Precise Management of n-Value Distribution at the 2D/3D Interface Enabling Efficient Wide-Bandgap Perovskite Solar Cells.

Introducing a 2D perovskite layer on the surface of 3D perovskite has been broadly recognized as an effective strategy to enhance the performance of perovskite solar cells (PSCs). However, the mechanism governing the 2D phase formation remains uncertain. In this work, the phase transitions of 2D perovskite during spin-coating and annealing processes has been investigated. Our findings reveal a dimensional phase shift from low to high n-value 2D phases, driven by the release of organic cations during annealing. Additionally, the spin-coating process exhibited concentration-dependent behavior, where higher n-octylamine hydrobromide (OABr) concentrations predominantly formed n = 1 phases. These observations highlight the complexity of the 2D phase composition at the 2D/3D interface. The coexistence of various 2D phases significantly influences device performance, as a conflict between n = 1 and n ≥ 2 phases was identified. Through forming a well-balanced proportion of different 2D phases, we achieved a wide-bandgap perovskite solar cell with a significantly improved power conversion efficiency of 19.29%. This work suggests the critical roles of phase dynamics and n-value distribution in optimizing 2D/3D interfaces for advancing high-performance wide-bandgap PSCs.