Solution-grown near-stoichiometry Cu2ZnSnS4 films: Optical transitions and defect levels

The preparation of device quality Cu2ZnSnS4 (CZTS) films by a simple and low-cost technique is of great importance for photovoltaic applications. In this work, stoichiometric films of CZTS could be synthesized by a successive dip coating method using a non-toxic methanol-based solution. The films' structure, composition, and surface morphology were studied by standard techniques. CZTS/CdS diodes with high rectification factors were fabricated and were used for electrical characterization of the films. The optical transmittance and photocurrent measurements were performed for optoelectronic characterization of the films. The results revealed the presence of two acceptor-type native defects with the activation energies of 114 meV and 300 meV. Five optical transitions were detected in the energy range of 1.34-3.2 eV and all could be identified on the basis of the two measured defect levels and the theoretical band diagrams of the stannite and kesterite structural phases of CZTS reported by Botti, Kammerlander, and Marques (2011). The results were in support of the co-existence of both structural phases in the films.