Solution-Processed Ge-Sn (1:1) Perovskite Solar Cells with a DMSO-Free Solvent System and GeCl2-Dioxane Additives

The toxicity of lead (Pb) has driven the quest for Pb-free halide perovskite solar cells, with tin (Sn) and germanium (Ge) emerging as promising alternatives due to their isoelectronic properties and lower toxicity. In this study, we report the rapid reactivity of dimethyl sulfoxide (DMSO) and GeI2, underscoring the necessity of a DMSO-free solvent system. Then, we introduce a novel solvent system, N,N-dimethylformamide:N-N'-dimethylpropyleneurea:4-tert-butylpyridine, that effectively curtails this reactivity, enabling the formation of pure-phase Ge-Sn (1:1) perovskite films, as confirmed by X-ray diffraction patterns. This approach provided an effective method to wet-coat Ge-rich films. Additionally, a GeCl2-dioxane complex (1:1)-alloying strategy was adopted, which slowed the crystallization rate (dioxane coordination with Sn), facilitated Ge integration into the vacant sites of the perovskite lattice, thus effectively improving the crystallinity and optoelectronic properties. This resulted in a Ge-Sn (1:1)-alloyed perovskite solar cell with an efficiency of 4.56%, the highest performance achieved using any solution method to date. Furthermore, the solar cells exhibited long-term operational stability, maintaining 91% of initial efficiency after 50 h continuous 1 sun illumination without the use of a UV filter.