Article | Published:

Understanding the role of selenium in defect passivation for highly efficient selenium-alloyed cadmium telluride solar cells

Abstract

Electricity produced by cadmium telluride (CdTe) photovoltaic modules is the lowest-cost electricity in the solar industry, and now undercuts fossil fuel-based sources in many regions of the world. This is due to recent efficiency gains brought about by alloying selenium into the CdTe absorber, which has taken cell efficiency from 19.5% to its current record of 22.1%. Although the addition of selenium is known to reduce the bandgap of the absorber material, and hence increase the cell short-circuit current, this effect alone does not explain the performance improvement. Here, by means of cathodoluminescence and secondary ion mass spectrometry, we show that selenium enables higher luminescence efficiency and longer diffusion lengths in the alloyed material, indicating that selenium passivates critical defects in the bulk of the absorber layer. This passivation effect explains the record-breaking performance of selenium-alloyed CdTe devices, and provides a route for further efficiency improvement that can result in even lower costs for solar-generated electricity.

Access optionsAccess options

Rent or Buy article

Get time limited or full article access on ReadCube.

from$8.99

All prices are NET prices.

Data availability

The data that support the plots within the paper and other findings of this study are available in the repository at Loughborough University (https://repository.lboro.ac.uk/) or from the corresponding author on reasonable request.

Additional information

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

References

  1. 1.

    Gloeckler, M., Sankin, I. & Zhao, Z. CdTe solar cells at the threshold to 20% efficiency. IEEE J. Photovolt. 3, 1389–1393 (2013).

  2. 2.

    Green, M. A. et al. Solar cell efficiency tables (version 51). Prog. Photovolt. Res. Appl. 26, 3–12 (2018).

  3. 3.

    Lazard’s Levelized Cost of Energy Analysis—Version 11.0 (Lazard, 2017).

  4. 4.

    Balderelli, J. Public Utilities Commission of Nevada Electronic Filing (2015).

  5. 5.

    Hagenorf, C. et al. Assessment of Performance, Environmental, Health and Safety Aspects of First Solar’s CdTe Photovoltaic Technology (Cener, 2016).

  6. 6.

    Paudel, N. R. & Yan, Y. Enhancing the photo-currents of CdTe thin-film solar cells in both short and long wavelength regions. Appl. Phys. Lett. 105, 183510 (2014).

  7. 7.

    Swanson, D. E., Sites, J. R. & Sampath, W. S. Co-sublimation of CdSexTe1− x layers for CdTe solar cells. Sol. Ener. Mater. Sol. Cells 159, 389–394 (2017).

  8. 8.

    Poplawsky, J. D. et al. Structural and compositional dependence of the CdTexSe1−x alloy layer photoactivity in CdTe-based solar cells. Nat. Commun. 7, 12537 (2016).

  9. 9.

    Munshi, A. H. et al. Polycrystalline CdSeTe/CdTe absorber cells with 28 mA/cm2 short-circuit current. IEEE J. Photovolt. 8, 310–314 (2018).

  10. 10.

    Kephart, J. M. et al. Sputter-deposited oxides for interface passivation of CdTe photovoltaics. IEEE J. Photovoltaics 8, 587–593 (2018).

  11. 11.

    Kuciauskas, D. et al. Recombination velocity less than 100 cm/s at polycrystalline Al2O3/CdSeTe interfaces. Appl. Phys. Lett. 112, 263901 (2018).

  12. 12.

    Dharmadasa, I. Review of the CdCl2 treatment used in CdS/CdTe thin film solar cell development and new evidence towards improved understanding. Coatings 4, 282–307 (2014).

  13. 13.

    Barnard, E. S. et al. 3D lifetime tomography reveals how CdCl2 improves recombination throughout CdTe solar cells. Adv. Mater. 29, 1603801 (2017).

  14. 14.

    Fiducia, T. A. M. et al. 3D distributions of chlorine and sulphur impurities in a thin-film cadmium telluride solar cell. MRS Adv. 3, 3287–3292 (2018).

  15. 15.

    Abbas, A. et al. The effect of cadmium chloride treatment on close-spaced sublimated cadmium telluride thin-film solar cells. IEEE J. Photovolt. 3, 1361–1366 (2013).

  16. 16.

    Munshi, A. H. Polycrystalline CdTe photovolaics with efficiency over 18% through improved absorber passivation and current collection. Sol. Ener. Mater. Sol. Cells 176, 9–18 (2018).

  17. 17.

    Munshi, A. H. et al. Effect of CdCl2 passivation treatment on microstructure and performance of CdSeTe/CdTe thin-film photovoltaic devices. Sol. Ener. Mater. Sol. Cells 186, 259–265 (2018).

  18. 18.

    Harrison, L. G. Influence of dislocations on diffusion kinetics in solids with particular reference to the alkali halides. Trans. Faraday Soc. 57, 1191–1199 (1961).

  19. 19.

    Moutinho, H. R. et al. Grain boundary character and recombination properties in CdTe thin films. In 2013 IEEE 39th Photovoltaic Specialists Conference 3249–3254 (IEEE, 2013).

  20. 20.

    Moseley, J. et al. Cathodoluminescence analysis of grain boundaries and grain interiors in thin-film CdTe. IEEE J. Photovolt. 4, 1671–1679 (2014).

  21. 21.

    Moseley, J. et al. Recombination by grain-boundary type in CdTe. J. Appl. Phys. 118, 025702 (2015).

  22. 22.

    Stechmann, G. et al. A correlative investigation of grain boundary crystallography and electronic properties in CdTe thin film solar cells. Sol. Ener. Mater. Sol. Cells 166, 108–120 (2017).

  23. 23.

    Lane, D. W. A review of the optical band gap of thin film CdSxTe1−x. Sol. Ener. Mater. Sol. Cells 90, 1169–1175 (2006).

  24. 24.

    Mendis, B. G., Bowen, L. & Jiang, Q. Z. A contactless method for measuring the recombination velocity of an individual grain boundary in thin-film photovoltaics. Appl. Phys. Lett. 97, 092112 (2010).

  25. 25.

    Mendis, B. & Bowen, L. Cathodoluminescence measurement of grain boundary recombination velocity in vapour grown p-CdTe. J. Phys. Conf. Ser. 326, 012017 (2011).

  26. 26.

    Lee, J., Giles, N. C., Rajavel, D. & Summers, C. J. Room-temperature band-edge photoluminescence from cadmium telluride. Phys. Rev. B 49, 1668–1676 (1994).

  27. 27.

    Yang, J.-H., Yin, W.-J., Park, J.-S., Ma, J. & Wei, S.-H. Review on first-principles study of defect properties of CdTe as a solar cell absorber. Semicond. Sci. Technol. 31, 83002 (2016).

  28. 28.

    Yang, J. H., Shi, L., Wang, L. W. & Wei, S. H. Non-radiative carrier recombination enhanced by two-level process: a first-principles study. Sci. Rep. 6, 21712 (2016).

  29. 29.

    Abbas, A. The Microstructure of Thin Film Cadmium Telluride Photovoltaic Materials. PhD thesis, Loughborough Univ. (2014).

Download references

Acknowledgements

The authors at Loughborough University are grateful to the EPSRC CDT in New and Sustainable Photovoltaics for providing T.F. with a studentship, RCUK for providing funding through the EPSRC SUPERGEN SuperSolar Hub (EP/J017361/1), and the Loughborough Materials Characterisation Centre for use of equipment. The authors at Colorado State University acknowledge support from NSF AIR, NSF I/UCRC and DOE SIPS programmes. The work at Colorado State University was supported by NSF award 1540007, NSF PFI:AIR-RA programme 1538733 and DOE SIPS award DE-EE0008177. K.L. acknowledges support from EPSRC grant M018237/1.

Author information

T.A.M.F. conceived this study and planned it along with J.M.W., K.L., C.R.M.G. and B.G.M. A.H.M. made the cells with assistance from K.B. and W.S.S. A.A. performed the TEM. T.A.M.F. prepared the samples for CL and SIMS characterization. B.G.M. carried out the CL measurements. K.L. performed the SIMS characterization with assistance from C.R.M.G. T.A.M.F. and L.D.W. performed the data analysis. T.A.M.F. wrote the manuscript with help from J.W.B. and J.M.W.

Competing interests

The authors declare no competing interests.

Correspondence to John M. Walls.

Supplementary information

  1. Supplementary Information

    Supplementary Figs. 1–4

Rights and permissions

To obtain permission to re-use content from this article visit RightsLink.

About this article

Fig. 1: CdTe luminescence efficiency improved by selenium alloying.
Fig. 2: Hyperspectral CL imaging shows that selenium is associated with a sub-bandgap emission peak.
Fig. 3: CdTe diffusion lengths improved by selenium alloying.
Fig. 4: Mapping selenium-induced bandgap changes in the absorber.