Hole Transport in Poly(phenylene vinylene)/Methanofullerene Bulk‐Heterojunction Solar Cells

Abstract 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 × 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 %.