Antiphase Boundaries Regulate Phase Stability and Performance in DMA⁺-Assisted CsPbI₃-Based Perovskites

CsPbI₃-based perovskites are promising absorbers for tandem solar cells owing to their optimal bandgap (∼1.7 eV). However, the phase transition from photoactive γ-CsPbI₃ to non-photoactive δ-CsPbI₃ remains a major obstacle and is strongly governed by microstructural defects formed during film growth. Among these, Ruddlesden-Popper antiphase boundaries (RP-APBs) are particularly prevalent and exhibit competing effects, relieving lattice strain while simultaneously facilitating moisture penetration, ion migration, and nonradiative recombination. Here, we systematically regulate RP-APB defects in γ-phase CsPbI₃ thin films and elucidate their decisive influence on both phase stability and optoelectronic performance. A compositional strategy based on PbI₂ excess effectively reduces RP-APB density but induces edge-sharing [PbI₆]^4- motifs that nucleate the δ phase. In contrast, a dimethylammonium (DMA⁺)-assisted phase-engineering strategy forms β-(DMA,Cs)PbI₃, which intrinsically suppresses RP-APB formation while preserving the photoactive perovskite framework. As a result, RP-APB-free β-phase films exhibit prolonged carrier lifetimes, strongly suppressed nonradiative recombination, and the lowest apparent trap densities, enabling a champion power conversion efficiency of 20.23% together with markedly enhanced operational, thermal, and ambient-air stability. This work demonstrates that regulating crystalline defects, exemplified by RP-APBs, plays a critical role in achieving both stable and efficient perovskite solar cells.