Charge separation in localized and delocalized electronic states in polymeric semiconductors

Conjugated polymers such as poly(p-phenylene vinylene)s (PPVs) allow low-cost fabrication of thin semiconducting films by solution processing onto substrates. Several polymeric optoelectronic devices have been developed in recent years, including field-effect transistors, light-emitting diodes, photocells and lasers. It is still not dear, however, whether the description of electronic excitations in these materials is most appropriately formulated within a molecular or semiconductor (band-theory) picture. In the former case, excited states are localized and are described as excitons in the latter they are delocalized and described as free electron-hole pairs. Here we report studies of the electronic states associated with optical excitations in the visible and ultraviolet range for the conjugated polymer poly(2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene) (MEH-PPV), by means of photocurrent measurements and quantum-chemical calculations. We find several photocurrent spectral features between 3 and 5 eV which are coupled with bands in the absorption spectrum. On modelling the excited states in this energy range, we have discovered an important feature that is likely to be general for materials composed of coupled molecular units: that mixing of delocalized conduction- and valence-band states with states localized on the molecular units produces a sequence of excited states in which positive and negative charges can be separated further at higher energies. In other words, these excited states facilitate charge separation, and provide a conceptual bridge between the molecular (localized) and semiconductor (delocalized) pictures.