Field effect on the singlet and triplet exciton formation in organic/polymeric light-emitting diodes

Recently, both experimental and theoretical evidence indicate that the electrically-generated singlet and triplet exciton formation ratio (r(S/T)) in conjugated polymers can exceed the 1:3 spin statistics limit. However, the extent that the electric field influences r(S/T) is in controversy. By measuring the rates of photo- and electro-phosphorescence and fluorescence, Wilson et al.(6) concluded that the ratio r(S/T) is independent of the external electric field; Lin et al.(8) found that the ratio increases monotonically with the electric field by measuring the relative densities of singlet and triplet exciton in EL and PL processes; from a quantum-dynamical calculation, Tandon et al.(11) observed an abrupt increase in the ratio with respect to the electric field. In this work, we apply a correlated quantum-chemical approach coupled with a first-order perturbation, to investigate the relationship between the formation ratio and the electric field. We have calculated the influences of the electric field on the singlet and triplet exciton states as well as on the electron-hole free pair (charge-transfer) state. It is found that for p-phenylene-vinylene oligomer, the formation ratio of singlet excitons with respect to triplet excitons increases with the electric field.