The mobility and decay kinetics of charge carriers in pulse-ionized microcrystalline PCBM powder

The mobility and decay kinetics of charge carriers produced by nanosecond pulsed ionization of a microcrystalline powder sample of phenyl-C-61-butyric acid methyl ester (PCBM) have been investigated by time-resolved microwave conductivity. The sum of the electron and hole trap-free mobilities within crystallites is between 0.04 and 0.3 cm(2) V-1 s(-1) at room temperature with an activation energy of 0.066 eV The decay of the conductivity, which takes place over a timescale of milliseconds at room temperature, is controlled mainly by charge-carrier trapping with an activation energy of 0.53 eV. An upper limit of 1 X 10(-18) m(3) s(-1) is estimated for the rate coefficient of charge recombination, which is more than four orders of magnitude lower than expected for diffusion-controlled recombination, indicating PCBM to be an "indirect bandgap" semiconductor.