Antiphase Boundaries Regulate Phase Stability and Performance in DMA ⁺ ‐Assisted CsPbI ₃ ‐Based Perovskites

CsPbI 3 ‐based perovskites are promising absorbers for tandem solar cells owing to their optimal bandgap (∼1.7 eV). However, the phase transition from photoactive γ‐CsPbI 3 to non‐photoactive δ‐CsPbI 3 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 CsPbI 3 thin films and elucidate their decisive influence on both phase stability and optoelectronic performance. A compositional strategy based on PbI 2 excess effectively reduces RP‐APB density but induces edge‐sharing [PbI 6 ] 4− motifs that nucleate the δ phase. In contrast, a dimethylammonium (DMA + )‐assisted phase‐engineering strategy forms β‐(DMA,Cs)PbI 3 , 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.