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18 May 2010 Improved photon harvesting by employing C70 in bulk heterojunction solar cells
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To achieve higher efficiencies in organic solar cells, ideally the open circuit voltage (VOC), fill factor (FF) as well as the short current density (JSC) have to be further improved. However, only a few suitable acceptor molecules, e.g. C60, are currently available for the photoactive layer. Despite a good electron mobility on the order of 1×10-3 cm2/Vs the absorption of C60 in the visible sun spectrum is low. From polymer based solar cells it is known that the fullerene derivative [70]PCBM used in the photoactive layer shows a significant enhancement in JSC compared to [60]PCBM. This work investigates the application of fullerene C70 as acceptor in comparison to the well known C60 in vacuum processed small molecule solar cells. C70 shows a broadened and red shifted absorption (abs. maximum around 500 nm) compared to C60. By fabricating p-i-i solar cells we show that the stronger absorption of C70 leads to enhanced photon harvesting and increased external quantum efficiency. The bulk heterojunction p-i-i solar cell containing C70 as acceptor and ZnPc as donor, co-evaporated with an optimized ratio of 2:1, and a layer thickness of 30 nm shows improved solar cell parameters: a 30% larger photocurrent of 10.1 mA/cm2 is obtained. The VOC of 0.56 V and FF of 55% remain comparable to C60-containing p-i-i solar cells. Therefore, the solar cell performance is mainly improved by JSC and leads to a mismatch corrected power conversion efficiency of 3.12%. Thus, we show that C70 is an alternative fullerene to C60 for solar cell applications.
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Steffen Pfuetzner, Jan Meiss, Selina Olthof, Moritz P. Hein, Annette Petrich, Lothar Dunsch, Karl Leo, and Moritz Riede "Improved photon harvesting by employing C70 in bulk heterojunction solar cells", Proc. SPIE 7725, Photonics for Solar Energy Systems III, 77250E (18 May 2010);

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