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

Nature Energy volume 9, pages 163–171 (2024)Cite this article

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Abstract

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.

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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.

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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.

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Acknowledgements

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.

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Authors and Affiliations

  1. INL–International Iberian Nanotechnology Laboratory, Braga, Portugal

    Deepanjan Sharma, Nicoleta Nicoara & Sascha Sadewasser

  2. Werkstoffe der Elektrotechnik, Universität Duisburg-Essen, Duisburg, Germany

    Deepanjan Sharma

  3. Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW), Stuttgart, Germany

    Philip Jackson, Wolfram Witte & Dimitrios Hariskos

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Contributions

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.

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Correspondence to Sascha Sadewasser.

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

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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.

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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). https://doi.org/10.1038/s41560-023-01420-7

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