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 {φ} and the sum of the charge carrier mobilities {Σ}{μ} was determined from the maximum (end-of-pulse) value of the transient photoconductivity. On excitation at 500 nm, {φ}{Σ}{μ} remained almost constant in going from WPCBM=0.2 to 0.6 with an average value of 0.6×10^-3cm²/Vs. Above WPCBM=0.6, {φ}{Σ}{μ} increased dramatically, reaching a maximum value of 83×10^-3cm²/Vs 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 {φ}{Σ}{μ} observed below WPCBM=0.6 is assigned mainly to mobile holes within the polymer component of the blend. Possible explanations for the decrease in {φ}{Σ}{μ} with increasing light intensity, found for all blend compositions, are discussed.