Light-Induced Effects Dominate Transconductance in OFETs
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then performed x-ray diffraction measurements and molecular modeling to demonstrate that the –3P SAM matched the molecules’ orientation and the packing of the nascent anthracene crystal in the c*-direction. Thus –3P SAMs are a nearly perfect template for the oriented nucleation of the anthracene crystals from the (001) plane. On the other hand, –CH3 SAMs have the least nucleation density of anthracene crystals and the least surface coverage. The researchers then addressed the possibility of controlling the location and the pattern of nucleation. To achieve a better selectivity of patterned growth, they designed the template with SAMs of two extremes bearing –CH3 and –3P. The research team used the microcontact printing (μCP) method to make micropatterned substrates with regions of –3P and –CH3 having different geometries and relative sizes. The team used both solvent evaporation and dip-coating methods to achieve selective growth of organic semiconductor molecules. The researchers observed highly localized crystal growth onto regions patterned with oligophenylene thiols. The surface coverage of crystals grown on micropatterned –3P regions increased to >95% versus 23% for the unpatterned –3P substrates. Using this method, the researchers obtained large oriented single crystals of anthracene overgrowing the entire underlying 100 μm × 100 μm square regions of –3P. The researchers said that the flux of molecules to the growing crystals in the –3P region will induce near-surface concentration gradients and deplete anthracene molecules from the less active alkylthiol areas. This is a promising way to achieve highly selective patterned growth of single-crystalline organic semiconductor with increased surface coverage, they said. TAO XU
Light-Induced Effects Dominate Transconductance in OFETs The physics of photoinduced charge transfer across organic interfaces in organic field-effect transistors (OFETs) is not well known, but is crucial to the development of devices such as active-matrix displays. Two researchers, V. Podzorov and M.E. Gershenson, from Rutgers University have now published their results for a mechanism of photoinduced charge transfer across the interface between an organic semiconductor (rubrene) and a polymer (parylene). In their article in the July 1 issue of Physical Review Letters (#016602), the researchers said that their discovery could be crucial for understanding optoelectronic devices such as active-matrix displays. They carried out their experiments on single-crystal rubrene OFETs with parylene as the gate dielectric. The researchers said that the frontgate FET design with a transparent dielectric and semitransparent gate electrode allows them to study the light-induced effects at the organic molecular crystal (OMC)–polymer interface over a wide spectral range that includes the absorption band of OMC. In the dark, the “as-prepared” single-crystal rubrene OFETs exhibit a very small field-effect threshold. The researchers said this indicates that the density of deep traps at the OMC–parylene inter
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