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

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.

Molecular-weight dependence of interchain polaron delocalization and exciton bandwidth in high-mobility conjugated polymers

“…Here the chain lengths are calculated from the number average molecular weight. Unlike the relatively low MW P3HT forming rigid nanorods composed of extended chain lamellae with a width approximate to single molecular length, 14 our result indicates that polymer chains above certain MW range tend to chain fold several times inside a nanoribbon. Some well-studied polymers such as polyethylene also show relatively little MW dependence of the crystal thickness under certain processing conditions.…”

“…[9][10][11] The mobility depends critically on manifold other factors such as molecular weight, interaction between polymer and solvent, and deposition conditions. 12 Recent research by McGehee et al 13,14 and Neher et al, 15 focusing on the effects of molecular weight ͑MW͒, has shown that MW is another dominant factor to affect mobilities by several orders of magnitude in a regime with mobilities less than 10 −2 cm 2 / V s. They found that the mobility increases with MW in spite of reduced crystallinity at higher MW, indicating that charge transport depends on factors other than the degree of crystallinity. Two possible explanations were proposed respectively.…”

“…Neher et al suggested that the mobility-MW relationship is mainly due to backbone conformation, and presented evidence that larger intrachain ring torsions tend to be present in low MW P3HT reducing the effective conjugation lengths for charge hopping. However, to date a microscopic understanding of the dependence of interchain interactions on molecular weight, and of the factors that limit the field-effect mobility in P3HT to values around 0.1 cm 2 / V s is still lacking, as most previous studies were performed in a mobility range of 10 −3 -10 −2 cm 2 /V s. [13][14][15] In this paper we present a systematic study of the correlation of the electrical, structural, and spectroscopic properties of P3HT as a function of MW in a high-mobility regime from 10 −3 to 10 −1 cm 2 / V s. A broad range of MW from 15 kD to 270 kD is investigated. In Sec.…”

Influence of blend microstructure on bulk heterojunction organic photovoltaic performance

“…4 In transistor studies of P3HT films over a range of MWs, it has been shown that higher MW P3HT has better quality crystallites which are more co-aligned and connected, possibly by individual polymer chains spanning between crystallites 5 leading to high field effect transistor (FET) mobility. Low MW polymer exhibits reduced transistor performance, which has been explained either by grain boundary trapping effects at lower MW, 4 or enhanced out-of-plane backbone twisting, 6 which results in a reduction in backbone planarity and a resulting decrease in effective conjugation length and the efficiency of charge hopping. Solution studies upon higher MW P3HT have also shown that physical entanglements can initiate polymer aggregation in some solvents, which then leads to gellation.…”

Influence of the Molecular Weight and Size Dispersion of the Electroluminescent Polymer on the Performance of Air-Stable Hybrid Light-Emitting Diodes

“…[1][2][3] This has been reported in significant detail for many organic electronic devices, in particular organic solar cells and organic transistors, 4 where it has been demonstrated that the behaviour of the polymer depends on its molecular weight, which can be difficult to control and can lead to large differences in device performance. 5 Furthermore, the chain length and its size distribution are also key factors that determine the packing of the polymer chains and their orientation towards the electrodes. 6 Indeed, the distance between the aromatic groups of adjacent chains also varies with the polymeric Mw resulting in different charge mobilities.…”