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 SiO 2 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.

1 . A high transmittance optical window comprising:
 a transparent substrate coated on both sides with a multiple layer coating; and   the multiple layer coating comprising a plurality of optical films, and the multiple layer coating defining a refractive index intermediate between the refractive index of the transparent substrate and air.   
     
     
         2 . The high transmittance window of  claim 1  wherein the transparent substrate comprises at least one of glass, quartz, and sapphire materials. 
     
     
         3 . The high transmittance window of  claim 1  wherein the multiple layer coating comprises at least one of SiO 2 , TiO 2 , Si 3 N 4 , BaF 2 , CdTe, and diamond like carbon materials. 
     
     
         4 . The high transmittance window of  claim 1  wherein the multiple layer coating comprises a transparent conductive oxide including at least one of indium tin oxide and zinc oxide. 
     
     
         5 . The high transmittance window of  claim 1  wherein the multiple layer coating is deposited by oblique-angle deposition. 
     
     
         6 . The high transmittance window of  claim 1  wherein the multiple layer coating comprises at least two layers having a similar chemical composition but a different porosity and thus a different refractive index. 
     
     
         7 . The high transmittance window of  claim 6  wherein the transparent substrate comprises sapphire and the index of refraction for each of the plurality of optical films is varied from 1.5 to 1.1 over two steps, with the plurality of deposited layers defining approximately 230 nm of dense SiO 2  (n˜1.46) and approximately 300 nm of porous SiO 2  (n˜1.18). 
     
     
         8 . The high transmittance window of  claim 1  wherein the multiple layer coating contains one of (i) at least one layer of the AR coating comprises a single dense material, (ii) at least one layer of the AR coating comprises a solid solution of two different dense materials, and (iii) at least one layer of the AR coating comprises a porous material. 
     
     
         9 . The high transmittance window of  claim 6  further comprising a pore closing coating. 
     
     
         10 . A photovoltaic device comprising:
 a glass window coated on a top, sun-facing surface with a multiple layer coating comprising a plurality of optical films, and the multiple layer coating defining a refractive index intermediate between the refractive index of the glass window (n˜1.5) and air (n˜1); and   an underlying semiconductor solar cell device.   
     
     
         11 . The photovoltaic device of  claim 10  wherein the glass window forms a cover glass that is attached to the underlying semiconductor device with transparent epoxy. 
     
     
         12 . The photovoltaic device of  claim 10  wherein the glass window forms a transparent superstrate upon which a semiconductor thin film solar cell structure is deposited. 
     
     
         13 . The photovoltaic device of  claim 10  wherein the multiple layer coating comprises at least two layers having a similar chemical composition but a different porosity and thus a different refractive index. 
     
     
         14 . The photovoltaic device of  claim 13  wherein the index of refraction in the topmost coating is varied from 1.5 to 1.1 over three steps, with the plurality of optical films defining approximately 192 nm of porous SiO 2  (n˜1.36), approximately 179 nm of porous SiO 2 (n˜1.19), and approximately 260 nm of porous SiO 2 (n˜1.10). 
     
     
         15 . The photovoltaic device of  claim 13  further comprising a pore closing coating covering the topmost layer in the antireflection coating. 
     
     
         16 . A method of manufacturing a thin film solar cell comprising:
 providing a transparent substrate having a front surface and a back surface; and   coating the transparent substrate on at least one side with a multiple layer optical coating comprising a plurality of optical films, and the multiple layer optical coating defining a refractive index intermediate between the refractive index of the transparent substrate and the refractive index of air.   
     
     
         17 . The method of  claim 16  wherein the step of coating the transparent substrate comprises the deposition of porous SiO 2  layers using oblique-angle deposition. 
     
     
         18 . The method of  claim 16  wherein the step of coating the transparent substrate comprises the deposition of porous TiO 2  layer using oblique-angle deposition. 
     
     
         19 . The method of  claim 16  wherein the step of coating the transparent substrate comprises the depositing of a porous layers consisting of SiO 2 , TiO 2 , Si 3 N 4 , BaF 2 , CdTe, and diamond like carbon materials using oblique-angle deposition. 
     
     
         20 . The method of  claim 16  wherein the multiple layer optical coating is applied on the front surface after forming a thin film solar cell device on the back surface of the transparent substrate.