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Charge transport in CdTe solar cells revealed by conductive tomographic atomic force microscopy

Nature Energy volume 1, Article number: 16150 (2016) Cite this article

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Abstract

The influence of microstructural defects on the device properties in CdTe remains largely unknown. This is partly because characterization techniques have been unable to image electrical pathways throughout three-dimensional grains and grain boundaries with nanoscale resolution. Here, we employ a conductive and tomographic variation of atomic force microscopy to study charge transport at the nanoscale in a functioning thin-film solar cell with 12.3% efficiency. Images of electric current collected through the device thickness reveal spatially dependent short-circuit and open-circuit performance, and confirm that grain boundaries are preferential pathways for electron transport. Results on samples with and without cadmium chloride treatment reveal little difference in grain structure at the microscale, with samples without treatment showing almost no photocurrent either at planar defects or at grain boundaries. Our results supports an energetically orthogonal transport system of grain boundaries and interconnected planar defects as contributing to optimal solar cell performance, contrary to the conventional wisdom of the deleterious role of planar defects on polycrystalline thin-film solar cells.

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Figure 1: Tomographic image of short-circuit current throughout a working CdTe solar cell.
Figure 2: Connectivity of planar defects at grain boundaries.
Figure 3: Planar defects.
Figure 4: Electron transport through grain boundaries.
Figure 5: Effects of cadmium chloride treatment.

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Acknowledgements

J.L. acknowledges support from the US Department of Energy, Office of Energy Efficiency and Renewable Energy (EERE) Postdoctoral Research Awards under the SunShot Solar Energy Technologies Program. Y.K. and B.D.H. recognize DOE-BES-ESPM project DE-SC0005037. This research used resources of the Center for Functional Nanomaterials, which is a US DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704. Authors acknowledge Kim Kisslinger for his assistance in cross-sectional TEM, and James Steffes for work on temperature dependence. Support from Jim Sites at Colorado State University is appreciated.

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Authors and Affiliations

  1. Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06268, USA

    Justin Luria, Yasemin Kutes & Bryan D. Huey

  2. Colorado State University, Ft. Collins, Colorado 80523, USA

    Andrew Moore

  3. Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, USA

    Lihua Zhang & Eric A. Stach

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  1. Justin Luria
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Contributions

J.L. performed CT-AFM experiments, assisted by Y.K.; A.M. was responsible for fabrication of samples. TEM work led by E.A.S., including work from L.Z. The manuscript was written by J.L. and B.D.H. All authors discussed the results and reviewed the manuscript.

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Correspondence to Justin Luria or Bryan D. Huey.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Information

Supplementary Notes 1–5, Supplementary Figures 1–11. (PDF 2334 kb)

Supplementary Video 1

Photoconductivity Tomography in a CdTe Thin-Film. Illuminated with 15 suns of intensity, the brightness contrast of these images are from 0 pA (dark) to 30 pA (bright) and z-scale at 2.2 microns. Although sampled from the top of the film to the bottom of the film, we present an array of side-profiles to show the capabilities of tomography. (AVI 2773 kb)

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Luria, J., Kutes, Y., Moore, A. et al. Charge transport in CdTe solar cells revealed by conductive tomographic atomic force microscopy. Nat Energy 1, 16150 (2016). https://doi.org/10.1038/nenergy.2016.150

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