For cost-effective and flexible multi-junction solar cells, the development of high-quality infrared-absorbing Ge layers on general insulating substrates is essential, yet has been hindered by small grain sizes and high defect densities in conventional polycrystalline films. In this study, we employed a solid-phase crystallization technique combined with subsequent grain-templated growth to control microstructure evolution. By suppressing interfacial nucleation, a large-grained seed layer was formed, enabling thick Ge growth while preserving the enlarged grain structure. The resulting Ge films exhibited significantly improved crystallinity, with large grain sizes and reduced acceptor defect density, leading to enhanced carrier transport. As a result, clear photoresponsivity was demonstrated in polycrystalline Ge layers on glass. Furthermore, this low-temperature process (< 500 °C) was extended to polyimide films, where Ge-related photoresponsivity up to 1.8 μm was observed, suggesting efficient infrared carrier generation and extraction. These results establish a viable route for realizing high-quality Ge thin films on low-cost and flexible substrates, providing a promising platform for next-generation multi-junction solar cells and infrared optoelectronic devices.
