Exploring the potential of BaS as a hole transport layer for BaSi2 solar cells: thin-film growth and defect characterization using first-principles calculation

Abstract

Barium disilicide (BaSi₂) is a promising non-toxic and abundant material for thin-film solar cells due to its large optical absorption coefficient and a suitable bandgap of 1.3 eV. However, due to its small electron affinity (EA = 3.2 eV), oxide-based hole transport layer (HTL) materials used in crystalline silicon solar cells cause a large valence band offset at the HTL/BaSi₂ interface, hindering hole transport. In this study, barium sulfide (BaS) is proposed as an effective HTL for BaSi₂ based solar cells. This paper reports the growth of polycrystalline BaS thin films for the first time using radio-frequency sputtering. Spectroscopic ellipsometry evaluation of the BaS absorption edge yielded an indirect bandgap of approximately 3.6 eV. Ultraviolet photoelectron spectroscopy results indicate an ionization potential of approximately 4.3 eV. These results indicate that BaS is a good HTL candidate for BaSi₂ solar cell applications. To better understand the origin of point defects in BaS, first-principles defect calculations were conducted under various Ba, S, and oxygen (O)-chemical potential conditions. The calculation results indicate that among various point defects interstitial atoms of Ba and O (O_i), and O substituted for S antisites (O_S) readily form; O_S does not alter the electronic properties of BaS, while O_i can be regarded as shallow acceptor defects.