Role of electron-electron interactions on spin effects in electron-hole recombination in organic light emitting diodes

We investigate the possibility that high-energy singlet and triplet excited states with large electron-hole separations are generated in electron-hole recombination process, over and above the lowest singlet and triplet excitons. Using a time-dependent calculation of the interchain/intermolecular charge-transfer within model Hamiltonians that explicitly include electron-electron interactions between the π-electrons, we show that there occurs a bifurcation of the electron-hole recombination path in each of the two spin channels that leads to the generation of both the lowest energy exciton and a specific-high-energy charge-transfer state, with the matrix elements favoring the lowest energy exciton and the energy difference factor favoring the higher energy state, resulting in an enhancement of the singlet:triplet yield ratio above the value of 0.25 predicted from statistical considerations.