Exciton Dissociation Dynamics in Model Donor-Acceptor Polymer Heterojunctions: I. Energetics and Spectra

TL;DR

This study investigates the electronic excited states in donor-acceptor polymer heterojunctions, analyzing how exciton binding energy and band off-set influence device behavior, with predictions on spectra and charge transfer processes.

Contribution

It introduces a model using a two-band exciton to predict vibronic spectra and explores the impact of vibrational relaxation and intersystem crossings on charge-transfer states.

Findings

Exciton binding energy determines whether the heterojunction acts as a light-emitting diode or facilitates charge transfer.

Vibrational relaxation significantly influences the formation of charge-transfer states.

Intersystem crossings may contribute to charge-transfer state formation after photoexcitation.

Abstract

In this paper we consider the essential electronic excited states in parallel chains of semiconducting polymers that are currently being explored for photovoltaic and light-emitting diode applications. In particular, we focus upon various type II donor-acceptor heterojunctions and explore the relation between the exciton binding energy to the band off-set in determining the device characteristic of a particular type II heterojunction material. As a general rule, when the exciton binding energy is greater than the band off-set at the heterojunction, the exciton will remain the lowest energy excited state and the junction will make an efficient light-emitting diode. On the other hand, if the off-set is greater than the exciton binding energy, either the electron or hole can be transferred from one chain to the other. Here we use a two-band exciton to predict the vibronic absorption and emission spectra of model polymer heterojunctions. Our results underscore the role of vibrational relaxation and suggest that intersystem crossings may play some part in the formation of charge-transfer states following photoexcitation in certain cases.