Charge Mobility and Transport Behavior in the Ordered and Disordered States of the Regioregular Poly(3-hexylthiophene)

We theoretically analyze the charge-transfer behavior of regioregular poly(3-hexylthiophene) (rr-P3HT) by quantum mechanical (QM) and molecular dynamics (MD) methods. In particular, we clarify the effects associated with the respective contribution from the ordered and disordered regions. In the ordered regions, the typical value of the hole mobility along the intrachain route is about 1 cm(2) V(-1) s(-1), which is significantly larger than that along the pi-pi interchain route, approximately 10(-2) cm(2) V(-1) s(-1). Our results indicate that the main charge-transfer route within the P3HT ordered lamellae is along the intrachain direction instead of the interchain direction. Moreover, the calculated hole mobility of 10(-2) cm(2) V(-1) s(-1) along the pi-pi interchain route is consistent with the experimental data measured in the P3HT single fibril. In the disordered regions, we propose a crossing-point/bridging-chain model to describe the charge-transport routes. In this model, the hole mobility can reach the limit of around 10(-2) and 1 cm(2) V(-1) s(-1) when the charge takes the interchain route through the crossing points and the intrachain route along the bridging chains, respectively. As expected, the resultant mobility in the disordered state is strongly affected by the ratio of the amount of crossing points and bridging chains. When considering the presence of both ordered and disordered regions, the average overall charge mobility is mainly dominated by the charge transport in the disordered regions. The fact that some of the experimentally measured hole mobility by varying the molecular weight is limited to a maximum value of 10(-2) cm(2) V(-1) s(-1) is mainly due to the presence of more crossing points instead of the bridging chains in the disordered regions. With increasing the amount of the bridging chains in the disordered regions, one can expect an enhancement of the charge mobility, such as to the experimentally obtained high value of 0.1 cm(2) V(-1) s(-1).