Structural characterization of tin halide perovskites for solar energy conversion

The growing global energy demand and the urgency of mitigating climate change have driven intense research toward sustainable materials for solar energy con- version. Among the various emerging semiconductors, metal halide perovskites (MHPs) with typical ABX3 structure have recently gained attentions thanks to their remarkable optical tunability and low-cost fabrication routes, therefore emerg- ing as promising materials for next generation optoelectronic and photocatalytic technologies. Within this class, tin-based perovskites (THPs) are considered the most viable alternative to toxic lead analogues. However, their widespread adop- tion remains limited by the intrinsic instability of the Sn2+ oxidation state, which rapidly converts into Sn4+ in the presence of oxygen , leading to structural and elec- tronic degradation. One strategy to increase the structural stability of perovskites is tuning their composition, particularly the A-site cation. In this work, we study the use of dimethylammonium (DMA+) as A-site cation to form DMASnBr3 and DMASnI3. These two compounds were prepared by planetary ball-milling, a green and scalable method that ensures high stoichiometric control, easy scalability and high reproducibility. Structural, optical, and thermal properties of the synthesized materials were analyzed using X-ray diffraction (XRD), UV-Vis spectroscopy, and thermogravimetric analysis (TGA). Particular attention was dedicated to assess- ing their stability under different atmospheric conditions, namely N2, O2, ambient air, and deionized water. Both compositions show exceptional structural stability toward O2 but reduced stability towards moisture/liquid water. DMASnI3 exhib- ited a distinct chromogenic transition, rapidly turning from yellow to black upon air exposure, associated with partial oxidation and amorphization, forming a hy- drated phase. In the presence of liquid water DMASnI3 transformed into SnI4 but reversibly returned to the hydrated phase when the material was dried. In con- trast, DMASnBr3 demonstrated overall higher structural stability than DMASnI3, moreover, the presence of moisture and liquid water generated a composite mate- rial where DMASnBr3 coexisted with (DMA)2SnBr6 phase. Results highlight the critical influence of environmental agents on the degradation pathways of tin halide perovskites and provide valuable insight into the mechanisms governing their oxida- tion behavior, which is fundamental to fabricate efficient materials for solar energy conversion and photocatalytic applications.