Anion‐Cation Synergistic Passivation for High‐Performance FA‐Cs Halide Perovskite Photodetectors

Formamidinium‐cesium (FA‐Cs) lead halide perovskites have emerged as promising materials for high‐performance photodetectors due to their superior thermal stability; however, intrinsic ionic size mismatch induces significant surface and bulk defects, limiting device performance. Here, we report an anion‐cation synergistic passivation (AC‐SP) strategy that enables spatially decoupled defect management in formamidinium‐cesium lead iodide (FACsPbI 3 ) films. By employing a Dion–Jacobson‐type organic spacer, N,N‐dimethyl‐1,3‐propanediamine dihydrochloride (DMePDACl 2 ), N,N‐dimethyl‐1,3‐propane diammonium (DMePDA 2+ ) cations preferentially accumulate at the surface and grain boundaries, while Cl − anions diffuse into the bulk lattice. This cooperative surface‐bulk passivation effectively suppresses trap‐assisted recombination and enhances carrier transport. As a result, the AC‐SP treated photodetectors exhibit a high responsivity of 72.88 A W −1 , a specific detectivity of 1.07 × 10 14 Jones, and a fast response time of 1.86 ms, outperforming conventional passivation approaches. Comprehensive characterization reveals reduced trap density, prolonged carrier lifetime, and improved film morphology, confirming the effectiveness of the spatially separated passivation mechanism. This work establishes a general defect‐engineering strategy based on anion‐cation functional decoupling, providing a viable approach for advancing high‐performance perovskite optoelectronic devices.