Single-step multiphonon emission model of spin-dependent exciton formation in organic semiconductors

The maximum efficiency in organic light-emitting diodes depends on the ratio, r=k(S)/k(T), where k(S) (k(T)) is the singlet (triplet) exciton formation rate. Several recent experiments found that r increases with increasing oligomer length starting from a value r approximate to 1 in monomers and short oligomers. Here, we model exciton formation as a single recombination step accompanied by multiphonon emission. Our model is based on two assertions. (i) More phonons are emitted in triplet formation than in singlet formation. (ii) The Huang-Rhys parameter for this phonon emission is smaller in long oligomers than in short ones. We justify these assertions based on recent experimental and theoretical data.