High-efficiency Perovskite Light-emitting Diodes Research Articles

Eliminating the Adverse Impact of Composition Modulation in Perovskite Light-Emitting Diodes toward Ultra-High Brightness and Stability.

Excess ammonium halides as composition additives are widely employed in perovskite light-emitting diodes (PeLEDs), aiming to achieve high performance by controlling crystallinity and passivating defects. However, an in-depth understanding of whether excess organoammonium components affect the film physical/electrical properties and the resultant device instability is still lacking. Here, the trade-off between the performance and stability in high-efficiency formamidinium lead iodide (FAPbI3)-based PeLEDs with excess ammonium halides is pointed, and the underlying mechanism is explored. Systematic experimental and theoretical studies reveal that excess halide salt-induced ion-doping largely alters the PeLEDs properties (e.g., carrier injection, field-dependent ion-drifting, defect physics, and phase stability). A surface clean assisted cross-linking strategy is demonstrated to eliminate the adverse impact of composition modulation and boost the operational stability without sacrificing the efficiency, achieving a high efficiency of 23.6%, a high radiance of 964 W sr-1 m-2 (The highest value for FAPbI3 based PeLEDs), and a prolong lifetime of 106.1 h at large direct current density (100mA cm-2), concurrently. The findings uncovered an important link between excess halide salts and the device performance, providing a guideline for rational design of stable, bright, and high efficiency PeLEDs.

Dual-functionalized poly(ethylene oxide) for interface modification and addition of rubidium-doped CsPbBr3 perovskite composite emission layer to enhance brightness of perovskite light-emitting diodes

Metal halide perovskite as an emitter has extremely high color purity, but the generally poor morphology of the perovskite layer restricts the performance of the emitter. The additive of a suitable polymer can greatly boost the uniformity of the spin-coated perovskite film. The all-inorganic perovskite layer deposited by the solution method has a multi-crystalline structure. Besides, there are a lot of defects and grain boundaries on the surface, and the carriers will undergo non-radiative recombination at these defects. In this study, Cs1-xRbxPbBr3 was prepared by introducing Rb+ cations with smaller ionic radius to partially replace Cs+ cations, and combined with poly(ethylene oxide) (PEO) and (1,3,5-benzinetriyl)tris(1-phenyl-1-H-benzimidazole) (TPBi) blended composite film as the light-emitting layer, that can increase the film coverage and radiative recombination efficiency of the perovskite film. The polymer PEO was introduced as a buffer layer deposited on the poly-TPD hole transport layer (HTL) to to enhance the hydrophilicity and passivate the poly-TPD HTL/perovskites interface in order to obtain high-efficiency perovskite light-emitting diodes. By adjusting the concentration of the PEO buffer layer, the champion luminance and external quantum efficiency are 25650 cd/m2 and 0.844 %, respectively.

Deep Learning for Additive Screening in Perovskite Light-Emitting Diodes.

Additive engineering with organic molecules is of critical importance for achieving high-performance perovskite optoelectronic devices. However, experimentally finding suitable additives is costly and time consuming, while conventional machine learning (ML) is difficult to predict accurately due to the limited experimental data available in this relatively new field. Here, we demonstrate a deep learning method that can predict the effectiveness of additives in perovskite light-emitting diodes (PeLEDs) with a high accuracy up to 96 % by using a small dataset of 132 molecules. This model can maximize the information of the molecules and significantly mitigate the duplicated problem that usually happened with previous models in ML for molecular screening. Very high efficiency PeLEDs with a peak external quantum efficiency up to 22.7 % can be achieved by using the predicated additive. Our work opens a new avenue for further boosting the performance of perovskite optoelectronic devices.

Deep Learning for Additive Screening in Perovskite Light‐Emitting Diodes

AbstractAdditive engineering with organic molecules is of critical importance for achieving high‐performance perovskite optoelectronic devices. However, experimentally finding suitable additives is costly and time consuming, while conventional machine learning (ML) is difficult to predict accurately due to the limited experimental data available in this relatively new field. Here, we demonstrate a deep learning method that can predict the effectiveness of additives in perovskite light‐emitting diodes (PeLEDs) with a high accuracy up to 96 % by using a small dataset of 132 molecules. This model can maximize the information of the molecules and significantly mitigate the duplicated problem that usually happened with previous models in ML for molecular screening. Very high efficiency PeLEDs with a peak external quantum efficiency up to 22.7 % can be achieved by using the predicated additive. Our work opens a new avenue for further boosting the performance of perovskite optoelectronic devices.

Synergistic effect of indium doping and chlorine surface passivation on CsPbI3 perovskite quantum dots for deep-red light-emitting devices.

Deep-red CsPbI3 perovskite quantum dots (PeQDs) are essential for high-efficiency perovskite light-emitting diodes (PeLEDs) because of their high color purity and high photoluminescence quantum yield (PLQY). The synergetic strategy of indium (In) doping and chlorine (Cl) surface passivation not only partially replaced Pb2+ ions with the smaller ionic In3+ but also filled I- vacancies by Cl- on the surface, maintaining the humidity stability for more than 24 days and yielding excellent PLQY. Benefiting from this synergetic strategy, deep-red (approximately at 683 nm) CsPbI3 PeLEDs showed a maximum luminance and external quantum efficiency (EQE) of 311 cd m-2 and 8.32%, respectively.

Robust transfer-printing method for perovskite films and nanostructures

Metal halide perovskites, as a promising semiconductor material, have been successfully used in electroluminescent devices because of their desirable characteristics, such as good conductivity, high color purity, tunable bandgap, low cost and solution process ability. In the past few years, significant progress has been made in the development of high-efficiency perovskite light-emitting diodes (PeLEDs). These efficient PeLEDs are mainly achieved by sophisticated spin-coating methods, which can easily control the perovskite's composition, film thickness, morphology and crystallinity. However, with the continuous development of PeLEDs, commercial production problems have to be solved, such as large area production, high resolution patterning and substrate diversity, which are difficult for the current spin-coating process.

Elucidating the Role of Substrates on Domain Distribution of Quasi-2D Perovskites for Blue Light-Emitting Diodes

Perovskite light-emitting diodes (PeLEDs) have been developed at a rapid pace with high external quantum efficiencies (EQEs) surpassing 20% for red, infrared, and green emissions; nevertheless, the performance of their blue-emitting cousins remains relatively quite low. Among various crucial factors that could impact these solution-processed devices, selection of a suitable underlayer (usually a hole-transport layer) is critical to the quality of resulting perovskite films. Particularly, the role of the substrate is highly important in the case of quasi-2D perovskite films where the emission occurs as a result of cascade energy transfer among various perovskite domains with different numbers of lead-halide octahedral layers (n) sandwiched between long-chain organic molecules. Herein, we explored the impact of the substrate on domain distribution of quasi-2D perovskite films, and a series of efficient and color-stable blue PeLEDs is attained through precise stoichiometric engineering. By employing a range of characterization tools, we thoroughly investigated the crystallization mechanism of quasi-2D perovskites on different substrates, including poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and poly[(9,9-dioctylfluorenyl-2,7-diyl)- co -(4,4′-( N -(4-sec-butylphenyl)diphenylamine)] (TFB). Perovskite films spin-coated on PEDOT:PSS exhibited cyan emission from the n ≥ 4 quasi-2D phase with uniformly distributed small grains. In contrast, perovskite layers formed on TFB featured an island morphology and showed blue emission from the quasi-2D phase with n-values between 2 and 4. In addition, we examined how the variation of A-site cations affects the optical properties of quasi-2D perovskites. Consequently, PEDOT:PSS-based champion PeLED exhibited cyan emission at 495 nm with a maximum EQE of 6.73%, whereas TFB-based counterparts manifested blue emission at 486 nm with a peak EQE of 3.95%. We anticipate that our work will provide a fundamental guideline to rationally selecting the substrates for the optimized growth of quasi-2D perovskites toward high-efficiency PeLEDs.

Manipulating crystallization dynamics through chelating molecules for bright perovskite emitters

Molecular additives are widely utilized to minimize non-radiative recombination in metal halide perovskite emitters due to their passivation effects from chemical bonds with ionic defects. However, a general and puzzling observation that can hardly be rationalized by passivation alone is that most of the molecular additives enabling high-efficiency perovskite light-emitting diodes (PeLEDs) are chelating (multidentate) molecules, while their respective monodentate counterparts receive limited attention. Here, we reveal the largely ignored yet critical role of the chelate effect on governing crystallization dynamics of perovskite emitters and mitigating trap-mediated non-radiative losses. Specifically, we discover that the chelate effect enhances lead-additive coordination affinity, enabling the formation of thermodynamically stable intermediate phases and inhibiting halide coordination-driven perovskite nucleation. The retarded perovskite nucleation and crystal growth are key to high crystal quality and thus efficient electroluminescence. Our work elucidates the full effects of molecular additives on PeLEDs by uncovering the chelate effect as an important feature within perovskite crystallization. As such, we open new prospects for the rationalized screening of highly effective molecular additives.

Excess Ion-Induced Efficiency Roll-Off in High-Efficiency Perovskite Light-Emitting Diodes.

Applying extensively excess ammonium halides in forming perovskites is a widely used approach to achieve high-performance perovskite light-emitting diodes (PeLEDs). However, most of these PeLEDs suffer from severe external quantum efficiency (EQE) roll-off at high current densities, thereby restricting the realization of high-brightness PeLEDs and laser diodes. In this work, we explore the underlying mechanism of the EQE roll-off in high-efficiency formamidinium lead iodide (FAPbI3)-based PeLEDs. By combining voltage-dependent electrical stress measurements and ex situ ion distribution analysis of PeLEDs, we found that the electric field-driven migration and local segregation of excess iodide ions, originated from nonstoichiometric precursors, trigger the EQE roll-off via promoting imbalanced charge injection. Based on this discovery, we introduced a simple wash-off treatment with chloroform to remove the excess iodides from the perovskite surface and demonstrated that the treatment is highly effective in suppressing the roll-off behavior. By combining the treatment and the use of an ultrathin poly(methyl methacrylate) (PMMA) interlayer, we achieved a high-brightness PeLED with an EQEmax of 19.6%, a critical current density of 1550 mA cm-2, and a radiancemax of 875 W sr-1 m-2. The study reveals the double-edge sword effect of precursor nonstoichiometry and highlights the importance of managing excess ions in perovskite films.

Gold nanoparticals modified indium tin oxide anode for high performance red perovskite light emitting diodes

Gold nanoparticles (Au NPs) play an important role in improving the external quantum efficiency of perovskite light emitting diodes (PeLED). To avoid direct contact between the Au NPs and the light emitting layer, the Au NPs@SiO<sub>2</sub> structure and blending the Au NPs into the hole transport layer (HTL) or electron transport layer (ETL) have been proposed previously. However, the Au NPs@SiO<sub>2</sub> is difficult to obtain and affects the charge transport. When the Au NPs is blended in poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT: PSS), the density of Au NPs is not easily controlled and the PEDOT:PSS is not an ideal HTL for PeLED. Therefore, the electrostatic adsorption is used in this work to uniformly disperse the ~20 nm-size Au NPs on the top of the ITO anode, and the Poly(9-vinylcarbazole) (PVK) is spin-coated as the HTL to achieve the high performance red PeLED based on the (NMA)<sub>2</sub>Cs<sub><i>n</i>–1</sub>Pb<sub><i>n</i></sub>I<sub>3<i>n</i>+1</sub>. After the Au NPs modification, the maximum luminous brightness rises from ~5.2 to ~83.2 cd/m<sup>2</sup>. Meanwhile, the maximum external quantum efficiency rises from ~0.255% to ~6.98%. Mechanism studies show that microcavity can be formed between the Au NPs-modified ITO anode and the Al cathode, and the transmitted light and the reflected light interfere with each other to improve the output couple efficiency of the PeLED. The photoluminescence (PL) spectrum and angle dependent PL intensity of the Au NPs-modified PeLED prove that the fluorescence enhancement of the (NMA)<sub>2</sub>Cs<sub><i>n</i>–1</sub>Pb<sub><i>n</i></sub>I<sub>3<i>n</i>+1</sub> perovskite is attributed mainly to the microcavity effect. Furthermore, the effects of Au NPs density on the performance of the PeLED are investigated, which reveals that the device with ~15 min adsorption is optimal. Finally, we rule out the contributions of Au NPs to the morphology, crystallization, electrical properties and localized surface plasmon resonance (LSPR) effects of (NMA)<sub>2</sub>Cs<sub><i>n</i>–1</sub>Pb<sub><i>n</i></sub>I<sub>3<i>n</i>+1</sub> perovskite films. In this work, the Au NPs are successfully applied to red PeLED for the first time, providing a feasible way of developing the low-cost and high-efficiency PeLED.

Metal halide perovskites for light-emitting diodes

Metal halide perovskites have shown promising optoelectronic properties suitable for light-emitting applications. The development of perovskite light-emitting diodes (PeLEDs) has progressed rapidly over the past several years, reaching high external quantum efficiencies of over 20%. In this Review, we focus on the key requirements for high-performance PeLEDs, highlight recent advances on materials and devices, and emphasize the importance of reliable characterization of PeLEDs. We discuss possible approaches to improve the performance of blue and red PeLEDs, increase the long-term operational stability and reduce toxicity hazards. We also provide an overview of the application space made possible by recent developments in high-efficiency PeLEDs.

In-situ passivation perovskite targeting efficient light-emitting diodes via spontaneously formed silica network

Perovskite materials are attractive candidates as emitting layers in light-emitting diodes due to their excellent optical and electrical properties. Effective charge radiative recombination is a key to target high-efficiency perovskite light-emitting diodes (PeLEDs). State-of-the-art effective passivation chemicals in PeLEDs mostly belong to organic chelating molecules, associated with like molecular detachment in the device operation, which simultaneously degrades the performance especially the operational stability of the devices. Here, a silane material tetraethoxysilane (TEOS), which can be crosslinkable to avoid any likely detachment from perovskite film, is incorporated into the perovskite film to enhance film radiative recombination and stability. An oxo-bridged silica network anchored with perovskite is formed after the TEOS in-situ crosslinking process. It is found that the lone pair electrons in TEOS network can coordinate with the undercoordinated Pb2+ of perovskite. Consequently, defect states in perovskite film are dramatically diminished, which enhances radiative recombination. The photoluminescence intensity of resultant perovskite-TEOS film is enhanced by 40% over that of the pristine one. The average photoluminescence lifetime of perovskite-TEOS film reaches 58 ns, enhanced by 65% over that of the pristine perovskite film of 35 ns.. As a result, a green PeLED achieved an external quantum efficiency of 16.6% with improved working stability. This work presents a facile strategy targeting efficient and stable perovskite devices via utilizing detachment-free self-crosslinked ligands.

High-Efficiency Perovskite Light-Emitting Diodes with Improved Interfacial Contact.

Unbalanced charge injection is one of the major issues that hampers the efficiency of perovskite light-emitting diodes (PeLEDs). Through engineering the device structure with multiple hole transport layers (HTLs), i.e., poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)diphenylamine) (TFB)/poly(9-vinylcarbazole) (PVK) and nickel oxide (NiOx)/TFB/PVK, efficient PeLED devices have been successfully demonstrated. However, in a typical solution-processed PeLED with multiple HTLs, the underlying conjugated HTL could be easily redissolved by the ink of the following one, which not only dramatically deteriorates the electrical property of HTLs but also influences the quality of the top perovskite films. In this work, through inserting a thin atomic layer-deposited aluminum oxide (Al2O3) layer between HTLs and the perovskite layer, an improved interfacial contact can be achieved, which enables us to obtain perovskite films with enhanced characteristics and balanced charge injection in the resultant PeLEDs. In addition, because of the proper refractive index (r), the presence of the Al2O3 layer also favors the light out-coupling of PeLEDs. As a result, we fabricate green PeLEDs with good repeatability and external quantum efficiency of 17.0%, which is approximately 60% higher than that of the control device without Al2O3. Our work provides a promising avenue to enhance interfacial contact between the charge transport layer and perovskite for efficient perovskite-based optoelectronic devices.

Improved Efficiency of Perovskite Light-Emitting Diodes Using a Three-Step Spin-Coated CH3NH3PbBr3 Emitter and a PEDOT:PSS/MoO3-Ammonia Composite Hole Transport Layer

High efficiency perovskite light-emitting diodes (PeLEDs) using PEDOT:PSS/MoO3-ammonia composite hole transport layers (HTLs) with different MoO3-ammonia ratios were prepared and characterized. For PeLEDs with one-step spin-coated CH3NH3PbBr3 emitter, an optimal MoO3-ammonia volume ratio (0.02) in PEDOT:PSS/MoO3-ammonia composite HTL presented a maximum luminance of 1082 cd/m2 and maximum current efficiency of 0.7 cd/A, which are 82% and 94% higher than those of the control device using pure PEDOT:PSS HTL respectively. It can be explained by that the optimized amount of MoO3-ammonia in the composite HTLs cannot only facilitate hole injection into CH3NH3PbBr3 through reducing the contact barrier, but also suppress the exciton quenching at the HTL/CH3NH3PbBr3 interface. Three-step spin coating method was further used to obtain uniform and dense CH3NH3PbBr3 films, which lead to a maximum luminance of 5044 cd/m2 and maximum current efficiency of 3.12 cd/A, showing enhancement of 750% and 767% compared with the control device respectively. The significantly improved efficiency of PeLEDs using three-step spin-coated CH3NH3PbBr3 film and an optimum PEDOT:PSS/MoO3-ammonia composite HTL can be explained by the enhanced carrier recombination through better hole injection and film morphology optimization, as well as the reduced exciton quenching at HTL/CH3NH3PbBr3 interface. These results present a promising strategy for the device engineering of high efficiency PeLEDs.

High-Efficiency Perovskite Light-Emitting Diodes with Synergetic Outcoupling Enhancement.

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.

Suppression of non-radiative recombination toward high efficiency perovskite light-emitting diodes

Nickel oxide (NiO) would be an alternative hole transport layer for perovskite light-emitting diodes (PeLEDs). However, the performances of NiO-based PeLEDs are still inferior due to the adverse non-radiative recombination at the interface. Here, a poly(9-vinlycarbazole) (PVK) layer is inserted between the perovskite and the NiO film. The photoluminescence quantum yield is dramatically enhanced from 23% to 54% in the presence of PVK layer owing to suppression of the non-radiative recombination. Combined with the favorable hole injection from the ladder energy band scheme of NiO/PVK layer, an external quantum efficiency of 11.2% for a green PeLED is achieved. This work demonstrates the importance of interface control to boost the radiative recombination rate for high performance PeLEDs.

Perovskite light-emitting diodes based on n-type nanocrystalline silicon oxide electron injection layer

Organometal halide perovskites featuring solution-processable characteristics, high photoluminescence quantum yield (PLQY), and color purity, are an emerging class of semiconductor with considerable potential applications in optoelectronic devices. Electron injection layer is an important component of perovskite light-emitting device, which determines the growth of perovskite film directly. In this paper, the perovskite light-emitting diodes (PeLEDs) based on n-type nanocrystalline silicon oxide (n-nc-SiO<sub><i>x</i></sub>:H) electron injection layer are designed and realized. This novel electron injecting material is prepared by the plasma enhanced chemical vapor deposition (PECVD), and its smooth surface and matched energy band result in superior perovskite crystallinity and low electron injection barrier from the electron injecting layer to the emissive layer, respectively. However, the external quantum efficiency (EQE) of PeLED is as low as 0.43%, which relates to defects and leakage current due to the incomplete surface coverage of perovskite film. The fast exciton emission decay (< 10 ns) stems from strong non-radiative energy transfer to the trap states, and represents a big challenge in fabricating high-efficiency PeLEDs. In order to obtain desirable perovskite film morphology, an excessive proportion of methylammonium bromide (MABr) is incorporated into the perovskite solution, and a volume of benzylamine (PMA) is added into the chlorobenzene antisolvent. The perovskite films suffer low PLQY and short PL lifetime if only MABr or PMA is introduced. When the molar ratio of MABr is higher than 60%, the luminescence quenching arising from Joule heating is depressed by employing PMA, contributing to a higher PLQY (> 30%) and a longer carrier lifetime. The synergistic effect of MABr and PMA increase the coverage and reduce the trap density of perovskite film, inhibit the luminescence quenching in the annealing process, and thus facilitating the perovskite film with higher quality. Finally, the n-i-p PeLED exhibits green-light emission with a maximum current efficiency of 7.93 cd·A<sup>-1</sup> and a maximum EQE up to 2.13% is obtained. These facts provide a novel electron injecting material and a feasible process for implementing the PeLEDs. With further optimizing the perovskite layer and device configuration, the performance of n-i-p type PeLEDs will be improved significantly on the basis of this electron injection material.

Theoretical perspective to light outcoupling and management in perovskite light-emitting diodes

Despite the rapid development of high-efficiency perovskite light-emitting diodes (PeLEDs) via material design and morphology control, a comprehensive evaluation of the power loss among various optical modes has seldom been conducted for the precise design of device architectures. Here, a quantitative analysis of light outcoupling efficiency (ηout) of PeLEDs is presented with the theoretical simulations by taking into account the key factors of refractive index and film thickness of various functional layers, and the emitter dipole orientation. For a planar PeLED, a rational design to enhance the ηout is predicted by constructing multilayer stack in series of transparent electrode/high-index transport layer/perovskite emitter/low-index transport layer/reflective electrode. Furthermore, the role of photonic structures on the outcoupling management is explored, and the potential of a substantial increase in ηout is demonstrated with an enhancement factor of over 1.75 by implementing a moth-eye nanostructure into the emitter. We anticipate that the findings shown here will provide the design guidelines for the optical optimization of PeLEDs.

High efficiency perovskite light-emitting diodes of ligand-engineered colloidal formamidinium lead bromide nanoparticles

Formamidinium (FA, CH(NH2)2) lead bromide perovskite (FAPbBr3) nanoparticles (NPs) are promising emitters due to their high stability and ability to emit pure green color in both film and solution states. Even though various types of metal halide NP emitters in solution have shown high photoluminescence quantum efficiencies (PLQEs), electroluminescence efficiencies of the light-emitting diodes (LEDs) using the NP films are still much poorer, possibly due to the insulating ligands which can impede the charge injection and transport in films. Therefore, the organic ligand of NPs should be designed to facilitate charge injection and transport in LEDs. Here, we synthesize ligand-engineered colloidal FAPbBr3 NPs at RT and demonstrate high efficiency perovskite NP LEDs based on the FAPbBr3 NPs. Control of ligand length reduces trap-assisted recombination of carriers at the surface traps, and thus maximizes the PLQE of FAPbBr3 NPs. Ligand engineering can also improve the charge injection and transport capability in FAPbBr3 NP films. With this ligand engineering method, we achieve maximum current efficiency of 9.16cd/A in LEDs based on FAPbBr3 NPs, which is the highest efficiency in FAPbBr3 NP-LEDs to date. The ligand engineering method reported here can be a simple way to improve the luminescence efficiency of optoelectronic devices based on perovskite NP LEDs.