A 19.9%-efficient ultrathin solar cell based on a 205-nm-thick GaAs absorber and a silver nanostructured back mirror

Abstract

Conventional photovoltaic devices are currently made from relatively thick semiconductor layers, ~150 µm for silicon and 2–4 µm for Cu(In,Ga)(S,Se)2, CdTe or III–V direct bandgap semiconductors. Ultrathin solar cells using 10 times thinner absorbers could lead to considerable savings in material and processing time. Theoretical models suggest that light trapping can compensate for the reduced single-pass absorption, but optical and electrical losses have greatly limited the performances of previous attempts. Here, we propose a strategy based on multi-resonant absorption in planar active layers, and we report a 205-nm-thick GaAs solar cell with a certified efficiency of 19.9%. It uses a nanostructured silver back mirror fabricated by soft nanoimprint lithography. Broadband light trapping is achieved with multiple overlapping resonances induced by the grating and identified as Fabry–Perot and guided-mode resonances. A comprehensive optical and electrical analysis of the complete solar cell architecture provides a pathway for further improvements and shows that 25% efficiency is a realistic short-term target.

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Fig. 1: Fabrication process for ultrathin GaAs solar cells with a nanostructured back mirror.
Fig. 2: Best ultrathin solar cell based on a 205-nm-thick GaAs absorber and a nanostructured Ag mirror.
Fig. 3: Optical analysis of the ultrathin GaAs solar cells.
Fig. 4: JV characteristics and loss analysis.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

The authors acknowledge discussions with J.-F. Guillemoles, electromagnetic simulation support from P. Lalanne, J.-P. Hugonin and C. Sauvan and financial support through French ANR project NANOCELL (grant no. ANR-15-CE05-0026) and the French Renatech network.

Author information

H.-L.C. carried out most of the fabrication steps for the solar cell experiments at C2N and performed optical modelling and results analysis. A.W.W., O.H., D.L. and F.D. designed the optimized GaAs solar cell layer structure, D.L. wrote the recipe for epitaxy growth and G.S. evaluated the characterization results in the Fraunhofer ISE CalLab. H.-L.C., A.C., R.D.L., M.F., N.V., J.G., B.B., C.D. and N.B. developed and optimized the fabrication process. A.C. and M.F. specifically developed the nanoimprint process for TiO2 sol–gel films. N.V. contributed to the design and modelling of the devices. A.C. and S.C. developed the concept of ultrathin solar cells with a nanostructured back mirror and supervised the project. H.-L.C. and S.C. wrote the manuscript. All authors participated in the discussions and improvements of the manuscript.

Correspondence to Stéphane Collin.

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Supplementary Figs. 1–18, Supplementary Notes 1–5, Supplementary Table 1, Supplementary refs. 1–8.

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