Route Toward High-Efficiency Single-Phase Cu$_{\bf 2}$ ZnSn(S,Se)$_{\bf 4}$ Thin-Film Solar Cells: Model Experiments and Literature Review

Thin-film chalcogenide kesterites Cu
₂
ZnSnS
₄
and Cu
₂
ZnSnSe
₄
(CZTSSe) are promising candidates for the next-generation thin-film solar cells. They exhibit a high natural abundance of Cu, Zn, Sn and S
₂
, a high absorption coefficient, and a tunable direct bandgap between 1.0 and 1.5 eV. A prerequisite for the use of CZTSSe as absorber layers in photovoltaic applications on large scales is a detailed knowledge of the formation reaction. Recently, we have shown that a decomposition/formation equilibrium governs the formation reaction. The presence of Sn(S,Se) during the high-temperature preparation steps is essential to prevent decomposition. This improves the solar cell efficiency from 0.02% to 6.1%. In this paper, we show that the decomposition is universal. Absorbers produced by high-temperature coevaporation and samples produced by low-temperature precursor fabrication followed by annealing in a tube furnace in S or Se atmosphere are compared in order to elucidate that in all cases, the loss of Sn(S,Se) forms a degraded surface region. We demonstrate that the degraded surface of CZTSe absorbers contains grains of ZnSe. These new insights can be used to explain why some of the synthesis routines described in the literature yield much better efficiencies than others.