Kang Wang1,Letian Dou1
Purdue University1
Quasi-two-dimensional (quasi-2D) halide perovskite are emerging candidates for the next-generation light-emitting diodes (LEDs) because of their superior optoelectronic properties with remarkable tunability. The external quantum efficiency of quasi-2D perovskite-based LEDs has recently exceeded 20% in both green and near-infrared regions. Nevertheless, owing to the weak ionic bonding of halide perovskites, the quasi-2D perovskites suffer from ion diffusion-controlled phase disproportionation that causes the formation of multiple-quantum-well structures with broad and unpredictable phase distributions. This limits device efficiency, stability, and reproducibility. Here, we report a molecular design strategy to suppress phase disproportionation. Specifically, we extend the pi-conjugation length and fine-tune the dihedral angle between two phenyl rings within the ligand molecules to increase the effective bulkiness. When incorporated into the quasi-2D perovskite lattice, the novel ligands substantially inhibit ion transport and kinetically stabilize metastable phases in the perovskite solids. This leads to high-quality thin films with narrow phase distributions, low defect densities, and enhanced radiative recombination efficiencies. Molecular dynamics simulations suggest that the free energy barrier for ion transport through the new organic ligand layer is twice that of the conventional ligands, in agreement with experimental observations. Using the improved materials, we demonstrate efficient, stable, and wavelength-tunable red LEDs with a record peak external quantum efficiency of 25.8% (average 22.7% among 60 devices). Importantly, in contrast to existing perovskite LEDs, our devices exhibit ideal current-voltage characteristics completely free from hysteresis and efficiency overshoot, suggesting suppressed ion migration and diffusion under operational conditions. These discoveries provide critical insights into the crystallization pathways of solution-processed low-dimensional perovskite semiconductors, thus opening up a new avenue towards high-performance optoelectronic devices via molecular engineering.