Intermediate-Phase Engineering of Thermally-Evaporated Lead-Free Halide Perovskites for Indoor Photovoltaics

Tin-based halide perovskite (THP) solar cells have emerged as promising candidates for indoor photovoltaics (IPVs). However, the complex crystallization kinetics of THP films during the vacuum evaporation represent a major challenge. Here, we introduce formamidine acetate (FAAc) as a vapor-deposited additive to regulate the solid-state reaction of thermally evaporated FASnI2Br. We found that FAAc can coordinate with SnI2 to form a metastable SnI2–FAAc intermediate phase, enabling a better control of the crystallization kinetics. Meanwhile, FAAc decreases the surface free energy of the SnI2 film, promoting the uniform deposition of the subsequent FABr layer, which in turn leads to significantly enhanced THP film quality and suppressed trap-assisted recombination in the assembled IPVs. The resulting IPV devices achieve a power conversion efficiency (PCE) of 16.36% under 1000 lx illumination and a stability for over 3000 h under N2 without encapsulation. This work demonstrates an effective intermediate-phase strategy for controlling solvent-free crystallization of wide-bandgap THPs for IPV applications.