Cu2ZnSnS4 thin-film solar cell absorbers illuminated by soft x-rays


In view of the complexity of thin-film solar cells, which are comprised of a multitude of layers, interfaces, surfaces, elements, impurities, etc., it is crucial to characterize and understand the chemical and electronic structure of these components. Because of the high complexity of the Cu2ZnSn(S,Se)4 compound semiconductor absorber material alone, this is particularly true for kesterite-based devices. Hence, this paper reviews our recent progress in the characterization of Cu2ZnSnS4 (CZTS) thin films. It is demonstrated that a combination of different soft x-ray spectroscopies is an extraordinarily powerful method for illuminating the chemical and electronic material characteristics from many different perspectives, ultimately resulting in a comprehensive picture of these properties. The focus of the article will be on secondary impurity phases, electronic structure, native oxidation, and the CZTS surface composition.

This is a preview of subscription content, access via your institution.

FIG. 1
FIG. 2
FIG. 3
FIG. 4


  1. 1.

    I. Repins, M.A. Contreras, B. Egaas, C. DeHart, J. Scharf, C.L. Perkins, B. To, and R. Noufi: 19·9%-efficient ZnO/CdS/CuInGaSe2 solar cell with 81·2% fill factor. Prog. Photovoltaics Res. Appl. 16, 235–239 (2008).

    CAS  Article  Google Scholar 

  2. 2.

    P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann, and M. Powalla: New world record efficiency for Cu(In, Ga)Se2 thin-film solar cells beyond 20%. Prog. Photovoltaics Res. Appl. 19, 894–897 (2011).

    CAS  Article  Google Scholar 

  3. 3.

    X. Wu, J.C. Keane, R.G. Dhere, C. Dehart, D.S. Albin, A. Duda, T.A. Gessert, S. Asher, D.H. Levi, and P. Sheldon: 16.5%-efficient CdS/CdTe polycrystalline thin-film solar cell. in Proceedings of the Seventeenth European Photovoltaic Solar Energy Conference, edited by M. McNelis. (Munich, Germany, 2001); pp. 995–1000.

    Google Scholar 

  4. 4.

    First Solar, Inc: News releases: “First Solar Announces Second Quarter 2011 Financial Results”, Aug. 8, 2011 and “First Solar Sets World Record for CdTe Solar PV Efficiency”, July 26, 2011. (accessed August 15, 2011).

    Google Scholar 

  5. 5.

    M.A. Green, K. Emery, Y. Hishikawa, W. Warta, and E.D. Dunlop: Solar cell efficiency tables (Version 38). Prog. Photovoltaics Res. Appl. 19, 565–572 (2011).

    Article  Google Scholar 

  6. 6.

    T.M. Friedlmeier, H. Dittrich, and H.-W. Schock: Growth and characterization of Cu2ZnSnS4 and Cu2ZnSnSe4 thin films for photovoltaic applications. Inst. Phys. Conf. Ser. 152A, p. 345 (1998).

    Google Scholar 

  7. 7.

    T. Tanaka, T. Nagatomo, D. Kawasaki, M. Nishio, Q. Guo, A. Wakahara, A. Yoshida, and H. Ogawa: Preparation of Cu2ZnSnS4 thin films by hybrid sputtering. J. Phys. Chem. Solids 66, 1978–1981 (2005).

    CAS  Article  Google Scholar 

  8. 8.

    H. Katagiri, K. Jimbo, S. Yamada, T. Kamimura, W.S. Maw, T. Fukano, T. Ito, and T. Motohiro: Enhanced conversion efficiencies of Cu2ZnSnS4-based thin film solar cells by using preferential etching technique. Appl. Phys. Express 1, 041201 (2008).

    Article  CAS  Google Scholar 

  9. 9.

    A. Weber, H. Krauth, S. Perlt, B. Schubert, I. Kötschau, S. Schorr, and H-W. Schock: Multistage evaporation of Cu2ZnSnS4 thin films. Thin Solid Films 517, 2524–2526 (2009).

    CAS  Article  Google Scholar 

  10. 10.

    J.J. Scragg, D.M. Berg, and P.J. Dale: A 3.2% efficient kesterite device from electrodeposited stacked elemental layers. J. Electroanal. Chem. 646, 52–59 (2010).

    CAS  Article  Google Scholar 

  11. 11.

    G.S. Babu, Y.K. Kumar, P.U. Bhaskar, and V.S. Raja: Effect ofpostdeposition annealing on the growth of Cu2ZnSnSe4 thin films for a solar cell absorber layer. Semicond. Sci. Technol. 23, 085023 (2008).

    Article  CAS  Google Scholar 

  12. 12.

    G.S. Babu, Y.K. Kumar, P.U. Bhaskar, and V.S. Raja: Effect of Cu/(Zn+Sn) ratio on the properties of coevaporated Cu2ZnSnSe4 thin films. Sol. Energy Mater. Sol. Cells 94, 221–226 (2010).

    Article  CAS  Google Scholar 

  13. 13.

    D.A. Barkhouse, O. Gunawan, T. Gokmen, T.K. Todorov, and D.B. Mitzi: Device characteristics of a 10.1% hydrazine-processed Cu2ZnSn(Se, S)4 solar cell. Prog. Photovoltaics Res. Appl. 20, 6–11 (2012).

    CAS  Article  Google Scholar 

  14. 14.

    S. Chen, X.G. Gong, A. Walsh, and S-H. Wei: Crystal and electronic band structure of Cu2ZnSnX4 (X = S and Se) photovoltaic absorbers: First-principles insights. Appl. Phys. Lett. 94, 041903 (2009).

    Article  CAS  Google Scholar 

  15. 15.

    J. Paier, R. Asahi, A. Nagoya, and G. Kresse: Cu2ZnSnS4 as a potential photovoltaic material: A hybrid Hartree-Fock density functional theory study. Phys. Rev. B 79, 115126 (2009).

    Article  CAS  Google Scholar 

  16. 16.

    M. Ichimura and Y. Nakashima: Analysis of atomic and electronic structures of Cu2ZnSnS4 based on first-principle calculation. Jpn. J. Appl. Phys. 48, 090202 (2009).

    Article  CAS  Google Scholar 

  17. 17.

    C. Persson: Electronic and optical properties of Cu2ZnSnS4 and Cu2ZnSnSe4. J. Appl. Phys. 107, 053710 (2010).

    Article  CAS  Google Scholar 

  18. 18.

    S. Chen, A. Walsh, J-H. Yang, X.G. Gong, L. Sun, P-X. Yang, J-H. Chu, and S-H. Wei: Compositional dependence of structural and electronic properties of Cu2ZnSn(S, Se)4 alloys for thin film solar cells. Phys. Rev. B 83, 125201 (2011).

    Article  CAS  Google Scholar 

  19. 19.

    G.H. Moh: Tin-containing mineral systems. Part II: Phase relations and mineral assemblages in the Cu-Fe-Zn-Sn-S system. Chem. Erde 34, 1–61 (1975).

    CAS  Google Scholar 

  20. 20.

    I.D. Olekseyuk, I.V. Dudchak, and L.V. Piskach: Phase equilibria in the Cu2S–ZnS–SnS2 system. J. Alloys Compd. 368, 135–143 (2004).

    CAS  Article  Google Scholar 

  21. 21.

    S. Schorr: The crystal structure of kesterite type compounds: A neutron and x-ray diffraction study. Sol. Energy Mater. Sol. Cells 95, 1482–1488 (2011).

    CAS  Article  Google Scholar 

  22. 22.

    H. Katagiri, N. Ishigaki, T. Ishida, and K. Saito: Characterization of Cu2ZnSnS4 thin films prepared by vapor phase sulfurization. Jpn. J. Appl. Phys. 40, 500–504 (2001).

    CAS  Article  Google Scholar 

  23. 23.

    P.A. Fernandes, P.M.P. Salomé, and A.F. da Cunha: Study of polycrystalline Cu2ZnSnS4 films by Raman scattering. J. Alloys Compd. 509, 7600–7606 (2011).

    CAS  Article  Google Scholar 

  24. 24.

    X. Fontané, L. Calvo-Barrio, V. Izquierdo-Roca, E. Saucedo, A. Pérez-Rodriguez, J.R. Morante, D.M. Berg, P.J. Dale, and S. Siebentritt: In-depth resolved Raman scattering analysis for the identification of secondary phases: Characterization of Cu2ZnSnS4 layers for solar cell applications. Appl. Phys. Lett. 98, 181905 (2011).

    Article  CAS  Google Scholar 

  25. 25.

    D. Abou-Ras, T. Kirchartz, and U. Rau: Advanced Characterization Techniques for Thin Film Solar Cells. (Wiley VCH Verlag GmbH & Co KGaA, Weinheim, Germany, 2011).

    Google Scholar 

  26. 26.

    M. Bär, B-A. Schubert, R.G. Wilks, B. Marsen, Y. Zhang, M. Blum, S. Krause, W. Yang, T. Unold, L. Weinhardt, C. Heske, and H-W. Schock: Identification of impurity phases in Cu2ZnSnS4 thin-film solar cell absorber material by soft x-ray absorption spectroscopy, in Compound Semiconductors for Energy Applications and Environmental Sustainability—2011, edited by L.D. Bell, F. Shahedipour-Sandvik, K.A. Jones, D. Schaadt, B.S. Simpkins, and M.A. Contreras (Mater. Res. Soc. Symp. Proc. 1324, Warrendale, PA, 2011); p. 91, DOI: 10.1557/opl.2011.842.

    Google Scholar 

  27. 27.

    M. Bär, B-A. Schubert, B. Marsen, S. Schorr, R.G. Wilks, L. Weinhardt, S. Pookpanratana, M. Blum, S. Krause, Y. Zhang, W. Yang, T. Unold, C. Heske, and H-W. Schock: Electronic structure of Cu2ZnSnS4 probed by soft x-ray emission and absorption spectroscopy. Phys. Rev. B 84, 035038 (2011).

    Article  CAS  Google Scholar 

  28. 28.

    M. Bär, B-A. Schubert, B. Marsen, S. Krause, S. Pookpanratana, T. Unold, L. Weinhardt, C. Heske, and H-W. Schock: Native oxidation and Cu-poor surface structure of thin film Cu2ZnSnS4 solar cell absorbers. Appl. Phys. Lett. 99, 112103 (2011).

    Article  CAS  Google Scholar 

  29. 29.

    J.J. Jia, T.A. Callcott, J. Yurkas, A.W. Ellis, F.J. Himpsel, M.G. Samant, J. Stoehr, D.L. Ederer, J.A. Carlisle, E.A. Hudson, L.J. Terminello, D.K. Shuh, and R.C.C. Perera: First experimental results from IBM/TENN/TULANE/LLNL/LBL undulator beamline at the advanced light source. Rev. Sci. Instrum. 66, 1394–1397 (1995).

    CAS  Article  Google Scholar 

  30. 30.

    B-A. Schubert, B. Marsen, S. Cinque, T. Unold, R. Klenk, S. Schorr, and H-W. Schock: Cu2ZnSnS4 thin film solar cells by fast coevaporation. Prog. Photovoltaics Res. Appl. 19, 93–96 (2011).

    CAS  Article  Google Scholar 

  31. 31.

    S. Schorr, A. Weber, V. Honkimäki, and H-W. Schock: In situ investigation of the kesterite formation from binary and ternary sulphides. Thin Solid Films 517, 2461 (2009).

    CAS  Article  Google Scholar 

  32. 32.

    A. Meisel, G. Leonhardt, and R. Szargan: X-Ray Spectra and Chemical Binding, Springer Series in Chemical Physics Vol. 37 (Springer, Berlin, 1989).

  33. 33.

    M. Shishkin and G. Kresse: Implementation and performance of the frequency-dependent GW method within the PAW framework. Phys. Rev. B 74, 035101 (2006).

    Article  CAS  Google Scholar 

  34. 34.

    M. Shishkin and G. Kresse: Self-consistent GW calculations for semiconductors and insulators. Phys. Rev. B 75, 235102 (2007).

    Article  CAS  Google Scholar 

  35. 35.

    F. Fuchs, J. Furthmüller, F. Bechstedt, M. Shishkin, and G. Kresse: Quasiparticle band structure based on a generalized Kohn-Sham scheme. Phys. Rev. B 76, 115109 (2007).

    Article  CAS  Google Scholar 

  36. 36.

    L. Hedin: New method for calculating the one-particle green’s function with application to the electron-gas problem. Phys. Rev. 139, A796–A823 (1965).

    Article  Google Scholar 

  37. 37.

    M. Bär, S. Nishiwaki, L. Weinhardt, S. Pookpanratana, O. Fuchs, M. Blum, W. Yang, J.D. Denlinger, W.N. Shafarman, and C. Heske: Depth-resolved band gap in Cu(In, Ga)(S, Se)2 thin films. Appl. Phys. Lett. 93, 244103 (2008).

    Article  CAS  Google Scholar 

  38. 38.

    M. Morkel, L. Weinhardt, B. Lohmüller, C. Heske, E. Umbach, W. Riedl, S. Zweigart, and F. Karg: Flat conduction-band alignment at the CdS/CuInSe2 thin-film solar-cell heterojunction. Appl. Phys. Lett. 79, 4482–4484 (2001).

    CAS  Article  Google Scholar 

  39. 39.

    L. Weinhardt, M. Morkel, Th. Gleim, S. Zweigart, F. Karg, C. Heske, and E. Umbach: Band alignment at the CdS/CuIn(S, Se)2 heterojunction in thin film solar cells, in Proceedings of the EPVSEC-17, Munich, Germany, October 22–26, 2001, p. 1261.

    Google Scholar 

  40. 40.

    NIST x-ray Photoelectron Spectroscopy Database, Version 3.5 (National Institute of Standards and Technology, Gaithersburg, 2003). (accessed August 15, 2011).

    Google Scholar 

  41. 41.

    E.M. Nanobashvili, T.G. Nemsadze, and A.S. Svanidze: Synthesis and properties thiostannates and thiostibites of zinc and cadmium. Soobs. Akad. Nauk. Gruz. SSR 63, 321–324 (1971).

    CAS  Google Scholar 

  42. 42.

    J.R. Tuttle, D.S. Albin, and R. Noufi: Thoughts on the microstructure of polycrystalline thin-film CuInSe2 and its impact on material and device performance. Sol. Cells 30, 21–38 (1991).

    CAS  Article  Google Scholar 

  43. 43.

    D. Schmid, M. Ruckh, F. Grunwald, and H-W. Schock: Chalcopyrite/defect chalcopyrite heterojunctions on the basis of CuInSe2. J. Appl. Phys. 73, 2902–2909 (1993).

    CAS  Article  Google Scholar 

  44. 44.

    A. Klein and W. Jaegermann: Fermi-level-dependent defect formation in Cu-chalcopyrite semiconductors. Appl. Phys. Lett. 74, 2283–2285 (1999).

    CAS  Article  Google Scholar 

  45. 45.

    H-W. Schock and U. Rau: The role of structural properties and defects for the performance of Cu-chalcopyrite-based thin-film solar cells. Physica B 308–, 1081–1085 (2001).

    Article  Google Scholar 

  46. 46.

    M. Turcu and U. Rau: Compositional trends of defect energies, band alignments, and recombination mechanisms in the Cu(In, Ga)(Se, S)2 alloy system. Thin Solid Films 431–, 158–162 (2003).

    Article  CAS  Google Scholar 

  47. 47.

    O. Gunawan, T.K. Todorov, and D.B. Mitzi: Loss mechanisms in hydrazine-processed Cu2ZnSn(Se, S)4 solar cells. Appl. Phys. Lett. 97, 233506 (2010).

    Article  CAS  Google Scholar 

  48. 48.

    K. Wang, O. Gunawan, T. Todorov, B. Shin, S.J. Chey, N.A. Bojarczuk, D. Mitzi, and S. Guha: Thermally evaporated Cu2ZnSnS4 solar cells. Appl. Phys. Lett. 97, 143508 (2010).

    Article  CAS  Google Scholar 

  49. 49.

    M. Turcu, O. Pakma, and U. Rau: Interdependence of absorber composition and recombination mechanism in Cu(In, Ga)(Se, S)2 heterojunction solar cells. Appl. Phys. Lett. 80, 2598–2600 (2002).

    CAS  Article  Google Scholar 

  50. 50.

    L. Weinhardt, O. Fuchs, M. Blum, M. Bär, M. Weigand, J. Denlinger, Y. Zubavichus, M. Zharnikov, M. Grunze, C. Heske, and E. Umbach: Resonant x-ray emission spectroscopy of liquid water: Novel instrumentation, high resolution, and the “map” approach. J. Electron Spectrosc. Relat. Phenom. 177, 206 (2010).

    CAS  Article  Google Scholar 

Download references


R.G. Wilks and M. Bär acknowledge the financial support by the Impuls- und Vernetzungsfonds of the Helmholtz-Association (VH-NG-423). The ALS is funded by the Department of Energy, Basic Energy Sciences, Contract No. DE-AC02-05CH11231. Furthermore, the authors thank J. Paier and G. Kresse for making their calculated DOS data15 available in electronic form.

Author information



Corresponding author

Correspondence to M. Bär.

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Bär, M., Schubert, BA., Marsen, B. et al. Cu2ZnSnS4 thin-film solar cell absorbers illuminated by soft x-rays. Journal of Materials Research 27, 1097–1104 (2012).

Download citation