Electroluminescence efficiencies and stabilities of quasi-two-dimensional halide perovskites are restricted by the formation of multiple-quantum-well structures with broad and uncontrollable phase distributions. Here, we report a ligand design strategy to substantially suppress diffusion-limited phase disproportionation, thereby enabling better phase control. We demonstrate that extending the π-conjugation length and increasing the cross-sectional area of the ligand enables perovskite thin films with dramatically suppressed ion transport, narrowed phase distributions, reduced defect densities, and enhanced radiative recombination efficiencies. Consequently, we achieved efficient and stable deep-red light-emitting diodes with a peak external quantum efficiency of 26.3% (average 22.9% among 70 devices and cross-checked) and a half-life of ~220 and 2.8 h under a constant current density of 0.1 and 12 mA/cm2, respectively. Our devices also exhibit wide wavelength tunability and improved spectral and phase stability compared with existing perovskite light-emitting diodes. These discoveries provide critical insights into the molecular design and crystallization kinetics of low-dimensional perovskite semiconductors for light-emitting devices.
Two‐dimensional (2D) halide perovskites can be regarded as natural organic‐inorganic hybrid quantum wells, which exhibit very promising light‐emitting applications due to their high photoluminescence quantum yield, narrow emission bandwidth, and large exciton binding energy. However, it remains a grand challenge to achieve reliable devices for both light‐emitting diodes (LEDs) and lasers utilizing phase‐pure 2D perovskites. Recently, exciting progresses have been made with respect to molecular design, optoelectronic property, and device fabrication for novel 2D perovskite hybrid quantum‐wells. In this article, we critically review the key challenges of exciton losses, charge injections, and triplet issues associated with the light‐emitting applications of such phase‐pure 2D perovskites after examining their recent breakthroughs in LEDs and lasers. Lastly, we provide a new perspective on molecular engineering strategies to address the above‐mentioned fundamental issues, which may open up a new avenue to the development of highly efficient quantum‐well emitters for solid‐state lighting and display.
Quasi-2D halide perovskites have attracted much interest as a promising material for light-emitting diodes (LEDs) due to their tunability in quantum confinement and halide alloy formation to modulate the energy bandgap and emission color. However, two-factor phase separations with respect to heterogeneous quantum-well thicknesses and halide segregation are still crucial issues in quasi-2D perovskite LEDs, leading to low external quantum efficiencies (EQEs) and color shifts. Herein, we compare quasi-2D perovskite films using different cations to unveil the key contributions from the chemical design of organic cations. While mixing halide ions in conventional quasi-2D perovskite films induces micrometerscale heterogeneity, new extended and twisted conjugated cations suppress the two-factor phase separations, leading to high EQEs of over 25% and controllable emission wavelengths across red and near-infrared regions. The fundamental insights in this work will provide guidance for advancing materials design and device performance in the future.
Commercialization of halide perovskites in the semiconductor industry is hindered by their short-term stability. The instability of perovskites is closely interlinked with ionic diffusion. Historically, attempts to study diffusion in 2D perovskites mostly utilized electrical characterizations, but these characterizations pose a challenge in deconvoluting the impact of device architecture, interlayers, and ionic species. In this Perspective, we focus our attention on simple optical characterizations employed in the literature to investigate halide diffusion in 2D perovskites using lateral and vertical heterostructure platforms. We review the various synthesis techniques used for fabrication of halide perovskite heterostructures and discuss the qualitative and quantitative diffusion studies performed using these platforms. We discuss the numerical methods used to validate and supplement the experimental halide diffusion kinetics. Finally, we highlight the need to conduct further research on the impact of device operating conditions, lattice structure, and vacancy concentration on halide diffusion. Through this Perspective, we aim to emphasize the need of developing a comprehensive understanding of halide diffusion in perovskites for their successful deployment in optoelectronics.
Two-dimensional perovskite crystals have attracted significant attention for their diverse optoelectronic characteristics, owing to their superior semiconducting properties. However, the majority of studies to date have focused on single crystals, which pose challenges for integration into device arrays due to their incompatibility with selective growth or conventional lithography techniques. Here, a facile one-step solution process for synthesizing 2D perovskite crystal arrays is proposed through meniscus-guided coating on patterned substrates. We further utilized this method for the synthesis of lateral heterostructure nanoplate arrays. Six different 2D perovskite nanoplate arrays, including epitaxial heterostructures, are successfully realized. Optical and crystallographic characterizations show the high optical performance and crystallinity of the nanoplates. Moreover, this method is further employed to prepare high-performance 2D perovskite nanoplate photosensor arrays. This strategy can be utilized as a guideline for the fundamental investigation of optical properties and the development of high-performance optoelectronics of perovskite materials including photosensors and displays.
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