Formation of High-Photoresponsivity\nBaSi<sub>2</sub> Films by Radio-Frequency Sputtering and Evaluation\nof a BaSi<sub>2</sub>/TiN/Glass Heterojunction by Transmission Electron\nMicroscopy

Barium disilicide (BaSi<sub>2</sub>) is a thin-film solar\ncell\nmaterial composed of abundant elements, and its application potential\nis further enhanced by its formation on inexpensive substrates, such\nas glass. The effect of the substrate temperature on the co-sputtering\nof BaSi<sub>2</sub> and Ba targets to form BaSi<sub>2</sub> films\non Si(111) and TiN/glass substrates was investigated. Contrary to\nexpectations, the photoresponsivity reached maximum values exceeding\n5 and 2 A W<sup>–1</sup>, respectively, the highest value ever\nreported for as-deposited samples formed at 750 °C, more than\n100 °C higher than those reported previously. Because the photoresponsivity\nis proportional to carrier lifetime, this result indicates that high-temperature\ngrowth can bring out the high performance of BaSi<sub>2</sub> as a\nlight-absorbing layer. Because amorphous SiC (a-SiC) has a larger\nforbidden band gap and electron affinity than BaSi<sub>2</sub>, it\nis considered suitable as an electron transport layer (ETL) material\nfor BaSi<sub>2</sub> solar cells. On the basis of this, the formation\nof BaSi<sub>2</sub> (absorption layer)/a-SiC (ETL)/TiN (electrode)/glass\nheterojunctions was also attempted, and the layered structure was\nexamined by cross-sectional transmission electron microscopy (TEM).\nPolycrystalline BaSi<sub>2</sub> films were found to be even on the\namorphous layer by TEM. A high photoresponsivity of over 2 A W<sup>–1</sup> was obtained. Therefore, the BaSi<sub>2</sub>/a-SiC/TiN\nstructure provides a guideline for the structural design of BaSi<sub>2</sub>-based thin-film solar cells on glass.