The semiconducting and metallic phases of conjugated polymers

Abstract

Recently a variety of $\pi$-conjugated polymers have been developed and essayed for a number of applications such as organic light-emitting diodes, organic field-effect transistors, organic photovoltaics, and sensors. Is central for these applications the semiconductor character of the pure materials, which can turn into metallic conductivity by local oxydation or reduction. Many very recent experiments show two characteristic peaks in the UV-Vis excitation spectra of the conjugated polymers of interest, both lying in the gap between the energies of the bonding $\pi$ and antibonding $\pi^*$ bands and having excitation energies in a ratio ranging from 1.4 to 1.7. The issue is that $\pi$-electrons are paired in covalent orbitals which interact strongly between them and with the ionic cores, thus being far from the extended quasi-free independent one-electron states assumed by the theory of inorganic semiconductors. A model yielding a mechanism for many-body conduction of charge and semiconducting properties of the undoped material is introduced here. The model yields two new flat bands of excited bonding states of Bose-Einstein statistics able of charge transport. The two bands explain well the characteristic pair of maxima of the UV-Vis excitation spectra of the conductive conjugated polymers.