Photoconductivity of polydiacetylene chains in polymer and monomer single crystals

Photocurrent spectra of fully polymerized polydiacetylene single crystals (DCHD and 4BCMU) are compared with those of chains diluted in the corresponding monomer crystals. Displacement currents in the dark and persistent photocurrents indicate the presence of long-lived carriers in deep traps which provide a reservoir for carrier emission and modify the response to changes of light intensity and applied voltage. Their influence on transport properties is most pronounced in the case of isolated chains where de currents show large fluctuations after illumination. All samples respond fast to changing light intensity, and the photocurrent spectra are very similar. Photocurrent requires that photo-excited carriers escape from a chain to recombine non-geminately on another chain. Such escape occurs with comparable probability for isolated chains and in polymer crystals and seems not activated because the photocurrent depends weakly on temperature. We propose tunneling of carriers to sidegroups of the polydiacetylene chain as initial step of pair separation. (C) 1999 Elsevier Science B.V. All rights reserved.