Cu2ZnSn(SxSe1−x)4 thin film solar cell with high sulfur content (x approximately 0.4) and low Voc deficit prepared using a postsulfurization process

High-efficiency (11.1%) Cu2ZnSn(SxSe1−x)4 (CZTSSe) solar cells have been obtained only with low-sulfur absorbers because the incorporation of high sulfur content is typically accompanied by a large open-circuit voltage (Voc) deficit. In this research, a sulfur content of 40% (x = 0.4) was obtained by postsulfurization. In comparison with a low-sulfur CZTSSe with x = 0.13, an efficiency of 9.8%, a band gap of 1.05 eV, and a Voc of 446 mV, the proposed cell had an efficiency of 11.1%, a band gap of 1.2 eV and a Voc of 578 mV; the post-sulfurization caused a very small increase in the Voc deficit (approximately 18 mV). Approximately 250-nm-thick S-rich CZTSSe layer was found near the surface and was close to the depletion width (approximately 238.5 nm) of the p-n junction, indicating a double-layered CZTSSe included a high-sulfur top layer was responsible for the high Voc. Admittance spectroscopy showed the activation energy of the bulk defect was 138 meV; this revealed some deep-level defects were associated with the low short-circuit current at long wavelengths. The activation energy of the interfacial defects was 1.08 eV, indicating the Voc deficits of future devices may be decreased by suitable surface treatment of high-sulfur-content CZTSSe devices.