Photo-excitation in conjugated polymers

Abstract

Conjugated polymers such as polyacetylene form a group of model semiconductors in which the 'one-dimensional' character of the polymer strongly modifies the behaviour of charges added to the chains from that usually observed in three-dimensionally bonded semiconductors. First, the polymer chain can undergo local reorganisation of the pi -electron bonding in the vicinity of a charge added to it. This has the effect of localising the charge in a state that can be described as a soliton for the particular case of trans-polyacetylene, and more generally as a polaron. Secondly, the motion of charged excitations is highly anisotropic, with strong confinement to the chain through the 'polaronic' structural relaxation of the chain. This anisotropy strongly affects charge transport and charge separation following photo-excitation. The authors review some of the recent work in this area and present polarisation-dependent measurements of photoluminescence (PL) and photo-induced absorption (PA) on highly oriented films of polyacetylene and poly(phenylenevinylene). They find that charge separation, detected through PA from photogenerated solitons or bipolarons, is the result of inter-chain electron transfer, whereas intra-chain excitation leads to charge confinement, and decay by PL (for poly(phenylenevinylene)) and by non-radiative channels. They find furthermore that these non-radiative channels become more efficient as the length of the conjugated polymer chain increases, and consider that this occurs through rapid motion of the excited state to recombination centres. They have identified both monomolecular and bimolecular kinetics for these processes.