We demonstrate that, by using circular array of electrode pattern and applying multi-level phase modulation in each zone, a high-efficiency switchable electro-optic diffractive lens using liquid crystal as the active medium can be produced as a switchable eyewear. The lens is flat and the thickness of the liquid crystal is 5 μm. Two different designs are presented. In one design, all the patterned electrodes are distributed in one layer with a 1-μm gap between the electrodes. In the other design, the odd- and even-numbered electrodes are separately patterned in two layers without any lateral gaps between the electrodes. In both cases, vias are made for interconnection between the electrodes and the conductive wires. With the one-layer electrode design, both 1-diopter and 2-diopter 8-level lenses are demonstrated with an aperture of 10 mm. With the two-layer electrode design, a 2-diopter, 15-mm, 4-level lens is demonstrated. The diffraction efficiency of the 8-level lens can be higher than 90%. The ON- and OFF-state of the electrically controlled lens allow near- and distance-vision respectively for presbyopic eyes. The focusing power of the lens can be adjusted to be either positive or negative. The focusing power of the 8-level lens can be adjusted for near-, intermediate-, and distance vision. The lens is compact and easy to operate with fast response time, low voltages and low power dissipation. This is the first demonstration of the switchable lenses that almost meet the requirements for spectacle lens.
We present a comprehensive study of short channel effects in organic field-effect transistors by measuring the electrical characteristics of devices with fixed channel width and varying channel length. Our studies are conducted on a p-type organic semiconductor, (E,E-2,5-bis-{4'-bis-(4''-methoxy-phenyl)amino-styryl}-3,4-ethylenedioxy-thiophene that is spin-coated from solution to form bottom contact organic field-effect transistors. Drain-source currents from transistors with a channel length of 50 μm show excellent agreement with the square law equations derived for crystalline Si MOSFETs in both the linear and saturation regimes. As the channel length is incrementally reduced to 1 μm, device characteristics such as saturation regime channel conductance, sub-threshold current and threshold voltage, behave in a manner similar to Si MOSFETs of decreasing channel length. Results of these studies indicate the presence of non-destructive current punch-through and in addition, both channel-length modulation and threshold-voltage roll-off, neither of which have previously been reported in OFETs.
Significant progress has been made in the area of p-type organic field-effect transistors while progress in developing n-type materials and devices has been comparatively lacking, a limiting factor in the pursuit to develop complementary organic electronic circuits. Given the need for n-type organic semiconductors we have carried out studies using two different fullerene molecules, C60 and C70. Here, we report mobilities for C60 ranging from 0.02 cm2/Vs up to 0.65 cm2/Vs (depending on channel length), and mobilities from 0.003 cm2/Vs up to 0.066 cm2/Vs for C70. All devices were fabricated with organic films deposited under high vacuum but tested at ambient pressures under nitrogen.
Molecules designed to possess large dipole moments have been synthesized and doped into liquid crystals for the purpose of reducing the threshold voltage associated with the Freedericksz transition in nematics. These molecules were tested in the commercial nematic mixture E7 at varying concentrations and the resultant mixtures were evaluated to determine their nematic-isotropic transition temperature, threshold voltage, dielectric anisotropy, and visco-elastic constant as a function of dopant concentration. We have shown that these materials reduce the threshold voltage by up to 25 % at the 10 wt.% doping level but do so via a reduction of the splay elastic constant, not by the increase of the dielectric anisotropy as was expected. As a consequence of this result these dopants have also shown to increase the response time of the dopant/liquid crystal mixture.
A series of soluble arylamine-based hole transporting polymers with glass transition temperatures in the range of 97-108 degree(s)C have been synthesized. The synthetic methodology allows substitution of the aryl groups on the amine with electron-withdrawing and electron-donating moieties, which permits tuning of the redox potential of the polymer. The TPD-based monomers have been copolymerized with cinnamate-based moieties to obtain photo-crosslinkable polymers. These polymers have been used as hole-transport layers (HTLs) in multi-layer light-emitting diodes ITO/HTL/AlQ3/Mg:Ag [ITO=indium tin oxide, AlQ3=tris(8-hydroxyquinolinato)aluminum]. The maximum external quantum efficiency of the device increases as the redox potential of the HTL is increased. A fluorinated hole- transport polymer with a relatively high oxidation potential (390 mV vs ferrocenium/ferrocene) yielded the device with the highest external quantum efficiency and the longest lifetime under constant current operation. UV cross-linking was optimized to obtain an insoluble hole-transport layer with stable performance. Processing of these materials is compatible with a standard mask aligner used for photolithography. Electroluminescent devices have also been fabricated by spinning a blend of polystyrene and AlQ3 on top of the crosslinked hole-transport layer.
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