In situ gel formation of high quality kesterite thin films

Conventional methods for fabricating chalcogenide-based solar cells involve vacuum processes, e.g. co-evaporation and sputtering, even if the most performing devices based on the kesterite compound Cu2ZnSn(S,Se)4 have been realized using a solution-based methodology. Moreover, non-vacuum processes present significant advantages as lower production costs, higher productivity and uniformity of the final stoichiometry composition. In this context, we present a new chemical procedure to obtain a superior quality Cu2ZnSnS4 and Cu2FeSnS4 films composed by highly soluble and inexpensive precursors in a non-toxic and environmentally friendly solvent. Therefore, the films were prepared by a Sol-Gel method, starting from the metal salts in organic solvent (dimethylsulfoxide); thiourea was used as a source of sulphur. The influence of the composition of solutions containing metal ions was studied: solutions of metal salts containing non-coordinating anions and coordinates ones were compared to evaluate the role of anions and their coordination with metal ions in the formation of a pure and homogeneous phase. To obtain information about the complex mechanism of interaction between the metals and the various components of the solution Raman measurements and electronic paramagnetic resonance (EPR) were performed. The film deposition was carried out by in situ gel formation on fluorine doped tin oxide coated glass. The subsequent heat treatment guarantees the formation of the correct crystalline phase, without sulfurization but in Argon atmosphere [and at temperature suitable also for flexible and plastic substrates]. The films obtained were characterized by Raman spectroscopy, X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDX), morphologically by electronic scanning electron microscopy (SEM). Raman and EPR measurements have shown that thiourea plays a primary role in coordinating metallic ions in solution, also highlighting the reductive power on copper; the coordinating effect of acetate ions, employed as precursors, plays a primary role in ensuring homogeneity in solution and consequently in the film. XRD confirms the absence of secondary phases. Therefore, the developed methodology has successfully identified an innovative way to achieve high quality kesterite thin films for photovoltaic applications; relate devices optimization is currently underway.