Formation mechanism of ZnS impurities and their effect on photoelectrochemical properties on a Cu2ZnSnS4 photocathode

Cu2ZnSnS4 (CZTS) nanocrystals were synthesized by a hot injection method. Ultraviolet (UV) Raman spectroscopy measurements showed that ZnS impurities existed in CZTS with a Zn/Sn precursor ratio of 1.2. Combining Raman spectra with energy-dispersive X-ray spectroscopy (EDS) characterization, the formation mechanism of CZTS and ZnS impurities were also investigated. We found that crystalline CZTS and Cu2SnS3, as well as amorphous ZnS, formed after sulfur-oleylamine was injected into the mixture solution. Amorphous ZnS reacted with Cu2SnS3 to form CZTS during sulfur annealing at high temperature. However, if the Zn/Sn ratio was too high, the excess ZnS could not be eliminated even after a longer reaction time. A relatively pure phase of CZTS was only obtained by decreasing the Zn/Sn precursor ratio to 0.6. Moreover, CZTS nanocrystals were assembled into thin films by electrophoretic deposition onto molybdenum-coated soda lime glass and the photoelectrochemical properties of CZTS photocathodes with and without ZnS impurities were investigated. A CZTS photocathode without ZnS demonstrated higher photocurrent than those with ZnS. The results will deepen the understanding of the CZTS formation process and be helpful to explore efficient CZTS solar conversion devices.