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.

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

With the aim of enhancing the field‐effect mobility of self‐assembled regioregular poly(3‐hexylthiophene), P3HT, by promoting two‐dimensional molecular ordering, the organization of the P3HT in precursor solutions is transformed from random‐coil conformation to ordered aggregates by adding small amounts of the non‐solvent acetonitrile to the solutions prior to film formation. The ordering of the precursor in the solutions significantly increases the crystallinity of the P3HT thin films. It is found that with the appropriate acetonitrile concentration in the precursor solution, the resulting P3HT nanocrystals adopt a highly ordered molecular structure with a field‐effect mobility dramatically improved by a factor of approximately 20 depending on the P3HT concentration. This improvement is due to the change in the P3HT organization in the precursor solution from random‐coil conformation to an ordered aggregate structure as a result of the addition of acetonitrile. In the good solvent chloroform, the P3HT molecules are molecularly dissolved and adopt a random‐coil conformation, whereas upon the addition of acetonitrile, which is a non‐solvent for aromatic backbones and alkyl side chains, 1D or 2D aggregation of the P3HT molecules occurs depending on the P3HT concentration. This state minimizes the unfavorable interactions between the poorly soluble P3HT and the acetonitrile solvent, and maximizes the favorable π–π stacking interactions in the precursor solution, which improves the molecular ordering of the resulting P3HT thin film and enhances the field‐effect mobility without post‐treatment.

We have demonstrated the influence of evaporation‐induced flow in a single droplet on the crystalline microstructure and film morphology of an ink‐jet‐printed organic semiconductor, 6,13‐bis((triisopropylsilylethynyl) pentacene (TIPS_PEN), by varying the composition of the solvent mixture. The ringlike deposits induced by outward convective flow in the droplets have a randomly oriented crystalline structure. The addition of dichlorobenzene as an evaporation control agent results in a homogeneous film morphology due to slow evaporation, but the molecular orientation of the film is undesirable in that it is similar to that of the ring‐deposited films. However, self‐aligned TIPS_PEN crystals with highly ordered crystalline structures were successfully produced when dodecane was added. Dodecane has a high boiling point and a low surface tension, and its addition to the solvent results in a recirculation flow in the droplets that is induced by a Marangoni flow (surface‐tension‐driven flow), which arises during the drying processes in the direction opposite to the convective flow. The field‐effect transistors fabricated with these self‐aligned crystals via ink‐jet printing exhibit significantly improved performance with an average effective field‐effect mobility of 0.12 cm2 V–1 s–1. These results demonstrate that with the choice of appropriate solvent ink‐jet printing is an excellent method for the production of organic semiconductor films with uniform morphology and desired molecular orientation for the direct‐write fabrication of high‐performance organic electronics.

Device performances of BHJ solar cells based on poly[(4,4‐didodecyldithieno[3,2‐b:2’,3’‐d]silole)‐2,6‐diyl‐alt‐(2,1,3‐benzoxadiazole)‐4,7‐diyl]and PC71BM improve by capping the chain ends with thiophene fragments. This structural modification yields materials that are more thermally robust and that can be used in devices with thicker films – important considerations for enabling the mass production of plastic solar cells.