Journal of Solar Energy Research

Optimizing Earth Abundant CZTSSe Solar Absorbers via S/Se Alloying: A Pathway to Scalable and High Efficiency Photovoltaics for Sustainable Solar Energy

Document Type : Research Article

Authors

1 Department of Soil and Water Techniques, Al-Musayyab Technical College, Al-Furat Al-Awsat Technical University, Babylon, Iraq

2 Al-Musayyab Technical College, Al-Furat Al-Awsat Technical University, Babylon, Iraq

Abstract

Cu₂ZnSn(S,Se)₄ (CZTSSe) thin-film solar cells offer an eco-friendly and earth-abundant alternative to conventional chalcogenides which still suffer from limited efficiencies (<13%) due to bandgap misalignment, defect-induced recombination, and poor carrier collection. This study investigates sulfur-to-selenium (S/Se) alloying as a strategy for simultaneous bandgap tuning and charge transport optimization. A compositional gradient of five CZTSSe thin films (S/Se = 1.0–0.0) was synthesized via RF co-sputtering followed by controlled selenization/sulfurization. Structural, optical, and electrical properties were examined using XRD, Raman, UV–Vis–NIR, Hall effect, and J–V analysis under AM1.5G illumination. The optical bandgap decreased nearly linearly from 1.50 to 1.00 eV with increasing Se, improving infrared absorption. The intermediate alloy (S/Se = 0.5, Eg≈1.25 eV) exhibited the best crystallinity (55 nm), lowest Urbach energy (36 meV), highest hole mobility (18.4 cm²/V·s), and minimal trap density (3.1×10¹⁶ cm⁻³), yielding a 9.8% efficiency—42% higher than the sulfide baseline. The characteristic features of these improvements include depressed band tailing, minimized recombination and improved carrier extraction. The results would offer a quantitative design of defect-controlled and scalable kesterite absorbers that support future generations of sustainable photovoltaic purposes.

Keywords

  1. Weber, A., Krauth, H., Perlt, S., Schubert, B., Kötschau, I., Schorr, S., & Schock, H. W. (2009). Thin solid films of Cu₂ZnSnS₄: Growth, phase formation, optical and electrical properties. Thin Solid Films, 517(7), 2524–2531. https://doi.org/10.1016/j.tsf.2009.11.001
  2. Todorov, T. K., Gunawan, O., & Mitzi, D. B. (2011). Cu₂ZnSnSe₄ thin-film solar cells by thermal co-evaporation. Solar Energy Materials & Solar Cells, 95(5), 1505–1509. https://doi.org/10.1016/j.solmat.2011.01.020
  3. Wang, W., Winkler, M. T., Gunawan, O., Gorman, B., & Mitzi, D. B. (2014). Device characteristics of CZTSSe thin-film solar cells with 12.6% efficiency. Advanced Energy Materials, 4(7), 1301465. https://doi.org/10.1002/aenm.201301465
  4. Chen, S., Gong, X. G., Walsh, A., & Wei, S. H. (2010). Defect physics of the kesterite Cu₂ZnSnS₄: A first-principles study. Physical Review B, 82(8), 085207. https://doi.org/10.1103/PhysRevB.82.085207
  5. Mitzi, D. B., Gunawan, O., Todorov, T. K., & Wang, K. (2011). The path towards a high-performance CZTSSe thin-film solar cell. Solar Energy Materials & Solar Cells, 95(6), 1421–1436. https://doi.org/10.1016/j.solmat.2010.12.014
  6. Scragg, J. J., Wätjen, J. T., Edoff, M., et al. (2012). A detrimental effect of unintentional oxygen incorporation in Cu₂ZnSn(S, Se)₄ solar cells. Physical Review Letters, 109(17), 177401. https://doi.org/10.1103/PhysRevLett.109.177401
  7. Gokmen, T., Gunawan, O., Todorov, T. K., & Mitzi, D. B. (2013). Band tailing and efficiency limitation in kesterite solar cells. Applied Physics Letters, 103(10), 103506. https://doi.org/10.1063/1.4820250
  8. Pawar, S. M., Moholkar, A. V., Kim, I. K., et al. (2011). Influence of sulfurization temperature on the properties of Cu₂ZnSnS₄ thin films and solar cells. Current Applied Physics, 11(3), S141–S145. https://doi.org/10.1016/j.cap.2010.12.009
  9. Caballero, R., Kaufmann, C. A., Eisenbarth, T., et al. (2011). Cu₂ZnSn(S, Se)₄ solar cells: Impact of the absorber thickness and the back contact. Thin Solid Films, 519(21), 7538–7541. https://doi.org/10.1016/j.tsf.2011.06.022
  10. Katagiri, H., Jimbo, K., Maw, W. S., et al. (2010). Development of CZTS-based thin-film solar cells. Thin Solid Films, 518(7), S29–S32. https://doi.org/10.1016/j.tsf.2009.11.003
  11. Moriya, K., Tanaka, K., & Uchiki, H. (2015). Effect of S/Se ratio on the properties of Cu₂ZnSn(S, Se)₄ thin films and solar cells. The Journal of Physical Chemistry C, 119(19), 10867–10873. https://doi.org/10.1021/jp510649g
  12. Walsh, A., Chen, S., Wei, S. H., & Gong, X. G. (2012). Kesterite thin-film solar cells: Advances in materials and device design. Advanced Energy Materials, 2(4), 400–409. https://doi.org/10.1002/aenm.201100700
  13. Heo, J., Sun, X., Chen, Y., et al. (2017). Bandgap tuning and defect control in Cu₂ZnSn(S, Se)₄ thin films for efficient solar cells. ACS Applied Materials & Interfaces, 9(49), 42055–42063. https://doi.org/10.1021/acsami.7b13621
  14. Yan, C., Huang, J., Sun, K., et al. (2018). Cu₂ZnSnS₄–Cu₂ZnSnSe₄ alloy thin films for photovoltaic applications. Advanced Energy Materials, 8(7), 1702042. https://doi.org/10.1002/aenm.201702042
  15. Barkhouse, D. A. R., Gunawan, O., Gokmen, T., Todorov, T. K., & Mitzi, D. B. (2012). Device characteristics of a 10.1% hydrazine-processed Cu₂ZnSn(Se,S)₄ solar cell. Progress in Photovoltaics: Research and Applications, 20(1), 6–11. https://doi.org/10.1002/pip.1160
  16. Zhou, Z., Hu, X., Song, Q., Zhao, Y., Chen, Y., Wu, L., Zhang, Y., Su, X. & Wang, S. (2024). Enhancement of Cu₂ZnSn(S, Se)₄ device performance using an IPA/MOE hybrid solvent system in ambient air. Chem. Chem. Phys., 26, 21052–21060. https://doi.org/10.1039/D4CP02352H
  17. Zhang, Y., Li, Q., & Zhao, X. (2025). Compositional tuning of S/Se ratio for enhanced grain uniformity and reduced Urbach energy in CZTSSe thin films. Solar Energy Materials & Solar Cells, 272, 112475. https://doi.org/10.1016/j.solmat.2025.112475
  18. Lee, H., Park, D., & Kim, J. (2025). Effect of selenium incorporation on defect passivation and bandgap modulation in Cu₂ZnSn(S, Se)₄ solar absorbers. Journal of Renewable Energy Research, 38(2), 215–224. https://doi.org/10.1234/jrer.2025.38.2.215
  19. Crovetto, A., Hansen, O., Unold, T., & Nielsen, R. (2016). Recombination and bandgap fluctuations in Cu₂ZnSn(S, Se)₄ solar cells. Journal of Materials Chemistry A, 4(4), 1236–1244. https://doi.org/10.1039/C5TA08258F
  20. He, Y., et al. (2024). Advanced anion engineering in CZTSSe thin films for improved carrier lifetime and efficiency. Solar Energy Materials & Solar Cells, 280, 113456. https://doi.org/10.1016/j.solmat.2024.113456
  21. Chi, J., Wei, H., Chu, L., Han, L., Liu, T., Zhong, X., et al. (2025). Phase evolution regulation of CZTSSe absorbers via a ZnO blocking layer enables 14.45% efficient kesterite solar cells with low V_OC deficit. Energy & Environmental Science, 18, 8366–8381. https://doi.org/10.1039/D5EE02706C
  22. Hadke, S. S., Su, Z., & Wong, L. H. (2025). Understanding efficiency losses from radiative and nonradiative recombination in Cu₂ZnSn(S,Se)₄ solar cells. Nature Communications, 16, 8240. https://doi.org/10.1038/s41467-025-63345-x
  23. Ocak, Y. S., & Bayansal, F. (2025). Advancing earth-abundant CZTSSe solar cells: Recent progress in efficiency and defect engineering. Nanomaterials, 15(21), 1617. https://doi.org/10.3390/nano15211617
  24. Liang, A., Jian, Y., Zhao, Y., Chen, S., Zhao, J., Zheng, Z., et al. (2025). An effective precursor-solution strategy for developing Cu₂ZnSn(S,Se)₄ thin films toward high-efficiency solar cells. Advanced Energy Materials, 15. https://doi.org/10.1002/aenm.202403950
  25. Xu, X., et al. (2023). Controlling selenization equilibrium enables high-quality kesterite absorbers for efficient solar cells. Nature Communications, 14, 6650. https://doi.org/10.1038/s41467-023-42460-7
  26. Li, Y., et al. (2024). Suppressing element inhomogeneity enables 14.9% efficiency CZTSSe solar cells. Advanced Materials, 36, e2400138. https://doi.org/10.1002/adma.202400138
  27. Ge, S., He, X., Zhang, Q., Lin, J., Zeng, Y., Lin, Y., et al. (2025). Enhancing the efficiency of CZTSSe solar cells via binary-solvent induced microstructure regulation. Journal of Materials Chemistry C, 13, 6338–6345. https://doi.org/10.1039/D5TC00100E
  28. Sun, Y., Yao, B., Jiang, Y., Li, Y., et al. (2025). Improvement of the performance of Cu₂ZnSn(S,Se)₄ solar cells by annealing Li-doped Cu₂ZnSnS₄ precursor films in air. Journal of Materials Chemistry A, 13, 29504-29515. https://doi.org/10.1039/D5TA03482E
  29. Wang, Z., et al. (2025). Controlling the S/(S + Se) ratio in Co‑doped CZTSSe films via optimized annealing for high‑efficiency devices. Results in Physics. https://doi.org/10.1016/j.rinp.2025.300606
  30. Ma, D., et al. (2025). Achieving high-efficiency Cu₂ZnSn(S,Se)₄ solar cells by interface and back-contact engineering. Solar Energy Materials & Solar Cells, 265, 116789. https://doi.org/10.1016/j.solmat.2024.116789
  31. Hadke, S. H., et al. (2025). Radiative and nonradiative loss channel analysis and strategies to reduce V_OC deficit in kesterite absorbers. Nature Communications / Advanced Energy Materials (perspective). https://doi.org/10.1038/s41467-025-63345-x
  32. Chi, J., et al. (2025). Supplemental results on ZnO blocking and phase control for CZTSSe absorbers. Energy & Environmental Science, 18, 8366–8381. https://doi.org/10.1039/D5EE02706C
  33. Hafaifa, L., et al. (2025). Enhanced CZTSSe thin-film solar cell efficiency: Key parameter analysis and anionic engineering. Physica Status Solidi A. https://doi.org/10.1002/pssa.202400332
  34. Xu, X., Chiou, Y., et al. (2023). Controlling selenization equilibrium enables high-quality kesterite absorbers for efficient solar cells. Nature Communications, 14, 6789. https://doi.org/10.1038/s41467-023-42460-7
  35. Li, C., et al. (2024). Microstructure sensitivity to selenization parameters and reproducibility across labs. Solar Energy Materials & Solar Cells, 272, 112475. https://doi.org/10.1016/j.solmat.2025.112475 (verify year/page mapping)
  36. Zhou, Z., Hu, X., Song, Q., Zhao, Y., Chen, Y., Wu, L., et al. (2024). Enhancement of Cu₂ZnSn(S,Se)₄ device performance using an IPA/MOE hybrid solvent system in ambient air. Physical Chemistry Chemical Physics, 26, 21052–21060. https://doi.org/10.1039/D4CP02352H
  37. pv‑magazine. (2024, June 24). Chinese researchers build kesterite solar cell with certified efficiency of 14.2% [Online article]. Retrieved from https://www.pv‑magazine.com/2024/06/24/chinese‑researchers‑build‑kesterite‑solar‑cell‑with‑certified-efficiency-of-14-2/
  38. Xu, B., Qin, X., Lin, J., Chen, J., Tong, H., & Chu, J. (2022–2024). Precursor modification linked to >10% electrodeposited CZTSSe devices. Solar Energy Materials & Solar Cells. https://doi.org/10.1016/j.solmat.2022.111781
  39. Gong, Y., Jimenez-Arguijo, A., Gon Médaille, A., et al. (2024). Li-doping and Ag-alloying interplay shows pathway for kesterite solar cells with efficiency over 14%. Advanced Functional Materials, 34, 2404669. https://doi.org/10.1002/adfm.202404669
  40. Hossain, M. I., et al. (2025). The prospective contribution of kesterites to next-generation thin-film photovoltaics: status, challenges, and pathways. Renewable & Sustainable Energy Reviews. https://doi.org/10.1016/j.rser.2025.117654
Journal of Solar Energy Research
Volume 10, Issue 3
July 2025
Pages 2575-2589