Excitons in conjugated polymers

The question how in conjugated polymers neutral optical excitations (excitons) dissociate into a pair of unbound charge carriers has been discussed. The analogous process in molecular crystals will be outlined briefly as well as experiments to delineate the bandgap in crystalline polydiacetylenes and the equivalent quantity in disordered conjugated polymers. Based upon studies on the prompt and delayed fluorescence in a ladder-type poly-para-phenylene under an applied electric field, it will be argued that, due to the spatial extension of the singlet exciton, on-chain excitons are virtually degenerate with off-chain geminate electron–hole pairs at adjacent chains. The exciton binding energy is by and large the coulombic energy of the pair. The question of intrinsic photogeneration of unbound charge carriers will be discussed.