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 (V-oc 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 V-oc of 446 mV, the proposed cell had an efficiency of 11.1%, a band gap of 1.2 eV and a V-oc of 578 mV; the postsulfurization 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 V-oc. 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 V-oc deficits of future devices may be decreased by suitable surface treatment of high-sulfur-content CZTSSe devices.