Kesterite, Cu2ZnSnS4 (CZTS), absorber layer based solar cells are unique owing to their earth-abundant, non-toxic elements; however, the record efficiency, ~12 %, is still lower than the calculated theoretical values [1,2]. The crystallographic properties of CZTS absorber layers, which are found to influence the solar cell efficiency, can be manipulated with a suitable element [3, 4]. Bismuth has been shown to work as a fluxing element and enhance the grain growth of CZTS crystals [5,6], however it has been studied only to a limited extent and further studies are required to investigate the influence of bismuth on the properties of CZTS thin films.
In this study, first we electrodeposited suitable Cu-Zn-Sn-Bi (CZTB) metallic precursors from a single deposition bath and synthesized CZTS layers with bismuth (CZTS-Bi) via sulfurization. Second, we investigated the crystallographic features of the resulting CZTS-Bi layers to inspect the doping role of Bi. Composition was determined by EDS and structural properties were analyzed with XRD and Raman Spectroscopy. Lastly, we presented the correlation between photoelectrochemical (PEC) performance and crystallographic properties of CZTS only and CZTS-Bi layers.
The incorporation of Bi into the CZTS lattice is found to be limited to ~0.5 at% Bi, and above this amount we observed Bi-rich secondary phases in the layers. We found that Bi additions do increase the grain size, suggesting that it is acting as a fluxing agent; in addition, the FWHM analyses of films with XRD and Raman spectra are in agreement with regard to the better crystallinity of CZTS-Bi films vs. CZTS only layers. Figure 1 shows the PEC response of the CZTS only and CZTS-Bi thin films a function of Zn/Sn ratio. The PEC response of the layers increased with the increasing Zn/Sn ratio for both of the CZTS only and CZTS only layers. The photoresponse of the CZTS-Bi layers is higher than the CZTS layers for Zn/Sn ratio above 0.90. CZTS-Bi layer also showed the highest PEC response in these set of samples, with a 2.65 mA/cm2 photoresponse at -0.95 V. The improvement of the PEC performance is attributed to two reasons; the increase of the grain size and the improvement of the chemical order of the CZTS lattice, which influences the electronic structure, hence band gap, of the CZTS material.

References
1. Wang, W., Winkler, M. T., Gunawan, O., Gokmen, T., Todorov, T. K., Zhu, Y., & Mitzi, D. B. (2014). Device characteristics of CZTSSe thin-film solar cells with 12.6% efficiency. Advanced Energy Materials, 4, 1–5. https://doi.org/10.1002/aenm.201301465
2. Wallace, S. K., Mitzi, D. B., & Walsh, A. (2017). The Steady Rise of Kesterite Solar Cells. ACS Energy Letters, 776–779. https://doi.org/10.1021/acsenergylett.7b00131
3. Altamura, G., Wang, M., & Choy, K.-L. (2016). Influence of alkali metals (Na, Li, Rb) on the performance of electrostatic spray-assisted vapor deposited Cu2ZnSn(S,Se)4 solar cells. Scientific Reports, 6(October 2015), 22109. https://doi.org/10.1038/srep22109
4. Ionkin, A. S., Fish, B. M., Marshall, W. J., & Senigo, R. H. (2012). Use of inorganic fluxes to control morphology and purity of crystalline késterite and related quaternary chalcogenides. Solar Energy Materials and Solar Cells, 104, 23–31. https://doi.org/10.1016/j.solmat.2012.04.042
5. Tong, Z., Zhang, K., Sun, K., Yan, C., Liu, F., Jiang, L., Li, J. (2016). Modification of absorber quality and Mo-back contact by a thin Bi intermediate layer for kesterite Cu2ZnSnS4 solar cells. Solar Energy Materials and Solar Cells, 144, 537–543. https://doi.org/10.1016/j.solmat.2015.09.066
6. Rawat, K., & Shishodia, P. K. (2016). Enhancement of photosensitivity in bismuth doped Cu 2 ZnSnS 4 thin films, 5(12), 1–5. https://doi.org/10.1002/pssr.201600335