Exceeding 20% efficiency with in situ group V doping in polycrystalline CdTe solar cells

Abstract

CdTe-based solar technology has achieved one of the lowest levelized costs of electricity among all energy sources as well as state-of-the-art field stability. Yet, there is still ample headroom to improve. For decades, mainstream technology has combined fast CdTe deposition with a CdCl2 anneal and Cu doping. The resulting defect chemistry is strongly compensated and limits the useful hole density to ~1014 cm−3, creating a ceiling for fill factor, photovoltage and efficiency. In addition, Cu easily changes energy states and diffuses spatially, creating a risk of instabilities that must be managed with care. Here, we demonstrate a significant shift by doping polycrystalline CdSexTe1 − x and CdTe films with As while removing Cu entirely from the solar cell. The absorber majority-carrier density is increased by orders of magnitude to 1016–1017 cm−3 without compromising the lifetime, and is coupled with a high photocurrent greater than 30 mA cm−2. We demonstrate pathways for fast dopant incorporation in polycrystalline thin films, improved stability and 20.8% solar cell efficiency.

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Fig. 1: Schematic of a state-of-the-art CdTe solar cell.
Fig. 2: Diffusion versus in situ doping of thin polycrystalline solar cells.
Fig. 3: As-doped Cu-free solar cell with 20.8% efficiency.
Fig. 4: Absorber hole densities and lifetime.
Fig. 5: The effect of As on GB potentials and recombination.
Fig. 6: Optoelectronic properties of the front interface and absorber.

Data availability

The data that support the plots within this article are available from the authors upon reasonable request.

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Acknowledgements

The authors thank P. Dippo for low-temperature photoluminescence measurements. This work was authored in part by the National Renewable Energy Laboratory, operated by the Alliance for Sustainable Energy, LLC, for the US Department of Energy under contract no. DE-AC36- 08GO28308. Funding was provided by the US Department of Energy Office of Energy Efficiency and Renewable Energy Solar Energy Technologies Office and CRD-13-507. The views expressed do not necessarily represent the views of the US Department of Energy or the US Government.

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S.G., E.C., X.L., R. Mallick, W.Z., R. Malik, J.K. and D.S.A. performed film and device synthesis as well as J–V, J–V versus temperature, EQE and C–V measurements. S.G., D.K., J.M., C.L.P., C.-S.J. and M.M.A. performed photoluminescence, TRPL, CL, XPS, KPFM and STEM measurements. W.K.M., D.L., G.X. and M.G. directed the research. All authors contributed to the design and interpretation of experiments and writing the manuscript.

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Correspondence to W. K. Metzger.

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S.G., D.L., X.L., R. Mallick, W.Z., R. Malik, J.K., G.X. and M.G. work at First Solar, which is a publicly traded company that manufactures CdTe solar panels and develops grid-connected photovoltaic power plants. Outside of this and the funding listed in the acknowledgments section, the authors declare no competing interests.

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Metzger, W.K., Grover, S., Lu, D. et al. Exceeding 20% efficiency with in situ group V doping in polycrystalline CdTe solar cells. Nat Energy 4, 837–845 (2019). https://doi.org/10.1038/s41560-019-0446-7

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