Joshua N. Haddock

Large-aperture switchable thin diffractive lens with interleaved electrode patterns

The authors report on a high-performance large-aperture switchable diffractive lens using nematic liquid crystal that can be used as an adaptive eyewear. The odd- and even-numbered ring electrodes are separated in two layers, avoiding the gaps between the neighboring electrodes and allowing high diffraction efficiency. It is easier to avoid shorts between neighboring conductive electrodes and fabricate lenses with larger aperture and smaller feature size. With a four-level phase modulation, a 15mm aperture, 2dpt lens with small aberrations and diffraction efficiency of above 75% could be demonstrated with low operating voltages. The thickness of the liquid crystal is only 5μm. The lens switching time is about 180ms. The on and off states of the electrically controlled lens allows near and distance vision, respectively. The focusing power of the lens can be adjusted to be either positive or negative. This structure can be extended to higher-level phase modulation with even higher efficiencies.

Comparison of Pentacene and Amorphous Silicon AMOLED Display Driver Circuits

Organic light-emitting diode (OLED) displays offer distinct advantages over liquid crystal displays for portable electronics applications, including light weight, high brightness, low power consumption, wide viewing angle, and low processing costs. They also are attractive candidates for highly flexible substrates. In active-matrix OLED (AMOLED) displays, a small transistor circuit is used to drive each OLED device. This paper compares the simulated performance of two state-of-the-art AMOLED drivers with a proposed 5 thin-film-transistor (TFT) voltage programmed driver circuit which combines the advantages of the first two configurations. A competitive evaluation is also done between amorphous silicon (alpha-Si) and organic TFTs (OTFTs,) using comparable empirical device models for alpha-Si) and pentacene OTFTs. The 5-TFT circuit is found to match the speed of the 2-TFT while achieving a stability closer to the 4-TFT circuits and demonstrating a better speed-stability tradeoff.

Electro-Optic Adaptive Lens as a New Eyewear

Adaptive lens with the capability of changing the focusing power has important applications in 3D imaging, optical information processing, and ophthalmology. We demonstrate a switchable electroactive lens with very high diffraction efficiencies using a circular array of electrode pattern filled with liquid crystal as the active medium to be used as an adaptive eyewear. Electrically adjustable lens allows the focal length to be voltage controlled without bulky and inefficient mechanical movement. Binary Fresnel zone plates using liquid crystal as active material have been demonstrated for imaging applications, but the diffraction efficiency is low. The lens is flat and the thickness of the liquid crystal is 5 μm. Diffraction efficiencies exceeding 90% has been achieved for an 8-level diffractive lens. The lens can be operated as both positive and negative lens. Design, modeling, fabrication, and characterization of the lens is presented. Using nematic liquid crystal, the lens is polarization dependent and two crossed lenses are integrated to form a complete lens. The ON- and OFF-state of the electrically controlled lens allow near- and distance-vision respectively for presbyopia eyes.

SPICE Optimization of Organic FET Models Using Charge Transport Elements

We report on a modeling technique that uses charge transport equations to calculate channel current in organic field effect transistors (OFETs) by numerical solution in the SPICE simulation program. SPICE is also used to optimize the model and achieve a fit to measured characteristics within 5% error. The overall modeling technique is a bridge between physical models of charge transport and a SPICE model useful in circuit simulation without requiring a closed-form drain-current equation. The automatic optimization of the simulation to measured curves will also allow, in the future, the empirical weighing of various charge transport effects in search of physical device operation, given sufficient empirical data. This modeling technique was applied to the measured characteristics of an OFET using pentacene in which the mobility was dependent on the voltage in the channel. The accuracy of the fit was better than 5% for 40 V > VDS > 7V and better than 20% for VDS > 7V. Simulation was completed within 3 min for this optimization on a modern personal computer.

A comparative study of charge mobility measurements in a diamine and in a hexaazatrinaphthylene using different techniques

A comparative study of hole-mobility in N,N′-diphenyl-N,N′-di(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) using three independent measurement techniques is presented. Time-of-flight (TOF), steady-state space charge-limited current (SCLC) experiments are carried-out in neat films of TPD, sandwiched between electrodes with different work functions. Organic field-effect transistors (OFET) are fabricated from TPD thin films and their electrical properties characterized. The comparative results show that all-electrical measurements that involve carrier injection from a metal electrode into the organic film yield effective mobility values that are one order of magnitude lower than those obtained from time-of-flight experiments in which carriers are created in the film optically. Similarly, SCLC and TOF experiments were used to measure electron-mobility in a discoid organic compound 2,3,8,9,14,15-hexakis-dodecylsulfanyl-5,6,11,12,17,18-hexaazatrinaphthylene (HATNA-(SC12H25)6).

Colorless Molecular Dopants for Low-Operating-Voltage Nematic Liquid Crystals

ABSTRACT In this paper we present a study of two similar, colorless molecular dopants designed to decrease the operating voltages of nematic liquid crystals by increasing the dielectric anisotropy. Experimental analysis shows that these materials reduced the threshold voltage (voltage associated with the Freedericksz transition) by up to 20%, but via a reduction in the liquid crystal order parameter. To gain insight into the decrease of the order parameter, we used density-functional and single-point MP2 theory to investigate the conformational preferences of our dopants; the calculations suggest that, because of steric interactions, the conformations adopted are rather rigid.

Thin-film fullerenes for organic field-effect transistors and complementary digital logic circuits

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.