With the aim of enhancing the field‐effect mobility by promoting surface‐mediated two‐dimensional molecular ordering in self‐aligned regioregular poly(3‐hexylthiophene) (P3HT) we have controlled the intermolecular interaction at the interface between P3HT and the insulator substrate by using self‐assembled monolayers (SAMs) functionalized with various groups (–NH2, –OH, and –CH3). We have found that, depending on the properties of the substrate surface, the P3HT nanocrystals adopt two different orientations—parallel and perpendicular to the insulator substrate—which have field‐effect mobilities that differ by more than a factor of 4, and that are as high as 0.28 cm2 V–1 s–1. This surprising increase in field‐effect mobility arises in particular for the perpendicular orientation of the nanocrystals with respect to the insulator substrate. Further, the perpendicular orientation of P3HT nanocrystals can be explained by the following factors: the unshared electron pairs of the SAM end groups, the π–H interactions between the thienyl‐backbone bearing π‐systems and the H (hydrogen) atoms of the SAM end groups, and interdigitation between the alkyl chains of P3HT and the alkyl chains of the SAMs.
To investigate the effects of the phase state (ordered or disordered) of self-assembled monolayers (SAMs) on the growth mode of pentacene films and the performance of organic thin-film transistors (OTFTs), we deposited pentacene molecules on SAMs of octadecyltrichlorosilane (ODTS) with different alkyl-chain orientations at various substrate temperatures (30, 60, and 90 degrees C). We found that the SAM phase state played an important role in both cases. Pentacene films grown on relatively highly ordered SAMs were found to have a higher crystallinity and a better interconnectivity between the pentacene domains, which directly serves to enhance the field-effect mobility, than those grown on disordered SAMs. Furthermore, the differences in crystallinity and field-effect mobility between pentacene films grown on ordered and disordered substrates increased with increasing substrate temperature. These results can be possibly explained by (1) a quasi-epitaxy growth of the pentacene film on the ordered ODTS monolayer and (2) the temperature-dependent alkyl chain mobility of the ODTS monolayers.
Obtaining control over the supramolecular organization of electronically active p-conjugated polymers [1] would make it possible to fine-tune and optimize their electrical properties for applications in organic field-effect transistors (OFETs) [2][3][4][5] and sensors. [6,7] Typically, it is far more challenging to obtain high-quality single crystals of conjugated polymers by facile solution processing than oligomers [8] and small molecules, [9] which are prepared by vacuum processes. However, 1D, highquality, single-crystal semiconductors comparable to inorganic single crystals, such as silicon nanowires, have not hitherto been observed for conjugated polymers. Self-organized poly(3-hexylthiophene) (P3HT), [10][11][12][13][14][15][16][17][18] with its supramolecular 2D structure, is of special interest because the 1D electronic properties of the p-conjugated polymer chains are modified by the increased interchain stacking that results from p-p interactions. Therefore, the possibility of achieving good electrical performance as a result of 2D transport (i.e., band-like transport) in self-organized single-crystal P3HT has spurred its use in enhanced polymer electronic devices (PEDs). By better control of structural anisotropy, and by developing P3HT structures with strongly p-p interacting building blocks coinciding with the direction of current flow in PEDs, optimized electrical performance and, possibly, a truly delocalized transport regime may be attained. We report here the preparation and properties of high-quality, 1D single-crystal P3HT microwires grown by a facile self-assembly process in dilute solution. Figure 1 outlines the fabrication steps for the preparation of low-voltage, gate-modulated PEDs based on well-faceted, 1D single-crystal P3HT microwires. Dense octadecyltrichlorosilane (ODTS) self-assembled monolayers (SAMs) (structure shown in Fig. 1A) possessing sufficient robustness are used as the molecular dielectrics to reduce the operating voltage of COMMUNICATIONS
Summary: Nanowire lengths and length‐to‐width aspect ratios in regioregular poly(3‐hexylthiophene) (P3HT) were simply controlled through changes in the solvent vapor pressure during solidification. It is demonstrated that the nanowires grew by rod‐to‐rod association, in which the molecular long axis of the P3HT chains appeared to be well‐oriented parallel to the silicon substrate (Si/SiOx). The formation of the nanowires took place by one dimensional self‐assembly, governed by π‐π stacking of the P3HT units.TEM high contrast images showing P3HT nanowires fabricated by spin‐coating under a solvent vapor pressure.magnified imageTEM high contrast images showing P3HT nanowires fabricated by spin‐coating under a solvent vapor pressure.
With the aim of improving the field-effect mobilities in poly(3-hexylthiophene) (P3HT) thin film transistors, we controlled the nanostructures of P3HT thin film by changing the solvent vapor pressure in a spin-coating chamber during solidification. The transistors with P3HT thin films spin-coated under a high solvent vapor pressure (56.5 KPa), showing the one-dimensional nanowire morphologies, resulted in the relatively high field-effect mobilities (0.02 cm2/(V.s)) that are typically more than 1 order of magnitude higher than those prepared under ambient conditions, showing the featureless morphologies. This can be attributed to the higher solvent vapor pressure during film formation, providing the solvent is allowed to evaporate slowly and the degree of ordering within the P3HT crystalline domains is dramatically improved.
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