Enhanced broadband and omni-directional performance of polycrystalline Si solar cells by using discrete multilayer antireflection coatings.

The performance enhancement of polycrystalline Si solar cells by using an optimized discrete multilayer anti-reflection (AR) coating with broadband and omni-directional characteristics is presented. Discrete multilayer AR coatings are optimized by a genetic algorithm, and experimentally demonstrated by refractive-index tunable SiO2 nano-helix arrays and co-sputtered (SiO2)x(TiO2)1₋x thin film layers. The optimized multilayer AR coating shows a reduced total reflection, leading to the high incident-photon-to-electron conversion efficiency over a correspondingly wide range of wavelengths and incident angles, offering a very promising way to harvest more solar energy by virtually any type of solar cells for a longer time of a day.

A near single crystalline TiO2 nanohelix array: enhanced gas sensing performance and its application as a monolithically integrated electronic nose.

We present high performance gas sensors based on an array of near single crystalline TiO(2) nanohelices fabricated by rotating oblique angle deposition (OAD). The combination of large surface-to-volume ratio, extremely small size (<30 nm) comparable to the Debye length, a near single crystallinity of TiO(2) nanohelices, together with the unique top-and-bottom electrode configuration hugely improves the H(2)-sensing performance, including ∼10 times higher response at 50 ppm, approximately a factor of 5 lower detection limit, and much faster response time than the conventional TiO(2) thin film devices. Beyond such remarkable performance enhancement, the excellent compatibility of the OAD method compared with the conventional micro-fabrication technology opens a new avenue for monolithic integration of high-performance chemoresistive sensors to fabricate a simple, low cost, reliable, yet fully functional electronic nose and multi-functional smart chips for in situ environmental monitoring.

Experimental and theoretical study of the optical and electrical properties of nanostructured indium tin oxide fabricated by oblique-angle deposition.

Oblique-angle deposition of indium tin oxide (ITO) is used to fabricate optical thin-film coatings with a porous, columnar nanostructure. Indium tin oxide is a material that is widely used in industrial applications because it is both optically transparent and electrically conductive. The ITO coatings are fabricated, using electron-beam evaporation, with a range of deposition angles between 0 degrees (normal incidence) and 80 degrees. As the deposition angle increases, we find that the porosity of the ITO film increases and the refractive index decreases. We measure the resistivity of the ITO film at each deposition angle, and find that as the porosity increases, the resistivity increases superlinearly. A new theoretical model is presented to describe the relationship between the ITO film's resistivity and its porosity. The model takes into account the columnar structure of the film, and agrees very well with the experimental data.

Improved color rendering and luminous efficacy in phosphor-converted white light-emitting diodes by use of dual-blue emitting active regions.

Conventional white-light sources suffer from a fundamental trade-off between color rendering index and the luminous efficacy; increasing one generally comes at the expense of the other. We demonstrate through simulation that dual-wavelength blue-emitting active regions in phosphor-converted white light sources maximize the output luminous flux while significantly increasing the color rendering ability. Our results indicate that such improvements can be achieved over a broad range of correlated color temperatures.

Design of multilayer antireflection coatings made from co-sputtered and low-refractive-index materials by genetic algorithm.

Designs of multilayer antireflection coatings made from co-sputtered and low-refractive-index materials are optimized using a genetic algorithm. Co-sputtered and low-refractive-index materials allow the fine-tuning of refractive index, which is required to achieve optimum anti-reflection characteristics. The algorithm minimizes reflection over a wide range of wavelengths and incident angles, and includes material dispersion. Designs of antireflection coatings for silicon-based image sensors and solar cells, as well as triple-junction GaInP/GaAs/Ge solar cells are presented, and are shown to have significant performance advantages over conventional coatings. Nano-porous low-refractive-index layers are found to comprise generally half of the layers in an optimized antireflection coating, which underscores the importance of nano-porous layers for high-performance broadband and omnidirectional antireflection coatings.

Encapsulation shape with non-rotational symmetry designed for extraction of polarized light from unpolarized sources.

A non-rotationally symmetric encapsulation shape - which takes advantage of the low reflection coefficient for transverse magnetic polarized light near Brewster's angle - designed to enhance extraction of a particular desired linear polarization from an unpolarized source is reported. The algorithm for optimization of the shape is described. Numerical ray-tracing simulations of the encapsulation shape are performed and predict an integrated enhancement of 8.3% in the ratio of desired polarization to undesired polarization when the refractive index of the encapsulant is 1.5. Experimental measurements of fabricated encapsulant shapes agree well with numerical predictions.

Linearly polarized emission from GaInN lightemitting diodes with polarization-enhancing reflector.

A polarization-enhancing reflector design, which is matched to the emission characteristics of GaInN/GaN 460 nm light-emitting diodes grown on (0001) oriented sapphire substrates, is reported. Side-emitted light from these devices is known to be highly polarized with the electric field in the plane of the active region. Through selective rotation of polarization by the reflector, the in-plane polarized side-emitted light is directed upwards with a single dominant linear polarization. Polarization ratios as high as 3.5:1 are measured in the farfield, and the average polarization ratio is 1.9:1. If only light that strikes the reflector is considered, the polarization ratio is 2.5:1. The concept of the polarization-enhancing reflector and the numerical algorithm used to generate the optimized shape are also described.