The compositional profile and the band alignment across the interface between the CdS buffer layer and Kesterite absorber layer were investigated on Cu2ZnSnS4 and Cu2ZnSnSe4 devices. While the Cu2ZnSnS4 submodules had conversion efficiency as high as 9.2% in our previous work, the Cu2ZnSn(S,Se)4 submodules showed S-shaped current-voltage curves with poor fill factor and efficiency, indicating some problems were involved in the devices, probably at the p/n junction. Meanwhile, the Cu2ZnSn(S,Se)4 submodules exhibit high potential (i.e., Voc × Jsc ≈ 16–17 mW/cm2) to gain more than 12% efficiency. The X-ray/ultraviolet photoelectron spectroscopy revealed the possible problems at the CdS/Cu2ZnSnSe4 interface; (i) a conspicuous interdiffusion of Cd, Zn, and Se, and (ii) a large positive conduction-band offset (spike). By modifying deposition process of CdS layer to suppress the interdiffusion, the S-shape was completely eliminated, and a new record efficiency of 10.8% has been achieved on a Cu2ZnSn(S,Se)4 submodule.
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