Organic single-crystal field-effect transistors are ideal tools for the study of charge transport in organic semiconductor materials. Their high performance and outstanding electrical characteristics make them promising candidates for electronic applications such as active matrix displays and sensor arrays. In order to minimize cross-talk between neighbouring devices, patterning of the active semiconductor layer is necessary. Therefore, the need to manually select and place individual crystals represents a critical limitation for producing single-crystal devices at high density and with reasonable throughput. While arrays of inorganic crystals have been patterned over large areas, successful patterning of discrete organic molecular crystals has not yet been reported. We present a materials-general method of fabricating large arrays of patterned organic single crystals. We find that by microcontact-printing thin film domains of octadecyltriethoxysilane (OTS) onto a clean Si/SiO₂ surface, the nucleation of vapour-grown organic single crystals is restricted to the printed OTS domains. This patterning method allows the growth of crystals directly onto transistor source-drain electrodes, yielding large arrays of high-performance organic single-crystal field effect transistors with mobilities as high as 2.4 cm²/Vs and on/off ratios greater than 10⁷. We anticipate the described approach to represent a significant step towards the realization of large arrays of high-performance organic field-effect transistors.

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(Highlighted in MRS Bulletin, July 2006)

(Highlighted in MRS Bulletin, March 2007)

There has been a recent surge of interest in the fabrication of electronic devices from high quality single crystals of organic semiconductors. The importance of these materials stems from the fact that intrinsic charge transport properties are not influenced by grain boundaries and molecular disorder – the defects commonly present in polycrystalline organic thin films. Such defects cause low mobilities and therefore reduce the performance of organic thin film transistors (OTFTs). In contrast, mobilities as high as 20 cm²/Vs have been reported for single crystal rubrene transistors and as high as 35 cm²/Vs for ultra pure pentacene single crystals from space-charge-limited current measurements. Despite the high mobilities reported for single crystal devices, there are many factors limiting their applications. In addition to the difficulties of fabricating good electrical contacts on single crystals, the most challenging task is handling the fragile crystals. Currently, single crystals are handpicked and made into an individual device but this method is impractical for fabricating a high density of devices over a large area. Our long-term goal is to understand the growth behavior of organic semiconductors in order to selectively grow oriented single crystalline organic semiconductors in designated locations directly in a device structure. We report a method for inducing site-specific growth of large oriented organic semiconductor crystals using micropatterned self-assembled monolayers (SAMs) bearing the regions of oligophenylene thiols as nucleation templates.

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Organic semiconductor nanostructures are potential next-generation materials for displays, integrated circuits, solar cells, memory elements, and actuators. The advances in molecular design and synthesis has made it possible to tune the molecular structures and properties of organic semiconductors to meet the technological requirements for fabricating practical devices. Studies on aromatic molecules have shown that self-assembly through strong p-p interactions can lead to the formation of one-dimensional (1D) nanostructures preferred for field-effect transistors. Examples of self-assembled nanostructures include (but are not limited to) nanofibers, nanowires, and nanoribbons synthesized from hexabenzocoronene derivatives, perylene tetracarboxylic diimide derivatives, macrocyclic aromatic molecules, C_60, and metal phthalocyanines. One-dimensional nanostructures predominantly self-assemble along the p-p stacking direction, which is believed to favor a high charge-carrier mobility as a result of the strong intermolecular coupling between the packed molecules. It has recently been shown that p-stacking in oligoacene derivatives can generate high carrier mobilities in transistor devices. Among the oligoacenes, pentacene and its derivatives have thus far exhibited the best-reported device performance. Despite these promising results, there are very few reports on 1D nanostructures (e.g., nanowires) synthesized from oligoacenes, let alone pentacene derivatives. A possible explanation for this slow development is the relative scarcity of p-stacking pentacene derivatives.

One of the greatest challenges for organic thin-film transistors is to achieve an ultimate control of both film morphology and degree of crystallinity. Even the best-performing organic semiconductors inevitably contain molecular disorder and grain boundaries, which effectively reduces the mobility of the material. Organic single crystals, on the other hand, are known to have superior performance as they are largely free of grain boundaries or molecular disorder. However, their fragility and brittleness makes it very difficult to work with and reports of fabricating devices from solution-processable single crystals are somewhat unusual. Solution deposition offers a cost-effective method for fabricating large-area electronic components from organic materials. While solution-deposition techniques have been reported with some level of success, there are still unresolved issues in controlling the crystal packing and film-forming properties. Therefore, there is a growing need to explore organic single-crystal nanostructures as solution-processable materials, which are expected to have a profound impact on the future of molecular and flexible electronics.

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See my review in Materials Today on Organic Nanowire Transistors

(Highlight in NanoToday)

Organic single crystal devices are predominantly employed in charge-transport studies, yet, despite of their high performance, these devices have seen limited use in flexible electronics and other wide-range applications. One key reason for this shortcoming is poor mechanical properties of the single crystals. We demonstrate the controlled growth of flexible and conformable organic single crystals as thin as 100 nm for fabricating mechanically bendable organic single-crystal field-effect transistors on low-cost plastic substrates. We report saturated field-effect mobilities as high as 4.6 cm²/Vs for a flexible rubrene single-crystal transistor, on/off ratio of ~10⁶, threshold voltage of -2.1 V, and a normalized subthreshold swing of 0.9 V·nF/decade·cm². Furthermore, we find that our devices can be bent to a radius of less than 1 cm without any significant loss in performance and recover more than 90% of their original performance after bending to a radius of 5.9 mm. In addition to demonstrating comparable device characteristics to amorphous silicon, these devices are the first to reach mainstream flexible organic single-crystal transistors. As such, devices fabricated from high-quality conformable single-crystals opens up possibilities in practical applications such as low-cost flexible displays and wearable electronics.

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One-dimensional (1-D) organic semiconductor nanostructures formed via self-assembly are regarded as a promising class of nanomaterials for use in solution-processable organic field-effect transistors. Although field-effect transistors with excellent performance have been demonstrated by various groups for 1-D nanostructures of p-type organic semiconductors, very few examples of OFETs with 1-D n-channel semiconductors have been published, and those reported required high temperature synthesis. Electron-transporting semiconductors and n-channel OFETs are highly desired as they will enable the fabrication of complementary inverters, ring oscillators, and shift registers for radio frequency identification (RFID) tags and complex circuits. Among n-type semiconductors, the perylenediimides (PTCDIs) are the most widely studied due to their commercial availability, low cost, chemical stability, and promising electronic and optoelectronic applications. In addition, PTCDIs have a propensity to self-assemble into 1-D nanostructures through p-p stacking. Many studies of PTCDI nanowires (NWs) have been reported, but we are unaware of any report of their incorporation in OFETs. Here, we report both n-channel OFETs based on PTCDI NWs and complementary inverters achieved by combining them with the p-type hexathiapentacene (HTP) NWs we recently reported. The results of this study show that one can easily fabricate transistors and basic logic elements that exhibit excellent electrical characteristics via solution-processing.

Perylenetetracarboxyldiimide (PTCDI) nanowires self-assembled from commercially available materials are demonstrated as the n-channel semiconductor in organic field-effect transistors (OFETs) and as a building block in high-performance complementary inverters. Devices based on a network of PTCDI nanowires have electron mobilities and current on/off ratios on the order of 10^-2 cm²/Vs and 10⁴, respectively. Complementary inverters based on n-channel PTCDI nanowire transistors and p-channel hexathiapentacene (HTP) nanowire OFETs achieved gains as high as 8. These results demonstrate the first example of the use of one-dimensional organic semiconductors in complementary inverters.

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