Zn_1-GeO Passivating Interlayers for BaSi₂ Thin-Film Solar Cells

BaSi₂ is a promising absorber material for next-generation thin-film solar cells (TFSCs). For high-efficiency TFSCs, a suitable interlayer should be found for every light absorber. However, such an interlayer has not been studied for BaSi₂. In this study, we investigated amorphous Zn_1-GeO films as interlayers for BaSi₂. The Zn/Ge atomic ratio in the Zn_1-GeO film and the optical band gap depend on the substrate temperature during sputtering deposition. A suitable i-Zn_1-GeO/BaSi₂ heterointerface with spike-type conduction band offset was achieved when Zn_1-GeO was deposited on BaSi₂ at 50 °C. Furthermore, photoresponsivity measurements revealed that Zn_1-GeO has an excellent surface passivation effect on BaSi₂. When the thickness of Zn_1-GeO was 2 nm, a high photoresponsivity of 0.9 A/W was obtained for a 500 nm thick BaSi₂ layer at a wavelength of 780 nm under an applied bias voltage of 0.5 V between the front and rear electrodes, where the photoresponsivity in the short-wavelength region was significantly improved compared with that of BaSi₂ capped with an amorphous Si layer. X-ray photoelectron spectroscopy revealed that the Zn_1-GeO films suppressed the oxidation of the BaSi₂ surface, decreasing the carrier recombination rate. This is the first demonstration of passivation interlayers for BaSi₂ with suitable band alignment for carrier transport and surface passivation effects.