Interface Engineering by Small Molecules toward Efficient Hole Transport Layer-Free Sn–Pb Perovskite Solar Cells with High Fill Factors

Low-bandgap (LBG) tin-lead (Sn-Pb) perovskites are essential for tandem solar cells but face challenges because the use of a hygroscopic PEDOT:PSS hole transport layer (HTL) reduces device stability. A HTL-free structure can overcome these issues but introduces new drawbacks like imbalanced carrier transport and severe recombination. This study introduces 3-amino-5-mercapto-1,2,4-triazole (AMTZ) as a rear interface passivator to simultaneously mitigate defects and stabilize Sn-Pb perovskite films. The aromatic triazole group of AMTZ coordinates with dangling Pb²⁺/Sn²⁺ cations to reduce interface trap states, while the reducing thiol (-SH) group suppresses Sn²⁺ oxidation. In addition, AMTZ post-treatment also modulates the energy-level alignments, promoting fluent charge transfer in HTL-free perovskite solar cells (PSCs). Consequently, we achieved efficient HTL-free LBG PSCs with a champion efficiency of 21.87% and an impressive fill factor of over 80%. Moreover, the optimized device maintained 80% of the initial efficiency upon 550 h of storage. This work demonstrates a viable strategy for developing efficient and stable HTL-free LBG PSCs through interfacial engineering.