Cooperative thermal crystallization enables interfacial homogenization in antisolvent- and HTL-free Sn-Pb perovskite solar cells

Low-bandgap Sn-Pb perovskite solar cells are indispensable building blocks for all-perovskite tandems, yet their development is constrained by challenging crystallization control, severe interfacial recombination, and reliance on costly hole transport layers (HTLs) and antisolvent processing involving hazardous and volatile organic solvents. Here, a GuaSCN-assisted vacuum crystallization strategy is developed to realize efficient HTL-free and antisolvent-free Sn-Pb perovskite solar cells. The in-situ photoluminescence evolution is consistent with a cooperative crystallization process, where SCN - regulates early-stage growth while Gua + facilitates later-stage structural relaxation, enabling sustained optoelectronic improvement during film formation. This synergistic regulation yields compact Sn-Pb perovskite films with a uniform buried interface, reduced structural and compositional heterogeneity, and a surface-associated residual region that may contribute to interfacial passivation. Cross-sectional chemical mapping further shows reduced local compositional fluctuations and interfacial irregularity within the analysed regions. These structural characteristics are accompanied by increased quasi-Fermi level splitting, reduced energetic disorder, and a higher attainable photovoltage of the neat perovskite film. Consequently, HTL-free devices deliver a champion efficiency of 21.4% with a fill factor exceeding 80%, among the highest values reported for HTL-free, antisolvent-free Sn-Pb perovskite solar cells. Furthermore, the GuaSCN-treated narrow-bandgap subcell is integrated into a monolithic all-perovskite tandem device, demonstrating its promise for simplified tandem photovoltaics.