Surface Engineering of Perovskite Films via Sequential Moisture Cooling and Passivation for Efficient Solar Cells

Defect density on the perovskite film surface significantly exceeds that found in the bulk, primarily due to the presence of dangling bonds and excessive strain. Herein, a synergistic surface engineering is reported aimed at reducing surface defects of perovskite films. This method involves subjecting the thermally-annealed perovskite films to a controlled cooling condition involving an ambient environment with regulated humidity, as opposed to a nitrogen environment, followed by phenethylammonium iodide (PEAI) passivation. The perovskite films treated with moisture cooling (MC) exhibit enhanced radiative recombination, prolonged charge carrier lifetime, and improved hole transport and extraction when in contact with the hole transport layer (HTL), alongside a significant reduction in strain. Notably, the passivation effect of PEAI on the MC-treated perovskite films is significantly amplified compared with the films subjected to nitrogen cooling (NC) treatment, as evidenced by a more uniform surface potential mapping and a markedly extended charge carrier lifetime. This enhanced passivation effect may arise from the higher ratio of newly-formed 2D perovskite phase PEA2FAPb2I7 to PEA2PbI4 in the MC-treated film. Consequently, the MC-based perovskite solar cell (PSC) achieves a champion power conversion efficiency (PCE) of 25.28%, surpassing that of the NC-treated device, which exhibits a PCE of only 24.01%.