A new light trapping technique has enabled more than 30% power enhancement of an ultrathin solar cell, according to BGU and Technion-Israel Institute of Technology researchers. The method was developed for hematite (iron-oxide, Fe2O3) cells meant for hydrogen production via water splitting, but could achieve its goal for other types of cells as well. In addition, only simple optical materials were used, adding to the sustainability and cost effectiveness of this approach.

Research was conducted by a joint team headed by Prof. Avner Rothschild from the Department of Material Science and Engineering of the Technion – Israel Institute of Technology, and by Dr. Avi Niv from the Alexandre Yersin Department of Solar Energy and Environmental Physics of BGU’s Jacob Blaustein Institutes for Desert Research. Their findings were just published in the Journal of Materials Chemistry A in an article entitled “Separation of Light Confinement and Absorption Sites for Enhancing Solar Water Splitting.”

“Finding a way to harness ray optics for boosting the absorption of ultrathin cells, as was done here, could have a large impact on the future of solar cells,” says Niv.

One of the ways of increasing the cost effectiveness of solar cells is by reducing the thickness of their active light absorbing layers, the part of the cell that turns light into electricity. The challenge is therefore to have a thinner absorber but to sustain significant overall absorptivity. To meet this challenge, optical trapping methods were devised. Trapping the light within the interior of the cell causes a longer effective beam path through the active layer, which in turn raises the absorptivity. For more than four decades now, improvement in light trapping went hand in hand with thinner cells that were cheaper and more efficient.

For the next generation of solar cells, however, more power at lower costs is expected. These demands are often met by considering active layers thinner than the typical wavelength of sunlight itself. The problem is that in such thin layers light behaves as a wave, rendering ray-based trapping ineffective. For this reason light trapping methods that are based on the wave nature of light emerged. While impressive progress has been made in the field, none of the results so far rival ray optics when it comes to utility. Thus, the researchers returned to ray optics and managed to develop a new method that combines ray-based trapping with wave optics absorption. This goal was achieved by structural separation of the trapping and absorption sites within the cell. Light is first trapped in a lossless thick substrate layer that later feeds the absorption in a deep subwavelength active hematite layer. Enhancements of more than 30% in the power production of the cell were shown using this approach. Researchers also predict that even higher enhancements are in fact possible, reaching more than 40%.

The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement no. [617516] and from the Israeli Nanotechnology Focal Technology Area on Nanophotonics for Detection. Measurements were conducted at the Technion’s Photovoltaic Laboratory, supported by the Nancy & Stephen Grand Technion Energy Program (GTEP) and by the Russell Berrie Nanotechnology Institute (RBNI), and at the Technion’s Hydrogen Technologies Research Laboratory (HTRL), supported by the Adelis Foundation and by the Solar Fuels I-CORE program of the Planning and Budgeting Committee and the Israel Science Foundation (Grant no. 152/11). This work was also supported by the Grand Technion Energy Program (GTEP), and constitutes part of The Leona M. and Harry B. Helmsley Charitable Trust reports on Alternative Energy series of the Technion, Israel Institute of Technology, and the Weizmann Institute of Science.

This research was carried out in the framework of the Energy Initiative Program and supported by the Adelis Foundation for renewable energy research.

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