3D organometal halide perovskite. [2] Due to the poor film morphology and strong trap-assisted nonradiative recombination, the device performance is modest, with a peak EQE of 0.76%. Many methods, including interfacial engineering, polymer additive, and antisolvent, have been used to improve the quality of perovskite films. [3,[13][14][15] However, due to the serious trap-assisted nonradiative recombination, the photoluminescence quantum efficiencies (PLQEs) of 3D perovskites at low excitations are quite low, limiting the further improvement of device performance.The emerged multiple-quantum-well (MQW) perovskite has the merits of good film morphology and high PLQE, which is promising to achieve high performance LEDs. The MQW perovskite can be defined as quasi-2D layered perovskite, which is composed of different layered perovskites with naturally formed quantum wells (QWs) (Figure 1). [16][17][18] Generally, the layered perovskite has a formula of L 2 (SMX 3 ) n−1 MX 4 , where L is the large organic cation, S is the small monovalent cation, M is the divalent metal cation, X is the halide anion, and n is the number of MX 4 2− sheets. [19][20][21] In layered perovskites, the MX 4 2− sheet acts as potential well and its number, n, determines well width and the bandgap, while the large organic layer acts as potential barrier and its ionic radius determines the barrier width. It was found that quasi-2D layered perovskite thin film can spontaneously form MQW structure by spin-coating process, which is a mixture of layered perovskites with different n numbers and different bandgaps. [16] The energy transfer process from large bandgap QWs to small bandgap QWs is fast and efficient, resulting in carrier localization and accumulation in low energy QWs. [16,18] Consequently, trap-induced nonradiative recombination can be suppressed and high PLQE can be obtained. Based on the MQWs, the EQE of perovskite LEDs first reaches >10% in 2016. [16,22] Recently, through further suppressing nonradiative recombination and enhancing outcoupling by polymer additive, the peak EQE of near-infrared (NIR) perovskite LEDs based on MQWs has reached 20%. [7] Here, we focus on the unique properties of MQW perovskite and address its potential for high performance LEDs. We then discuss how to control the MQW structure and its effect on perovskite LED performance. Why MQW Perovskites are Promising for High Performance LEDsFor perovskite LEDs, the EQE is intrinsically limited by the properties of perovskite film, which also determine the stability Light-emitting diodes (LEDs) based on solution-processed metal halide perovskites have shown great application potential in energy-efficient lighting and displays. Multiple-quantum-well (MQW) perovskites simultaneously possess high photoluminescence quantum efficiency and good film morphology and stability, making it attractive for high-performance perovskite LEDs. Here, merits of MQW perovskites and the progress in MQW perovskite LEDs are reviewed. Challenges and future directions of perovskite LEDs are ...

Solution-processed metal halide perovskites have been recognized as one of the most promising semiconductors, with applications in light-emitting diodes (LEDs), solar cells and lasers. Various additives have been widely used in perovskite precursor solutions, aiming to improve the formed perovskite film quality through passivating defects and controlling the crystallinity. The additive’s role of defect passivation has been intensively investigated, while a deep understanding of how additives influence the crystallization process of perovskites is lacking. Here, we reveal a general additive-assisted crystal formation pathway for FAPbI3 perovskite with vertical orientation, by tracking the chemical interaction in the precursor solution and crystallographic evolution during the film formation process. The resulting understanding motivates us to use a new additive with multi-functional groups, 2-(2-(2-Aminoethoxy)ethoxy)acetic acid, which can facilitate the orientated growth of perovskite and passivate defects, leading to perovskite layer with high crystallinity and low defect density and thereby record-high performance NIR perovskite LEDs (~800 nm emission peak, a peak external quantum efficiency of 22.2% with enhanced stability).

Solution-processed metal-halide perovskites are emerging as one of the most promising materials for displays, lighting and energy generation. Currently, the best-performing perovskite optoelectronic devices are based on lead halides and the lead toxicity severely restricts their practical applications. Moreover, efficient white electroluminescence from broadband-emission metal halides remains a challenge. Here we demonstrate efficient and bright lead-free LEDs based on cesium copper halides enabled by introducing an organic additive (Tween, polyethylene glycol sorbitan monooleate) into the precursor solutions. We find the additive can reduce the trap states, enhancing the photoluminescence quantum efficiency of the metal halide films, and increase the surface potential, facilitating the hole injection and transport in the LEDs. Consequently, we achieve warm-white LEDs reaching an external quantum efficiency of 3.1% and a luminance of 1570 cd m−2 at a low voltage of 5.4 V, showing great promise of lead-free metal halides for solution-processed white LED applications.

Metal halide perovskite light-emitting diodes (LEDs) have achieved great progress in recent years. However, bright and spectrally stable blue perovskite LED remains a significant challenge. Three-dimensional mixed-halide perovskites have potential to achieve high brightness electroluminescence, but their emission spectra are unstable as a result of halide phase separation. Here, we reveal that there is already heterogeneous distribution of halides in the as-deposited perovskite films, which can trace back to the nonuniform mixture of halides in the precursors. By simply introducing cationic surfactants to improve the homogeneity of the halides in the precursor solution, we can overcome the phase segregation issue and obtain spectrally stable single-phase blue-emitting perovskites. We demonstrate efficient blue perovskite LEDs with high brightness, e.g., luminous efficacy of 4.7, 2.9, and 0.4 lm W-1 and luminance of over 37,000, 9,300, and 1,300 cd m-2 for sky blue, blue, and deep blue with Commission Internationale de l’Eclairage (CIE) coordinates of (0.068, 0.268), (0.091, 0.165), and (0.129, 0.061), respectively, suggesting real promise of perovskites for LED applications.