The photovoltaic (PV) market nowadays is dominated by 1^st generation solar cells made of crystalline silicon. But silicon solar cells generate power from only a small portion of the electromagnetic spectrum and demonstrate efficiencies of about 25% in laboratory [1]. 2^nd generation solar cells based on thin films have an efficiency theoretical upper limit of about 30% [2].

Nano-antennas coupled to rectifying diodes (known as rectennas) are developped for 3rd generation PV solar cells. They directly convert the electromagnetic waves into electricity from far infrared to visible wavelengths. With a theoretical conversion efficiency of 85% [3] and a technology compatible with low costs fabrication techniques on flexible substrates, rectennas appear to be promising candidates for next generation PV solar cells.

In the present study, pyramidal structures were nano-imprinted on a flexible substrate (polyethylene terephthalate: PE) and then covered by a thin metallic layer. We have investigated the morphology, structure and chemistry at each step of the fabrication process via SEM techniques, in order to highlight structural and/or chemical defects at nanometer scale which can affect the overall physical and electrical properties of the device. The results were obtained with a Zeiss GeminiSEM 500 ultra-high resolution FESEM. This system is equipped with In-lens SE and the Energy Selective Backscattered detector (EsB), as well as variable pressure (VP) SE and VPBSE detectors to observe non-conductive samples. For the chemical and crystallographic analyses, EDS, WDS, and EBSD accessories are additionally installed on this microscope achieving the full imaging linked to the analytical capabilities. The low voltage acquisition was suitable for surface studies since it increases topographic contrast and reduces specimen charging. The VP mode was also employed, which features not only a very restricted skirt effect under high gas pressures, but also enables the detection of pure secondary electron (SE) signal using all in-lens detectors [4].

Figure 1 shows a cross-section image of a pyramidal nano-antenna array, taken with the VP SE detector at 140 Pa and 5 kV. The charging effect on the PE polymer is completely eliminated with the use of the VP mode. The morphology and wetting of the silver metallic layer could be clearly observed. Figure 2 displays a top view of one nanopyramid. The image was taken with the in-lens SE detector at 1 kV and at a very short working distance (0.4 mm), which greatly improves the resolution. Low kV imaging allows the observation of the silver layer structure on the pyramid facets. Crystal grains and defects such as grain boundaries and twins are acutely visible.

This work highlights the use of SEM high resolution imaging, at low kV and even in variable pressure mode, to improve the understanding of structural and chemical properties of hybrid nanomaterials as plastic/metal pyramidal nano-antennas.

References

[1] M.A. Green, K. Emery, Y. Hishikawa, W. Warta and E. D. Dunlop, Prog. Photovolt: Res. Appl. 20:12–20 (2012)

[2] W. Shockley and H. J. Queisser, Journal of Applied Physics, Volume 32:. 510-519 (1961)

[3] D. K. Kotter, S. D. Novack, S. D. Slafer, P. Pinhero, Proceedings of the 2^nd International Conference on Energy Sustainability, ES 2008, 2, 409 (2009)

[4] L. Han, C. Berger, M. Boese, A. Thesen, F. Zhou, S. Meyer, Erik Essers, Microsc. Microanal. 21, Suppl 3 (2015)

Acknowledgments

This study is part of “Energy Storage by Direct Conversion of Electromagnetic Radiation Captured within Nano-Antennas” (ETNA) project funded by the Amidex Fundation (PR2I projects)

Figures:

Figure1. VP SE image of a cross-section from a pyramidal-shaped nano-antenna, taken at 140 Pa and 5 kV. The polymer and the metal layer can be cleary observed.

Figure 2. In-lens SE image at 1 kV and 0.4 mm working distance, unveiling silver grains on a PE nanopyramid.

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EMC Abstracts - https://emc-proceedings.com/abstract/high-resolution-and-variable-pressure-imaging-of-pyramidal-nano-antennas-in-a-sem/