Probing the Relation Between Charge Transport and Supramolecular Organization Down to Ångström Resolution in a Benzothiadiazole‐Cyclopentadithiophene Copolymer

Abstract

Molecular modeling shows that longitudinal displacement of the backbones by a couple of ångströms has a profound impact on the electronic coupling mediating charge transport in a conjugated copolymer. These changes can be probed by monitoring the calculated X-ray scattering patterns and NMR chemical shifts as a function of sliding of the polymer chains and comparing them to experiment.

Solid-State Organization and Ambipolar Field-Effect Transistors of Benzothiadiazole-Cyclopentadithiophene Copolymer with Long Branched Alkyl Side Chains

“…Solid-state NMR is a unique method that offers access to molecular packing details for polymers and other organic-based materials [29][30][31][32][33] and to details about the donor-acceptor packing in CDT-BTZ polymers [34,35]. For CDT-BTZ-C16, we were able to show from a combination of X-ray diffraction and solid-state NMR that the CDT-BTZ polymer chains self-assemble into π-stacks, where the single polymer chain includes alternating CDT units, pointing in opposite directions (packing in a staggered fashion) and BTZ moieties stacked in an antiparallel configuration along the π-stacking direction [13].…”

“…To correlate the translational shift between π-stacked CDT-BTZ-C14,10 polymer chains to the charge-carrier transport properties, we compare the current results of the calculated electronic coupling in terms of the charge-transfer integral between stacked polymer chains in CDT-BTZ-C16 [35]. To recall, these calculations showed a cosine-like behavior of the HOMO-HOMO (hole-) and LUMO-LUMO (electron-) transfer integrals as a function of the translational shift between CDT-BTZ polymer chains with local minima at −1.0 and −1.8 Å, respectively.…”

“…Further, the amplitude and period of the HOMO-HOMO charge-transfer integral is larger and shorter, respectively, compared to the LUMO-LUMO charge-transfer integral. The different shapes of the HOMOs and LUMOs and their corresponding wave-function overlap can explain these differences in amplitude and period: the HOMOs are strongly localized on the BTZ acceptor groups, whereas the LUMOs are spread along the polymer backbone [35]. Although the charge-transfer integral calculations were performed at a shorter π-stacking distance of 3.6 Å for CDT-BTZ-C16 [35], we expect the trends to be comparable for CDT-BTZ-C14,10 at a stacking distance of 4.0 Å, however, with a reduced charge-transfer integral.…”

“…The different shapes of the HOMOs and LUMOs and their corresponding wave-function overlap can explain these differences in amplitude and period: the HOMOs are strongly localized on the BTZ acceptor groups, whereas the LUMOs are spread along the polymer backbone [35]. Although the charge-transfer integral calculations were performed at a shorter π-stacking distance of 3.6 Å for CDT-BTZ-C16 [35], we expect the trends to be comparable for CDT-BTZ-C14,10 at a stacking distance of 4.0 Å, however, with a reduced charge-transfer integral. Thus, the origin of the ambipolarity for CDT-BTZ-C14,10 is a result of increased donor-acceptor interactions caused by sliding of the CDT-BTZ polymer chains away from co-facially stacked BTZ acceptors.…”

Simulating charge transport in organic semiconductors and devices: a review

“…Examples such as this may give the impression that control or engineering of structural order is vital to enhancing mobility of OSCs, and indeed there are many examples of this [271][272][273][274][275]. However, the form of a percolation network is not only due to alignment (or not) or electronic transfer integrals between molecules, but also due to spatial variations in energetic disorder.…”