Excitonic photoconductivity of 4BCMU polydiacetylene single crystals

Thermal annealing of 4BCMU single crystals increases the transition energy of excitons from 2.0 to 2.3 eV and a similar increase is observed for the free carrier gap resolved by electroabsorption spectra. This modification leads to a significant change of the photocurrent threshold which after the transformation shifts to the excitonic absorption edge. Since the exciton binding energy of 0.5 eV is not altered, this excitonic photoconductivity cannot be due to dissociation of excitons. We propose Auger excitation of trapped carriers into free carrier states which acquire energy from recombination of free excitons. The crucial step is the escape from the chain which requires sufficient excess energy of these excited carriers. This energy is provided by excitons of 2.3 eV but not by those of 2 eV.