Excitons in disordered polymers

TL;DR

This paper studies how different types of disorder in conjugated polymers affect exciton properties, including density of states, localization, and optical absorption spectra, using large-scale numerical simulations.

Contribution

It introduces a model for excitons in disordered polymers considering angular disorder and analyzes its effects on exciton localization and spectra.

Findings

Segmental disorder causes large fluctuations in transfer integrals.

Worm-like disorder results in small, Gaussian-distributed angular fluctuations.

Disorder influences the optical absorption spectra and exciton localization.

Abstract

We investigate the effects of disorder on Frenkel excitons in disordered conjugated polymers with allowed rotations about single bonds. In these materials, the principal effect of the disorder is to modify the transfer integrals appearing in the exciton Hamiltonian by changing the angle of rotation between single-bonded segments in a random manner. It is assumed the integrals have the form Jk,k+1 cos(qk-qk+1) where Jk,k+1 is taken to be a constant and qk denotes the orientation angle of the kth planar segment. Two types of disorder can be present: segmental disorder which is characterized by infrequent, large fluctuations in qk-qk+1 and a worm-like disorder marked by repeated, small fluctuations in the angular difference that are governed by a Gaussian distribution. We calculate the density of states and the localization lengths for the exciton modes in chains of 5x10^6 segments by mode counting techniques and the optical absorption spectra by direct matrix diagonalization. Both types of disorder are investigated. Particular emphasis is placed on relating the features of the absorption spectra to the distribution and localization of the underlying exciton modes.