In‐Situ Hot Oxygen Cleansing and Passivation for All‐Inorganic Perovskite Solar Cells Deposited in Ambient to Breakthrough 19% Efficiency

All‐inorganic perovskite CsPbI 3 has attracted extensive attention recently because of its excellent thermal and chemical stability. However, its photovoltaic performance is hindered by large energy losses ( E loss ) due to the presence of point defects. In addition, hydroiodic acid (HI) is currently employed as a hydrolysis‐derived precursor of intermediate compounds, which often leads to a small amount of organic residue, thus undermining its chemical stability. Herein, an in‐situ hot oxygen cleansing with superior passivation (HOCP) for the triple halide‐mixed CsPb(I 2.85 Br 0.149 Cl 0.001 ) perovskite solar cells (abbreviated as CsPbTh 3 ) deposited in an ambient atmosphere to reduce the E loss to as low as 0.48 eV for the power conversion efficiency (PCE) to reach 19.65% is demonstrated. It is found that the hot oxygen treatment effectively removes the organic residues. Meanwhile, it passivates halide vacancies, hence reduces the trap states and nonradiative recombination losses within the perovskite layer. As a result, the PCE is increased significantly from 17.15% to 19.65% under 1 sun illumination with an open‐circuit voltage enlarged to 1.23 from 1.14 V, which corresponds to an E loss reduction from 0.57 to 0.48 eV. Also, the HOCP‐treated devices exhibit better long‐term stability. This insight should pave a way for decreasing nonradiative charge recombination losses for high‐performance inorganic perovskite photoelectronics.