Optical charge carrier generation in a conjugated polymer

Abstract

Stationary photoconduction was measured on 100 nm thick films of poly(methylimino-1,4-phenylene-1-phenyl-1,2-ethenylene-2,5-dimethoxy-1,4-phenylene-2-phenyl-1,2-ethenylene-1,4-phenylene) sandwiched between indium tin oxide and aluminium electrodes. Bulk photoionization was observed upon irradiating through the positively biased aluminium electrode. For photon energies 2.6 eV < hv < 3.2 eV either defect catalyzed or intrinsic dissociation of relaxed singlet excitations prevails. For hv > 3.2 eV the carrier yield increases with excess photon energy due to intrinsic photoionization. The functional dependencies on electric field and photon energy are in accord with the 3D version of Onsager's theory of geminate pair dissociation implying a geminate pair distance that increases with hv. As an ionizing event, coupling between the non-relaxed initial Franck-Condon state with an mAg state is involved. Further increase of the yield correlates with the onset of optical absorption to the third optically allowed Bu state.