Promoting Phase Transition in Quasi‐2D Perovskites For High‐Performance Pure‐Red LEDs

Layered quasi-two-dimensional (quasi-2D) halide perovskites have emerged as a promising platform for high-efficiency electroluminescence. Narrowing the multi-quantum well distribution and eliminating wide-bandgap 2D phases are crucial for achieving a flat energy landscape, minimizing energy loss, and ensuring high-color-purity emission. Here, it is demonstrated that solution-processed quasi-2D perovskite films with broad component distributions arise from an incomplete kinetic transition from low-n (n, quantum well thickness) to high-n phases. To address this, an acetate anion treatment strategy is proposed, which induces competitive coordination between the acetate anion, the bulky spacer cation, and the inorganic layer, thereby facilitating the insertion of octahedral precursor ions and promoting phase transition. This treatment results in quasi-2D films with enhanced color purity, efficient energy transfer, and high photoluminescence quantum yield. The fabricated perovskite light emitting diode (PeLED) exhibits an emission peak at 641 nm and a peak external quantum efficiency (EQE) of 25.3%, representing one of the most efficient pure-red PeLEDs. The strategy also showcases the versatility of quasi-2D films for different emission wavelengths.