Grain Size Modulation and Interfacial Engineering of CH3 NH3 PbBr3 Emitter Films through Incorporation of Tetraethylammonium Bromide

Bifacial passivation towards efficient FAPbBr3-based inverted perovskite light-emitting diodes

A unique technique to passivate both bottom and top sides of perovskite has been successfully developed to achieve highly efficient inverted perovskite light-emitting diodes (PeLEDs). For the bottom passivation, an organic/inorganic hybrid electron transporting layer (ETL) replaces the widely adopted inorganic ETL to overcome the disadvantages of the pure inorganic ETL. The ZPM (ZnO-in-polymer matrix) ETL, which consists of ZnO nanoparticles blended into polyvinylpyrrolidone, not only passivates the surface defects of ZnO nanoparticles, but also improves the morphology and stability of FAPbBr₃ film. For the top passivation, smaller grains and a FAPbBr₃/PEA₂PbBr₄ 3D/2D hybrid structure are obtained by applying a small amount of PEABr solution. The synergetic interplay of organic/inorganic hybrid ETL and organic halide salt surface modification substantially shrinks the grain size to facilitate radiative recombination, and suppresses non-radiative recombination both at the interface of ETL/perovskite and HTL/perovskite, and in the perovskite layer. As a result, the highly efficient green PeLED sets a new record of device performance for FAPbBr₃-based inverted PeLEDs, with current efficiency of 39.7 cd A^-1, external quantum efficiency of 9.0%, power efficiency of 46.4 lm W^-1, maximum luminance of 6.03 × 10⁴ cd m^-2, and half-lifetime of 297 minutes at an initial brightness of ∼100 cd m^-2.

Stabilizing lead halide perovskites with quaternary ammonium cations: the case of tetramethylammonium lead iodide

Organoammonium lead halide perovskites, especially methylammonium lead iodide CH3NH3PbI3, are promising photovoltaic materials, but they are far from commercial applications due in particular to their thermal instability and moisture sensitivity. Here, we present a multitechnique study aimed at investigating the kinetic and thermodynamic stability of the simplest quaternary ammonium lead iodide, tetramethylammonium lead iodide N(CH3)4PbI3. The kinetics of thermal decomposition was studied by X-ray powder diffraction of samples treated in air at different temperatures combined with Rietveld quantitative phase analysis, and by the isoconversional analysis of differential thermal analysis measurements. Evidence for first order kinetics was obtained, with an activation energy of 280-290 kJ mol-1, suggesting that the breaking of the C-N bond is the rate determining step. The composition of the gas phase released under heating was investigated by Knudsen Effusion Mass Spectrometry, giving evidence for the occurrence of the process N(CH3)4PbI3(s) = PbI2(s) + N(CH3)3(g) + CH3I(g), consistent with the kinetic results. Decomposition pressures and thermodynamic properties were derived by Knudsen effusion mass loss experiments, obtaining values of 391.5 ± 2.0 kJ mol-1 and -577.4 ± 4.0 kJ mol-1 for the decomposition and formation enthalpies at 298 K, respectively. The reactivity towards water of N(CH3)4PbI3 was checked by XRD after total and prolonged immersion in water at room temperature. Overall, N(CH3)4PbI3 was found to be thermally much more stable than CH3NH3PbI3, both kinetically and thermodynamically, and much less prone to water-induced degradation, suggesting that the use of a quaternary ammonium cation may be an effective strategy in order to produce more stable materials.

Conjugated Polyelectrolytes as Multifunctional Passivating and Hole-Transporting Layers for Efficient Perovskite Light-Emitting Diodes

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.

Versatile Defect Passivation Methods for Metal Halide Perovskite Materials and their Application to Light-Emitting Devices

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.