Rubidium as an Alternative Cation for Efficient Perovskite Light-Emitting Diodes.

Incorporation of rubidium (Rb) into mixed lead halide perovskites has recently achieved record power conversion efficiency and excellent stability in perovskite solar cells. Inspired by these tremendous advances in photovoltaics, this study demonstrates the impact of Rb incorporation into MAPbBr-based light emitters. Rb partially substitutes MA (methyl ammonium), resulting in a mixed cation perovskite with the formula MARb PbBr. Pure MAPbBr crystallizes into a polycrystalline layer with highly defective sub-micrometer grains. However, the addition of a small amount of Rb forms MARb PbBr nanocrystals (10 nm) embedded in an amorphous matrix of MA/Rb Br. These nanocrystals grow into defect-free sub-micrometer-sized crystallites with further addition of Rb, resulting in a 3-fold increase in exciton lifetime when the molar ratio of MABr/RbBr is 1:1. A thin film fabricated with a 1:1 molar ratio of MABr/RbBr showed the best electroluminescent properties with a current efficiency (CE) of 9.45 cd/A and a luminance of 7694 cd/m. These values of CE and luminance are, respectively, 19 and 10 times larger than those achieved by pure MAPbBr devices (0.5 cd/A and 790 cd/m). We believe this work provides important information on the future compositional optimization of Rb-based mixed cation perovskites for obtaining high-performance light-emitting diodes.