Mobility and decay kinetics of charge carriers in photoexcited PCBM/PPV blends

The transient photoconductivity of blends of a highly soluble C60 derivative (PCBM) and a dialkoxy-phenylene-vinylene polymer (MDMO-PPV) has been studied using the electrodeless flash-photolysis time-resolved microwave conductivity technique (FP-TRMC). Films approximately 100 nm thick on a quartz substrate were prepared by spin-coating PCBM/PPV solutions with PCBM weight fractions (WPCBM) from 0.2 to 0.95. For all blends, the wavelength dependence of the photoconductivity in the range 420–700 nm closely resembled the photon attenuation spectrum, indicating that photoexcitation of both components contributes to mobile charge carrier formation. The product of the quantum yield for charge separation f and the sum of the charge carrier mobilities Sµ was determined from the maximum (end-of-pulse) value of the transient photoconductivity. On excitation at 500 nm, fSµ remained almost constant in going from WPCBM=0.2 to 0.6 with an average value of 0.6×10-3 cm2/V s. Above WPCBM=0.6, fSµ increased dramatically, reaching a maximum value of 83×10-3 cm2/V s for WPCBM=0.85. This effect is attributed to the occurrence of phase separation above WPCBM=0.6 and to the resulting formation of highly mobile electrons within PCBM-rich aggregates. The much lower value of fSµ observed below WPCBM=0.6 is assigned mainly to mobile holes within the polymer component of the blend. Possible explanations for the decrease in fSµ with increasing light intensity, found for all blend compositions, are discussed.