Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
6457000 | Solar Energy Materials and Solar Cells | 2017 | 8 Pages |
â¢We investigate the properties of band gap front-graded CZTSSe thin films.â¢The precursors are annealed by a modified single-step sulfo-selenization process.â¢The CZTSSe thin-film solar cell exhibits power conversion efficiency of 10.33%.â¢The high Voc and less loss of Jsc are attributed to the band gap front-grading.
In this study, we investigate the electrical, structural, and optical properties of band gap front-graded Cu2ZnSn(S,Se)4 (CZTSSe) thin films grown by a modified single-step sulfo-selenization process from copper-poor and zinc-rich precursor metallic stacks prepared by co-evaporation. To investigate how the bandgap was graded in connection with the compositional distribution, we calculated the bandgap energy distribution along the film thickness, based on the transmission electron microscopy and energy-dispersive X-ray spectroscopy composition profile. The band gap of the CZTSSe phase with high S content near the surface layer is determined to be 1.161Â eV. From the surface to the bottom, there is a decrease in the S content of the CZTSSe phase, and the band gap subsequently decreases to, 1.029Â eV, close to the value of CZTSe. From the results of dimpling-Raman and scanning transmission electron microscopy line scanning, we confirm that the S content drastically increases from the bottom to the top surface of the CZTSSe thin film. The CZTSSe thin-film solar cell exhibits a power conversion efficiency (PCE) of 10.33%, with an open-circuit voltage (Voc) of 0.505 V, short-circuit current density (Jsc) of 31.61Â mA/cm2, fill factor (FF) of 64.6%, and Voc deficit of 525Â mV. Compared with the performance of the CZTSe solar cell, which had PCE of 7.23%, Voc of 0.424 V, Jsc of 32.83Â mAÂ cmâ2, FF of 51.9%, and Voc deficit of 576Â mV, the Voc and Voc deficit of the CZTSSe cell improved considerably. The high Voc, low Voc deficit, and less loss of Jsc are attributed to the effect of band gap front-grading induced by S grading into the CZTSSe thin film.
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