Sidney S. Yang

Cesium carbonate as a functional interlayer for polymer photovoltaic devices

The device characteristics of polymer solar cells with cesium carbonate (Cs2CO3) as an electron-injection interlayer have been investigated. It is found that the insertion of Cs2CO3 at the cathode interface improves the device power conversion efficiency from 2.3% to 3.1%. In order to further understand the mechanism, the interfacial interaction between the active organic layer and the cathode was studied by x-ray photoemission spectroscopy (XPS). The results of XPS measurement indicate the fact that a portion of electrons transfer from the interlayer into the organic layer, resulting in n-type doping. The n-doping effect enhances the efficiency of electron injection and collection. Further, the maximum open-circuit voltage (Voc) was determined from its temperature dependence. For the device with Cs2CO3, the maximum Voc is extremely close to the corresponding value of the energy difference between the highest occupied molecular orbital of the electron donor and the lowest unoccupied molecular orbital of t...

Chemical and Electrical Properties of Low-Temperature Solution-Processed In–Ga–Zn-O Thin-Film Transistors

We have fabricated a bottom-gate thin-film transistor (TFT) with the active layer of indium-gallium-zinc-oxide (IGZO) nanoparticles prepared from water solution. Polycrystallized InGaZn2O5 films were formed by a spin-coating method and postbaked at low temperature (95°C). The morphology of the IGZO film indicates that not only are the grain size and shape uniform but also the cross section of the film appears dense, considering that the films were fabricated by a solution process at low temperature. The TFT device shows good switching ability, high drain current (> 105 A), and low threshold voltage (0.9 V). Its field-effect mobility of 2.3 cm2/V · s and ON-OFF current ratio over 106 are very promising for TFT applications.

Characteristic Enhancement of Solution-Processed In–Ga–Zn Oxide Thin-Film Transistors by Laser Annealing

We applied laser annealing on indium-gallium-zinc oxide (IGZO) thin-film transistors (TFTs) to improve their electrical characteristics. The IGZO nanoparticles were prepared from a water solution, and the films were fabricated by the spin-coating method and postbaked at a low postbake temperature (95°C). The crystallinity and compositions of the as-deposited film and the laser-annealed films were analyzed by X-ray diffraction. The film morphology and the electrical characteristics of TFTs at various irradiation dosages are also presented. Results show that the field-effect mobility can be improved to 7.65 cm2/V·s.

Mass transport following impulsive optical excitation

Transition from reverse-saturable absorption to saturable absorption of the chloroaluminum phthalocyanine solution excited by a giant laser pulse is ascribed not just to the saturation of excited state absorption, but also to the outward migration of the solute molecules at the laser beam center. While the saturation of excited state absorption occurs within a single picosecond laser pulse, the beam center population decrease is sustained much longer than the pulse duration. We distinguish these two mechanisms with the Z-scan technique, utilizing picosecond pulses with pulse-to-pulse separations ranging from 0.1 to 5.0 s.

Pi-shape gate polycrystalline silicon thin-film transistor for nonvolatile memory applications

In this work, we studied a pi-shape gate polycrystalline silicon thin-film transistor poly-Si TFT with silicon-oxide-nitride-oxide-silicon SONOS layers and nanowire channels for the application of electric driver and nonvolatile memory. The proposed pi-gate TFT-SONOS has superior transfer characteristics and its output characteristic also exhibits the high driving current and the suppression of the kink effect. For memory application, the device can provide high program/erase efficiency and large threshold voltage shift under adequate bias operation. The enhanced performance for the pi-gate TFT-SONOS is attributed to the larger effective channel width and the number of channel corners.

Wavelength tunable single mode Nd:GdVO 4 laser using a volume Bragg grating fold mirror

A Nd:GdVO4 laser was built with a volume Bragg grating as the fold mirror, wavelength selector and spectral narrowing element. Over 2 nm tuning with wavelength centered at 1063.39 nm was achieved by angular tuning.

Z-scan study of thermal nonlinearities in silicon naphthalocyanine-toluene solution with the excitations of the picosecond pulse train and nanosecond pulse.

Using the Z-scan technique, we studied the nonlinear absorption and refraction behaviors of a dilute toluene solution of a silicon naphthalocyanine (Si(OSi(n-hexyl)(3))(2), SiNc) at 532 nanometer with both a 2.8-nanosecond pulse and a 21-nanosecond (HW1/eM) pulse train containing 11 18-picosecond pulses 7 nanosecond apart. A thermal acoustic model and its steady-state approximation account for the heat generated by the nonradiative relaxations subsequent to the absorption. We found that when the steady-state approximation satisfactorily explained the results obtained with a 21-nanosecond pulse train, only the thermal-acoustic model fit the 2.8-nanosecond experimental results, which supports the approximation criterion established by Kovsh et al.

Z-scan study of optical nonlinearity in C60-toluene solution

Nonlinear absorption and refraction of C60 dissolved in toluene are investigated using the Z-scan technique with trains of picosecond laser pulses extending over ∼70 nanoseconds. Analysis of the experimental results, with a five-state model, assures contributions of the triplet–triplet absorption and the thermal effect induced by nonradiative relaxation, in addition to the singlet–singlet excitation verified in a previous picosecond study.

Control of Thermal Lensing Effect in Transparent Liquids by Femtosecond Laser Pulses

The thermal lensing effect in transparent (linear and nonlinear) molecular liquids can be modulated by disrupting continuously output 82 MHz 28 fs laser pulses at 800 nm to form trains of various widths (τt) with respect to the thermal diffusivity time τth (~2 ms). We present nonlinear refraction results for CHBr3 (bromoform) obtained by the Z-scan technique. The results show that the thermal lensing effect increases with τt when τt is less than τth, but becomes steady when τt exceeds τth and reaches 30 ms. The proposed technique of modulating the thermal lensing effect by varying τt has great application potential in information photonics and optoelectronic devices, such as variable optical attenuators, holographic recording media, optical limiters and optical switches.

Effect of a diffuser on distortion reduction for a virtual image projector

Distortion aberration of a virtual image generated by a projection lens system is reduced by a diffuser. This technique is implemented by putting a diffuser on the intermediate image plane in the imaging system. Theoretical simulation and experimental imaging results of the proposed technique are analyzed and demonstrated.