Morphological and charge transport properties of amorphous and crystalline P3HT and PBTTT: insights from theory

We explore the relation between the morphological and the charge transport properties of poly(3-hexylthiophene) (P3HT) and poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) semiconductor polymers in both amorphous and crystalline phases. Using molecular dynamics to simulate bulk supercells and the Marcus theory to analyze the transport properties we found that amorphous systems display a reduced hole mobility due to the loss of nematic order and π-π stacking leading to a reduction in the electronic coupling between two chains. In the crystal phase, PBTTT displays a larger charge mobility than P3HT due to the interdigitation of the side chains enhancing the stability of the conjugated rings on the backbones. This more stable π-π stacking reduces the energetic disorder with respect to P3HT and increases the electronic coupling. In contrast, in the amorphous phase, PBTTT displays a reduced charge mobility with respect to P3HT due to the absence of side chains attached to the thienothiophenes, which increases their fluctuations and the energetic disorder. In addition, we show that it is possible to calculate the reorganization energy neglecting the side chains of the polymers and thus saving computational time. Within this approximation, we obtained mobility values matching the experimental measurements, thus confirming that the side chains are crucial to shape the morphology of the polymeric systems but are not involved in the charge transport process.