Synergistic Enhancement of Tin–Lead Perovskite Solar Cells Using NH4SCN and Guanidinium-Based Additive for Superior Efficiency and Stability

Tin–lead mixed perovskites have emerged as promising absorber materials for tandem and low-bandgap photovoltaics, yet their development is hindered by rapid crystallization and severe oxidation of Sn2+. In this work, we propose a synergistic dual-additive strategy using ammonium thiocyanate (NH4SCN) and 4-guanidinobenzoic acid hydrochloride (GBAC) to simultaneously modulate crystallization kinetics and suppress defect formation in FA0.7MA0.3Pb0.5Sn0.5I3 perovskite films. We demonstrate that NH4SCN coordinates with Sn2+ to create a uniform crystalline scaffold by suppressing random nucleation, while GBAC not only passivates iodine vacancies and inhibits Sn2+ oxidation through its multifunctional groups but also efficiently promotes large-grained growth via hydrogen-bonding interactions upon this optimized scaffold. The resulting perovskite films exhibit enhanced crystallinity, reduced trap density, and suppressed nonradiative recombination. Consequently, the inverted tin–lead perovskite solar cells (PSCs) achieve a champion power conversion efficiency (PCE) of 20.41% with negligible hysteresis and significantly improved operational stability, retaining over 91% of their initial efficiency after 1500 h in nitrogen atmosphere. This work provides a deeper understanding of sequential and synergistic additive-driven crystallization and passivation mechanisms, offering a practical route toward efficient and stable low-bandgap perovskite photovoltaics.