Chain-length dependence of singlet and triplet exciton formation rates in organic light-emitting diodes

The operation and efficiencies of molecular or polymer organic light-emitting diodes depend on the nature of the excited species that are formed. The lowest singlet and triplet excitons display different characteristics that impact on the quantum yields achievable in the devices. Here, by performing correlated quantum-chemical calculations that account for both the electronic couplings and energetics of the charge-recombination process from a pair of positive and negative polarons into singlet and triplet excitons, we show that the formation rates for singlet over triplet excitons vary with chain length and favor singlet excitons in longer chains. Thus, in polymer devices, the resulting singlet/triplet fraction can significantly exceed the spin-statistical limit.