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.

  • Article
  • Published:

Charge-carrier-concentration inhomogeneities in alkali-treated Cu(In,Ga)Se2 revealed by conductive atomic force microscopy tomography


Photovoltaic power conversion using polycrystalline light-absorbing semiconductors enables low-cost electricity generation. Cu(In,Ga)Se2 (CIGS) are among the best performing thin-film solar cells with notable recent improvements upon an alkali-fluoride (AlkF) post-deposition treatment (PDT). Here we show that the success of this treatment can be hampered by spatial inhomogeneities in the conductivity. We apply an emerging conductive atomic force microscopy (C-AFM) tomography technique and obtain three-dimensional conductivity maps, enabling imaging of the carrier concentration grain by grain on the submicrometre scale. We find that a solar cell with KF PDT shows a stronger inhomogeneity of charge-carrier concentration, while RbF and CsF lead to narrow distributions at higher charge-carrier concentrations. The CIGS charge-carrier concentration and its homogeneity influence directly the open-circuit voltage of solar cells, thereby impacting device performance. Our insights support the development of higher efficiency thin-film photovoltaics through optimized AlkF PDTs. Moreover, the C-AFM tomography method is widely applicable to energy materials.

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

Access options

Buy this article

Prices may be subject to local taxes which are calculated during checkout

Fig. 1: C-AFM tomography on non-PDT CIGS.
Fig. 2: Analysing currents from C-AFM tomography on non-PDT CIGS.
Fig. 3: Model for C-AFM tomography.
Fig. 4: Mapping conductivities and charge-carrier concentrations on AlkF PDT CIGS.
Fig. 5: Statistical analysis of conductivity and charge-carrier concentrations.
Fig. 6: Impact on solar cell performance.

Similar content being viewed by others

Data availability

Source data are provided with this paper. All other datasets generated and analysed in the present study are available from the corresponding author upon reasonable request.


  1. Green, M. A. et al. Solar cell efficiency tables (version 59). Prog. Photovoltaics Res. Appl. 30, 3–12 (2022).

    Article  Google Scholar 

  2. Zhang, L. et al. Effect of copassivation of Cl and Cu on CdTe grain boundaries. Phys. Rev. Lett. 101, 155501 (2008).

    Article  ADS  PubMed  Google Scholar 

  3. Poplawsky, J. D. et al. Direct imaging of Cl- and Cu-induced short-circuit efficiency changes in CdTe solar cells. Adv. Energy Mater. 4, 1400454 (2014).

    Article  Google Scholar 

  4. Alberi, K. et al. Measuring long-range carrier diffusion across multiple grains in polycrystalline semiconductors by photoluminescence imaging. Nat. Commun. 4, 2699 (2013).

    Article  ADS  CAS  PubMed  Google Scholar 

  5. Shao, Y., Xiao, Z., Bi, C., Yuan, Y. & Huang, J. Origin and elimination of photocurrent hysteresis by fullerene passivation in CH3NH3PbI3 planar heterojunction solar cells. Nat. Commun. 5, 5784 (2014).

    Article  ADS  CAS  PubMed  Google Scholar 

  6. Zhang, W., Eperon, G. E. & Snaith, H. J. Metal halide perovskites for energy applications. Nat. Energy 1, 16048 (2016).

    Article  ADS  CAS  Google Scholar 

  7. DeQuilettes, D. W. et al. Impact of microstructure on local carrier lifetime in perovskite solar cells. Science 348, 683–686 (2015).

    Article  ADS  CAS  PubMed  Google Scholar 

  8. Zuo, L. et al. Polymer-modified halide perovskite films for efficient and stable planar heterojunction solar cells. Sci. Adv. 3, e1700106 (2017).

    Article  ADS  PubMed  PubMed Central  Google Scholar 

  9. Nicoara, N. et al. Direct evidence for grain boundary passivation in Cu(In,Ga)Se2 solar cells through alkali-fluoride post-deposition treatments. Nat. Commun. 10, 3980 (2019).

    Article  ADS  PubMed  PubMed Central  Google Scholar 

  10. Siebentritt, S. et al. Heavy alkali treatment of Cu(In,Ga)Se2 solar cells: surface versus bulk effects. Adv. Energy Mater. 10, 1903752 (2020).

    Article  CAS  Google Scholar 

  11. Cojocaru-Mirédin, O., Raghuwanshi, M., Wuerz, R. & Sadewasser, S. Grain boundaries in Cu(In,Ga)Se2: a review on composition–electronic property relationships by atom probe tomography and correlative microscopy. Adv. Funct. Mater. 31, 2103119 (2021).

    Article  Google Scholar 

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

    Article  ADS  PubMed  Google Scholar 

  13. Jackson, P., Hariskos, D., Wuerz, R., Wischmann, W. & Powalla, M. Compositional investigation of potassium doped Cu(In,Ga)Se2 solar cells with efficiencies up to 20.8%. Phys. Status Solidi Rapid Res. Lett. 8, 219–222 (2014).

    Article  ADS  CAS  Google Scholar 

  14. Ochoa, M., Yang, S.-C., Nishiwaki, S., Tiwari, A. N. & Carron, R. Charge carrier lifetime fluctuations and performance evaluation of Cu(In,Ga)Se2 absorbers via time-resolved-photoluminescence microscopy. Adv. Energy Mater. 12, 2102800 (2022).

    Article  CAS  Google Scholar 

  15. Pianezzi, F. et al. Unveiling the effects of post-deposition treatment with different alkaline elements on the electronic properties of CIGS thin-film solar cells. Phys. Chem. Chem. Phys. 16, 8843–8851 (2014).

    Article  CAS  PubMed  Google Scholar 

  16. Lin, T.-Y. et al. Alkali-induced grain boundary reconstruction on Cu(In,Ga)Se2 thin-film solar cells using cesium fluoride post-deposition treatment. Nano Energy 69, 104299 (2020).

    Article  Google Scholar 

  17. Laemmle, A., Wuerz, R. & Powalla, M. Efficiency enhancement of Cu(In,Ga)Se2 thin-film solar cells by a post-deposition treatment with potassium fluoride. Phys. Status Solidi Rapid Res. Lett. 7, 631–634 (2013).

    Article  ADS  CAS  Google Scholar 

  18. Laemmle, A. et al. Investigation of the diffusion behavior of sodium in Cu(In,Ga)Se2 layers. J. Appl. Phys. 115, 154501 (2014).

    Article  ADS  Google Scholar 

  19. Vilalta-Clemente, A. et al. Rubidium distribution at atomic scale in high efficient Cu(In,Ga)Se2 thin-film solar cells. Appl. Phys. Lett. 112, 103105 (2018).

    Article  ADS  Google Scholar 

  20. Schöppe, P. et al. Overall distribution of rubidium in highly efficient Cu(In,Ga)Se2 solar cells. ACS Appl. Mater. Interfaces 10, 40592–40598 (2018).

    Article  PubMed  Google Scholar 

  21. Stokes, A., Al-Jassim, M., Diercks, D., Clarke, A. & Gorman, B. Impact of wide-ranging nanoscale chemistry on band structure at Cu(In,Ga)Se2 grain boundaries. Sci. Rep. 7, 14163 (2017).

    Article  ADS  PubMed  PubMed Central  Google Scholar 

  22. Raghuwanshi, M. et al. Influence of RbF post-deposition treatment on heterojunction and grain boundaries in high efficient (21.1%) Cu(In,Ga)Se2 solar cells. Nano Energy 60, 103–110 (2019).

    Article  CAS  Google Scholar 

  23. Sadewasser, S. & Glatzel, T. (eds) Kelvin Probe Force MicroscopyMeasuring and Compensating Electrostatic Forces (Springer, 2012).

  24. Hui, F. & Lanza, M. Scanning probe microscopy for advanced nanoelectronics. Nat. Electron. 2, 221–229 (2019).

    Article  Google Scholar 

  25. Celano, U. et al. Mesoscopic physical removal of material using sliding nanodiamond contacts. Sci. Rep. 8, 2994 (2018).

    Article  ADS  PubMed  PubMed Central  Google Scholar 

  26. Luria, F. et al. Charge transport in CdTe solar cells revealed by conductive tomographic atomic force microscopy. Nat. Energy 1, 16150 (2016).

    Article  ADS  CAS  Google Scholar 

  27. Song, J., Zhou, Y., Padture, N. P. & Huey, B. D. Anomalous 3D nanoscale photoconduction in hybrid perovskite semiconductors revealed by tomographic atomic force microscopy. Nat. Commun. 11, 3308 (2020).

    Article  ADS  CAS  PubMed  PubMed Central  Google Scholar 

  28. Chintala, R. C. et al. Nanoscale 3D characterisation of soft organic material using conductive scanning probe tomography. AIP Adv. 9, 025105 (2019).

    Article  ADS  Google Scholar 

  29. Hertz, H. On the contact of elastic solids. J. Reine Angew. Math. 92, 156–171 (1881).

    Google Scholar 

  30. Johnson, K. L., Kendall, K. & Roberts, A. D. Surface energy and the contact of elastic solids. Proc. R. Soc. Lond. A 324, 301–313 (1971).

    Article  ADS  CAS  Google Scholar 

  31. Derjaguin, B. V., Muller, V. M. & Toporov, Y. P. Effect of contact deformations on the adhesion of particles. J. Colloid Interface Sci. 53, 314–326 (1975).

    Article  ADS  CAS  Google Scholar 

  32. Adams G. G. in Encyclopedia of Tribology (eds Wang, Q. J. & Chung, Y. W.) 3560–3565 (Springer, 2013).

  33. Nanosensors. NanoAndMore USA (2022).

  34. Clarysse, T., Vanhaeren, D., Hoflijk, I. & Vandervorst, W. Characterization of electrically active dopant profiles with the spreading resistance probe. Mater. Sci. Eng. R 47, 123–206 (2004).

    Article  Google Scholar 

  35. Jensen, S. A. et al. Beneficial effect of post-deposition treatment in high-efficiency Cu(In,Ga)Se2 solar cells through reduced potential fluctuations. J. Appl. Phys. 120, 063106 (2016).

    Article  ADS  Google Scholar 

  36. Khatri, I., Fukai, H., Yamaguchi, H., Sugiyama, M. & Nakada, T. Effect of potassium fluoride post-deposition treatment on Cu(In,Ga)Se2 thin films and solar cells fabricated onto sodalime glass substrates. Sol. Energy Mater. Sol. Cells 155, 280–287 (2016).

    Article  CAS  Google Scholar 

  37. Karki, S. et al. Analysis of recombination mechanisms in RbF treated CIGS solar cells. IEEE J. Photovoltaics 9, 313–318 (2019).

    Article  Google Scholar 

  38. Kodalle, T. et al. Elucidating the mechanism of an RbF post-deposition treatment in CIGS thin-film solar cells. Sol. RRL 2, 1800156 (2018).

    Article  Google Scholar 

  39. Ishizuka, S. & Fons, P. J. Role of the Cu-deficient interface in Cu(In,Ga)Se2 thin-film photovoltaics with alkali-metal doping. Phys. Rev. Appl. 15, 054005 (2021).

    Article  ADS  CAS  Google Scholar 

  40. Lin, T.-Y. et al. Alkali-induced grain boundary reconstruction on Cu(In,Ga)Se2 thin-film solar cells using cesium fluoride post-deposition treatment. Nano Energy 68, 104299 (2020).

    Article  CAS  Google Scholar 

  41. Burgelman, M., Nollet, P. & Degrave, S. Modelling polycrystalline semiconductor solar cells. Thin Solid Films 361362, 527–532 (2000).

    Article  ADS  Google Scholar 

  42. Jackson, P. et al. Effects of heavy alkali elements in Cu(In,Ga)Se2 solar cells with efficiencies up to 22.6%. Phys. Status Solidi Rapid Res. Lett. 10, 583–586 (2016).

    Article  ADS  CAS  Google Scholar 

  43. Jackson, P. et al. Properties of Cu(In,Ga)Se2 solar cells with new record efficiencies up to 21.7%. Phys. Status Solidi Rapid Res. Lett. 9, 28–31 (2015).

    Article  ADS  CAS  Google Scholar 

  44. Nakamura, M. et al. Cd-free Cu(In,Ga)(Se,S)2 thin-film solar cell with record efficiency of 23.35%. IEEE J. Photovoltaics 9, 1863–1867 (2019).

    Article  Google Scholar 

  45. Sozzi, G. et al. Influence of conduction band offsets at window/buffer and buffer/absorber interfaces on the roll-over of JV curves of CIGS solar cells. In 44th Photovoltaic Specialist Conference 2205–2208 (IEEE, 2017).

  46. Frisk, C. et al. Optimizing Ga-profiles for highly efficient Cu(In, Ga)Se2 thin-film solar cells in simple and complex defect models. J. Phys. D 47, 485104 (2014).

    Article  Google Scholar 

  47. Spear, K. E. & Dismukes, J. P. (eds) Synthetic Diamond: Emerging CVD Science and Technology Vol. 25 (John Wiley & Sons, 1994).

  48. Buzás, A. & Geretovszky, Z. Nanosecond laser-induced selective removal of the active layer of CuInGaSe2 solar cells by stress-assisted ablation. Phys. Rev. B 85, 245304 (2012).

    Article  ADS  Google Scholar 

  49. Gerthoffer, A. et al. CIGS solar cells on ultra-thin glass substrates: determination of mechanical properties by nanoindentation and application to bending-induced strain calculation. Sol. Energy Mater. Sol. Cells 166, 254–261 (2017).

    Article  CAS  Google Scholar 

Download references


The CIGS samples for this work were prepared as part of the project Sharc25, funded through the European Union’s Horizon 2020 Research and Innovation programme under grant agreement number 641004 (N.N., P.J., W.W., D.H., S.S.). E. Bertin is acknowledged for support in the initial phases of the development of the C-AFM technique on CIGS samples. We thank I. Khatri, D. Colombara and G. Bacher for helpful discussions.

Author information

Authors and Affiliations



S.S. and N.N. conceived the study. D.S., N.N. and S.S. developed the experimental methodology and data analysis. D.S. performed the experiments. D.S., N.N. and S.S. performed the data analysis. P.J., W.W. and D.H. prepared the samples. D.S. and S.S. wrote the paper. All authors discussed the results and revised the paper.

Corresponding author

Correspondence to Sascha Sadewasser.

Ethics declarations

Competing interests

The authors declare no competing interests.

Peer review

Peer review information

Nature Energy thanks Bryan Huey and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Additional information

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Supplementary Information

Supplementary Figs. 1–13, Discussions 1 and 2, and Tables 1–5.

Reporting Summary

Supplementary Video 1

Top view of current maps through RbF PDT CIGS sample from top to bottom of C-AFM tomography experiment.

Supplementary Video 2

Video of vertical slice through the 3D C-AFM current volume for the RbF PDT CIGS sample, illustrating the rich information obtained from C-AFM tomography experiments.

Supplementary Data 1

Source data for Supplementary Fig. 5.

Supplementary Data 2

Source data for Supplementary Fig. 8.

Supplementary Data 3

Source data for Supplementary Fig. 9.

Supplementary Data 4

Source data for Supplementary Fig. 10.

Supplementary Data 5

Source data for Supplementary Fig. 11.

Supplementary Data 6

Source data for Supplementary Fig. 12.

Source data

Source Data Fig. 2

Source data for grains 1–4 and data for fit curves shown in panel b.

Source Data Fig. 5

Statistical source data.

Source Data Fig. 6

Statistical source data.

Rights and permissions

Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Sharma, D., Nicoara, N., Jackson, P. et al. Charge-carrier-concentration inhomogeneities in alkali-treated Cu(In,Ga)Se2 revealed by conductive atomic force microscopy tomography. Nat Energy 9, 163–171 (2024).

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI:

This article is cited by


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