Two‐Pronged Maneuver of Inner Regulation and Surface Repair for Stable Inorganic Perovskite Solar Cells with Ultralow Open‐Circuit Voltage Deficit

Despite inorganic perovskites are suitable for application into tandem solar cells, the unsatisfactory crystal quality and inevitable surface defects of low‐temperature fabricated inorganic perovskites lead to undesirable open‐circuit voltage ( V OC ) deficit, which limits the further development of inorganic perovskite solar cells (IPSCs). Herein, dimethylammonium chloride (DMACl) was introduced into perovskite precursor to prolong the solid‐state reactions between intermediate phases of DMAPbX 3 and Cs 4 PbX 6 , and mediate crystallization process of inorganic perovskite, leading to obtain high‐quality CsPbI 2.75 Br 0.25 perovskite films without DMA + residue in perovskite films after annealing. Additionally, the energy level position of perovskite slightly downshift after adding DMACl, narrowing the band offsets between perovskite and carrier transport layers. To further reduce V OC deficit and improve the performance of IPSCs, cesium iodide (CsI) thin layer is evaporated onto the perovskite surface to repair inevitable I‐related defects. Ultimately, the V OC and performance of IPSCs are improved without introducing organic components into final perovskite films. A champion photoelectric conversion efficiency of 20.75% is achieved, and the V OC deficit is significantly decreased to 0.382 V, which is the minimum V OC deficit of IPSCs at present. Additionally, the stability of the perovskite films and IPSCs has also been remarkably enhanced.