Constructing a Soft-Hard Interpenetrating Network in Flexible Quasi-2D Perovskite Solar Cells Through FABF₄-Driven Phase Redistribution

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 (FABF₄)-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.