High transmittance optical windows and method of constructing the same

Abstract

Designs for ultra-high, broadband transmittance through windows over a wide range of incident angles are disclosed. The improvements in transmittance result from coating the windows with a new class of materials consisting of porous nanorods. A high transmittance optical window comprises a transparent substrate coated on one or both sides with a multiple layer coating. Each multiple layer coating includes optical films with a refractive index intermediate between the refractive index of the transparent substrate and air. The optical coatings are applied using an oblique-angle deposition material synthesis technique. The coating can be performed by depositing porous SiO2 layers using oblique angle deposition. The high transmittance window coated with the multiple layer coating exhibits reduced reflectance and improved transmittance, as compared to an uncoated transparent substrate.

Description

RELATED APPLICATIONS[0001]This application claims the benefit of co-pending U.S. Provisional Patent Application Ser. No. 61 / 293,469, filed on Jan. 8, 2010 entitled EFFICIENT SOLAR CELL EMPLOYING MULTIPLE ENERGY-GAP LAYERS AND LIGHT-SCATTERING STRUCTURES AND METHODS FOR CONSTRUCTING THE SAME, which is expressly incorporated herein by reference.GOVERNMENT SUPPORT[0002]This invention was supported in part by Small Business Innovative Research (SBIR) contract # W31P4Q-08-C-0300 from the Defense Advanced Research Projects Agency (DARPA) to Magnolia Optical Technologies, Inc., 52 B Cummings Park, Suite 314, Woburn, Mass. 01801. The government may have certain rights in this invention.FIELD OF THE INVENTION[0003]This invention relates to transparent optical windows for detectors, sensors, and other optical devices; and to semiconductor-based photovoltaic energy converters, also known as “solar cells,” and to the design and fabrication of the same.BACKGROUND OF THE INVENTION[0004]Transparen...

AI Technical Summary

Benefits of technology

This patent describes a method for improving the transmittance of optical windows by using antireflection structures. These structures reduce reflection losses and increase the amount of light that can pass through the window. The antireflection coatings are made up of multiple layers with varying refractive indices, and can be designed to have lower reflectivity than continuous graded coatings. The coatings can be applied using oblique angle deposition and can be used on a variety of optical windows, such as glass and sapphire. The technical effects of this patent include increased transmittance through optical windows, reduced reflection losses, and improved performance of optical devices.

Problems solved by technology

Because these materials have very low absorption coefficients over a wide range of photon energies, optical transmittance through glass, sapphire, and quartz windows is typically limited by reflection losses.

Although Fresnel reflection losses are typically relatively low at normal incidence, they can become quite substantial for off-angle light incidence.

This reflection is undesirable in many applications as it can degrade the efficiency of the underlying device (e.g. efficiency of a solar photovoltaic cell), reduce signal-to-noise ratio (e.g. in a photodetector), and cause glare (e.g. from LCD screens, computer monitors, and televisions).

Often due to unavailability of materials with the desired, exact value of the refractive index, the performance of such λ / 4 AR coatings deviates from the optimum.

There is no conventional inorganic material that has such a low refractive index.

Also, fundamentally, these single-layer λ / 4 AR coatings can minimize reflection only for one specific wavelength at normal incidence and they are inherently unable to exhibit spectrally broadband reduction in reflectance over wide range of angles-of-incidence.

Optimization of multi-layer AR coatings is a difficult challenge because of the extremely large and complex dimensional space of possible solutions.

Analytical methods to optimize AR coatings are not feasible due to the complexity of the problem.

Until recently, however, the unavailability of materials with desired refractive indices, particularly materials with very low refractive indices below n=1.2, prevented the implementation of high-performance step graded refractive index designs.

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