Increasing the rigidity of the thiophene monomer through the use of an alkyl-substituted core that consists of four fused thiophene rings is shown to be a promising route toward high-performance organic semiconductors. We report on a dialkylated tetrathienoacene copolymer that can be deposited from solution to yield ordered films with a short pi-pi distance of 3.76 A and with a field-effect hole mobility that exceeds 0.3 cm2/V.s. This polymer enables simple transistor fabrication at relatively low temperatures, which is particularly important for the realization of large-area, mechanically flexible electronics.

A family of conjugated polymers with fused structures consisting of three to five thiophene rings and with the same alkyl side chains has been synthesized as a means to understand structure-property relationships. All three polymers showed well-extended conjugation through the polymer backbone. Ionization potentials (IP) ranged from 5.15 to 5.21 eV; these large values are indicative of their excellent oxidative stability. X-ray diffraction and AFM studies suggest that the polymer with the even number of fused thiophene rings forms a tight crystalline structure due to its tilted side chain arrangement. On the other hand, the polymers with the odd number of fused thiophene rings packed more loosely. Characterization in a field-effect transistor configuration showed that the mobility of the polymer with the even number of rings is 1 order of magnitude higher than its odd-numbered counterparts. Through this structure-property study, we demonstrate that proper design of the molecules and properly arranged side chain positions on the polymer backbone can greatly enhance polymer electronic properties.

We demonstrate that small-molecule organic thin films of pentacene deposited from thermal and supersonic molecular beam sources can undergo significant reorganization under vacuum or in N 2 atmosphere, beginning immediately after deposition of thin films onto SiO 2 gate dielectric treated with hexamethyldisilazane (HMDS) and fluorinated octyltrichlorosilane (FOTS). Films deposited on bare SiO 2 remain unchanged even after extended aging in vacuum. The changes observed on low-energy surfaces include the depletion of molecules in the interfacial monolayer resulting in the population of upper layers via upward interlayer transport of molecules, indicating a dewetting-like behavior. The morphology of pristine, as-deposited thin films was determined during growth by in situ real-time synchrotron X-ray reflectivity and was measured again, ex situ, by atomic force microscopy (AFM) following aging at room temperature in vacuum, in N 2 atmosphere, and in ambient air. Important morphological changes are observed in ultra-thin films (coverage < 5 ML) kept in vacuum or in N 2 atmosphere, but not in ambient air. AFM measurements conducted for a series of time intervals reveal that the rate of dewetting increases with decreasing surface energy of the gate dielectric. Films thicker than $5 ML remain stable under all conditions; this is attributed to the fact that the interfacial layer is buried completely for films thicker than $5 ML. This work highlights the propensity of small-molecule thin films to undergo significant molecular-scale reorganization at room temperature when kept in vacuum or in N 2 atmosphere after the end of deposition; it should serve as a cautionary note to anyone investigating the behavior of organic electronic devices and its relationship with the initial growth of ultra-thin molecular films on low-energy surfaces.Small-molecule thin films of pentacene exhibit among the highest field effect mobilities reported to date, in large partly because of the propensity of pentacene to form highly ordered thin films with efficient p-stacking along the channel of the organic thin film transistor (OTFT). 1-3 The charge carrier mobility in pentacene thin films is presumed to be severely affected by defects and trap sites at grain boundaries located near the semiconductordielectric interface. 4-7 The first 2 or 3 monolayers (ML) of pentacene are especially important to OTFT applications, as they account for the majority of charge transport during transistor operation. 8 Attempts have been made to improve charge transport by chemically modifying the surface of the dielectric (typically SiO 2 ) with self-assembled monolayers (SAMs) and interfacial organic layers, 9,10 or by employing hydrophobic polymers as gate dielectrics. 11,12 The use of these hydrophobic coatings, such as hexamethyldisilazane (HMDS), octadecyltrichlorosilane (ODTS) and others, has improved device performance significantly, resulting in systematically higher values of field effect mobility for pentacene films deposited from both thermal and superson...