To develop high-efficiency solar cells on inexpensive substrates, the direct synthesis of polycrystalline III–V compound semiconductor films has been investigated for decades. Here, we propose a synthesis technique for largegrained GaAs films using recently-developed Ge seed layer technologies, including advanced solid-phase crystallization and metal-induced layer exchange. The 500-nm-thick GaAs films epitaxially grown from the Ge seed layers (grain size: 1‒300 μm) at 550 °C inherited the grain size (grain boundaries) in Ge. With increasing grain size, the photoresponsivity corresponding to GaAs increased from 10−3000 mA W⁻¹ under a bias voltage of 0.3 V. The maximum photoresponsivity of the GaAs film reached 3000 mA W⁻¹, which was close to that of the GaAs film formed on a single-crystal Ge wafer. Furthermore, the GaAs film grown on a flexible plastic film (highly heat-resistant polyimide) at 500 °C exhibited the photoresponsivity reaching 980 mA W⁻¹. These achievements will pave the way for high-efficiency and versatile solar cells based on polycrystalline III–V compound semiconductors.

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