Theory of the singlet exciton yield in light-emitting polymers.pdf

a r X i v : p h y s i c s / 0 4 1 0 2 5 2 v 1 [ p h y s i c s .c h e m - p h ] 2 6 O c t 2 0 0 4 Theory of the singlet exciton yield in light-emitting polymers William Barford ?Department of Physics and Astronomy, University of Sheffield, Sheffield, S3 7RH, United Kingdom Cavendish Laboratory, University of Cambridge, Cambridge, CB3 0HE, United Kingdom The internal electroluminescent quantum efficiency of organic light emitting diodes is largely deter- mined by the yield of singlet excitons formed by the recombination of the injected electrons and holes. Many recent experiments indicate that in conjugated polymer devices this yield exceeds the statisti- cal limit of 25% expected when the recombination is spin-independent. This paper presents a possible explanation for these results. We propose a theory of electron-hole recombination via inter-molecular inter-conversion from inter-molecular weakly bound polaron pairs (or charge-transfer excitons) to intra-molecular excitons. This theory is applicable to parallel polymer chains. A crucial aspect of the theory is that both the intra-molecular and inter-molecular excitons are effective-particles, which are described by both a relative-particle wavefunction and a center-of-mass wavefunction. This im- plies two electronic selection rules. (1) The parity of the relative-particle wavefunction implies that inter-conversion occurs from the even parity inter-molecular charge-transfer excitons to the strongly bound intra-molecular excitons. (2) The orthonormality of the center-of-mass wavefunctions ensures that inter-conversion occurs from the charge-transfer excitons to the lowest branch of the strongly bound exciton families, and not to higher lying members of these families. The inter-conversion is then predominately a multi-phonon process, determined by the Franck-Condon factors. These fac- tors are exponentially smaller for the triplet manifold than the singlet manifold because of the large exchange energy. As a co