Publication date: 22nd July 2026
In the last 15 years lead halide perovskites have raised considerable interest for photovoltaic (PV) applications, with power conversion efficiencies in a single junction now exceeding 27%. However, the presence of lead in soluble form constitutes a major obstacle for their broad deployment. More generally, the imperatives in terms of environmental responsibility require exploring lead-free alternatives. In this context, our study focuses on mixed Sn/Ge-based homovalent Pb substitution, namely compounds of type (MA,FA,Cs)SnxGe1-x(I, Br)3. The band gap values of (MA,FA,Cs)Ge(I, Br)3 and (MA,FA,Cs)Sn(I, Br)3 compounds, ranging from 1.2 to 3.1 eV, make them promising candidates for PV applications. Despite potential instability due to the sensitivity of Sn2+ and Ge2+ to oxidation, promising PV results have already been reported with CsSn0.5Ge0.5I3. However, there is a lack of systematic experimental studies on the structural properties of (MA, FA, Cs)SnxGe1-x(I, Br)3 compounds.
Here, we investigate the temperature-dependent evolution of the structural and optical properties of selected bulk materials (A1yA21-ySnxGe1-xI3 with A1, A2 = FA, Cs) in order to map the phase stability, identify phase transitions, and determine the thermal expansion coefficients. We further use these results to distinguish between intrinsic compositional instability from degradation that originates from a given synthetic process or the thermal history. Based on these results, the most promising compositions are selected for thin film deposition. We will present the results of the investigation of the crystallization process of the thin films and of their resulting microstructure. Overall, the applied two-step approach strongly adds to the fundamental understanding of the Sn/Ge-based halide perovskite family and paves the way for their practical applications, thus contributing to the quest for eco-friendly solar technologies.
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