Researchers at the University of Electro-Communications (UEC) in Tokyo and Madan Mohan Malviya University of Technology (MMMUT) in India have developed a new solvent and additive system that enables solution-processed, lead-free germanium-tin (Ge-Sn, 1:1) perovskite solar cells, reaching 4.56% efficiency, which the team describes as the highest performance achieved with any solution-based method for this composition to date.
Germanium and tin are both being explored as isoelectronic, lower-toxicity alternatives to lead in halide perovskite solar cells, but Ge-Sn alloyed perovskites have been difficult to process from solution: the team found that dimethyl sulfoxide (DMSO), a solvent commonly used in perovskite fabrication, reacts too rapidly with the germanium precursor GeI2, preventing the formation of clean, pure-phase films. To get around this, the researchers developed a DMSO-free solvent system based on a mixture of N,N-dimethylformamide, N,N′-dimethylpropyleneurea, and 4-tert-butylpyridine, which suppresses that unwanted reactivity and allows Ge-rich perovskite films to be wet-coated while forming a pure-phase Ge-Sn (1:1) structure, confirmed by X-ray diffraction.
The team also introduced a germanium chloride-dioxane complex (GeCl2-dioxane, 1:1) as an alloying additive. The dioxane component coordinates with tin in a way that slows the film's crystallization rate, giving germanium more opportunity to integrate into vacant sites within the perovskite lattice, which improved both the crystallinity and optoelectronic properties of the resulting films. Devices built with this combined solvent-and-additive approach also showed strong operational stability, retaining 91% of their initial efficiency after 50 hours of continuous illumination under 1-sun conditions without the use of a UV filter.
The authors frame the work primarily as a processing breakthrough rather than an efficiency record in absolute terms, addressing a specific chemical incompatibility that has limited solution-based fabrication of Ge-Sn perovskites and opening a path toward further improving this lead-free composition.