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.

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.

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.

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).

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.