Sheng-Hui Chen

Efficiency enhancement in GaAs solar cells using self-assembled microspheres

In this study we develop an efficient light harvesting scheme that can enhance the efficiency of GaAs solar cells using self-assembled microspheres. Based on the scattering of the microspheres and the theory of photonic crystals, the path length can be increased. In addition, the self-assembly of microspheres is one of the simplest and the fastest methods with which to build a 2D periodic structure. The experimental results are confirmed by the use of a simulation in which a finite-difference time-domain (FDTD) method is used to analyze the absorption and electric field of the 2D periodic structure. Both the results of the numerical simulations and the experimental results show an increase in the conversion power efficiency of GaAs solar cell of about 25% when 1 microm microspheres were assembled on the surface of GaAs solar cells.

Low-temperature synthesis of graphene on Cu using plasma-assisted thermal chemical vapor deposition

Plasma-assisted thermal chemical vapor deposition (CVD) was carried out to synthesize high-quality graphene film at a low temperature of 600°C. Monolayer graphene films were thus synthesized on Cu foil using various ratios of hydrogen and methane in a gaseous mixture. The in situ plasma emission spectrum was measured to elucidate the mechanism of graphene growth in a plasma-assisted thermal CVD system. According to this process, a distance must be maintained between the plasma initial stage and the deposition stage to allow the plasma to diffuse to the substrate. Raman spectra revealed that a higher hydrogen concentration promoted the synthesis of a high-quality graphene film. The results demonstrate that plasma-assisted thermal CVD is a low-cost and effective way to synthesis high-quality graphene films at low temperature for graphene-based applications.

Improvement of the optical coating process by cutting layers with sensitive monitoring wavelengths

For costly optical coatings, a precise monitoring method is necessary. A new monitoring method based on the selection of the most sensitive monitor wavelength is proposed. The most sensitive monitor wavelength is easy to find by a numerical analysis. The equation for the thickness compensation when a layer is over-shot or under-shot was derived. Several examples, including narrow-band pass filters, have been given to demonstrate that this new method is superior to the turning point method in the coating process.

The effect of annealing on nanothick indium tin oxide transparent conductive films for touch sensors

This study aims to discuss the sheet resistance of ultrathin indium tin oxide (ITO) transparent conductive films during the postannealing treatment. The thickness of the ultrathin ITO films is 20 nm. They are prepared on B270 glass substrates at room temperature by a direct-current pulsed magnetron sputtering system. Ultrathin ITO films with high sheet resistance are commonly used for touch panel applications. As the annealing temperature is increased, the structure of the ultrathin ITO film changes from amorphous to polycrystalline. The crystalline of ultrathin ITO films becomes stronger with an increase of annealing temperature, which further leads to the effect of enhanced Hall mobility. A postannealing treatment in an atmosphere can enhance the optical transmittance owing to the filling of oxygen vacancies, but the sheet resistance rises sharply. However, a higher annealing temperature, above 250°C, results in a decrease in the sheet resistance of ultrathin ITO films, because more Sn ions become an effective dopant. An optimum sheet resistance of 336Ω/sqr was obtained for ultrathin ITO films at 400°C with an average optical transmittance of 86.8% for touch sensor applications.

Influence of hydrogen on the properties of Al and Ga-doped ZnO films at room temperature

Low resistivity and high transmittance of Al-doped ZnO (AZO) and Ga-doped ZnO (GZO) transparent conductive thin films have been achieved by use of pulsed dc magnetron sputtering in a hydrogen environment at room temperature. The addition of hydrogen to the sputtering gas can reduce the resistivity of the films and improve their electrical properties compared to those prepared without H2, because the hydrogen acts a shallow donor. The average transmittance was over 85% in the visible region, and the lowest resistivity of the AZO and GZO films was 4.01×10(-4) (Ω-cm) and 4.39×10(-4) (Ω-cm), respectively.

Optical monitoring and real time admittance loci calculation through polarization interferometer

A simple, low coherence, vibration insensitive, polarization Fizeau interferometer is employed in this novel optical monitoring system proposed to extract the temporal phase change of the reflection coefficient of the growing film stacks. This system can directly detect fluctuating reflection coefficient and obtain the corresponding optical admittance of the growing film in real time.

Nanoimprinting Pre-Patterned Effects on Anodic Aluminum Oxide

Periodic concaves have been fabricated on the initial aluminum surface using nanoimprint and anodization processes to obtain high order nanostructured anodic aluminum oxide (AAO) arrays. The effect of the pattern design process on the one- and two-dimensional structures is investigated. Both the experimental and simulation results show that the interpore distance and the location of dissolutions in the aluminum sheet affect the strength and are dependent on the pattern design under high applied voltage. The results show that the interpore distance in the AAO is limited and the initial sites of dissolution are restricted to the inner corner/electrolyte interface.

Absorption coefficient modeling of microcrystalline silicon thin film using Maxwell-Garnett effective medium theory

Considering the Mott-Davis density of state model and Rayleigh scattering effect, we present an approach to model the absorption profile of microcrystalline silicon thin films in this paper. Maxwell-Garnett effective medium theory was applied to analyze the absorption curves. To validate the model, several experimental profiles have been established and compared with those results from the model. With the assistance of the genetic algorithm, our results show that the absorption curves from the model are in good agreement with the experiments. Our findings also indicate that, as the crystal volume fraction increases, not only do the defects in amorphous silicon reduce, but the bulk scattering effect is gradually enhanced as well.

Transparent Conductive Distributed Bragg Reflectors Composed of High and Low Refractive Index Transparent Conductive Films

This article has shown that the co-sputtering method and post-annealing treatment have successfully fabricated the transparent conductive distributed Bragg reflectors (TCDBR). The transparent conducting film, anatase Nb-doped TiO2 (TNO), which has a high refractive index can be combined with Al-doped ZnO (AZO) low refractive index transparent conducting films to make a TCDBR capable of transferring the current from electrode to semiconductor while still maintaining the advantage for the micro resonant cavity. The eight-period stack of AZO/TNO achieves a reflectivity of up to 90% centered at 550 nm and a resistivity of 1.88×10-3 Ω cm.

Coating uniformity improvement for a dense-wavelength-division-multiplexing filter by use of the etching effect

The uniformity of optical narrow-bandpass filters for dense wavelength division multiplexing (DWDM) has been improved by control of the coating parameters of electron guns and the ion source. The optical film was deposited by the electron gun and was etched by the ion source during the ion-assisted deposition process. The uniformity of the coating of a 100 GHz DWDM filter is better than +/- 0.003% over a circular area of 50 mm in diameter when such an etching process is used.