Theoretical Evaluation of Cu-Sn-S and Cu-Sb-S Based Solar Absorbers for Earth-Abundant Thin-Film Solar Cells

Current thin-film solar absorbers such as Cu(In/Ga)Se$_2$ or CdTe, although remarkably efficient, incorporate limited-supply elements like indium or tellurium. Meeting the cost competiveness criterion necessary for a large-scale deployment of thin-film PV technologies requires development of new earth-abundant solar absorbers. In an effort to accelerate such development we combine first principles theory and high throughput experiments to explore In-free ternary copper chalcogenides. As part of the theoretical evaluation, we study the Cu$_2$SnS$_3$, Cu$_4$SnS$_4$, CuSbS$_2$ and Cu$_3$SbS$_3$ based compounds formed by isovalent alloying on Sn, Sb, and S sites. For this set of materials we predict band-structures and optical absorption coefficients and demonstrate the feasibility of achieving the optimal band gap of 1.3 eV for a single junction cell and a high optical absorption of $\sim 10^4$ cm$^{-1}$ at $E_g$+0.2 eV. We additionally perform defect studies to elucidate the doping trends within this class of materials.