Dependence of Regioregular Poly(3-hexylthiophene) Film Morphology and Field-Effect Mobility on Molecular Weight

TLDR

Morphological studies have linked the charge‑carrier mobility of regioregular poly(3‑hexylthiophene) thin films to their number‑average molecular weight. Atomic‑force microscopy, X‑ray diffraction, and grazing‑incidence scattering reveal that low‑MW films form highly ordered nanorods while high‑MW films adopt a less ordered isotropic nodule structure; altering morphology at constant MW markedly improves mobility in low‑MW films but not in high‑MW films, and although in‑plane π‑stacking correlates with mobility for fixed MW, it does not explain the overall mobility–MW relationship, which the authors attribute to disordered domain boundaries and chain‑length effects on electronic structure.

Abstract

Morphological characterization has been used to explain the previously observed strong correlation between charge carrier mobility measured with thin-film transistors and the number-average molecular weight (MW) of the conjugated polymer regioregular poly(3-hexylthiophene). Atomic force microscopy and X-ray diffraction show that the low-mobility, low-MW films have a highly ordered structure composed of nanorods and the high-mobility, high-MW films have a less ordered, isotropic nodule structure. Modifying the morphology for a constant MW by changing the casting conditions or annealing the samples strongly affects the charge transport and morphology in the low-mobility, low-MW films, but has little effect on the high-MW films. In-plane grazing incidence X-ray scattering shows the in-plane π-stacking peak increases when the mobility increases for a constant MW. When the MW is changed, this correlation breaks down, confirming that in-plane π-stacking does not cause the mobility−MW relationship. We believe a combination of disordered domain boundaries and inherent effects of chain length on the electronic structure cause the mobility−MW relationship.