Solar-powered windows: Transparent reaction

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  • Published: Aug 1, 2012
  • Author: David Bradley
  • Channels: Infrared Spectroscopy
thumbnail image: Solar-powered windows: Transparent reaction

Photoactivity

UCLA transparent solar cell courtesy of Yang Yang 

US researchers have developed a polymer that is photoactive in the near-infrared (NIR) rather than the visible region of the electromagnetic spectrum and so might be used as a transparent layer for converting ordinary windows into in situ solar panels.

Writing in the journal ACS Nano, a team from the University of California Los Angeles explains how balancing transparency and photovoltaic activity is all about a shift in spectrum. Their new material, they explain, generates electricity by absorbing mainly in the near-infrared while continuing to allow about 66% of visible light (optimised maximum in the team's tests at 550 nm) through. Such windows used on the sunny side of the street might not only allow office workers to power up the desktop devices but could offer additional shade.

"These results open the potential for visibly transparent polymer solar cells as add-on components of portable electronics, smart windows and building-integrated photovoltaics and in other applications," explains research leader Yang Yang of UCLA, who is also director of the Nano Renewable Energy Center at California NanoSystems Institute (CNSI). "Our new polymer solar cells (PCSs) are lightweight and flexible," he adds. "More importantly, they can be produced in high volume at low cost." The PCS is based on poly(2,6'-4,8-bis(5-ethylhexylthienyl)benzo[1,2-b;3,4-b]dithiophene-alt-5-dibutyloctyl-3,6-bis(5-bromothiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4-dione) (PBDTT-DPP), designed and synthesized by graduate student Letian Dou.

There has been a great deal of effort in the field of polymer solar cells not least because plastics are usually far less expensive and much less fragile than the purified and crystalline semiconductor materials require for solar panels. Researchers are also hoping that transparent polymeric photoactive materials might have broader applications in other areas off-limits to conventional semiconductor technology, in portable electronics, for instance, perhaps integrating a solar charger with the device's display.

Efficiency boost

Until now, efforts to make PSCs at once both photoactive and transparent have led only to dark films that transmit very little visible light or if they are more transparent have only very low efficiencies. The UCLA team believes it has found a way around this dichotomy by using composite films made from NIR-sensitive polymer with silver nanowires and titanium dioxide nanoparticles as the top transparent electrode. The silver nanowire and titanium dioxide nanoparticles conductor is almost totally transparent and so could replace the usual opaque metal electrodes used in earlier technology. Using a composite electrode also keeps fabrication cost down by enabling solution processing. The team has so far demonstrated a useful 4% power-conversion efficiency for their PCS. "These results open the potential for visibly transparent polymer solar cells as add-on components of multijunction photovoltaic devices, smart windows, and building-integrated photovoltaics and in other applications," they suggest.

Yang worked under Paul Weiss and alongside Rui Zhu, Chun-Chao Chen, Letian Dou, Choong-Heui Chung, Tze-Bin Song and Steve Hawks as well as Gang Li and Yue Bing Zheng in research supported by the Henry Samueli School of Engineering and Applied Science, the Office of Naval Research, and The Kavli Foundation.

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Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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