In this study, thin films of Cu2ZnSnS4 (CZTS) were synthesized via a low cost, wet chemical technique of chemical bath deposition (CBD) without any requirement of additional sulfurization. The aims of the study were: to achieve kesterite stoichiometry in the films without the need of post-synthesis sulfurization; to control the band structure and work function of the material only through precursor variation and to determine the correlation between Cu/Zn ratio and the photoresponse by the real-time, surface analysis technique of Kelvin probe force microscopy (KPFM).
Films having different sulfur content were deposited by varying the ratio of S/(Cu+Zn+Sn) while keeping the Cu/(Zn + Sn) and Zn/Sn ratios constant. Detailed electrical and optical characterization demonstrated that a controlled variation of the sulfur precursor allowed the formation of CZTS films with S/(Cu+Zn+Sn) ratio of ~1.1. As sulfur incorporation increased within the lattice, unwanted secondary phases decreased, band gap decreased, work function increased and the Fermi level moved closer to the valence band maxima; all of which point towards the material becoming more p-type in nature. To further study the effect of the Cu and Zn content on the band structure and photoresponse, the Cu/Zn ratio was varied keeping all other precursor ratios constant. Analysis showed that as the films transitioned from Cu-rich to Zn-rich, the band gap increased, work function decreased and the Fermi level moved away from the valence band maxima indicating a decrease in the hole population due to a decrease in CuZn type acceptor defects. At the same time, a significant improvement in photoresponse was observed in the KPFM mapping of the samples as Cu/Zn decreased and this was attributed to removal of secondary phases and atomic substitutions within the existing kesterite structure due to the Zn-rich environment. This mapping of the shifting surface potential (and therefore work function) of CZTS is unique and demonstrates an extremely simple method for mapping the changing band structure in a material without the requirement of any complex technique or equipment.
Thus, in addition to removing the need for post-synthesis sulfurization, the findings reveal the capability of significantly improving the photoresponse of CZTS by carefully controlling the Cu and Zn precursors which in turn has a direct correlation to the open-circuit voltage when used as an absorber layer in a device. This study demonstrates that optimizing the deposition conditions and the ability to control the band structure of the CZTS films prepared, allows them to be useful in a wide range of photovoltaic and photoelectrochemical devices.