Significantly enhancing the stability of a Cu2ZnSnS4 aqueous/ethanol-based precursor solution and its application in Cu2ZnSn(S,Se)4 solar cells

The stability of a CZTS precursor solution plays an important role for potential industrial applications, and could provide insight into the understanding of the solubility mechanism of the raw materials. In this work, we report a systematic investigation on the stability of a metal/thiourea aqueous/ethanol-based precursor solution used for fabricating Cu2ZnSnS4 (CZTS) thin films. It is found that the metal/thiourea aqueous/ethanol solution is metastable, and easily produced a mass of precipitation when the solution was prepared for few hours. This could cause an element to be missed out of the film, giving a very bad film-processing ability, and effect the application of such a method. So we studied in detail the crystalline precipitation through SEM, EDS and FTIR, and found that they mainly contain copper ion and thiourea coordination complexes. Furthermore, we demonstrated that the low-toxicity 3-mercaptopropionic acid (MPA) can play an important role as an auxiliary ligand in the precursor solution. Addition of MPA into the precursor solution can avoid the precipitation and noticeably improve the stability of the precursor solution from similar to 3 hours to 1 week. Using the precursor solution with added MPA, we fabricated a single phase of a CZTS film, and prepared a large-grain and hole-free Cu2ZnSn(S,Se)(4) (CZTSSe) film. More importantly, a CZTSSe film solar cell with a competitive power conversion efficiency of 7.25% has been reported in this work.