Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Potassium-induced surface modification of Cu(In,Ga)Se2 thin films for high-efficiency solar cells


Thin-film photovoltaic devices based on chalcopyrite Cu(In,Ga)Se2 (CIGS) absorber layers show excellent light-to-power conversion efficiencies exceeding 20% (refs 1, 2). This high performance level requires a small amount of alkaline metals incorporated into the CIGS layer, naturally provided by soda lime glass substrates used for processing of champion devices3. The use of flexible substrates requires distinct incorporation of the alkaline metals, and so far mainly Na was believed to be the most favourable element, whereas other alkaline metals have resulted in significantly inferior device performance4,5. Here we present a new sequential post-deposition treatment of the CIGS layer with sodium and potassium fluoride that enables fabrication of flexible photovoltaic devices with a remarkable conversion efficiency due to modified interface properties and mitigation of optical losses in the CdS buffer layer. The described treatment leads to a significant depletion of Cu and Ga concentrations in the CIGS near-surface region and enables a significant thickness reduction of the CdS buffer layer without the commonly observed losses in photovoltaic parameters6. Ion exchange processes, well known in other research areas7,8,9,10,11,12,13, are proposed as underlying mechanisms responsible for the changes in chemical composition of the deposited CIGS layer and interface properties of the heterojunction.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Figure 1: Elemental depth profiling.
Figure 2: Surface chemical analysis.
Figure 3: Variation of CdS film thickness.
Figure 4: Characteristics of the best device.


  1. 1

    Green, M. A., Emery, K., Hishikawa, Y., Warta, W. & Dunlop, E. D. Solar cell efficiency tables (version 41). Prog. Photovolt. 21, 1–11 (2013).

    Article  Google Scholar 

  2. 2

    Jackson, P. et al. New world record efficiency for Cu(In,Ga) Se2 thin-film solar cells beyond 20%. Prog. Photovolt. 19, 894–897 (2011).

    CAS  Article  Google Scholar 

  3. 3

    Hedstrom, J. et al. 23rd IEEE Phot. Spec. Conf. (PVSC) 364–371 (IEEE, 1993).

    Google Scholar 

  4. 4

    Contreras, M. A. et al. 26th IEEE Phot. Spec. Conf. (PVSC) 359–362 (IEEE, 1997).

    Google Scholar 

  5. 5

    Bodegård, M., Granath, K. & Stolt, L. Growth of Cu(In,Ga) Se2 thin films by coevaporation using alkaline precursors. Thin Solid Films 361, 9–16 (2000).

    Article  Google Scholar 

  6. 6

    Contreras, M. A. et al. Optimization of CBD CdS process in high-efficiency Cu(In,Ga) Se2-based solar cells. Thin Solid Films 403–404, 204–211 (2002).

    Article  Google Scholar 

  7. 7

    Jenny, H. Studies on the mechanism of ionic exchange in colloidal aluminium silicates. J. Phys. Chem. 36, 2217–2258 (1932).

    CAS  Article  Google Scholar 

  8. 8

    Clearfield, A. Role of ion exchange in solid-state chemistry. Chem. Rev. 88, 125–148 (1988).

    CAS  Article  Google Scholar 

  9. 9

    Rivest, J. B. & Jain, P. K. Cation exchange on the nanoscale: An emerging technique for new materials synthesis, device fabrication, and chemical sensing. Chem. Soc. Rev. 42, 89–96 (2013).

    CAS  Article  Google Scholar 

  10. 10

    Nordberg, M. E. et al. Strengthening by ion exchange. J. Am. Ceram. Soc. 47, 215–219 (1964).

    CAS  Article  Google Scholar 

  11. 11

    Zou, J. et al. Two-step K+–Na+ and Ag+–Na+ ion-exchanged glass waveguides for C-band applications. Appl. Opt. 41, 7620–7626 (2002).

    CAS  Article  Google Scholar 

  12. 12

    Das, S. R. et al. The preparation of Cu2S films for solar cells. Thin Solid Films 51, 257–264 (1978).

    CAS  Article  Google Scholar 

  13. 13

    Ramanathan, K. et al. Proc. 2nd World Conf. on Photovoltaic Solar Energy Conversion 477–481 (Joint Research Centre, European Commission, 1998).

  14. 14

    Wolden, C. A. et al. Photovoltaic manufacturing: Present status, future prospects, and research needs. J. Vac. Sci. Technol. A 29, 030801 (2011).

    Article  Google Scholar 

  15. 15

    Chopra, K. L., Paulson, P. D. & Dutta, V. Thin-film solar cells: An overview. Prog. Photovolt. 12, 69–92 (2004).

    CAS  Article  Google Scholar 

  16. 16

    Grätzel, M. Photoelectrochemical cells. Nature 414, 338–344 (2001).

    Article  Google Scholar 

  17. 17

    Bernede, J. C. Organic photovoltaic cells: History, principle and techniques. J. Chil. Chem. Soc. 53, 1549–1564 (2008).

    CAS  Article  Google Scholar 

  18. 18

    Reinhard, P. et al. Review of progress toward 20% efficiency flexible CIGS solar cells and manufacturing issues of solar modules. IEEE J. Photovolt. 3, 572–580 (2013).

    Article  Google Scholar 

  19. 19

    Kessler, F. & Rudmann, D. Technological aspects of flexible CIGS solar cells and modules. Sol. Energ. 77, 685–695 (2004).

    CAS  Article  Google Scholar 

  20. 20

    Chirilă, A. et al. Highly efficient Cu(In,Ga) Se2 solar cells grown on flexible polymer films. Nature Mater. 10, 857–861 (2011).

    Article  Google Scholar 

  21. 21

    Rudmann, D. et al. Sodium incorporation strategies for CIGS growth at different temperatures. Thin Solid Films 480, 55–60 (2005).

    Article  Google Scholar 

  22. 22

    Jackson, P. et al. High quality baseline for high efficiency, Cu(In1−x,Gax)Se2 solar cells. Prog. Photovolt. 15, 507–519 (2007).

    CAS  Article  Google Scholar 

  23. 23

    Ishizuka, S., Yamada, A., Fons, P. & Niki, S. Flexible Cu(In,Ga) Se2 solar cells fabricated using alkali-silicate glass thin layers as an alkali source material. J. Renew. Sustain. Energ. 1, 013102 (2009).

    Article  Google Scholar 

  24. 24

    Cojocaru-Mirédin, O. et al. Characterisation of grain boundaries in Cu(In,Ga) Se2 films using atom-probe tomography. IEEE J. Photovolt. 1, 207–212 (2011).

    Article  Google Scholar 

  25. 25

    Abou-Ras, D. et al. Confined and chemically flexible grain boundaries in polycrystalline compound semiconductors. Adv. Energy. Mater. 2, 992–998 (2012).

    CAS  Article  Google Scholar 

  26. 26

    Wuerz, R. et al. CIGS thin-film solar cells and modules on enamelled steel substrates. Sol. Energ. Mater. Sol. Cells 100, 132–137 (2012).

    CAS  Article  Google Scholar 

  27. 27

    Klein, A. et al. Fermi level-dependent defect formation at Cu(In,Ga) Se2 interfaces. Appl. Surf. Sci. 166, 508–512 (2000).

    CAS  Article  Google Scholar 

  28. 28

    Rau, U. et al. Oxygenation and air-annealing effects on the electronic properties of Cu(In,Ga) Se2 films and devices. J. Appl. Phys. 86, 497–505 (1999).

    CAS  Article  Google Scholar 

  29. 29

    Nakada, T. & Kunioka, A. Direct evidence of Cd diffusion into Cu(In,Ga) Se2 thin films during chemical-bath deposition process of CdS films. Appl. Phys. Lett. 74, 2444–2446 (1999).

    CAS  Article  Google Scholar 

  30. 30

    Kiss, J. et al. Theoretical study on the structure and energetics of Cd insertion and Cu depletion of CuIn5Se8 . J. Phys Chem. C 117, 10892–10900 (2013).

    CAS  Article  Google Scholar 

Download references


This work was supported by the Swiss National Science Foundation and the Swiss Federal Office of Energy. The laboratory for Nanoscale Materials Science and the Laboratory for Electronics/Metrology/Reliability at Empa are acknowledged for SIMS and scanning and transmission electron microscopy measurements, respectively.

Author information




A.C., P.R., F.P., P.B., S.B., S.N. and A.N.T. designed the research and experiments. A.C., P.R. and P.B. fabricated the solar cells. A.C., P.R., F.P., S.B., A.R.U., C.F., C.G., H.H., D.J., L.K., D.K., R.E. and A.N.T. performed the characterization and analysis. A.C., P.R., F.P., S.B. and A.N.T. wrote the paper. All authors contributed to discussions.

Corresponding author

Correspondence to Adrian Chirilă.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Supplementary information

Supplementary Information

Supplementary Information (PDF 892 kb)

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Chirilă, A., Reinhard, P., Pianezzi, F. et al. Potassium-induced surface modification of Cu(In,Ga)Se2 thin films for high-efficiency solar cells. Nature Mater 12, 1107–1111 (2013).

Download citation

Further reading


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing