Thermodynamic stability and optoelectronic properties of Cu(Sb/Bi)(S/Se)<inf>2</inf> ternary chalcogenides: Promising ultrathin photoabsorber semiconductors

We used density functional theory based calculations to investigate the structural and optoelectronic properties of copper-based ternary chalcogenide Cu-M-X2 (M: Sb, Bi & X: S, Se). These form orthorhombic crystallographic structure with Pnma space group. The relative thermodynamic stability of these structures is supported by their phonon band dispersions. The calculated electronic band structure is indirect for all these compounds in conjunction with a close direct band gap transition. The calculated effective mass of electrons and holes are (0.074, 0.732), (0.053, 0.297), (0.039, 0.655) and (0.031, 0.514) for CuSbS2, CuSbSe2, CuBiS2 and CuBiSe2, respectively. Interestingly, a very high optical absorption coefficient above 105 cm−1 above band gap values is noticed for these materials, making them suitable for ultrathin solar cell absorbers.