Reduced humidity sensitivity of the perovskite fabrication via intermediate treatment enabling stable perovskite solar cells

High-efficiency formamidinium lead iodide (FAPbI₃)-based perovskite solar cells (PSCs) typically involve annealing in humid air during the fabrication process of perovskite films. However, the combined effects of humidity and relatively high temperature often result in the uncontrollable formation of a detrimental PbI₂ phase in the perovskite films. As a result, the annealing process of perovskite films is highly sensitive to the relative humidity fluctuations of the environment. Under solar illumination, the undesired PbI₂ tends to decompose, accelerating the degradation of perovskite materials and severely compromising the light stability of PSCs. This issue is particularly critical for the buried interface and bulk of the perovskite films, as these regions absorb the majority of the incident light. Pre-treatment and post-treatment strategies are generally confined to address the PbI₂ issues at the buried interface and on the surface of the perovskite films, respectively. However, effectively addressing the effects of excess PbI₂ at buried interface and grain boundaries within bulk in a single step remains challenging. In this study, we propose an intermediate-treatment strategy using phthalylglycyl chloride (PTC), which involves treating the wet films with PTC prior to annealing during the formation process of the perovskite films. This approach protects the grain boundaries of polycrystalline perovskite films in advance, effectively preventing moisture-induced degradation of the perovskites and thus significantly broadening the relative humidity window of annealing process. Our results demonstrate that this strategy can successfully suppress the formation of PbI₂ at the grain boundaries and buried interface of perovskite films, thereby eliminating the PbI₂-induced degradation pathways. Our strategy significantly reduces the sensitivity to humidity fluctuations during annealing for fabricating stable PSCs, ensuring more consistent fabrication of stable PSCs. Consequently, the resulting PSCs achieve a champion power conversion efficiency of 26.1% and demonstrate excellent light stability.