Harvesting light energy with optical rectennas

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As predicted by Schockley and Queisser in the 60’s, the photoconversion phenomenon involved in mono junctions solar cells has an upper limit of 33% [1]. Other concepts, such as high efficiency tandem solar cells designed to perform under concentrated light have been developed to overcome this limitation [2], but this technology remains expensive and difficult to process to an industrial scale. The concept of optical rectennas goes back to the 70’s when Bailey proposed that a nano-scale antenna coupled with a rectifier could harvest electromagnetic waves in the visible and infrared region [3]. Later, Joshi and Moddel built a photon assisted theory and predicted conversion efficiencies close to 100% for a monochromatic rectenna in the visible range. As for the performance under a realistic sun illumination, it was found to be 44% for a single rectenna geometry [4]. Unlike the Schockley-Queisser limit, this 44% limit does not rely on the material, but strongly depends on the rectenna geometry. This feature makes rectenna arrays good candidates for tandem-like structures based on several antennas sizes and shapes, resulting in a promising low cost process. Today, nano-imprint technologies enable the fabrication of sufficiently small 3D structures to act as optical antennas. As for the rectifier element, MIM structures with a very small RC time could be a suitable solution [5], as well as organic molecular electronics which recently shown state of the art rectification ratio up to 1000 [6]. Recent work have already demonstrated power production originating from optical rectennas [7], but research in this field remains at stage of proof of concept. In this work, we study and develop rectennas solar cells composed of plasmonic nano-antennas associated with rectifying self-assembled molecular diodes (fig.1) First, we generate light trapping by printing nano-pyramid shaped structures into PMMA substrates using a nano-imprint technic. Then we deposit a gold bottom electrode (fig.2) on the top of which we build a self-assembled monolayer (SAM) of ferrocene alkanethiols molecules. The stack is then completed by successive deposition of ZnO and of a gold top electrode. The role of the molecules is to rectify the alternative current which arises from plasmonic modes within the stack (fig.3,4). The obtained DC current is eventually collected at the top electrode. So far, our work focus on electrical and chemical characterizations of rectenna samples, as well as a first theoretical model based on effective mass and transfer matrix methods to describe the charge transport within such a hybrid metal/organic structure.

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Journal: TechConnect Briefs
Volume: 2, Materials for Energy, Efficiency and Sustainability: TechConnect Briefs 2017
Published: May 14, 2017
Pages: 45 - 48
Industry sector: Energy & Sustainability
Topic: Solar Technologies
ISBN: 978-0-9975117-9-6