The 28th European Photovoltaic Solar Energy Conference and Exhibition had many different topics regarding solar energy. From the latest design of cell structure to the marketing of new concepts, every attendee should have seen some recent developments in the field of photovoltaic solar energy.

For me, a synthetic chemist, I was most interested in talks regarding organic photovoltaics. Luckily, the keynote speech about organic photovoltaics this year was given by  Prof. Jan Cornelis Hummelen, a world renowned researcher in this field. First, Prof. Hummelen reported some recent progress made in different subdivisions of organic solar cells. Heliateck achieved 12.0 % photo conversion efficiency (PCE) on tandem structures using small molecule organic semiconductors. Compared to the 10.7% they reported last year, this is a fairly impressive achievement.

It was also noted in the speech that the all-polymer cell had reached 6.4% efficiency while the triple junction tandem cell had made its way to 9.6%. The tandem cell using standard polymer-fullerene bulk heterojunction made in Yang yang’s group remained at 10.6%, the same number Prof. Yang published during his keynote speech at the EUPVSEC last year. If he hasn’t made any progress on this type of tandem cell, it would be challenging to achieve the 15% goal by the year 2015 as he touted in the talk last year.

Professor Hummelen then moved on to talk briefly some research results made by FOM focus group in University of Groningen which included the invention of stretchable OPV, morphological analysis, quantum chemical study, and perovskite hybrid sensitized solar cells. The speech then proceeded to a short review of the development of the p-type polymeric materials and fullerene derivatives from 1996 to now with emphasis on  recent achievements on organic semiconductors with ultrahigh carrier mobility. The future direction for research on organic photovoltaic materials, as Professor Hummelen perceived, should aim at increasing the dielectric constant. This specific physical property of organic semiconductors was what he believed to be limiting the performance of OPVs. Interestingly, as a co-author of the first paper reporting a bulk heterojuction in 1995, Professor Hummelen now said we could eliminate this structure and go with an old fashion single junction and achieve even higher efficiency if we made an organic material with a dielectric constant high enough to allow free charge excitation.

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Ben Zhang
Simon Fraser University, British Columbia