Electronic structure and morphology of organic semiconductors and the impact of molecular modifications

Abstract

Conjugated copolymers consisting of alternating donor and acceptor units play a prominent role in organic electronics, as their properties can be flexibly adapted to the respective field of application by chemical modifications and exchange of donors or acceptors. A prerequisite for understanding how these materials function is a deep knowledge of their structure-function relationships. This requires methods with a molecular resolution. TREPR spectroscopy in particular is suitable for investigating the electronic structure and morphology of these materials. In this work, DFT calculations were carried out in addition and necessary evaluation routines were developed. First, the electronic structure and morphology of the polymer PCDTBT and fragments of different lengths were investigated. Based on these results, the influence of additional hexyl side chains was investigated in detail. For the dominant unit TBT, triplet formation pathways could be elucidated, in particular the direct excitation from the singlet ground state into the triplet state. Furthermore, by linking TBT with the already known acceptor NDI, the influence of two competing acceptors on the electronic structure was detected. Finally, two potential repeat units, NDI-T2 and T-NDI-T, were investigated for the polymer PNDIT2 and NDI-T2 was unequivocally identified as the dominant chromophore in the polymer.