Device model for the operation of polymer/fullerene bulk heterojunction solar cells

We have developed a numerical device model that consistently describes the current-voltage characteristics of polymer:fullerene bulk heterojunction solar cells. Bimolecular recombination and a temperature- and field-dependent generation mechanism of free charges are incorporated. It is demonstrated that in poly[2-methoxy-5-(${3}^{\ensuremath{'}},{7}^{\ensuremath{'}}$-dimethyloctyloxy)-$p$-phenylene vinylene]- ($\mathrm{O}{\mathrm{C}}_{1}{\mathrm{C}}_{10}\text{\ensuremath{-}}\mathrm{PPV}$-) and [6,6]-phenyl ${\mathrm{C}}_{61}$-butyric acid methyl ester- (PCBM-) $(1:4\phantom{\rule{0.3em}{0ex}}\mathrm{wt.}\phantom{\rule{0.2em}{0ex}}%)$ based solar cells space-charge effects only play a minor role, leading to a relatively constant electric field in the device. Furthermore, at short-circuit conditions only 7% of all free carriers are lost due to bimolecular recombination. The model predicts that an increased hole mobility together with a reduction of the acceptor strength of $0.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ will lead to a maximum attainable efficiency of 5.5% in the PPV/PCBM-based solar cells.