Frank W. Mont

Realization of a near-perfect antireflection coating for silicon solar energy utilization

To harness the full spectrum of solar energy, Fresnel reflection at the surface of a solar cell must be eliminated over the entire solar spectrum and at all angles. Here, we show that a multilayer nanostructure having a graded-index profile, as predicted by theory [J. Opt. Soc. Am. 66, 515 (1976); Appl. Opt. 46, 6533 (2007)], can accomplish a near-perfect transmission of all-color of sunlight. An ultralow total reflectance of 1%-6% has been achieved over a broad spectrum, lambda = 400 to 1600 nm, and a wide range of angles of incidence, theta = 0 degrees-60 degrees . The measured angle- and wavelength-averaged total reflectance of 3.79% is the smallest ever reported in the literature, to our knowledge.

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.

High-refractive-index TiO2-nanoparticle-loaded encapsulants for light-emitting diodes

A high-refractive-index (high-n) encapsulant is highly desirable because it can result in enhancement of light-extraction efficiency from high-n semiconductor light-emitting diode (LED) chips. A uniform dispersion of TiO2 nanoparticles in epoxy for LED encapsulation is demonstrated for surfactant-coated TiO2 nanoparticles by drying, mixing with a solvent, refluxing, centrifuging, and mixing with epoxy. The refractive index of surfactant-coated TiO2-nanoparticle-loaded epoxy is 1.67 at 500nm, significantly higher than that of conventional epoxy (n=1.53). Theoretical analysis of optical scattering in nanoparticle-loaded encapsulants reveals that the diameter of nanoparticles and the volume loading fraction of nanoparticles are of critical importance for optical scattering. Quasispecular transparency of the encapsulant film can be achieved if the thickness of the film is kept below the optical scattering length. A graded-refractive-index multilayer encapsulation structure with the thickness of each layer bei...

Quantification of porosity and deposition rate of nanoporous films grown by oblique-angle deposition

We propose an analytic model that accurately predicts the porosity and deposition rate of nanoporous films grown by oblique-angle deposition. The model employs a single fitting parameter and takes into account geometrical factors as well as surface diffusion. We have determined the porosity and deposition rate from the measured refractive index and thickness of SiO2 and indium tin oxide nanoporous films deposited at various incident angles. Comparison of experimental data with the model reveals excellent agreement. The theoretical model allows for the predictive control of refractive index, porosity, and deposition rate for a wide range of deposition angles and materials.

Broadband omnidirectional antireflection coatings optimized by genetic algorithm.

An optimized graded-refractive-index (GRIN) antireflection (AR) coating with broadband and omnidirectional characteristics--as desired for solar cell applications--designed by a genetic algorithm is presented. The optimized three-layer GRIN AR coating consists of a dense TiO2 and two nanoporous SiO2 layers fabricated using oblique-angle deposition. The normal incidence reflectance of the three-layer GRIN AR coating averaged between 400 and 700 nm is 3.9%, which is 37% lower than that of a conventional single-layer Si3N4 coating. Furthermore, measured reflection over the 410-740 nm range and wide incident angles 40 degrees -80 degrees is reduced by 73% in comparison with the single-layer Si3N4 coating, clearly showing enhanced omnidirectionality and broadband characteristics of the optimized three-layer GRIN AR coating.

Effects of the refractive index of the encapsulant on the light-extraction efficiency of light-emitting diodes

We investigate the effects of the refractive index of the encapsulant on the light-extraction efficiency (LEE) of light-emitting diodes (LEDs) for GaN LEDs (n ≈ 2.5) and AlGaInP LEDs (n ≈ 3.0). For non-absorbing rectangular parallelepiped LED chips, as the refractive index of the encapsulant increases, the LEE first increases quasi-linearly, then increases sub-linearly, and finally a saturation is reached. Furthermore, LEDs with a dual-layer graded-refractive-index (GRIN) encapsulant (n(encapsulant 1) = 1.57 and n(encapsulant 2) = 1.41) is fabricated through a two-step curing process. We demonstrate that such an LED further enhances the LEE by reducing Fresnel reflection loss at the encapsulant/air interface by 35% compared with an LED encapsulated with a single-layer encapsulant (n(encapsulant) = 1.57).

Elimination of total internal reflection in GaInN light-emitting diodes by graded-refractive-index micropillars

A method for enhancing the light-extraction efficiency of GaInN light-emitting diodes (LEDs) by complete elimination of total internal reflection is reported. Analytical calculations show that GaInN LEDs with multilayer graded-refractive-index pillars, in which the thickness and refractive index of each layer are optimized, have no total internal reflection. This results in a remarkable improvement in light-extraction efficiency. GaInN LEDs with five-layer graded-refractive-index pillars, fabricated by cosputtering TiO2 and SiO2, show a light-output power enhanced by 73% and a strong side emission, consistent with analytical calculations and ray-tracing simulations.

On resonant optical excitation and carrier escape in GaInN/GaN quantum wells

Recently, photoluminescence studies using resonant optical excitation in GaInN layers have been used to investigate the physical origin of efficiency droop in GaInN/GaN light-emitting diodes. In these studies, it has been assumed that in the case of resonant excitation, where electron-hole pairs are generated in the GaInN layers only, carrier transport effects play no role. We report that in contrast to this assumption, carrier escape from quantum wells does take place and shows strong dependence upon the duration of excitation and bias conditions. We also discuss the time scales required to reach steady-state conditions under pulsed optical excitation.

Refractive-Index-Matched Indium–Tin-Oxide Electrodes for Liquid Crystal Displays

Refractive-index-matched indium–tin-oxide (ITO) electrode for thin-film transistor liquid crystal displays is presented to reduce optical losses caused by Fresnel reflections. Simulations show a 24% improvement in optical transmittance when the conventional dense ITO is replaced with the refractive-index-matched ITO in a stack of glass/ITO/liquid crystal/ITO/glass. The refractive-index-matched ITO, fabricated by oblique-angle deposition technique, shows higher optical transmittance and smaller dependency on film thickness and wavelength than conventional dense ITO.

Very high quality AlN grown on (0001) sapphire by metal-organic vapor phase epitaxy

Very high quality AlN epitaxially grown on (0001) sapphire by metal-organic vapor phase epitaxy is investigated by atomic force microscopy, x-ray diffraction, and photospectrometry. A clear and continuously linear step-flow pattern with sawtooth shaped terrace edges is observed in atomic force microscopic images. Triple-axis x-ray rocking curves show a full width at half maximum of 11.5 and 14.5arcsec for the (002) and (004) reflections, respectively. KOH etching reveals an etch-pit density of 2×107cm−2, as deduced from atomic force microscopy measurements. The optical transmission spectrum shows a sharp absorption edge with a band gap energy of 6.10eV.