Yield of exciton dissociation in a donor-acceptor system

TL;DR

This paper presents a simple theoretical model for exciton dissociation in bulk heterojunction solar cells, analyzing how physical parameters influence charge separation efficiency and decay processes.

Contribution

It introduces a model that describes exciton dissociation and subsequent electron dynamics considering various physical parameters in OPVs.

Findings

Dissociation depends on tunneling amplitude and energetics.

Charge loss processes include interface escape and recombination.

Model highlights the importance of LUMO/LUMO offset.

Abstract

A simple model is constructed to describe dissociation of charge transfer excitons in bulk heterojunction solar cells, and its dependence on the physical parameters of the system. In bulk heterojunction organic photovoltaics (OPVs), exciton dissociation occurs almost exclusively at the interface between the donor and acceptor, following one-electron initial excitation from the HOMO to the LUMO levels of the donor, and charge transfer to the acceptor to make a charge-transfer exciton. After exciton breakup, and neglecting the trapping of individual carriers, the electron may undergo two processes for decay: one process involves the electron and/or hole leaving the interface, and migrating to the electrode. This is treated here as the electron moving on a set of acceptor sites. The second loss process is radiationless decay following recombination of the acceptor electron with the donor cation; this is treated by adding a relaxation term. These two processes compete with one another. We model both the exciton breakup and the subsequent electron motion. Results depend on tunneling amplitude, energetics, disorder, Coulomb barriers, and energy level matchups, particularly the so-called LUMO/LUMO offset.