Effects of SnSe2 secondary phases on the efficiency of Cu2ZnSn(Sx,Se1 − x)4 based solar cells

In this work, we studied the effect of annealing on crystallization behavior of Cu2ZnSn(Sx,Se1 − x)4 (CZTSSe) absorbers. Cu-Zn-Sn-S precursors were deposited on glass/Mo substrate by co-sputtering of Cu, ZnS, and SnS. In order to have a better understanding of the mechanisms of formation of CZTSSe and secondary phases during annealing, the effect of the variation of the annealing temperature between 300 and 600 °C and of the annealing time between 1 and 60 min on structural and compositional properties of the materials and cells has been studied. For the best annealing conditions, efficiencies up to 7% were obtained. The surface morphology studies revealed that the samples annealed at higher temperatures, and longer times exhibited well-crystallized phases of CZTSSe and SnSe2. A comparison of material and solar cell properties studied by chemical, structural and opto-electronic characterizations shows that for all the annealing conditions, there is a simultaneous formation of CZTSSe and SnSe2 phases. We showed that these binaries are detrimental for the performance of solar cells. They are responsible for a strong shunt effect leading to a decrease of the open circuit voltage and fill factor.