Photophysics of charge-transfer excitons in thin films of \pi-conjugated polymers

TL;DR

This paper develops a theoretical framework to understand the electronic structure and photophysical behavior of c0-conjugated polymer thin films, explaining experimental observations of excitons and charge-transfer states.

Contribution

It extends existing theories to describe interchain excitons and charge-transfer states, linking single-chain and film photophysics.

Findings

Charge-transfer excitons contribute to ultrafast photoinduced absorption in films.

The theory explains the coexistence of excitonic features and absence of free charges.

A one-to-one correspondence exists between single-chain and interchain excited states.

Abstract

We develop a theory of the electronic structure and photophysics of interacting chains of \pi- conjugated polymers to understand the differences between solutions and films. While photoexcitation generates only the intrachain exciton in solutions, the optical exciton as well as weakly allowed charge-transfer excitons are generated in films. We extend existing theories of the lowest polaronpair and charge-transfer excitons to obtain descriptions of the excited states of these interchain species, and show that a significant fraction of ultrafast photoinduced absorptions in films originate from the lowest charge-transfer exciton. Our proposed mechanism explains the simultaneous observation of polaronlike induced absorption features peculiar to films in ultrafast spectroscopy and the absence of mobile charge carriers as deduced from other experiments. We also show that there is a 1:1 correspondence between the essential states that describe the photophysics of single chains and of interacting chains that constitute thin films.