Efficient Flexible Inorganic Perovskite Light-Emitting Diodes Fabricated with CsPbBr3 Emitters Prepared via Low-Temperature in Situ Dynamic Thermal Crystallization

Advances and Challenges in Large-Area Perovskite Light-Emitting Diodes

Metal halide perovskite light-emitting diodes (PeLEDs) have shown promise for high-definition displays and flat-panel lighting because of their wide color gamut, narrow emission band, and high brightness. The external quantum efficiency of PeLEDs increased rapidly from ≈1% to more than 25% in the past few years. However, most of these high-performance devices are fabricated using a spin coating method with a small device area of <0.1 cm2, limiting their commercial applications. Recently, large-area PeLEDs have attracted growing attention and significant breakthroughs have been reported. This perspective first introduces the pros and cons of each technique in making large-area PeLEDs. The advances in the fabrication of large-area PeLEDs are then summarized using spin coating and mass-production methods such as inkjet printing, blade coating, and thermal evaporation. Moreover, the challenging issues will be discussed that are urgent to be solved for large-area PeLEDs.

Unraveling the Luminescence Quenching Mechanism in Strong and Weak Quantum-Confined CsPbBr3 Triggered by Triarylamine-Based Hole Transport Layers

Luminescence quenching by hole transport layers (HTLs) is one of the major issues in developing efficient perovskite light-emitting diodes (PeLEDs), which is particularly prominent in blue-emitting devices. While a variety of material systems have been used as interfacial layers, the origin of such quenching and the type of interactions between perovskites and HTLs are still ambiguous. Here, we present a systematic investigation of the luminescence quenching of CsPbBr3 by a commonly employed hole transport polymer, poly[(9,9-dioctylfluorenyl-2,7diyl)-co-(4,4'-(N-(4-sec-butylphenyl) diphenylamine)] (TFB), in LEDs. Strong and weak quantum-confined CsPbBr3 (nanoplatelets (NPLs)/nanocrystals (NCs)) are rationally selected to study the quenching mechanism by considering the differences in their morphology, energy level alignments, and quantum confinement. The steady-state and time-resolved Stern-Volmer plots unravel the dominance of dynamic and static quenching at lower and higher concentrations of TFB, respectively, with a maximum quenching efficiency of 98%. The quenching rate in NCs is faster than that in NPLs owing to their longer PL lifetimes and weak quantum confinement. The ultrafast transient absorption results support these dynamics and rule out the involvement of Forster or Dexter energy transfer. Finally, the 1D 1H and 2D nuclear overhauser effect spectroscopy nuclear magnetic resonance (NOESY NMR) study confirms the exchange of native ligands at the NCs surface with TFB, leading to dark CsPbBr3-TFB ensemble formation accountable for luminescence quenching. This highlights the critical role of the triarylamine functional group on TFB (also the backbone of many HTLs) in the quenching process. These results shed light on the underlying reasons for the luminescence quenching in PeLEDs and will help to rationally choose the interfacial layers for developing efficient LEDs.

Perovskite Light-Emitting Diode Display Based on MoS2 Backplane Thin-Film Transistors

The uniform deposition of perovskite light-emitting diodes (PeLEDs) and their integration with backplane thin-film transistors (TFTs) remain challenging for large-area display applications. Herein, an active-matrix PeLED display fabricated via the heterogeneous integration of cesium lead bromide LEDs and molybdenum disulfide (MoS2 )-based TFTs is presented. The single-source evaporation method enables the deposition of highly uniform perovskite thin films over large areas. PeLEDs are integrated with MoS2 TFTs to fabricate an active-matrix PeLED display with an 8 × 8 array, which exhibits excellent brightness control capability and high switching speed. This study demonstrates the potential of PeLEDs as candidates for next-generation displays and presents a novel approach for fabricating optoelectronic devices via the heterogeneous integration of 2D materials and perovskites, thereby paving the way toward the fabrication of practical future optoelectronic systems.

Perovskite Light-Emitting Devices Based on Solid-State Diffusion In Situ Dynamic Thermal Crystallization

Due to the excellent photonic and electrical properties of metal halide perovskite materials, perovskite light-emitting devices have the potential to replace OLED devices as the next-generation of commercial light-emitting devices. In this article, we controlled the surface morphology of PbBr2 using an in situ dynamic thermal crystallization process, which increased the specific surface area of the films and promoted the solid-state diffusion rate. The CsPbBr3 PeLEDs prepared using this method achieved a maximum current efficiency of 7.1 cd/A at the voltage of 5 V, which was 200% higher than devices prepared using traditional spin-coating processes. These results proved that the in situ thermal dynamic crystallization process effectively improved the film quality of perovskite materials.

High-Efficiency CsPbBr3 Light-Emitting Diodes using One-Step Spin-Coating In Situ Dynamic Thermal Crystallization

All-inorganic perovskite materials (such as CsPbBr₃) have received widespread attention because of their better stability than hybrid counterparts, but their poor film morphology and crystalline quality limit their application in perovskite light-emitting devices (PeLEDs). Some previous studies have attempted to improve the morphology and crystalline quality of perovskite films by heating the substrate, but there are still some problems such as inaccurate temperature control, excessive temperature is not conducive to flexible applications, and the mechanism of action is not clear. In this work, we used a one-step spin-coating, low-temperature in situ thermally assisted crystallization process, in which the temperature was accurately monitored using a thermocouple in the range of 23-80 °C, and explored the effect of the in situ thermally assisted crystallization temperature on the crystallization of the all-inorganic perovskite material CsPbBr₃ and the performance of PeLEDs. In addition, we focused on the influence mechanism for the in situ thermally assisted crystallization process on the surface morphology and phase composition of the perovskite films and promote its possible application in inkjet printing and scratch coating methods.

Bionic electroluminescent perovskite light-emitting device

A perovskite light-emitting-diode (PeLED) displaying green color is combined with a brown fluorescent coating (FC) layer to form a hybrid FC-PeLED system. The FC-PeLED system can simulate the natural process of bionics of plant colors from green to brown through a low energy (<0.6 mW) input, promoting the development of future low-cost and low-power consumption bionics technology.

Collaborative Passivation for Dual Charge Transporting Layers Based on 4-(chloromethyl)benzonitrile Additive toward Efficient and Stable Inverted Perovskite Solar Cells

Poor carrier transport capacity and numerous surface defects of charge transporting layers (CTLs), coupled with misalignment of energy levels between perovskites and CTLs, impact photoelectric conversion efficiency (PCE) of inverted perovskite solar cells (PSCs) profoundly. Herein, a collaborative passivation strategy is proposed based on 4-(chloromethyl) benzonitrile (CBN) as a solution additive for fabrication of both [6,6]-phenyl-C61-butyric acid methylester (PCBM) and poly(triarylamine) (PTAA) CTLs. This additive can improve wettability of PTAA and reduce the agglomeration of PCBM particles, which enhance the PCE and device stability of the PSCs. As a result, a PCE exceeding 20% with a remarkable short circuit current of 23.9 mA cm^-2 , and an improved fill factor of 81% is obtained for the CBN- modified inverted PSCs. Devices maintain 80% and 70% of the initial PCE after storage under 30% and 85% humidity ambient conditions for 1000 h without encapsulation, as well as negligible light state PCE loss. This strategy demonstrates feasibility of the additive engineering to improve interfacial contact between the CTLs and perovskites for fabrication of efficient and stable inverted PSCs.

Photopatternable and Highly Conductive PEDOT:PSS Electrodes for Flexible Perovskite Light-Emitting Diodes

Flexible perovskite light-emitting diodes (PeLEDs) constitute an emerging technology opening new opportunities in the fields of lighting and display for portable and wearable electronics. Poly(3,4-ethylenedioxythiophene):poly(stryrenesulfonate) (PEDOT:PSS) as one of the most promising flexible electrode materials has attracted extensive attention. However, the patterning and conductivity issues of PEDOT:PSS electrodes should be addressed primarily. Here, a photopolymerizable additive is proposed to endow the PEDOT:PSS electrodes with photopatternability. Moreover, this additive can also improve the conductivity of the PEDOT:PSS electrode from 0.16 to 627 S/cm because of the phase separation between PEDOT and PSS components and conformation transition of PEDOT chains. Eventually, highly conductive PEDOT:PSS electrodes with various patterns are applied in flexible PeLEDs, demonstrating a high luminance of 25972 cd/m² and a current efficiency of 25.1 cd/A. This work provides a facile and effective method of patterning and improving the conductivity of PEDOT:PSS electrodes simultaneously, demonstrating the great potential of PEDOT:PSS electrodes in flexible perovskite optoelectronics.

Vacuum-Deposited Inorganic Perovskite Light-Emitting Diodes with External Quantum Efficiency Exceeding 10% via Composition and Crystallinity Manipulation of Emission Layer under High Vacuum

Although vacuum-deposited metal halide perovskite light-emitting diodes (PeLEDs) have great promise for use in large-area high-color-gamut displays, the efficiency of vacuum-sublimed PeLEDs currently lags that of solution-processed counterparts. In this study, highly efficient vacuum-deposited PeLEDs are prepared through a process of optimizing the stoichiometric ratio of the sublimed precursors under high vacuum and incorporating ultrathin under- and upper-layers for the perovskite emission layer (EML). In contrast to the situation in most vacuum-deposited organic light-emitting devices, the properties of these perovskite EMLs are highly influenced by the presence and nature of the upper- and presublimed materials, thereby allowing us to enhance the performance of the resulting devices. By eliminating Pb° formation and passivating defects in the perovskite EMLs, the PeLEDs achieve an outstanding external quantum efficiency (EQE) of 10.9% when applying a very smooth and flat geometry; it reaches an extraordinarily high value of 21.1% when integrating a light out-coupling structure, breaking through the 10% EQE milestone of vacuum-deposited PeLEDs.

Halide Perovskite Crystallization Processes and Methods in Nanocrystals, Single Crystals, and Thin Films

Halide perovskite semiconductors with extraordinary optoelectronic properties have been fascinatedly studied. Halide perovskite nanocrystals, single crystals, and thin films have been prepared for various fields, such as light emission, light detection, and light harvesting. High-performance devices rely on high crystal quality determined by the nucleation and crystal growth process. Here, the fundamental understanding of the crystallization process driven by supersaturation of the solution is discussed and the methods for halide perovskite crystals are summarized. Supersaturation determines the proportion and the average Gibbs free energy changes for surface and volume molecular units involved in the spontaneous aggregation, which could be stable in the solution and induce homogeneous nucleation only when the solution exceeds a required minimum critical concentration (C_min ). Crystal growth and heterogeneous nucleation are thermodynamically easier than homogeneous nucleation due to the existent surfaces. Nanocrystals are mainly prepared via the nucleation-dominated process by rapidly increasing the concentration over C_min , single crystals are mainly prepared via the growth-dominated process by keeping the concentration between solubility and C_min , while thin films are mainly prepared by compromising the nucleation and growth processes to ensure compactness and grain sizes. Typical strategies for preparing these three forms of halide perovskites are also reviewed.

Highly Tunable Enhancement and Switching of Nonlinear Emission from All-Inorganic Lead Halide Perovskites via Electric Field

Herein, we demonstrate a highly tunable enhancement and switching of nonlinear emission from all-inorganic metal halide perovskites based on an asymmetrically biased metal-insulator-semiconductor (MIS) structure. We achieve 2 orders of magnitude enhancement of the two-photon-pumped photoluminescence (TPL) from CsPbBr₃ microplates with the MIS structure, due to comprehensive effects including localized field effect, trap-filling effect, and collection enhancement. In particular, taking advantage of electric-field-induced passivation/activation of Br vacancies, we realize highly tunable TPL enhancement, ranging from ∼61.2-fold to ∼370.3-fold. Moreover, we demonstrate an efficient modulation of the two-photon-pumped lasing from the MIS structure, which exhibits electric field induced switching with a high on/off ratio of 67:1. This work has opened new avenues for steering carrier transport and nonlinear emission in lead halide perovskites, which shows great promise for realizing high-efficiency and tunable nonlinear nanophotonic devices.

Efficient Inorganic Perovskite Light-Emitting Diodes by Inducing Grain Arrangement via a Multifunctional Interface

Accumulating evidence shows that metal halide perovskite light-emitting diodes (PeLEDs) are well-described to show broad application prospects in lighting and display due to the wide color gamut and high color purity. However, it is still a great challenge to prepare high-quality all-inorganic PeLEDs by a solution method. For example, it is difficult to obtain all-inorganic perovskite films with good crystallinity and high grain orientation because of too fast and uncontrollable crystallization of all-inorganic perovskite films. Here, we demonstrated a multifunctional interface of formamide (FA)-doped PEDOT:PSS, which improved the crystallinity of all-inorganic perovskite films by inducing grain arrangement. As a result, a highly crystalline, ordered, and defect-passivated CsPbBr₃ film was obtained by the multiple roles of FA, and the CsPbBr₃-based PeLED treated with FA achieved both high brightness and high efficiency: the peak external quantum efficiency (EQE) reaches 9.61%, and the maximum brightness is 185,000 cd/m². In addition, Tween 80, used as a passivator of perovskite films, reduced the defect states and suppressed ion migration. Under the synergistic effect of FA interface treatment and Tween 80 passivation treatment, efficient CsPbBr₃-based PeLEDs were obtained with an EQE of 15.02% and an operation lifetime of 182.5 min at an initial brightness of 1000 cd/m², which is among the best reported lifetimes under high brightness. Our study provides a simple and effective strategy for the realization of all-inorganic PeLEDs with high efficiency, high brightness, and ultralong operation lifetime.

Phthalide and 1-Iodooctadecane Synergistic Optimization for Highly Efficient and Stable Perovskite Solar Cells

The carrier non-radiative recombination and instability of device caused by the inherent defects are main factors limiting development of perovskite solar cells (PSCs). During the fabrication process of a PSC device, perovskite films often produce Pb⁰ and I⁰ defects. This paper reports a strategy for synergistic optimization of perovskite films by defects passivation and surface modification. The doping of phthalide (PT) in the Pb-rich (CH(NH₂ )₂ )_1-x (CH₃ NH₃ )_x PbI₃ film can passivate lead cation defects, and the modification of 1-iodooctadecane (1-IO) can reduce halogen anion defects and improve stability of PSCs owing to its hydrophobicity. The PT and 1-IO optimized device achieves a power conversion efficiency (PCE) of 22.27%. The optimized PSCs remain 93.2% of the initial PCE when placed in air environment (relative humidity of 10%, 25 °C) more than 70 days. The PT and 1-IO synergistic optimization provides a novel strategy for improving the performance and stability of PSCs.

Advances in Flexible Memristors with Hybrid Perovskites

Hybrid organic-inorganic metal halide perovskite (HOIP)-based memristors have captured strong attention not only as an emerging candidate for next-generation high-density information storage technology but also for use in healthcare technology and the Internet of Things (IoT) because of their unique properties: low weight, flexibility, compatibility, stretchability, and low power consumption. In this Perspective, we review the recent advances of various aspects of flexible memristors focusing on the selection of the flexible substrates, materials, interfaces, several resistive switching mechanisms, and different methodologies of perovskite growth. The current state of the art of the memristor as an artificial synapse, light-induced resistive switching, and logic gates is comprehensively and systematically reviewed. Finally, we briefly discuss the stability factors of perovskites and present the conclusion with a broad outlook on the progress and challenges in the field of perovskite-based flexible memristors.

Mobile ions determine the luminescence yield of perovskite light-emitting diodes under pulsed operation

The external quantum efficiency of perovskite light-emitting diodes (PeLEDs) has advanced quickly during the past few years. However, under pulsed operation, an operation mode which is important for display and visible light communication, the performance of PeLEDs changes a lot and requires in-depth understanding to facilitate these applications. Here, we report the response of PeLEDs under pulsed operation in the range of 10 Hz to 20 kHz. Beyond transient effects in the low frequencies, we find that for higher frequencies (>500 Hz) the transient electroluminescence intensity depends strongly on the duty cycle. This feature is much more pronounced and of different origin than that in conventional LEDs. We rationalise our experimental observations using a mathematical model and assign these features to the effect of mobile ionic charges in the perovskite. Our work also provides important implications for the operation of PeLEDs under the steady state, where accumulation of mobile ions at the interfaces could be beneficial for high electroluminescence yields but harmful for the long-term stability.

Highly Efficient Vacuum-Evaporated CsPbBr3 Perovskite Light-Emitting Diodes with an Electrical Conductivity Enhanced Polymer-Assisted Passivation Layer

Highly efficient vacuum-deposited CsPbBr₃ perovskite light-emitting diodes (PeLEDs) are demonstrated by introducing a separate polyethylene oxide (PEO) passivation layer. A CsPbBr₃ film deposited on the PEO layer via thermal co-evaporation of CsBr and PbBr₂ exhibits an almost 50-fold increase in photoluminescence quantum yield intensity compared to a reference sample without PEO. This enhancement is attributed to the passivation of interfacial defects of the perovskite, as evidenced by temperature-dependent photoluminescence measurements. However, direct application of PEO to an LED device is challenging because of the electrically insulating nature of PEO. This issue is solved by doping PEO layers with MgCl₂. This strategy results in an enhanced luminance and external quantum efficiency (EQE) of up to 6887 cd m^-2 and 7.6%, respectively. To the best of our knowledge, this is the highest EQE reported to date among vacuum-deposited PeLEDs.

Efficient and large-area all vacuum-deposited perovskite light-emitting diodes via spatial confinement

With rapid advances of perovskite light-emitting diodes (PeLEDs), the large-scale fabrication of patterned PeLEDs towards display panels is of increasing importance. However, most state-of-the-art PeLEDs are fabricated by solution-processed techniques, which are difficult to simultaneously achieve high-resolution pixels and large-scale production. To this end, we construct efficient CsPbBr3 PeLEDs employing a vacuum deposition technique, which has been demonstrated as the most successful route for commercial organic LED displays. By carefully controlling the strength of the spatial confinement in CsPbBr3 film, its radiative recombination is greatly enhanced while the nonradiative recombination is suppressed. As a result, the external quantum efficiency (EQE) of thermally evaporated PeLED reaches 8.0%, a record for vacuum processed PeLEDs. Benefitting from the excellent uniformity and scalability of the thermal evaporation, we demonstrate PeLED with a functional area up to 40.2 cm2 and a peak EQE of 7.1%, representing one of the most efficient large-area PeLEDs. We further achieve high-resolution patterned perovskite film with 100 μm pixels using fine metal masks, laying the foundation for potential display applications. We believe the strategy of confinement strength regulation in thermally evaporated perovskites provides an effective way to process high-efficiency and large-area PeLEDs towards commercial display panels.

Efficient Thermally Evaporated Perovskite Light-Emitting Devices via a Bilateral Interface Engineering Strategy

Physical vapor deposition has emerged as a promising strategy for efficient and stable all-inorganic perovskite light-emitting devices (PeLEDs). However, the thermally evaporated PeLEDs still suffer from unsatisfactory optoelectrical performance because of the massive nonradiative defects. Herein, we demonstrate an efficient bilateral interfacial defect-passivation strategy toward high-performance PeLEDs with a thermally deposited CsPbBr₃ emissive layer (EML). Specifically, the nonradiative defects from the bulk as well as the EML/charge transport layer (CTL) interface are significantly suppressed by implementing the 3-amino-1-propanol (3AP)-modified PEDOT:PSS and introducing ammonium salts, respectively. Simultaneously, both the 3AP induced less-conductive Cs₄PbBr₆ and ammonium salts can balance the charge injection into the EML effectively. As a result, we achieved efficient PeLEDs based on thermally evaporated CsPbBr₃ with a luminance of 15745 cd/m², current efficiency of 32 cd/A, external quantum efficiency of 8.86%, and lifetime of 3.74 h. The strategy proposed here may shed light on the development of highly efficient thermally evaporated PeLEDs.

Improving the Photovoltaic Performance of Flexible Solar Cells with Semitransparent Inorganic Perovskite Active Layers by Interface Engineering

Inorganic perovskite CsPbBr₃ has broad application prospects in photovoltaic windows, tandem cells, and other fields due to its intrinsic semitransparency, excellent photoelectric properties, and stability. In this work, a high-quality semitransparent CsPbBr₃ film was prepared by a sequential vacuum evaporation deposition method without high-temperature annealing and successfully used as the active layer of flexible perovskite solar cells (F-PSCs) for the first time, achieving a power conversion efficiency (PCE) of 5.00%. By introducing an energy-level buffer layer of Cu₂O between CsPbBr₃ and Spiro-OMeTAD, the champion PCE has been further improved to 5.67% owing to the reduction of electron-hole recombination and enhanced charge extraction. The optimized devices present higher stability, which can maintain more than 95% of the initial efficiency even after continuous heating at 85 °C for 240 h. Moreover, the F-PSCs also exhibit excellent mechanical durability, and 90% of the original PCE can be retained after 1000 bending cycles at a curvature radius of 3 mm.

Stable and Self-Powered Solar-Blind Ultraviolet Photodetectors Based on a Cs3Cu2I5/β-Ga2O3 Heterojunction Prepared by Dual-Source Vapor Codeposition

Self-powered solar-blind ultraviolet (UV) photodetectors have drawn worldwide attention in recent years because of their important applications in military and civilian areas. In this study, a dual-source vapor codeposition technique was employed, for the first time, to prepare a nontoxic copper halide Cs₃Cu₂I₅, which was integrated with the β-Ga₂O₃ wafer to construct a type-II heterojunction for photodetection applications. By optimizing the annealing conditions, high-quality Cs₃Cu₂I₅ films with dense morphology, high crystallinity, and a long carrier lifetime of 1.02 μs were acquired. Because of the high material integrity of Cs₃Cu₂I₅ films and effective interfacial carrier transfer from Cs₃Cu₂I₅ to β-Ga₂O₃, a heterojunction device demonstrates a good solar-blind UV response property and operates at zero bias. Typically, the photodetector presents a low dark current (∼1.2 pA), a high solar-blind/UVA rejection ratio (∼1.0 × 10³), a relatively fast photoresponse speed (37/45 ms), and a high photo-to-dark current ratio (∼5.1 × 10⁴) at zero bias. Moreover, even after 12-h continuous working and 2-month storage without encapsulation in ambient air, the photodetection ability of the device can almost be maintained, demonstrating outstanding air stability. Our results suggest that nontoxic Cs₃Cu₂I₅ is able to serve as a prospective candidate for stable solar-blind UV photodetection.

Lead-Free Perovskite Materials for Solar Cells

The toxicity issue of lead hinders large-scale commercial production and photovoltaic field application of lead halide perovskites. Some novel non- or low-toxic perovskite materials have been explored for development of environmentally friendly lead-free perovskite solar cells (PSCs). This review studies the substitution of equivalent/heterovalent metals for Pb based on first-principles calculation, summarizes the theoretical basis of lead-free perovskites, and screens out some promising lead-free candidates with suitable bandgap, optical, and electrical properties. Then, it reports notable achievements for the experimental studies of lead-free perovskites to date, including the crystal structure and material bandgap for all of lead-free materials and photovoltaic performance and stability for corresponding devices. The review finally discusses challenges facing the successful development and commercialization of lead-free PSCs and predicts the prospect of lead-free PSCs in the future.

Synthesis of highly luminescent Mn-doped CsPbCl3 nanoplatelets for light-emitting diodes

An atmospheric approach is reported to synthesize Mn-doped CsPbCl₃ nanoplatelets with high PLQYs, and their applications in soft polymers and LEDs.

Low-temperature architecture of a cubic-phase CsPbBr3 single crystal for ultrasensitive weak-light photodetectors

We present the first report of a water-regulated method for obtaining a cubic-phase CsPbBr3 single crystal that could be frozen at low temperature with a CsBr/PbBr2 ratio of 1 : 1. The cubic CsPbBr3 single-crystal photodetector exhibits a superior responsivity of 278 A W-1, an EQE of 6.63 × 104%, and an ultrahigh detectivity of 4.36 × 1013 Jones under low-power 520 nm irradiation at 3 V.