Carboxylate Pseudo-Halide-Assisted crystallization and antioxidant strategy for stable wide bandgap tin perovskite photovoltaics

The facile oxidation of Sn²⁺ to Sn⁴⁺ and uncontrolled crystal growth are significant challenges in the development of wide-bandgap (WBG) tin perovskite solar cells (TPSCs), affecting their performance and stability. This study introduces three strategically synthesized carboxylate pseudo-halides—3,3-diphenylpropylammonium trifluoroacetate (D-TFA), 3,3-diphenylpropylammonium acetate (D-Ac), and 3,3-diphenylpropylammonium formate (D-Fo), —as additives to mitigate Sn²⁺/Sn⁴⁺ oxidation in WBG TPSCs. These pseudo-halides coordinate with Sn²⁺, reducing electron density at the carboxylate carbon and forming robust bonds with SnI₂(DMSO)₃, thus impeding Sn²⁺ oxidation through oxygen adsorption. Among them, the formate ion (Fo^-) in D-Fo exhibited the most significant deshielded peak, indicating the strongest coordination with Sn²⁺ and superior antioxidation effects by reducing reactivity with ambient oxygen. These interactions, along with 3,3-diphenylpropylammonium (DPA⁺), aid in controlling crystal growth, enhancing the formation of highly crystalline thin films, and improving overall stability. Consequently, D-Fo-assisted WBG tin perovskites demonstrated strong suppression of phase segregation under prolonged illumination of the air mass 1.5 spectrum, retaining 90 % of their initial characteristics. Ultimately, the D-Fo-enhanced WBG TPSC achieved an exemplary power conversion efficiency of 10.69 % with minimal hysteresis, surpassing the control cell's 7.43 %. Furthermore, the unencapsulated D-Fo device maintained nearly 87 % of its initial performance after 960 h under ambient conditions.