Two-Factor Phase Separations in Mixed-Halide Quasi-2D Perovskite LEDs: Dimensionality and Halide Segregations

Quasi-2Dhalide perovskites have attracted much interest as a promisingmaterial for light-emitting diodes (LEDs) due to their tunabilityin quantum confinement and halide alloy formation to modulate theenergy bandgap and emission color. However, two-factor phase separationswith respect to heterogeneous quantum-well thicknesses and halidesegregation are still crucial issues in quasi-2D perovskite LEDs,leading to low external quantum efficiencies (EQEs) and color shifts.Herein, we compare quasi-2D perovskite films using different cationsto unveil the key contributions from the chemical design of organiccations. While mixing halide ions in conventional quasi-2D perovskitefilms induces micrometer-scale heterogeneity, new extended and twistedconjugated cations suppress the two-factor phase separations, leadingto high EQEs of over 25% and controllable emission wavelengths acrossred and near-infrared regions. The fundamental insights in this workwill provide guidance for advancing materials design and device performancein the future.