organic inorganic hybrid perovskite materials have been developed for advanced optoelectronic applications. However, the toxicity of lead and the chemical instability of lead based perovskite materials have so far demonstrated to be an overwhelming challenge. The discovery of perovskite materials based on low toxic elements, such as, Sn, Bi, Sb, Ge and Cu, with superior optoelectronic properties provides alternative approach to realize high performance perovskite optoelectronics. In this review, recent advances in the aspects of low toxic perovskite solar cells, photodetectors, light emitting diodes and thermoelectric devices are highlighted. The anti oxidation stability of metal cation and the crystallization process of the low toxic perovskite materials are discussed. In the last part, the outlook towards addressing various issues required further attention in the development of low toxic perovskite materials is outlined.

title: Thermal Management Enables Bright and Stable Perovskite Light Emitting Diodes.
abstract: The performance of lead halide perovskite light emitting diodes (LEDs) has increased rapidly in recent years. However, most reports feature devices operated at relatively small current densities <500 mA cm 2 with moderate radiance <400 W sr 1 m 2 Here, Joule heating and inefficient thermal dissipation are shown to be major obstacles toward high radiance and long lifetime. Several thermal management strategies are proposed in this work, such as doping charge transport layers, optimizing device geometry, and attaching heat spreaders and sinks. Combining these strategies, high performance perovskite LEDs are demonstrated with maximum radiance of 2555 W sr 1 m 2 peak external quantum efficiency (EQE) of 17% considerably reduced EQE roll off (EQE 10% to current densities as high as 2000 mA cm 2 and tenfold increase in operational lifetime (when driven at 100 mA cm 2 Furthermore, with proper thermal management, a maximum current density of 2.5 kA cm 2 and an EQE of 1% at 1 kA cm 2 are shown using electrical pulses, which represents an important milestone toward electrically driven perovskite lasers.

title: Synergistic Cascade Carrier Extraction via Dual Interfacial Positioning of Ambipolar Black Phosphorene for High Efficiency Perovskite Solar Cells.
abstract: 2D black phosphorene (BP) carries a stellar set of physical properties such as conveniently tunable bandgap and extremely high ambipolar carrier mobility for optoelectronic devices. Herein, the judicious design and positioning of BP with tailored thickness as dual functional nanomaterials to concurrently enhance carrier extraction at both electron transport layer/perovskite and perovskite/hole transport layer interfaces for high efficiency and stable perovskite solar cells is reported. The synergy of favorable band energy alignment and concerted cascade interfacial carrier extraction, rendered by concurrent positioning of BP, delivered a progressively enhanced power conversion efficiency of 19.83% from 16.95% (BP free) Investigation into interfacial engineering further reveals enhanced light absorption and reduced trap density for improved photovoltaic performance with BP incorporation. This work demonstrates the appealing characteristic of rational implementation of BP as dual functional transport material for a diversity of optoelectronic devices, including photodetectors, sensors, light emitting diodes, etc.

title: Blue perovskite light emitting diodes (LEDs) A minireview
abstract: Metal perovskite halides have led to the development of light emitting diodes due to their excellent optical and electrical properties. However, compared to near infrared, red, and green perovskite light emitting diodes, blue emitting light emitting diodes lag far behind in development due to poor stability and low quantum efficiency. In this review, the latest advances in blue perovskite light emitting diodes are described, including three dimensional perovskites, two dimensional/quasi two dimensional perovskites, and perovskite nanocrystals. In the three dimensional perovskite section, the choices of halide perovskite composition, electron transport layer and hole transport layer enable tunable light emission from sky blue to dark blue. In the two dimensional/quasi two dimensional perovskite discussion, the co surfactant colloid chemistry, and component engineering precisely control the stacking number, which produces a significant blue shift by quantum confinement effects. In the perovskite nanocrystal section, the selection of organic ligands and inorganic passivation are the key strategies to keep the nanocrystal structure stable and fully passivate surface defects. In each section, the present situation and major challenges are overviewed. Next, the evolutions of the methods to overcome these challenges and the most important breakthroughs are highlighted. In conclusion, future directions that may improve the performance of blue perovskite light emitting diodes are discussed.

title: A zinc non halide dopant strategy enables efficient perovskite CsPbI3 quantum dot based light emitting diodes
abstract: Metal ion doping has been considered as one of the most effective methods to achieve highly efficient perovskite light emitting diodes (LEDs) which is significant for future high definition displays and high quality lighting. However, its exact function is still uncertain because the doping of metal ions is also accompanied by the introduction of halide ions and halide rich circumstances, which can enhance the optoelectronic properties as well. In this work, a zinc non halide dopant strategy was employed to specifically study the effect of Zn2+ on CsPbI3 quantum dots (QDs) We confirmed that Zn2+ was doped into the perovskites interstitially via X ray diffraction (XRD) and pair distribution function (PDF) analysis. Zn2+ doped CsPbI3 QDs exhibited higher emission properties with 120% enhancement in the photoluminescence quantum yield (PLQY) compared with pristine CsPbI3 QDs. The Zn2+ doped CsPbI3 QD based LEDs (QLEDs) showed nearly two fold increase in the external quantum efficiency (EQE) versus the control device, and it improved from 7.5% to 14.6% Besides, a maximum current efficiency of 0.83 cd A 1 and the highest luminance of 378 cd m 2 were achieved. These results certify that Zn2+ doping enables high performance QLEDs without the introduction of halide ions, which is beneficial for the further research on doped perovskite fields and lays the foundation for the future practical applications of QLEDs.

title: Blue quantum dot based electroluminescent light emitting diodes
abstract: Quantum dot light emitting diodes (QD LEDs) have been considered as one of the promising candidates for light sources in display applications owing to their good color saturation, high brightness, spectral tunability, and low processing cost. Thus far, the performance of red and green QD LEDs has been significantly improved by optimizing QD materials and device architectures and by gaining a further understanding of their luminescence mechanism. However, as one of the three primary color sources, the performance of blue QD LEDs still lags behind that of red and green QD LEDs. Herein, we review the recent research progress in blue QD LEDs, including Cd based QD LEDs, perovskite based LEDs (PeLEDs) and heavy metal free QD LEDs. The advances in Cd based blue QD LEDs are firstly introduced, and the efficiency has been improved from less than 0.1% to ~20% through the optimization of the components and surface ligands of QDs as well as the optimization of device architecture and interfacial engineering, and the aging mechanism of blue QD LEDs is also discussed. Next, we review PeLEDs based on different types of perovskite materials, including two dimensional (2D)/quasi 2D perovskites included in organic inorganic halide perovskites and all inorganic perovskites. Currently, state of the art PeLEDs fabricated from quasi 2D organic inorganic perovskites exhibit the maximum external quantum efficiency (eEQE) exceeding 11% Furthermore, we summarize the research progress in heavy metal free blue QD LEDs, which are fabricated from ZnSe, InP, Cu based QDs and carbon dots. Although there is a huge gap in the device performance of heavy metal free LEDs as compared to the other two types of blue LEDs, the heavy metal free blue LEDs stand for a promising achievement toward their practical application. Finally, some challenges and possible development directions of blue QD LEDs are suggested, which may speed up the further progress.

title: Suppressing Efficiency Roll Off at High Current Densities for Ultra Bright Green Perovskite Light Emitting Diodes.
abstract: Perovskite light emitting diodes (PeLEDs) have undergone rapid development in the last several years with external quantum efficiencies (EQE) reaching over 21% However, most PeLEDs still suffer from severe efficiency roll off (droop) at high injection current densities, thus limiting their achievable brightness and presenting a challenge to their use in laser diode applications. In this work, we show that the roll off characteristics of PeLEDs are affected by a combination of charge injection imbalance, nonradiative Auger recombination, and Joule heating. To realize ultrabright and efficient PeLEDs, several strategies have been applied. First, we designed an energy ladder to balance the electron and hole transport. Second, we optimized perovskite materials to possess reduced Auger recombination rates and improved carrier mobility. Third, we replaced glass substrates with sapphire substrates to better dissipate joule heat. Finally, by applying a current focusing architecture, we achieved PeLEDs with a record luminance of 7.6 Mcd/m2. The devices can be operated at very high current densities (J) up to 1 kA/cm2. Our work suggests a broad application prospect of perovskite materials for high brightness LEDs and ultimately a potential for solution processed electrically pumped laser diodes.

title: Photolithographic Patterning of Perovskite Thin Films for Multicolor Display Applications.
abstract: Metal halide perovskites are emerging as attractive materials for light emitting diode (LED) applications. The external quantum efficiency (EQE) has experienced a rapid progress and reached over 21% comparable to the state of art organic and quantum dot LEDs. For metal halide perovskites, their simple solution processing preparation, facile bandgap tunability and narrow emission linewidth provide another attractive route to harness their superior optoelectronic properties for multicolor display applications. In this work, we demonstrate a high resolution, large scale photolithographic method to pattern multicolor perovskite films. This approach is based on a dry lift off process which involves the use of parylene as an intermediary and the easy mechanical peeling off of parylene films on various substrates. Using this approach, we successfully fabricated multicolor patterns with red and green perovskite pixels on a single substrate, which could be further applied in liquid crystal displays (LCDs) with blue backlight. Besides, a prototype green perovskite micro LED display under current driving has been demonstrated.

title: Advances in two dimensional organic inorganic hybrid perovskites
abstract: Two dimensional (2D) perovskites have attracted considerable interest for their promising applications for solar cells and other optoelectronics, such as light emitting diodes, spintronics, and photodetectors. Here, we review the recent achievements of 2D perovskites for various optoelectronic applications. First, we discuss the basic structure and optoelectronic properties of 2D perovskites, including band structure, optical properties, and charge transport. We then highlight recent achievements using 2D perovskites in solar cells and beyond solar cells, including progress on various synthesis strategies and their impact on structural and optoelectronic properties. Finally, we discuss current challenges and future opportunities to further develop 2D perovskites for various applications.