Synergistic Optimization of Buried Interface via Hydrochloric Acid for Efficient and Stable Perovskite Solar Cells

Abstract Incorporating chlorine into the SnO 2 electron transport layer (ETL) has proven effective in enhancing the interfacial contact between SnO 2 and perovskite in perovskite solar cells (PSCs). However, previous studies have primarily focused on the role of chlorine in passivating surface trap defects in SnO 2 , without considering its influence on the buried interface. Here, hydrochloric acid (HCl) is introduced as a chlorine source into commercial SnO 2 to form Cl‐capped SnO 2 (Cl‐SnO 2 ) ETL, aiming to optimize the buried interface of the PSC. The incorporation of HCl into the SnO 2 precursor solution works in two key ways. First, it converts the detrimental KOH stabilizer into KCl through an acid‐base reaction. Second, it regulates the crystallization process of the perovskite, reducing PbI 2 residues and voids at the buried interface. As a result, the efficiency of the PSC increases from 21.93% to 25.39%, with a certified efficiency of 25.69%, the highest efficiency reported for Cl‐SnO 2 ETL‐based PSCs. Moreover, the target PSC exhibits excellent air stability, retaining 90% of its initial efficiency after 2900 h of air exposure, compared to only 56.1% for the control PSC. This investigation highlights the effectiveness of HCl in the synergistic optimization of the buried interface in PSCs.