Enhancement of Field‐Effect Mobility Due to Surface‐Mediated Molecular Ordering in Regioregular Polythiophene Thin Film Transistors

Abstract

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.

Carrier Transport Enhancement in Conjugated Polymers through Interfacial Self-Assembly of Solution-State Aggregates

“…13,17,66,[69][70][71][72] As mentioned above, the structural and optical analyses are bulk measurements, whereas transport measurements in the context of OTFTs is far more sensitive to the polymer order and conformation at the interface. 11 Since surface functionalization has an enhancing effect limited to within approximately 10 nm of the modified interface, 11,40 we speculate that the measured improvements in local order averaged over the entire film depth may be even more pronounced at the interface, perhaps explaining the dramatic enhancement of carrier mobility by nearly two orders of magnitude.…”

“…11 Importantly it has been suggested that surface functionalization has an enhancing effect on interfacial order, with the effect limited to within approximately 10 nm of the modified interface, 11,40 which means that the increased molecular order of polymer film at the interface with the OTS-treated substrate may arise from the increased interfacial order in very thin (less than ca. 10 nm) P3HT layer.…”

“…10 nm) P3HT layer. Such benefits may arise from the interactions of the alkyl chains in the polymer with those of OTS, 40 and this calls for deeper investigation through in situ methods to reveal whether polymer adsorption is responsible for this. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 …”

“…[33][34][35][36] In case of polymers, the effects have been attributed to minimizing polar moieties 37,38 and inducing molecular orientation and interfacial order at the dielectric surface. [39][40][41][42] We therefore take the view that the enhanced mobility of species in a disentangled polymer solution can be synergistically coupled with appropriate chemical modifications of the substrate to enhance self-assembly and formation of high quality semicrystalline conjugated polymers.…”

Fine fibrillar and rectangular/hexagonal ordered grains of poly(3-hexyl thiophene) and poly(ethylene glycol) developed by seeding technique

“…In the realm of semiconductive polymers, large single rectangular crystals of 3-hexyl thiophene (3HT) 8 [35,36], nanowires of conjugated poly(p-phenylene ethynylene) derivatives [37], poly(3-hexyl thiophene) (P3HT) nanowhiskers [38], one-dimensional (1D) microwire P3HT single crystals [39], and self-organized two-dimensional (2D) P3HT supramolecules [40,41] were reported. Nascent lateral habits of solution-grown single crystals [42], surface-patterned single crystals [43][44][45][46][47], and conductive-dielectric-conductive single crystals [48][49][50] have also been reported.…”