Interaction of intermediate excited states and formation of free charge carriers under laser excitation of a conjugated polymer

Picosecond time-resolved technique, based on correlated action of periodically repeated pairs of femtosecond laser pulses of 400 nm wavelength and recording of average photocurrent, was applied for studying the mechanism, of free charge carrier formation in ladder type poly(p-phenylene) (MeLPPP). The technique distinguishes selectively components of the photoconductivity caused by second-order processes between intermediate species formed under, the light absorption and which are responsible for cw photoconductivity. It was shown that though intra-chain excitons take part in those processes, an essential part of charge carriers is generated very quickly (within 1 ps). These primarily formed charge carriers are however not free, belonging to polaron pairs. The latter are demonstrated to be precursors of free polarons. It is because of their ability to suffer electric-field-assisted dissociation and to take part in random recombination reactions that generate free polarons though at a price of destroying more polarons inside the pairs. General scheme of free charge carrier formation, based on the results obtained, is presented. (C) 2002 Elsevier Science B.V. All rights reserved.