Hole transport in poly(phenylene vinylene)/methanofullerene bulk-heterojunction solar cells

A fundamental limitation of the photocurrent of solar cells based on a blend of poly(2-methoxy-5-(3',7'-dimethyloctyloxy)-p-phenylene vinylene) (MDMO-PPV) and [6,6]-phenyl C-61-butyric acid methyl ester (PCBM) is caused by the mobility of the slowest charge-carrier species, the holes in the MDMO-PPV. In order to allow the experimentally observed photocurrents electrostatically, a hole mobility of at least 10(-8) m(2) V-1 s(-1) is required, which exceeds the observed hole mobility in pristine MDMO-PPV by more than two orders of magnitude. However, from space-charge-limited conduction, admittance spectroscopy, and transient electroluminescence measurements, we found a hole mobility of 2 x 10(-8) m(2) V-1 s(-1) for the MDMO-PPV phase in the blend at room temperature. Consequently, the charge-carrier transport in a MDMO-PPV:PCBM-based solar cell is much more balanced than previously assumed, which is a necessary requirement for the reported high fill factors of above 50%.