Crystal structure and electronic properties of Sr-substituted barium disilicide Ba1-xSrxSi2 for solar cells: Computational and experimental studies

Abstract

The effects of Sr substitution in BaSi2, a promising photoabsorber for thin-film solar cells, were examined computationally and experimentally. The calculations showed that Ba0.5Sr0.5Si2 with Sr atoms at the A1 site (A1-Sr-BSS) is more stable than that with Sr atoms at the A2 site (A2-Sr-BSS). Both materials are indirect bandgap semiconductors. The indirect and direct bandgaps of A1-Sr-BSS are larger than those of BaSi2, while those of A2-Sr-BSS are smaller. The indirect bandgap of A1-Sr-BSS is 0.01 eV higher than that of BaSi2, while that of A2-Sr-BSS is 0.06 eV lower. This trend is attributable to the difference in the electronic properties around the conduction band minimum, which is determined by the type of atoms (Ba or Sr) that occupy the A1 site. Single-crystal X-ray diffraction and diffuse reflectance measurements of Ba1-xSrxSi2 (0.0 ≤ x ≤ 0.8) revealed that the Ba atoms at the crystallographic site A1 are preferentially substituted by Sr atoms and that the bandgap, Eg, decreases with x (1.24 eV at x = 0 and 1.15 eV at x = 0.65). The preferential occupation of the A1 site by Sr atoms is consistent with the computational result that A1-Sr-BSS is more stable than A2-Sr-BSS. However, the experimentally observed decrease in Eg with x is closer to the decrease of −0.06 eV in the indirect bandgap of A2-Sr-BSS than to the 0.01 eV increase in the case of A1-Sr-BSS. The reason for this discrepancy is discussed.