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


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.

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

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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

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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.