Thin film solar cell technologies are mainly based on polycrystalline absorber layers, which is also the case for kesterite-based photovoltaic devices. An alternative technology, which is promising and low cost, is based on kesterite-type Cu2ZnSn(S,Se)4 (CZTSSe) monograins as absorbers, which are fixed in a polymer matrix to form a flexible solar cell. The large band tailing observed in Cu-based kesterite-type semiconductors is believed to cause voltage losses, limiting the efficiency of kesterite-based devices. Cu/Zn disorder, which is always present in these compounds, is discussed in literature as a possible reason for band tailing. The experimental determination and quantification of Cu/Zn disorder is possible by in-depth analysis of neutron diffraction data. This work reveals that the purity of copper used in the synthesis of CZTSSe monograins has an influence on the degree of Cu/Zn disorder in the semiconductor and thus on optical and PV parameters. Comparing CZTSSe monograins, less Cu/Zn disorder was observed for the monograins synthesized using copper with higher purity; the respective monograin-based solar cell shows a higher power conversion efficiency. On the other hand, the band gap energy as well as the photoluminescence maximum (PLmax ) of both monograins are the same. Applying a low-temperature annealing procedure allowed us to increase the quality of monograins synthesized using 5N copper, very close to the one grown using 6N copper. The PLmax slightly shifts into higher energy, which is most likely an indication of the decreased Cu/Zn disorder, either moving the defect states toward the valence band or that it reduces the formation of the tail states near the conduction band minimum.
