| Streszczenie: | To achieve higher efficiencies in organic solar cells, ideally the open circuit voltage (V<sub>OC</sub>), fill factor (FF) as well as the short current density (J<sub>SC</sub>) have to be further improved. However, only a few suitable acceptor molecules, e.g. C<sub>60</sub>, are currently available for the photoactive layer. Despite a good electron mobility on the order of 1×10<sup>−3</sup> cm<sup>2</sup>/Vs the absorption of C<sub>60</sub> 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 J compared to [60]PCBM. This work investigates the application of fullerene C<sub>70</sub> as acceptor in comparison to the well known C<sub>60</sub> in vacuum processed small molecule solar cells. C<sub>70</sub> shows a broadened and red shifted absorption (abs. maximum around 500 nm) compared to C<sub>60</sub>. By fabricating p-i-i solar cells we show that the stronger absorption of C<sub>70</sub> leads to enhanced photon harvesting and increased external quantum efficiency. The bulk heterojunction p-i-i solar cell containing C<sub>70</sub> 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/cm<sup>2</sup> is obtained. The V<sub>OC</sub> of 0.56 V and FF of 55% remain comparable to C<sub>60</sub>-containing p-i-i solar cells. Therefore, the solar cell performance is mainly improved by J<sub>SC</sub> and leads to a mismatch corrected power conversion efficiency of 3.12%. Thus, we show that C<sub>70</sub> is an alternative fullerene to C<sub>60</sub> for solar cell applications. © 2010 Copyright SPIE - The International Society for Optical Engineering.
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