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

Abstract

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 ${(W}_{\mathrm{PCBM}})$ 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 \ensuremath{\varphi} and the sum of the charge carrier mobilities \ensuremath{\Sigma}\ensuremath{\mu} was determined from the maximum (end-of-pulse) value of the transient photoconductivity. On excitation at 500 nm, \ensuremath{\varphi}\ensuremath{\Sigma}\ensuremath{\mu} remained almost constant in going from ${W}_{\mathrm{PCBM}}=0.2$ to 0.6 with an average value of $0.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}{\mathrm{cm}}^{2}/\mathrm{V}\mathrm{}\mathrm{s}.$ Above ${W}_{\mathrm{PCBM}}=0.6,$ \ensuremath{\varphi}\ensuremath{\Sigma}\ensuremath{\mu} increased dramatically, reaching a maximum value of $83\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}{\mathrm{cm}}^{2}/\mathrm{V}\mathrm{}\mathrm{s}$ for ${W}_{\mathrm{PCBM}}=0.85.$ This effect is attributed to the occurrence of phase separation above ${W}_{\mathrm{PCBM}}=0.6$ and to the resulting formation of highly mobile electrons within PCBM-rich aggregates. The much lower value of \ensuremath{\varphi}\ensuremath{\Sigma}\ensuremath{\mu} observed below ${W}_{\mathrm{PCBM}}=0.6$ is assigned mainly to mobile holes within the polymer component of the blend. Possible explanations for the decrease in \ensuremath{\varphi}\ensuremath{\Sigma}\ensuremath{\mu} with increasing light intensity, found for all blend compositions, are discussed.