Metal halide perovskites (MHPs) have emerged as promising emitters because of their excellent optoelectronic properties, including high photoluminescence quantum yields (PLQYs), wide‐range color tunability, and high color purity. However, a fundamental limitation of MHPs is their low exciton binding energy, which results in a low radiative recombination rate and the dependence of PLQY on the excitation intensity. Under the operating conditions of light‐emitting diodes (LEDs), the injected current densities are typically lower than the trap density, leading to a low actual PLQY. Moreover, the defects not only initiate the decomposition of MHPs caused by extrinsic factors, but also intrinsically stimulate ion migration across the interface and lead to the corrosion of electrodes due to interaction between those electrodes, even under inert conditions. The passivation of defects has proven to be effective for mitigating the effects of defects in MHPs. Herein, the origins and theoretical calculations of the defect tolerance in MHPs and the impact of defects on both the performance and stability of perovskite LEDs are reviewed. The passivation methods and materials for MHP bulk films and nanocrystals are discussed in detail. Based on the currently reported advances, specific requirements and future research directions for display applications are suggested.

Metal halide perovskites (MHPs) have emerged as promising materials for light‐emitting diodes owing to their narrow emission spectrum and wide range of color tunability. However, the low exciton binding energy in MHPs leads to a competition between the trap‐mediated nonradiative recombination and the bimolecular radiative recombination. Here, efficient and stable green emissive perovskite light‐emitting diodes (PeLEDs) with an external quantum efficiency of 14.6% are demonstrated through compositional, dimensional, and interfacial modulations of MHPs. The interfacial energetics and optoelectronic properties of the perovskite layer grown on a nickel oxide (NiOx) and poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate hole injection interfaces are investigated. The better interface formed between the NiOx/perovskite layers in terms of lower density of traps/defects, as well as more balanced charge carriers in the perovskite layer leading to high recombination yield of carriers are the main reasons for significantly improved device efficiency, photostability of perovskite, and operational stability of PeLEDs.

Recently, metal halide perovskites (MHPs) have attracted increasing attention as promising emitting materials for use in light-emitting diodes (LEDs), because of their high color purities and facile color tunabilities. [1][2][3][4][5][6][7] However, the low exciton binding energy of MHPs results in the thermal dissociation of the excitons at room temperature, [8][9][10] leading to poor geminate radiative recombination. The generated free charges are trapped in defect states rather than undergoing radiative decay, resulting in trap-mediated nonradiative charge recombination. To overcome the poor radiative recombination yield, several strategies, including reduction in perovskite grain size [7,11,12] and use of quasi-2D perovskites, [5,6,13,14] have been employed to promote bimolecular radiative recombination. Defect passivation in MHPs is a potential solution for enhancing the luminescent properties by employing various passivating materials containing amine (NH 2 ), [4,15,16] hydroxyl (OH), [17,18] phosphine oxide (PO), [6,19,20] carbonyl (CO), carboxyl (COOH), [21,22] and ammonium bromide Metal halide perovskites (MHPs) have attracted significant attention as light-emitting materials owing to their high color purities and tunabilities.A key issue in perovskite light-emitting diodes (PeLEDs) is the fabrication of an optimal charge transport layer (CTL), which has desirable energy levels for efficient charge injection while blocking opposite charges and enabling perovskite layer growth with reduced interfacial defects. Herein, two poly(fluorene-phenylene)-based anionic conjugated polyelectrolytes (CPEs) with different counterions (K + and tetramethylammonium (TMA + )) are presented as multifunctional passivating and hole-transporting layers (HTLs). The crystal growth of MHPs grown on different HTLs is investigated through X-ray photoelectron spectroscopy, X-ray diffraction, and density functional theory calculation. The CPE bearing the TMA + counterions remarkably improves the growth of perovskites with suppressed interfacial defects, leading to significantly enhanced emission properties and device performance. The luminescent properties are further enhanced via aging and electrical stress application with effective rearrangement of the counterions on the interfacial defects in the perovskites. Finally, efficient formamidinium lead tribromide-based quasi-2D PeLEDs with an external quantum efficiency of 10.2% are fabricated. Using CPEs with varying counterions as a CTL can serve as an effective method for controlling the interfacial defects and improving perovskite-based optoelectronic device properties. Perovskite Light-Emitting DiodesThe ORCID identification number(s) for the author(s) of this article can be found under https://doi.

as a Postdoc (2007-2009). His main research fields are organic/polymeric materials and organic/metal-halideperovskite-based optoelectronic devices. Han Young Woo received his Ph.D. degree in chemistry from KAIST, Republic of Korea in 1999. After postdoctoral training at University of California, Santa Barbara, USA, he joined the Pusan National University, Republic of Korea, as an assistant professor. In 2015, he moved to Korea University and he is currently a professor in the Department of Chemistry, Korea University. His current researches focus on conjugated polymers and polyelectrolytes for applications in organic optoelectronic devices.

A series of anionic conjugated polyelectrolytes (CPEs) is synthesized based on poly(fluorene‐co‐phenylene) by varying the side‐chain ionic density from two to six per repeat units (MPS2‐TMA, MPS4‐TMA, and MPS6‐TMA). The effect of MPS2, 4, 6‐TMA as interlayers on top of a hole‐extraction layer of poly(bis(4‐phenyl)‐2,4,6‐trimethylphenylamine (PTAA) is investigated in inverted perovskite solar cells (PeSCs). Owing to the improved wettability of perovskites on hydrophobic PTAA with the CPEs, the PeSCs with CPE interlayers demonstrate a significantly enhanced device performance, with negligible device‐to‐device dependence relative to the reference PeSC without CPEs. By increasing the ionic density in the MPS‐TMA interlayers, the wetting, interfacial defect passivation, and crystal growth of the perovskites are significantly improved without increasing the series resistance of the PeSCs. In particular, the open‐circuit voltage increases from 1.06 V for the PeSC with MPS2‐TMA to 1.11 V for the PeSC with MPS6‐TMA. The trap densities of the PeSCs with MPS2,4,6‐TMA are further analyzed using frequency‐dependent capacitance measurements. Finally, a large‐area (1 cm2) PeSC is successfully fabricated with MPS6‐TMA, showing a power conversion efficiency of 18.38% with negligible hysteresis and a stable power output under light soaking for 60 s.