Free charge carrier formation in polymers under illumination

Abstract

In addition to many extrinsic processes, free charge carriers in polymers can be photogenerated intrinsically via an intermediate stage of bound ion-pairs formed in consequence of band-to-band transitions or exciton autoionization. The excitation can be realized within the framework of (i) a chemical bond in the polymer backbone or (ii) a side group skeleton. An electron-hole pair with a short separation distance created by light on the same polymer chain or on the same side group usually recombines geminately very fast and does not contribute to the photocurrent. An inter- or intrachain charge transfer or transfer of the electron from the main chain to a side group is necessary to form a more stable ion-pair. Dissociation of the ion-pairs by Brownian motion subjected to a combination of Coulomb and applied electrical field, into free charge carriers, can be described in terms of the Onsager theory of geminate recombination. The photogeneration yield can be improved by an electron acceptor doping or by an electron accepting groups attached to side chains.