Formation of High-Photoresponsivity BaSi₂ Films by Radio-Frequency Sputtering and Evaluation of a BaSi₂/TiN/Glass Heterojunction by Transmission Electron Microscopy

Barium disilicide (BaSi₂) is a thin-film solar cell material composed of abundant elements, and its application potential is further enhanced by its formation on inexpensive substrates, such as glass. The effect of the substrate temperature on the co-sputtering of BaSi₂ and Ba targets to form BaSi₂ films on Si(111) and TiN/glass substrates was investigated. Contrary to expectations, the photoresponsivity reached maximum values exceeding 5 and 2 A W^-1, respectively, the highest value ever reported for as-deposited samples formed at 750 °C, more than 100 °C higher than those reported previously. Because the photoresponsivity is proportional to carrier lifetime, this result indicates that high-temperature growth can bring out the high performance of BaSi₂ as a light-absorbing layer. Because amorphous SiC (a-SiC) has a larger forbidden band gap and electron affinity than BaSi₂, it is considered suitable as an electron transport layer (ETL) material for BaSi₂ solar cells. On the basis of this, the formation of BaSi₂ (absorption layer)/a-SiC (ETL)/TiN (electrode)/glass heterojunctions was also attempted, and the layered structure was examined by cross-sectional transmission electron microscopy (TEM). Polycrystalline BaSi₂ films were found to be even on the amorphous layer by TEM. A high photoresponsivity of over 2 A W^-1 was obtained. Therefore, the BaSi₂/a-SiC/TiN structure provides a guideline for the structural design of BaSi₂-based thin-film solar cells on glass.