Multiphonon emission model of spin-dependent exciton formation in organic semiconductors
Markus Wohlgenannt
Abstract
The maximum efficiency in organic light-emitting diodes (OLEDs) depends on the ratio, r=kS/kT, where kS (kT) is the singlet (triplet) exciton formation rate. Several recent experiments found that r increases with increasing oligomer length from a value r ≈ 1 in monomers and short oligomers. Here, we model exciton formation as a multi-phonon emission process. 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.
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