Alkoxy Side Chains in Low Band-Gap Co-Polymers: Impact on Conjugation and Frontier Energy Levels

Abstract

The molecular design of electron donor conjugated polymers with accurate optoelectronic properties for efficient bulk heterojunction photovoltaic devices has attracted a lot of attention worldwide. Ideally, the polymer should have i) appropriate photon harvesting properties, ii) frontier orbitals that allow both, efficient exciton dissociation and a high open circuit voltage, iii) high charge carrier mobilities, iv) high solubility and v) high molecular weights. Charge transfer D/A copolymers are promising materials that have allowed significant progress in this field. However, although a lot of attention has been given to the design of their molecular conjugated backbone, only few investigations have considered the influence of their solubilizing side chains. The aim of the present work is to show how the nature and the position of side chains may influence above mentioned polymer properties. For that purpose, our team has designed and characterized a series of donor-acceptor alternating conjugated polymers using 2,1,3- benzothiadiazole as electron-deficient unit, and thieno[3,2-b]thiophene as well as thiophenes moieties as electron-rich units. Various side-chains were introduced onto different positions along the same conjugated backbone and the molecular structure/polymer properties relationships have been investigated systematically.