Perovskite Emission Research Articles

White PeLED with the composite emission layer of organic small molecule and perovskite materials preparedby thermal evaporation

<p indent="0mm">Metal halide perovskite material has received considerable attention in recent years because of its unique properties, such as high photoluminescence quantum yield, adjustable bandgap, high carrier mobility, and long diffusion length, which provide promising research and application potential in the following optoelectronic fields: Photovoltaic, light-emitting diode, and photodetector. By improving emission layer morphology and structure quality as well as controlling emission materials’ dimensional size, the external quantum efficiency (EQE) of the sole color light-emitting PeLED has already been over 20%, which is comparable to the performance of the optimal organic light-emitting diode. However, the development of PeLED still faces serious challenges, such as poor temperature, long-term stability, and hard reproducibility of the fabrication. However, metal halide PeLED demonstrates superior stability compared with routine hybrid PeLED owing to the replacement of organic cations like MA⁺ and FA⁺ at the A site of the ABX₃ molecular structure by monovalent metal cations such as Cs⁺, Ag⁺, and Na⁺, which provides a promising substitute to the routine hybrid PeLED. There are few reports regarding PeLED fabrication using physical vapor deposition methods such as thermal evaporation, which can be attributed to its typically higher cost and lower light-emitting efficiency. However, the thermal evaporation procedure has distinct advantages, such as precise control of the element content and layer thickness, which is especially important for achieving a superthin layer. In this study, the fabrication and EL performance of the white PeLED with the structure of ITO/MoO₃/TAPC/TCTA/CsPbBr₃/mCP/TmPyPb/LiF/Al have been investigated, in which the CsPbBr₃/mCP is employed as emission layer. Consequently, all functional layers are fabricated through thermal evaporation, while the CsPbBr₃ layer is formed through the dual-source coevaporation of CsBr and PbBr₂. The passivation effects of the organic small molecule material mCP on the perovskite emitter are observed with considerably reduced trap densities in the emission layer. Meanwhile, as the mCP thickness increases, the trap density decreases. Furthermore, the thickness of the mCP strongly affects the ratio of the different emitting colors in white light and the shifting of the exciton recombination zone. Therefore, the white PeLED can be realized while the thicknesses of CsPbBr₃ and mCP are kept at <sc>20 nm</sc> and within <sc>~10–30 nm,</sc> respectively, in which the 20-nm mCP will yield the highest quality white light with the highest color rending index (CRI) of 89 and optimal CIE coordinates of (0.33, 0.34) due to the balanced emission of blue, green, and red light. The optimal EL performance can be obtained at the mCP thickness of <sc>30 nm,</sc> with the maximum current efficiency, brightness, EQE, and CRI of <sc>0.35 cd/A,</sc> <sc>341 cd/m²,</sc> 0.42%, and 82, respectively. Therefore, the improvement of the EL performance with the increasing mCP layers thickness <sc>(~10–30 nm)</sc> can be demonstrated. The blue-purple and red light at 380 and 620-nm wavelengths are attributed to the mCP, whereas the green light at <sc>520 nm</sc> is attributed to CsPbBr₃. The research demonstrates a practical method for developing white PeLEDs with high-quality white light emission and a simple structure.

Perspective on single-emissive-layer white-LED based on perovskites

Perovskite emitters have attracted great attention in recent years owing to their excellent photoelectric performance with low-cost potential. Some very recent studies have revealed that perovskites not only have narrow emitting peak with high purity but also have multi-color or broad spectrum in entire visible light, even some single emissive layer-based white perovskite light-emitting diodes (SEL-WPeLEDs) have been confirmed, showing promise in lighting and display. In this Perspective, we summarized the issues and challenges faced by these perovskite-based SEL-WLEDs and proposed some feasible suggestions in improving their light-emitting performance, hoping to promote their applications in lighting and other optoelectronic devices.

Temperature-Independent Dielectric Constant in CsPbBr3 Nanocrystals Revealed by Linear Absorption Spectroscopy.

Fundamental photophysical behavior in CsPbBr3 nanocrystals (NCs), especially at low temperatures, is under active investigation. While many studies have reported temperature-dependent photoluminescence, comparatively few have focused on understanding the temperature-dependent absorption spectrum. Here, we report the temperature-dependent (35-300 K) absorption and photoluminescence spectra of zwitterionic ligand-capped CsPbBr3 NCs with four different edge lengths (d = 4.9, 7.2, 8.1, and 13.2 nm). The two lowest-energy excitonic transitions are quantitatively modeled over the full temperature range within the effective mass approximation considering the quasi-cubic NC shape and nonparabolicity of the electronic bands. Significantly, we find that the effective dielectric constant determined from the best fit model parameters is independent of temperature. Moreover, we observe a temperature-dependent Stokes shift that saturates at a finite value of Δ ≈ 10 meV at low temperatures for d = 7.2 nm NCs, which is absent in bulk CsPbBr3 films. Overall, these observations highlight differences between the temperature-dependent dielectric behavior of NC and bulk perovskites and point to the need for a more unified theoretical understanding of absorption and emission in halide perovskites.

Minimizing Optical Energy Losses for Long‐Lifetime Perovskite Light‐Emitting Diodes

AbstractRegardless of the rapid advance on perovskite light‐emitting diodes (PeLEDs), the lack of long‐term operational stability hinders the practicality of this technology. Particularly, thermal management is indispensable to control the Joule heating induced by charge transport and parasitic re‐absorption of internally confined photons. Herein, a synergetic device architecture is proposed for minimizing the optical energy losses in PeLEDs toward high efficiency and long lifetime. By adopting a carefully modified perovskite emitter in combination with an improved light outcoupling structure, red PeLEDs emitting at 666 nm achieve a peak external quantum efficiency of 21.2% and an operational half‐lifetime of 4806.7 h for an initial luminance of 100 cd m‐2. The enhanced light extraction from trapped modes can efficiently reduce the driving current and suppress optical energy losses in PeLEDs, which in turn ameliorate the heat‐induced device degradation during operation. This work paves the way toward high‐performance PeLEDs for display and lighting applications in the future.

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.

Color‐Stable Deep‐Blue Perovskite Light‐Emitting Diodes Based on Organotrichlorosilane Post‐Treatment

AbstractRecent studies of sky‐blue perovskite light‐emitting diodes (PeLEDs) have extensively promoted optimal device design to achieve an external quantum efficiency (EQE) above 12%. However, the development of thin‐film deep‐blue PeLEDs lags dramatically behind, especially with regards to meeting the latest Rec. 2020 standard. A trichloro(3,3,3‐trifluoropropyl) silane post‐treatment that drives the emission of perovskite into the deep‐blue region, ranging from 440 to 460 nm, which meets the Rec. 2020 standard, is proposed. The chlorine ions released from the organotrichlorosilane molecules during their polycondensation reaction provide an addition halide source to fine tune the composition of the mixed halide perovskite films, leading to increase of bandgap and deep‐blue emission. In addition, hydrogen bonds between the hydroxy groups of silane molecules and halide anions in perovskite can suppress ion migration for improving emission stability. As a result, an optimal PeLED is developed with deep‐blue emission at 458 nm and excellent color stability, which yields an EQE and luminance of 1.1% and 130 cd m−2, respectively, representing a state‐of‐the‐art result for thin‐film PeLEDs in this emission region. This work paves the way to achieve high‐performance deep‐blue PeLEDs with stable emissions to meet the demand for potential applications such as full‐color display.

Bright all-solution-processed CsPbBr3 perovskite light emitting diodes optimized by quaternary ammonium salt

Perovskite light-emitting diodes (PeLEDs) prepared by the all-solution-process are gradually coming into view due to their low cost and flexible production process. However, the performance of CsPbBr3 device is limited by the high non-radiative recombination losses due to incomplete surface coverage and grain defects. Here a quaternary ammonium salt, tetrabutylammonium hexafluorophosphate (TBA-PF6) was simultaneously introduced into perovskite emission layers (CsPbBr3) and electron transport layer (TPBi (1,3,5-Tris(1-phenyl-1H-benzimidazol-2-yl) benzene) dissolved in ethyl acetate). As a result, the morphology and luminescence of CsPbBr3 films were improved, and the energy level of TPBi was more conducive to charge transport. Consequently, the maximum luminance and current efficiency of the modified green-emitting PeLEDs are improved. Furthermore, the optimized device had an operating life of more than 20 min at an initial luminance of 1230 cd/m2. This work provides a simple and easy method to be scaled up for the development of low-cost all-solution-processed PeLEDs.

Aligning Transition Dipole Moment toward Light Amplification and Polarized Emission in Hybrid Perovskites

AbstractOrientational manipulation of transition dipole moment (TDM) plays an important role in controlling the polarization of excited states in light emission as well as lasing actions. The present work discovers vertically aligned TDMs in hybrid perovskite films through angle‐resolved photoluminescence (PL) measurements, which show enhanced emission through the film edge. With increasing excitation intensity, the edge emission induced by these vertically aligned TDMs becomes dominant and eventually leads to amplified spontaneous emission (ASE) through the edge view. Meanwhile, polarized emission of both PL and electroluminescence (EL) provides further evidence for vertically aligned TDMs. Surprisingly, the degree of polarization (DOP) through the film edge is increased when grain boundary defects are passivated through either stochiometric engineering or self‐passivation by mobile ions under working conditions. With increasing DOP, ASE threshold of the perovskite film is reduced owing to enhanced collective behaviors of light‐emitting states. This work presents a useful method to manipulate TDMs in organic–inorganic hybrid perovskites.

Ultra-bright pure green perovskite light-emitting diodes

Inorganic perovskites, such as CsPbX3 (X = Cl, Br, I), have attracted attention due to their excellent thermal stabilities and high photoluminescence quantum efficiencies. Here, we report on all-inorganic light-emitting diodes (LEDs), including inorganic perovskite emitters (CsPbBr3) based on the heterostructure of n-ZnO/Al2O3/CsPbBr3/p-GaN. The effects of the CsPbBr3 and Al2O3 interlayers on the electroluminescence performance have been systematically investigated. It is observed that the Al2O3 interlayer plays an important role in determining the emission performance. By introducing the Al2O3 layers, the electron–hole recombination can almost be limited within the perovskite. The emission peaks of the heterojunction LEDs change from blue-green to pure green with the increased luminous intensities and decreased leakage currents. It provides an idea for the preparation of all-inorganic perovskite LEDs.

Phase Aggregation Suppression of Homogeneous Perovskites Processed in Ambient Condition toward Efficient Light‐Emitting Diodes

AbstractPerovskite light‐emitting diodes (PeLEDs) have attracted attention because of their high efficiencies. However, due to the sensitivity of perovskites to ambient condition, perovskite emitter layers are generally fabricated under an inert gas environment (e.g., dry N2), which increases processing complexity and cost. Here, air‐prepared quasi‐2D perovskites are reported for efficient PeLEDs. It is found that the phase aggregation is the major obstacle deteriorating the characteristics of air‐prepared perovskites. Through antisolvent engineering to modulate the nucleation and growth characteristics of perovskite films from precursor solution, phase aggregations are restrained. Confocal laser scanning fluorescence microscopy results demonstrate homogeneous perovskite films with uniform photoluminescence distributions. Traps at grain boundaries are passivated, and exciton transfer among perovskite phases becomes effective. Finally, efficient green PeLEDs based on air‐prepared perovskites are realized with an external quantum efficiency of 15.4%. This work provides a promising strategy to fabricate cost‐effective perovskite devices in ambient air condition.

Unraveling the Role of Crystallization Dynamics on Luminescence Characteristics of Perovskite Light‐Emitting Diodes

AbstractMetal halide perovskites are one of the most promising materials for optoelectronic applications owing to their unique optoelectronic properties. In the pursuit of achieving the efficient perovskite light‐emitting diodes (PeLEDs), the critical role of crystallization dynamics on luminescence properties of cesium lead bromide (CsPbBr3) perovskite films has been clarified based on the characterizations of in situ photoelectron spectroscopy, synchrotron‐based grazing incidence X‐ray diffraction, and device fabrication. The crystallinity and crystal orientation of CsPbBr3 perovskite films has been effectively controlled when tuning the underlayer of poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) with the ethanolamine modification. The fast crystallization with the formation of pure cubic phase (α‐CsPbBr3) reveals a substantial boost in luminance and device efficiency of PeLEDs, whereas the performance degradation occurs due to the phase transition from α‐CsPbBr3 to orthogonal phase (γ‐CsPbBr3) along with the appearance of PbBr2. The decomposition of organic additives in the perovskite films is a key factor that results in this phase transition, which changes the absorption and band gap as confirmed by the density functional theory calculation. These experimental and theoretical findings provide a better understanding of the crystallization dynamics of perovskite emitters and their influence on device performance by tuning the substrate properties.

Strategies Toward Efficient Blue Perovskite Light‐Emitting Diodes

AbstractSubstantial progress has been made in blue perovskite light‐emitting diodes (PeLEDs). In this review, the strategies for high‐performance blue PeLEDs are described, and the main focus is on the optimization of the optical and electrical properties of perovskites. In detail, the fundamental device working principles are first elucidated, followed by a systematical discussion of the key issues for achieving high‐quality perovskite nanocrystals (NCs) and quasi‐2D perovskites. These involve ligand optimization and metal doping in enhancing the carrier transport and reducing the traps of perovskite NCs, as well as the perovskite phase modulation and defect passivation in improving energy transfer and emission efficiency of quasi‐2D perovskites. The strategies for efficient 3D mixed‐halide perovskite and lead‐free perovskite blue LEDs are then briefly introduced. After that, other strategies, including effective charge transport layer, efficient perovskite emission system, and effective device architecture for high light outcoupling efficiency, are further discussed to boost the blue PeLED performances. Meanwhile, the testing standard of blue PeLED lifetime is suggested to enable the direct comparisons of the device operational stability. Finally, challenges and future directions for blue PeLEDs are addressed.

62‐3: Red Perovskite Emitters for Color Conversion and LEDs

Perovskites are promising light‐emitting materials for colour conversion and direct electroluminescence, with the potential to enable the most prominent advanced TV architectures. While early commercial development has focused on green, red emitters are needed for both in‐pixel and backlight film colour conversion. Here we report on the recent developments on red emitters tailored for wide‐colour gamut and power efficiency. The materials show very high colour stability and are suitable for use in films, pixels, and LEDs.

Strongly Luminescent Dion–Jacobson Tin Bromide Perovskite Microcrystals Induced by Molecular Proton Donors Chloroform and Dichloromethane

AbstractLead‐free 2D perovskites based on tin halide octahedron slabs with Dion–Jacobson (DJ) phases have drawn attention due to their improved stability; still, reports on light‐emitting DJ lead‐free perovskites are scarce. Herein, a room‐temperature ligand assisted re‐precipitation method is used to produce ODASnBr4 perovskite microcrystals (ODA denotes protonated 1,8‐octanediamine). After incorporating molecular dopants chloroform and dichloromethane, not only the crystallinity of the DJ perovskite phase improves, but their emission becomes much stronger due to the formation of hydrogen bonds between [SnBr6]4− octahedra and acidic CH proton donors. ODASnBr4 microcrystals doped with these molecules show a high photoluminescence quantum yield (PLQY) approaching 90%, and their emission remains stable under a continuous UV irradiation, with less than 10% loss in intensity over 6 h. Moreover, by tuning the pristine ODASnBr4 with various degrees of exposure to the molecular dopants, the maximum of their self‐trapped exciton emission can be fine‐tuned over a spectral range of 570–608 nm while maintaining high PLQYs of 83–88%. This provides a convenient way to adjust the spectral position of DJ perovskite emission without changing halides or A‐site spacers. Thus, stable and strongly emitting lead‐free DJ perovskite materials have been developed.

Recent Progress in Electrochemiluminescence of Halide Perovskites.

Halide perovskites are a rapidly developing class of solution-processable semiconductors which, to date, have a huge impact across several scientific communities. The remarkable photophysical attributes of halide perovskites illustrate their considerable potential in the electrochemiluminescence (ECL) realm. Over the past 4 years, great progress has been achieved in using halide perovskites as ECL emitters. In this mini-review, the basic characteristics, synthetic approaches, and ECL mechanisms for halide perovskite emitters are first introduced. To the best of our knowledge, most of the reported ECL-active halide perovskites and their disclosed unique features are detailly summarized. Stabilization and interface manipulation strategies for desirable ECL performance are further highlighted. The preliminary halide perovskites-related ECL applications are finally discussed, and prospects are also anticipated.

Lead-Free Organic-Perovskite Hybrid Quantum Wells for Highly Stable Light-Emitting Diodes.

Two-dimensional perovskites that could be regarded as natural organic-inorganic hybrid quantum wells (HQWs) are promising for light-emitting diode (LED) applications. High photoluminescence quantum efficiencies (approaching 80%) and extremely narrow emission bandwidth (less than 20 nm) have been demonstrated in their single crystals; however, a reliable electrically driven LED device has not been realized owing to inefficient charge injection and extremely poor stability. Furthermore, the use of toxic lead raises concerns. Here, we report Sn(II)-based organic-perovskite HQWs employing molecularly tailored organic semiconducting barrier layers for efficient and stable LEDs. Utilizing femtosecond transient absorption spectroscopy, we demonstrate the energy transfer from organic barrier to inorganic perovskite emitter occurs faster than the intramolecular charge transfer in the organic layer. Consequently, this process allows efficient conversion of lower-energy emission associated with the organic layer into higher-energy emission from the perovskite layer. This greatly broadened the candidate pool for the organic layer. Incorporating a bulky small bandgap organic barrier in the HQW, charge transport is enhanced and ion migration is greatly suppressed. We demonstrate a HQW-LED device with pure red emission, a maximum luminance of 3466 cd m-2, a peak external quantum efficiency up to 3.33%, and an operational stability of over 150 h, which are significantly better than previously reported lead-free perovskite LEDs.

Modulating low-dimensional domains of self-assembling quasi-2D perovskites for efficient and spectra-stable blue light-emitting diodes

Abstract Perovskite light-emitting diodes (PeLEDs) with green and red emissions have made tremendous progress in recent years. However, the realization of efficient and spectra-stable blue-emission PeLEDs remains challenging, which greatly limits their future commercialization. Here, a self-assembling strategy is proposed to achieve efficient blue PeLEDs with stable emission by modulating low-dimensional domains of pure-bromide perovskites to Bohr’s radii. A spacer of butylamine with polyamine is used as a modifier to facilitate the formation of homogeneous perovskite films with precious tailoring of the numbers (N) of metal halide octahedron PbBr64− layers. The cubic-phase quasi-2D perovskite films featuring the improved domain distributions and the enhanced luminance are realized, and their emissions can be modulated from violet (~436 nm) to greenish region (~500 nm), corresponding to the emission from N = 2 to N > 3 layered perovskites. The PeLEDs with deep-blue, pure-blue, and sky-blue emissions are obtained with the colour coordinates of (0.13, 0.06), (0.11, 0.13), and (0.08, 0.25), respectively, yielding the maximum external quantum efficiencies of 1.74%, 2.44%, and 5.71%. The approach demonstrated in this work paves the way for constructing efficient, endurable, spectra-stable perovskite emitters for the next-generation display and lighting technologies.

Using steric hindrance to manipulate and stabilize metal halide perovskites for optoelectronics.

The chemical instability of metal halide perovskite materials can be ascribed to their unique properties of softness, in which the chemical bonding between metal halide octahedral frameworks and cations is the weak ionic and hydrogen bonding as in most perovskite structures. Therefore, various strategies have been developed to stabilize the cations and metal halide frameworks, which include incorporating additives, developing two-dimensional perovskites and perovskite nanocrystals, etc. Recently, the important role of utilizing steric hindrance for stabilizing and passivating perovskites has been demonstrated. In this perspective, we summarize the applications of steric hindrance in manipulating and stabilizing perovskites. We will also discuss how steric hindrance influences the fundamental kinetics of perovskite crystallization and film formation processes. The similarities and differences of the steric hindrance between perovskite solar cells and perovskite light emission diodes are also discussed. In all, utilizing steric hindrance is a promising strategy to manipulate and stabilize metal halide perovskites for optoelectronics.

Electroluminescent materials toward near ultraviolet region.

Near ultraviolet (NUV) light-emitting materials and devices are significant due to unique applications in anti-counterfeit, manufacturing industries, and hygienic treatments. However, the development of high-efficiency NUV electroluminescent devices encounters great challenges and is far behind their RGB emitter counterparts. Besides the photoluminescence quantum yields (PLQYs) of NUV materials being higher than 40%, charge injection and lopsided carrier transport also determine the device performance, leading to great efforts in optimizing the frontier molecular orbitals to fit the adjacent function layer. In the exploration of NUV materials, organic molecules are one of the primary candidates, given their preparative facility and structural variability. Recently, all-inorganic quantum-dot light-emitting diodes (QLEDs) of Cd-based, ZnSe, graphene and inorganic perovskite emitters and organic-inorganic hybrid lead halide perovskite nanocrystals (NCs) were demonstrated for achieving NUV electroluminescence. Owing to the great efforts devoted to NUV material engineering and device configuration, NUV materials and devices have achieved great advances over the last two decades. In this review, we retrospect the development of NUV materials and devices covering all promising systems, which may inspire the enthusiasm of researchers to explore the huge potential in the NUV region.

Interface and bulk controlled perovskite nanocrystal growth for high brightness light-emitting diodes [Invited

Halide perovskites have attracted great attention due to their high color purity, high luminance yield, low non-radiative recombination rate, and solution processability. Although the external quantum efficiency of perovskite light-emitting diodes (PeLEDs) is comparable with that of the organic light-emitting diodes (OLEDs) and quantum-dots light-emitting diodes (QLEDs), the brightness is still low compared with the traditional OLEDs and QLEDs. Herein, we demonstrate high brightness and high-efficiency CsPbBr3-based PeLEDs using interface and bulk controlled nanocrystal growth of the perovskite emission layer. The interface engineering by ethanolamine and bulk engineering by polyethylene glycol led to highly crystallized and cubic-shaped perovskite nanocrystals with smooth and compact morphology. As a result, PeLEDs with a high brightness of 64756 cd/m2 and an external quantum efficiency of 13.4% have been achieved.