Theory of singlet exciton yield in light-emitting polymers

The internal electroluminescent quantum efficiency of organic light-emitting diodes is largely determined 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 statistical 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 intermolecular interconversion from intermolecular weakly bound polaron pairs (or charge-transfer excitons) to intramolecular excitons. This theory is applicable to parallel polymer chains. A crucial aspect of the theory is that both the intramolecular and intermolecular excitons are effective particles, which are described by both a relative-particle wave function and a center-of-mass wave function. This implies two electronic selection rules. (i) The parity of the relative-particle wave function implies that interconversion occurs from the even parity intermolecular charge-transfer excitons to the strongly bound intramolecular excitons. (ii) The orthonormality of the center-of-mass wave functions ensures that interconversion 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 interconversion is then predominately a multiphonon process, determined by the Franck-Condon factors. These factors are exponentially smaller for the triplet manifold than the singlet manifold because of the large exchange energy. As a consequence, the interconversion rate in the triplet manifold is significantly smaller than that of the singlet manifold, and indeed it is comparable to the intersystem crossing rate. Thus, it is possible for the singlet exciton yield in conjugated polymers to be considerably enhanced over the spin-independent recombination value of 25%.