Multiphonon emission model of spin-dependent exciton formation in organic semiconductors

TL;DR

This paper presents a model for exciton formation in organic semiconductors, showing how multiphonon emission influences the ratio of singlet to triplet excitons, which affects OLED efficiency.

Contribution

It introduces a multiphonon emission model that explains the increase in exciton formation ratio with oligomer length, based on phonon emission differences.

Findings

Triplet exciton formation involves more phonons than singlet formation.

The Huang-Rhys parameter decreases with increasing oligomer length.

The model aligns with recent experimental data.

Abstract

The maximum efficiency in organic light-emitting diodes (OLEDs) 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 from a value $r \approx 1$ 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.