Role of Electron--Electron Interactions on Spin Effects in Electron--Hole Recombination in Organic Light Emitting Diodes

TL;DR

This paper investigates how electron-electron interactions influence spin effects in electron-hole recombination in organic LEDs, revealing that these interactions can generate high-energy states and enhance the singlet:triplet ratio beyond statistical predictions.

Contribution

It extends existing theories by including electron-electron interactions, showing their role in producing high-energy states and increasing the singlet:triplet yield ratio.

Findings

High-energy charge-transfer states can be generated during recombination.

Electron-electron interactions favor higher singlet:triplet yield ratios.

Recombination involves tunneling influenced by matrix elements and energy differences.

Abstract

We extend our theory of electron--hole recombination in organic light emitting diodes to investigate the possibility that high energy singlet and triplet excited states with large electron--hole separations are generated in such processes, over and above the lowest singlet and triplet excitons. Our approach involves a time-dependent calculation of the interchain / intermolecular charge--transfer within model Hamiltonians that explicitly include electron-electron interactions between the $\pi$-electrons. We show that the electron--hole recombination reaction can be viewed as a tunneling process whose cross section depends on both the matrix element of the interchain part of Hamiltonian and the energy difference between the initial polaron--pair state and the final neutral states. 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. The overall effect of the electron--electron interactions is to enhance the singlet:triplet yield ratio over the value of 0.25 predicted from statistical considerations that are valid only within noninteracting models.