Cadmium Free Cu₂ZnSnS₄ Solar Cells with 9.7% Efficiency

Cu 2 ZnSnS 4 (CZTS) thin‐film solar cell absorbers with different bandgaps can be produced by parameter variation during thermal treatments. Here, the effects of varied annealing time in a sulfur atmosphere and an ordering treatment of the absorber are compared. Chemical changes in the surface due to ordering are examined, and a downshift of the valence band edge is observed. With the goal to obtain different band alignments, these CZTS absorbers are combined with Zn 1− x Sn x O y (ZTO) or CdS buffer layers to produce complete devices. A high open circuit voltage of 809 mV is obtained for an ordered CZTS absorber with CdS buffer layer, while a 9.7% device is obtained utilizing a Cd free ZTO buffer layer. The best performing devices are produced with a very rapid 1 min sulfurization, resulting in very small grains.