The dependence of the performance of OC1C10-PPV:PCBM (poly(2-methoxy-5-(3',7'-dimethyloetyloxy)-p-phenylene vinylene):methanofullerene [6,6]-phenyl C-61-butyric acid methyl ester)-based bulk heterojunction solar cells on their composition has been investigated. With regard to charge transport, we demonstrate that the electron mobility gradually increases on increasing the PCBM weight ratio, up to 80 wt.-%, and subsequently saturates to its bulk value. Surprisingly, the hole mobility in the PPV phase shows an identical behavior and saturates beyond 67 wt.-% PCBM, a value which is more than two orders of magnitude higher than that of the pure polymer. The experimental electron and hole mobilities were used to study the photocurrent generation of OC1C10-PPV:PCBM bulk-heterojunction (BHJ) solar cells. From numerical calculations, it is shown that for PCBM concentrations exceeding 80 wt.-% reduced light absorption is responsible for the loss of device performance. From 80 to 67 wt.-%, the decrease in power conversion efficiency is mainly due to a decreased separation efficiency of bound electron-hole (e-h) pairs. Below 67 wt.-%, the performance loss is governed by a combination of a reduced generation rate of e-h pairs and a strong decrease in hole transport.