Synergistic Enhancement of Tin–Lead Perovskite Solar Cells Using NH ₄ SCN 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 FA_0.7MA_0.3Pb_0.5Sn_0.5I₃ 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.