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The green layer represents the photonic crystal The device in the back does not include the photonic crystal

Researchers at The Institute of Photonic Sciences (ICFO), Barcelona, Spain, have created a highly transparent, highly efficient organic solar cell with promising potential for integration into windows and displays. Reaching 5.6% efficiency, the cell’s performance approaches that of its opaque counterparts.

The ICFO cell’s active layer is a blend of a polymer (PTB7) and a derivative of fullerene (PC71BM) and has a transparency close to 50%. By comparison, a typical transparency value for sunglasses is about 15%. A standard OPV would not be transparent to visible light, because the device incorporates a metal electrode that is completely opaque. In case of the ICFO cell, the electrode is made of silver. The group thinned that material to just 10 nm. “This gives the device a homogenous semi-transparency that ranges from 30 to 40%,” says Jordi Martorell, UPC Professor at ICFO and leader of the study.

Thinning the electrode would normally compromise the device’s ability to absorb the near-infrared and near-ultraviolet photons not visible to the eye, which would decrease the overall conversion efficiency. The ICFO group solved this dilemma by incorporating a photonic crystal that increases the amount of infrared and ultraviolet light absorbed by the cell. “The photonic crystal we designed sends these photons back to the active material but is transparent to visible photons,” Martorell explains, adding that the cell’s photon-to-charge conversion efficiency is close to 80% of that of the original opaque device. “To the best of our knowledge, such a level of efficiency when comparing 30% semitransparent to opaque devices has never been achieved before.”

“What makes it even more interesting is that one may tune the photonic crystal to optimise sun photon absorption for vertical PV installations, which is the natural type of installation in big cities, since the largest amount of transparent surface is vertical instead of horizontal,” Martorell elaborates. “No other PV technology, including other types of OPVs, can achieve the same goal so effectively.” What is more, to make the design even more appealing for such BIPV applications, the colour of the cells can be tuned by simply changing the layer configuration of the photonic crystal.

The expert agrees that integrated transparent solar cells, such as the one his group debuted, could soon revolutionise the way people use solar energy. “In my opinion semitransparent OPVs will represent a major step in the integration of energy production with energy consumption,” Martorell says. “With the technology developed at ICFO, which is patent-pending, we take such integration one step further.” And with an overall power conversion efficiency of 5.6%, he believes his device “is already acceptable for practical applications.”

Asked when ICFO’s device could become commercially viable, though, Martorell says a definite answer is “difficult,” citing several “strictly technical” challenges, such as scalability or stability. While his group knows how to address these challenges, and will focus on that next, there are “many other issues” out of the researchers’ control: “… for instance, capital investment, a favourable legislation, production costs compared to other type of windows, etc., which have little to do with the technology itself but that may play a larger role in terms of when we will see commercial PV windows.”

As it stands, this type of integrated organic solar cell would mostly find application in BIPV, Martorell says. “However, we intend to increase the transparency up to levels that make the technology also applicable for the industry of tablets, smart phones, and displays, which require levels of transparency close to 90%.” To increase the level of transparency, the ICFO group will focus on new materials and alternative device configurations going forward.

The results of this study are detailed in the paper “Transparent polymer solar cells employing a layered light-trapping architecture,” published in Nature Photonics.

Labels: Institute of Photonic Sciences,ICFO,polymer solar cells,OPV,Jordi Martorell,BIPV,photonic crystals,organic photovoltaics

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