Theory of molecular excitons in the phenyl-based organic semiconductors

Starting from the Pariser-Parr-Pople model of π conjugated systems, a model of the low-lying excitations of poly(paraphenylene) (PPP) and poly(para-phenylenevinylene) (PPV) is introduced, based on the bonding HOMO and LUMO states of the repeat units. The inter-phenyl hybridisation is treated as a phenomenological parameter. The model is solved using exact diagonalisation and the density matrix renormalisation group method. The predicted exciton energies for PPV agree well with absorption experiments on short oligomers and thin films. For a 15 phenylene-unit oligomer, an exciton gap of 2.7 eV is predicted, with a binding energy of ca. 1.4 eV. For PPP the exciton gap is 3.0 eV and the binding energy is ca. 1.7 eV.