ABSTRACT Spontaneous segregation of 2D perovskite phases near the substrate in quasi‐2D perovskite films impedes charge transport and compromises the mechanical reliability of quasi‐2D flexible perovskite solar cells (f‐PSCs). To address this challenge, a formamidinium tetrafluoroborate (FABF 4 )‐mediated recrystallization strategy is introduced to transform the initial vertically stacked 2D–3D layered architecture into a 2D–3D interpenetrating network. This post‐treatment induces redistribution of 2D‐ and 3D‐rich phases throughout the film via synergistic FA + /methylammonium (MA + )cation exchange and Ostwald ripening processes. The resulting interpenetrating structure integrates a continuous FA‐rich 3D framework that enables efficient charge transport with embedded, soft 2D‐rich domains that dissipate stress and suppress crack propagation. Consequently, the treated films exhibit a markedly reduced Young's modulus and significantly suppressed non‐radiative recombination. The resulting rigid devices achieve a champion power conversion efficiency (PCE) of 21.36%, while the flexible devices yield PCE of 19.67%. Notably, unencapsulated f‐PSCs retain 88% of their initial efficiency after 10 000 bending cycles at a 5 mm bending radius in nitrogen and maintain 82% under 60% ± 5% relative humidity. This work elucidates the critical relationship between phase distribution and mechanical durability in quasi‐2D perovskite thin films, providing a structure–property–oriented design principle for high‐performance flexible photovoltaics.
Shi et al. (Wed,) studied this question.