Preparing Cu2ZnSn(SxSe1−x)4 thin-film solar cells with front band gap grading by plasma sulfurization

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In this paper, Cu2ZnSn(SxSe1−x)4(CZTSSe) thin-films with a front-graded band gap were prepared by plasma sulfurization. The method of selenization followed by low-temperature plasma sulfurization was adopted to effectively control the uneven diffusion of S in the absorption layer. By investigating and analyzing the grazing incidence X-ray diffraction, Raman spectroscopy, and PL spectra, the CZTSSe absorption layer with an S-rich top and S-poor bottom layer could be obtained by plasma sulfurization. The inhomogeneous distribution of S led to the formation of band gap gradients. Based on that, the band gap near the top of the absorption layer was determined to be 1.13 eV and the bottom band gap was 1.03 eV. Approximately 300 nm thick sulfur-rich CZTSSe films were found near the surface favoring an increase in the band gap at the interface, which could be attributed to a high open circuit voltage (Voc). The establishment of the band gap gradient formed a barrier towards the motion of holes towards the interface and increased the effective interfacial recombination band gap, both of which significantly reduced the interfacial recombination. Finally, the CZTSSe thin-film solar cells with a band gap gradient were prepared by plasma sulfurization exhibited a power conversion efficiency (PCE) of 8.11%. Compared with CZTSSe thin-film solar cells without post plasma sulfurization, which had a PCE of 6.98%, the Voc of the CZTSSe solar cell improved considerably from 471.37 mV to 513.93 mV; however, it had little effect on short-circuit current density (Jsc).