For cost-effective multi-junction solar cells, developing high-quality infrared-absorbing Ge layers on general insulating substrates is essential. We employed a solid-phase crystallization technique combined with subsequent epitaxial growth. By controlling interfacial nucleation, a large-grained seed layer was formed, enabling epitaxial thickening while preserving the enlarged grain size. The resulting Ge films exhibited high crystallinity with large grains and a low acceptor defect density, showing clear photoresponsivity on glass. This low-temperature (<500 °C) process also demonstrated Ge-related photoresponsivity below 1.8 μm even on polyimide films, indicating its potential for flexible and low-cost multi-junction solar cells.
