Scaleable photovoltaic absorber materials within the Cu-Sb-S system

The Cu-Sb-S system contains four ternary compounds which may hold promise for scalable, non-toxic, and efficient solar photoconversion. Like similar compounds CuInSe$_2$, CIGS, and CZTS, the Cu-Sb-S compounds are predicted to offer high absorption coefficients, and electrically benign grain boundaries. Antimony, instead of indium or gallium, has the advantage of lower cost and greater availability, as well as theoretically predicted better photon absorption. It also has a potential advantage over CZTS, as the Cu-V-S compound avoids the deep traps associated with antisite defects. Here, we synthesize two compounds within the Cu-Sb-S system, Cu$_{12}$Sb$_4$S$_{14}$ (tetrahedrite) and CuSbS$_2$ (chalcostibite), by combinatorial RF magnetron co-sputtering from Cu$_2$S and Sb$_2$S$_3$ targets. Chalcostibite films were found to have good optical and electrical properties, with a steep absorption onset at 1.5eV, high absorption coefficient ($>10^{5}$cm$^{-1}$), good carrier concentration ($p=10^{17}$cm$^{-3}$) and mobility (0.2 cm$^2$/V-s). Chalcostibite growth conditions were therefore further optimized and it was found that an overflux of vapor phase Sb$_2$S$_3$ allowed strict control of stoichiometry for better device integration.