The role of the initial charge separation in the geminate pair recombination in disordered molecular systems

Geminate charge pair recombination in sensitised photoconducting polymers was investigated by means of Monte - Carlo simulations. It was found that the initial charge separation distance and the hole recombination rate in the charge transfer (CT) complex, formed between a matrix molecule and a sensitiser, are the main parameters determining the initial part of the recombination kinetics. By comparing the simulation results to the experimentally observed kinetics of sensitised carbazolyl-containing polymers, it has been concluded that part of the charge pairs remain at the CT complexes and recombine exponentially. For the rest, the holes are separated from the CT complexes and cause much slower nonexponential decay. The time scale for the hole hopping steps was estimated to be of the order of 10 divided by 50ps. A model of dynamical scattering of the mobile charges by sensitiser molecules at the initial stage of charge separation is suggested to explain faster recombination at high concentration of the sensitiser.