A thin-film solar cell based on Cu${}_2$ZnSn(S,Se)${}_4$ (CZTSSe) alloy was recently found to exhibit a light to electricity conversion efficiency of $10\%$, making it competitive with the more mature Cu(In,Ga)Se${}_2$ based technologies. We study the compositional dependence of the physical properties of CZTSSe alloys through first-principles calculations and find that these mixed-anion alloys are highly miscible with low enthalpies of formation, and the cations maintain the same ordering preferences as the parent compounds Cu${}_2$ZnSnS${}_4$ and Cu${}_2$ZnSnSe${}_4$. The band gap of the CZTSSe alloy decreases with the Se content almost linearly, and the band alignment between Cu${}_2$ZnSnS${}_4$ and Cu${}_2$ZnSnSe${}_4$ is of type I, which allows for more facile $n$-type and $p$-type doping for alloys with high Se content. Based on these results we analyze the influence of composition on the efficiency of CZTSSe solar cells and explain the high efficiency of the cells with high Se content.

• Received 15 December 2010

DOI:https://doi.org/10.1103/PhysRevB.83.125201