Multifunctional Buried-Interface Engineering for Efficient Low- <i>n</i> Quasi-2D Tin Perovskite Solar Cells

Although quasi-2D tin-based perovskite solar cells (TPSCs) have emerged as a promising and environmentally benign photovoltaic technology, their practical advancement remains hindered by high trap-state density, pronounced interfacial nonradiative recombination, and severe current–voltage hysteresis. In this study, we introduced a multifunctional buried-interface layer, 4-guanidinobutyric acid chloride (4GBACl), to regulate crystallization and passivate defects in quasi-2D tin perovskites. The 4GBACl interlayer suppressed the lower dimensional phases in the perovskite layer, enhanced vertical film densification, and reduced the interfacial trap density within the devices. Consequently, the extraction of charges was improved, and nonradiative recombination and ion migration were effectively suppressed. Inverted 4GBACl-interlayer-incorporated TPSCs achieved a champion power conversion efficiency of 7.66%, with an open-circuit voltage, fill factor, and ultralow hysteresis index of 0.619 V, 70.4%, and 0.013, respectively. This study highlights buried-interface engineering as an effective strategy for improving the efficiency and reliability of lead-free TPSCs.