Quantum well engineering in quasi-2D perovskites unlocks efficiency-stability synergy for photovoltaics

Quasi-2D perovskites offer enhanced stability for photovoltaic applications but suffer from compromised charge transport due to uncontrolled phase separation and nonideal quantum well (QW) distribution. Contrary to the prevailing belief that maximizing 3D-like phases optimizes performance, we demonstrate that strategic narrowing QW distribution into high-n phases while avoiding undesirable graded configurations (common in quasi-2D systems) unlocks unprecedented efficiency-stability synergy. Using 1,4-cyclohexanedimethanamine (CDMA) as a crystallization-directing spacer, we realize three advances. (a) High-n phases with minimized n-value dispersion establish a uniform energy landscape, significantly reducing interphase charge transfer barriers and suppressing voltage losses. (b) Lattice matching between adjacent high-n phases substantially alleviates microstrain accumulation, thereby mitigating defect generation and non-radiative recombination. (c) Strategic predominance of robust high-n phases achieves dual optimization: it excludes hygroscopic 3D-like phases and eliminates mobility-limiting low-n phases, thereby ensuring efficient charge dynamics while fortifying the perovskite’s intrinsic phase stability. The synergy of these advances enables the CDMA PSCs to achieve a champion efficiency of 19.02 %, making them among the highest-efficiency quasi-2D DJ PSCs. The unencapsulated device demonstrated remarkable stability, maintaining 92 % of its initial PCE after 5000 h in air. This work redefines phase engineering priorities, demonstrating that manipulating QW distribution in quasi-2D perovskites has the potential to address the efficiency-stability trade-off.