Fabrication of high-mobility poly(3-hexylthiophene) transistors at ambient conditions

We report on the fabrication of high-mobility organic thin-film transistors (OTFTs) made and tested under ambient conditions. A bottom gate, bottom contact architecture was used with a layer of poly(3-hexylthiophene) deposited on a 50 nm thick Al2O3 dielectric with pre-patterned Au source and drain electrodes. Fluoroalkyl trichlorosilane treatment of the Al 2O3 dielectric was found to significantly improve device performance. The field-effect hole mobility reproducibly reached 0.2 cm 2 V-1 s-1 (best device 0.29 cm2 V-1 s-1) with an on/off ratio of 104. Electrical and synchrotron X-ray scattering characterization shows that an interaction at the FOTS/P3HT interface is responsible for the high performance of these devices. The fabrication method described here is carried out under ambient conditions and does not require any post-deposition annealing or vacuum drying steps for the organic film; therefore it can simplify the manufacturing of OTFTs. © 2010 Elsevier B.V. All rights reserved

Solvent vapor annealing of an insoluble molecular semiconductor

Solvent vapor annealing has been proposed as a low-cost, highly versatile, and room-temperature alternative to thermal annealing of organic semiconductors and devices. In this article, we investigate the solvent vapor annealing process of a model insoluble molecular semiconductor thin film - pentacene on SiO 2 exposed to acetone vapor - using a combination of optical reflectance and two-dimensional grazing incidence X-ray diffraction measurements performed in situ, during processing. These measurements provide valuable and new insight into the solvent vapor annealing process; they demonstrate that solvent molecules interact mainly with the surface of the film to induce a solid-solid transition without noticeable swelling, dissolving or melting of the molecular material. © 2010 The Royal Society of Chemistry

Room-temperature preparation of crystalline TiO<inf>2</inf> thin films and their applications in polymer/TiO<inf>2</inf> hybrid optoelectronic devices

Highly homogeneous crystalline TiO2 thin films with very low surface roughness were prepared by spin-coating of a TiO2 nanoparticle aqueous dispersion at room temperature without further heat treatment. Using these films as electron transporting layers, inverted structure hybrid photovoltaic cells (TiO2/P3HT) and light-emitting diodes (TiO2/F8BT) were demonstrated. The photovoltaic cells exhibited an energy conversion efficiency of 0.26%, which is similar to that of cells utilizing TiO2 layers sintered at high temperatures. Moreover, the light-emitting diodes showed an efficiency of 0.65 cd/A, which is higher than obtained from a reference inverted diode (0.1 cd/A). These measurements demonstrate that a properly designed nanoparticle casting route can help avoid high temperature crystallization or sintering steps for TiO2 thin films, paving the road for their use in conjunction with plastic substrates. © 2011 Elsevier B.V. All rights reserved

Structure vs. Property relationship in high mobility fused thiophene polymers

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. X-ray diffraction study of the polymer thin films suggests 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 one 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. © 2009 SPIE

Post-deposition reorganization of pentacene films deposited on low-energy surfaces

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. © 2009 The Royal Society of Chemistry

Tetrathienoacene copolymers as high mobility, soluble organic semiconductors

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 π-π distance of 3.76 Å 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. Copyright © 2008 American Chemical Society

Alkylsubstituted thienothiophene semiconducting materials: Structure-property relationships

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 fieldeffect 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. © 2009 American Chemical Society

Importance of C<inf>2</inf> symmetry for the device performance of a newly synthesized family of fused-ring thiophenes

We investigated the relationship between molecular structure and field-effect hole mobility in a family of fused-ring polythiophene copolymers that we designed recently. The results suggest that a repeat unit that possesses a C2-axis perpendicular to the conjugation plane is important to achieve a high mobility. Our finding is supported by a review of literature data. Many polymer semiconductors showing a hole or electron mobility of >0.1 cm2/(V s) feature a repeat unit with C2 symmetry; however, exceptions have been found from some push-pull polymer structures. © 2010 American Chemical Society