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 FASnI 2 Br. We found that FAAc can coordinate with SnI 2 to form a metastable SnI 2 –FAAc intermediate phase, enabling a better control of the crystallization kinetics. Meanwhile, FAAc decreases the surface free energy of the SnI 2 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 N 2 without encapsulation. This work demonstrates an effective intermediate-phase strategy for controlling solvent-free crystallization of wide-bandgap THPs for IPV applications.