Abstract:
A photovoltaic device and method of manufacture provides a P-N junction formed between doped semiconductor materials and adapted to produce photovoltaic current in response to radiant energy reaching the P-N junction, and a silicon dioxide protective window layer located in proximity to doped semiconductor material and adapted to allow radiant energy to pass therethrough en route to the P-N junction, the protective layer including a high optical transparency layer of amorphous silica, having a silicon dioxide chemistry greater than 75 molar percent (75 mol %). A photovoltaic window provides a planar photovoltaic device being at least semi-transparent; and a pair of protective window layers sandwiched around the planar photovoltaic device and adapted to allow radiant energy to reach the photovoltaic device through both protective window layers, wherein at least one protective window layer is a high optical transparency layer of amorphous silica, having a silicon dioxide chemistry greater than 75 molar percent (75 mol %).

Description:
RELATED APPLICATIONS 
     The present application claims priority from U.S. Provisional Patent Application 60/761,725. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to photovoltaic devices having a high-transparency silicon dioxide environmental enclosure and the method to make the same. 
     BACKGROUND 
     U.S. Pat. No. 6,027,826 to de Rochemont, et al., disclose articles and methods to form oxide ceramic on metal substrates to form laminate, filament and wire metal-ceramic composite structures using liquid aerosol spray techniques. U.S. Pat. Nos. 6,323,549 and 6,742,249 to de Rochemont, et al., disclose articles that comprise, and methods to construct, an interconnect structure that electrically contacts a semiconductor chip to a larger system using at least one discrete wire that is embedded in silica ceramic, as well as methods to embed passive components within said interconnect structure. U.S. Pat. Nos. 5,707,715 and 6,143,432 to de Rochemont, et al., (the &#39;715 and &#39;432 patents), disclose articles and methods to relieve thermally-induced mechanical stress in metal-ceramic circuit boards and metal-ceramic and ceramic-ceramic composite structures. The contents of each of these references are incorporated herein by reference as if laid out in their entirety. 
     McMillan et al. (U.S. Pat. Nos. 5,456,945; 5,540,772; 5,614,252; 5,759,923; 5,888,583, hereinafter referred collectively as McMillan et al.) disclose methods and apparatus for disposing liquid precursor films by flowing a mist of liquid metalorganic precursors over a substrate contained within a deposition chamber, where both the substrate and the deposition chamber are held at substantially ambient temperatures. Although this art instructs the use of liquid precursors comprising wet chemistry techniques that include carboxylic acid and alkloxide chemistries to form silicon dioxide and other oxide dielectrics, such as barium strontium titanate (BST), on integrated circuit substrates, the inventors repeatedly advise that heating the deposition chamber and substrate during the deposition process leads to inferior quality films. Under McMillan et al., ambient temperatures must be maintained within the deposition chamber, which may alternatively be held under vacuum or at atmospheric pressure during the deposition process. General ambient temperatures are clearly defined as ranging between −50° C. and 100° C., preferably ranging between 15° C. and 40° C. The initial deposit is a liquid film that is subsequently dried and treated to form a solid oxide layer. Treatment of the liquid film is defined as meaning one or any combination of the following process steps: exposure to vacuum, application of ultraviolet (UV) radiation, electrical poling, drying, heating and annealing. Ultraviolet radiation is applied to the mist during the deposition process to accelerate dissociation of the precursor flowing over the substrate and electrical poling is believed to increase the dwell time of the precursor mist over the substrate. Solvents contained within the liquid film are primarily extracted from the deposit using vacuum techniques. Furthermore, in U.S. Pat. No. 5,759,923, McMillan et al. only instruct on a need for water-free alkoxide chemistries when depositing silicon dioxide materials, suggesting that silicon carboxylic acid chemistries can be exposed to water-containing chemical species or atmospheric environments having relatively humidity, such as ambient air. Additional prior art that instructs the application of a liquid film to a substrate by means of an aerosol spray, followed by solvent extraction and subsequent treatment is cited by Hayashi et al. (U.S. Pub. No. 2002/0092472 A1) 
     R. Khun et al., “Charcterization of Novel Mono- and Bifacially Active Semi-Transparent Crystalline Silicon Solar Cells”, IEEE Transactions on Electron Devices, 46(10), October 1999, p. 2013-2017, disclose the use of mechanical saws to cut groves into a silicon photovoltaic device to render it semi-transparent for the purpose of developing architectural solar cell devices. Kalkan et al, U.S. Pat. No. 6,919,119, and Sager et al, U.S. Pub. No. US/2004/0084080 A1, disclose the use nano-architected (corrugated) surface topologies to increase the electrically active surface area per unit volume of photovoltaic device media. Scher et al, U.S. Pub. No. US/2004/0118404 disclose the use of nano-particle P-N junctions embedded in organic media or assembled within a void existing between two electrodes to form solar cell devices. Nano-particle P-N junction embodiments comprising semiconductor compounds defined as Group II-VI, Group III-V, and Group IV semiconductors are incorporated herein by way of reference. 
     SUMMARY OF THE INVENTION 
     In one embodiment of the present invention, a photovoltaic device comprises a P-N junction formed between doped semiconductor materials and adapted to produce photovoltaic current in response to radiant energy reaching the P-N junction, and a silicon dioxide protective window layer located in proximity to doped semiconductor material and adapted to allow radiant energy to pass therethrough en route to the P-N junction the protective layer including a high optical transparency layer of amorphous silica, having a silicon dioxide chemistry greater than 75 molar percent (75 mol %). 
     The protective window layer may include a high optical transparency layer of amorphous silica having a silicon dioxide chemistry greater than or equal to 90 mol %. The protective window layer may consist of a high optical transparency layer of amorphous silica having a silicon dioxide chemistry greater than or equal to, and, more preferably greater than 99 mol %. 
     The protective window layer may be formed on one or more of the doped semiconductor materials, or the doped semiconductor materials are formed on the protective window layer. 
     In another embodiment of the present invention, a photovoltaic window comprises a planar photovoltaic device being at least semi-transparent; and a pair of protective window layers sandwiched around the planar photovoltaic device and adapted to allow radiant energy to reach the photovoltaic device through both protective window layers, wherein at least one protective window layer is a high optical transparency layer of amorphous silica, having a silicon dioxide chemistry greater than 75 molar percent (75 mol %). 
     The at least one protective window layer may include a high optical transparency layer of amorphous silica having a silicon dioxide chemistry greater than or equal to 90 mol %. 
     The at least one protective window layer may include a high optical transparency layer of amorphous silica consisting of a silicon dioxide chemistry greater than or equal to 99 mol %. 
     In yet another embodiment of the present invention, a method for fabricating a photovoltaic device, comprises the steps of forming a P-N junction between doped semiconductor materials adapted to produce photovoltaic current in response to radiant energy reaching the P-N junction; and forming a silicon dioxide protective window layer in conjunction with the doped semiconductor material and adapted to allow radiant energy to pass therethrough en route to the P-N junction, the protective window layer including a high optical transparency layer of amorphous silica, having a silicon dioxide chemistry greater than 75 molar percent. 
     The step of forming a silicon dioxide protective window layer may be performed either before or after the step of forming a P-N junction. 
     The method may further comprise the step of first forming the silicon dioxide protective window layer on a sacrificial substrate prior to forming the P-N junction on the silicon dioxide protective layer 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       For a better understanding of the present invention, together with other and further aspects thereof reference is made to the following description taken in conjunction with the accompanying figures of the drawing, wherein: 
         FIG. 1  depicts the layered structure of a photovoltaic (PV) device; 
         FIGS. 2A-2B  depict TOP and SIDE VIEWS of a photovoltaic (PV) module; 
         FIG. 3  is a schematic representation of a deposition chamber configured to spray a liquid aerosol of encapsulating silicon dioxide; 
         FIG. 4  provides characteristic sequencing of control parameters used to deposit amorphous silica; 
         FIGS. 5A-5C  depict architectural solar cell embodiments wherein a high-transparency amorphous silica (silicon dioxide) window layer is used as a substrate upon which a photovoltaic P-N junction is subsequently formed; and 
         FIGS. 6A-6B  depict architectural solar cell embodiments wherein nano-particle P-N junctions are embedded in one or more layers of transparent conducting oxides formed on a window layer of high transparency amorphous silica. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This application incorporates by reference all matter contained in co-pending U.S. patent application Ser. No. 11/243,422, filed Oct. 3, 2005 and entitled “CERAMIC ANTENNA MODULE AND METHODS OF MANUFACTURE THEREOF” (the &#39;422 application), Ser. No. 11/479,159, filed Jun. 30, 2006 and entitled “ELECTRICAL COMPONENT AND METHOD OF MANUFACTURE” (the &#39;159 application), and Ser. No. 11/620,042, filed Jan. 6, 2007 and entitled “POWER MANAGEMENT MODULES AND METHOD OF MANUFACTURE” (the &#39;042 application). In the &#39;422 application, de Rochemont, discloses articles and methods that are used to form silicon dioxide-based layers and meta-material bodies on the surface of a semiconductor die using low-temperature liquid or powder aerosol sprays. 
     Photovoltaic (PV) cells, more popularly known as solar cells, convert sunlight to direct current (DC) electrical energy that can be used to charge batteries, power a variety of microelectronic devices, or drive DC motors. PV systems can also be configured with power inverters to deliver alternating current (AC) electrical energy that is compatible with the function of an electrical utility grid used to drive the operation of general electrical appliances. 
     The output power generated by a PV device is directly proportional to its circuit efficiency and physical size. Cell circuit efficiency will be affected by the integrity of one or more P-N junctions formed within silicon semiconductor and the total amount of sunlight transmitted to that interface. 
     High power PV modules comprise multiple PV cells sealed in an environmentally protective laminate. Therefore methods and material compositions that provide a means to form a mechanically hard, high transparency, low-reflectance protective laminate over large surface areas without altering the integrity of an electrical interface forming a P-N junction are desirable. 
     Reference is now made to  FIGS. 1 ,  2 A and  2 B, which describe a photovoltaic (PV) device  101  and PV module  111  consisting of a semiconductor  103 , comprising an ultra-thin layer of N-type semiconductor  105  in direct electrical contact with a P-type semiconductor layer  107 , with an environmentally protective laminate  109 . A PV module  111  comprises an array of discrete PV devices  113  that are electrically connected in series and/or in parallel through a wiring harness  115  to generate higher currents, voltages, and power levels. In the case of the PV module  111 , it is desirable to apply the environmentally protective laminate  117  to the entire assembly and encapsulate the entire panel  119 . 
     Within each PV device  101 / 113 , an intrinsic electrical field is generated at the electrical interface  121  that forms the P-N junction between the doped semiconductor layers. The region in which the intrinsic electrical field is contained is depleted of conductive charges. Incident sunlight penetrating the surface of the PV cell will stimulate conduction electrons within the P-N junction&#39;s depletion zone. The intrinsic electrical field imparts directional momentum to the light-stimulated electrical charge when the PV device is connected to an electrical load in electrical communication with conductive electrodes  122 A,  122 B located on opposite sides of the electrical interface  121  forming the P-N junction. The strength of the intrinsic electrical field and device efficiency are directly proportional to the P-N junction&#39;s doping profile and the quantity of light energy penetrating into the depletion region. It is therefore desirable to avoid exposing the semiconductor  103  to process temperatures (&gt;600° C.) that would stimulate thermally-activated diffusion processes within and between the doped layers and degrading the dopant profile forming the P-N junction. It is also desirable to utilize an ultra-high transparency window material as the encapsulation layer 
     Silicon dioxide is a highly desirable material for use as an encapsulation layer. It has high optical transparency ranging from near-infrared, across the visible, and into the near ultraviolet optical spectrum, which makes it useful for multi-junction PV devices. It is mechanically hard and has an excellent coefficient of expansion match with silicon. However, conventional methods used to apply silicon dioxide to the surface of a silicon PV device impose limitations harmful to device performance. The historical method, which forms silicon dioxide surface layers by flowing de-ionized water vapor over a silicon surface heated to temperatures exceeding 850° C. degrades the dopant profile in the ultra-thin phosphorous-doped layer. Spin-on glass (SOG) techniques spin-coat a liquid film layer of metalorganic precursors that is subsequently converted into silicon dioxide by low-temperature heat treatment (350-600° C.). The heat treatment converts the metalorganic compounds into silicon dioxide by thermal pyrolysis. While the SOG methods are suitable for producing high-quality silicon dioxide films, individual SOG layer thickness is typically much less than 1 micron, which is not sufficiently thick for use as an environmentally protective laminate. Multiple layers can be built up using the SOG technique, however, this approach is not efficient from a manufacturing perspective, and can not be used in non-planar PV module assemblies that might include assembly components  123  that frame individual PV devices  113  within the PV module  119 . 
     In one embodiment, amorphous silicon dioxide is deposited on the surface of a semiconductor wafer at low temperatures (&lt;430° C.) using liquid aerosol sprays. Liquid aerosols comprise a metalorganic solution of silicon precursors that can be applied at atmospheric pressures with a controlled-gas ambient. This method is preferred because its ability to deposit high quality silicon dioxide layers to arbitrary thicknesses at atmospheric pressures is easily adapted to high productivity manufacturing environments. A variety of aerosolizing techniques can be applied to form an aerosol spray from the liquid precursor solution. While other nebulization methods, (such as ultrasonic and thermo-resistive heating techniques, among others), can be used to form the aerosol spray, the use of pressurized resonant air-cavity nozzles is the preferred technique to blanket coat large surface areas because of its ability to aerosolize high viscosity solutions and to establish reactive-gas atmospheres that influence the dynamics of efficient precursor decomposition. 
     The silicon dioxide is deposited on to the surface of a semiconductor wafer at low temperatures (&lt;430° C.) using liquid aerosol sprays. Liquid aerosols comprise a metalorganic solution of silicon precursors that can be applied at atmospheric pressures with a controlled-gas ambient. This method is preferred because its ability to deposit high quality silicon dioxide layers to arbitrary thicknesses at atmospheric pressures is easily adapted to high productivity manufacturing environments. A variety of aerosolizing techniques can be applied to form an aerosol spray from the liquid precursor solution. While other nebulization methods, (such as ultrasonic and thermo-resistive heating techniques, among others), can be used to form the aerosol spray, the use of pressurized resonant air-cavity nozzles is the preferred technique to blanket coat large surface areas because of its ability to aerosolize high viscosity solutions and to establish reactive-gas atmospheres that influence the dynamics of efficient precursor decomposition. 
     Making reference to  FIG. 3 , a liquid aerosol spray station consists of a deposition chamber  125  filled to atmospheric pressure with air or a controlled mixture of inert and process gases  127 , exhaust vents  129  that are used to draw  130  vaporized waste products out of the deposition chamber  125 , at least one spray nozzle  131  that is supplied with one or more process gases  133  that have pressure and flow rates regulated by mass flow controllers  134 . It is preferred to mix the process gases in a gas manifold  135  before supplying them to the spray nozzle(s)  131 . The gas manifold  135  is used to regulate a gas mixture consisting of an inert gas carrier (for instance, dry nitrogen, argon, helium, among others), and an oxidizing agent (such as oxygen or a mixture of carbon monoxide and carbon dioxide). The oxidizing agent should have a partial pressure that ranges between 0.05% and 20%, preferably 2-10% of the inert carrier gas. 
     The spray nozzle(s)  131  is (are) additionally supplied by one or more liquid precursors  137  and the precursor flow rate delivered to the spray nozzle is regulated by one or more mass flow controllers  138 . The precursor delivery system may optionally include a liquid manifold  139  that is used to blend individual liquid precursors in a controlled manner to introduce a compositional gradient that varies a physical property of the silicon dioxide deposit, such as the refractive index gradient, with layer thickness to enhance the deposited layer&#39;s anti-reflective properties. 
     The deposition chamber  125  contains a heated pedestal  141  upon which the silicon dioxide is formed on the PV device  143  or substrate as the case may be. The chamber may optionally include infrared or ultraviolet lamps  145 , such as an excimer lamp, that expose the sprayed deposits to radiant wavelengths  146  that improve precursor decomposition processes. Additionally, a pyrometer  147 , or similar thermally sensing device, is used to measure the surface temperature of the article upon which the silicon dioxide film is deposited. 
     Control loops  148 A,  148 B,  148  C,  148  D,  148 E,  148 F,  148 G managed by a central processing unit (CPU)  149  may be used to control process parameters. Control of deposition temperature may be accomplished by regulating the surface temperature of the substrate or PV device  143 .  FIG. 4  shows a time chart of how certain process parameters may be pulsated during the deposition process in a coordinated fashion by CPU  149 , including the chamber exhaust  151 , liquid precursor feed  153 , gas feed  155 , and UV lamp exposure  157 . 
     An embodiment of the invention forms the silicon dioxide encapsulation layer  159  directly on the surface of semiconductor layer  103  adjacent to the P-N junction as shown in  FIG. 1 . The electrical interface  121  of the P-N junction need not be planar as depicted, and could, in fact, have any topology demonstrated to improve photovoltaic efficiency. Reference is now made to  FIGS. 5A ,  5 B,  5 C and  6 A,  6 B to illustrate another embodiment of the invention, wherein liquid aerosol spray deposition forms an encapsulating high-transparency amorphous silica layer  161  having thickness ranging between 1 micron and 1 centimeter or more, preferably thickness ranging from 250 micron to 1 millimeter or more, on the surface of a sacrificial substrate  163 . A first conducting medium  165  is then applied to the surface to the encapsulating amorphous silica layer to function as an electrode for the PV device. The first conducting medium  165  may be a transparent oxide layer, for instance, an indium-tin oxide layer formed using liquid aerosol spray deposition, or it may comprise an array of thin metallic fingers  167  patterned on the surface so as not to obscure light transmission into additional layers applied to its surface, At least one semiconductor absorbing layer  169 , constructed to function as a photovoltaic device, is applied to the surface of the amorphous silica layer  161  and the conducting medium  165 . Copper indium gallium selenide (CIGS) is a preferred semiconductor compound for its intrinsic ability to function as a glass tint and to form a nanoscale “percolation network” that accelerates the transfer of electrons generated by light energy absorbed in the semiconductor absorbing layer  169 . A second conducting medium  171  is applied to the surface of the semiconductor absorbing layer  169  to function as a second electrode for the PV device. Although this second conducting medium  171  does not need to be transparent, a transparent conducting medium is preferred when the photovoltaic structure is to be used as a tinted solar cell window or as a sheet of architectural glass. In this instance, the second conducting medium  171  may comprise a semi-transparent metallic film, a transparent oxide, or a transparent electrically conducting polymer. A pane of tinted solar cell window or architectural glass  173  is competed by separating the high-transparency amorphous silica layer  161  from the sacrificial substrate  163  thereby exposing the amorphous silica layer  161  as a high transparency protective layer maximizing the quantity of light energy allowed to penetrate into the semiconductor absorbing layer  169 . An alternative preferred embodiment for use as a tinted solar cell window or pane of architectural glass attaches a thick pane of lower cost float glass  175  ( FIG. 5C ) to the multilayer structure forming the tinted solar cell window or architectural glass  173  through a transparent adhesive layer  176 , preferably a transparent polymer adhesive, to mechanically reinforce the structure. 
     An alternative preferred embodiment of the present invention, depicted in  6 A,  6 B, utilizes liquid aerosol sprays to embed absorbing semiconductor nanoparticles  177 , wherein each nanoparticle comprises a nanoscale P-N junction, within a transparent conducting oxide layer  179 , such as an indium-tin oxide layer. A first conducting medium  181  and a second conducting medium  183  are attached to the transparent conducting oxide layer  179  to make electrical contact with the nanoparticle P-N junctions  177  embedded within the transparent conducting layer  179 . The nanoparticle P-N junctions are formed from semiconductor compounds that can withstand spray deposition temperatures in the range of 200° C. and 430° C., such as gallium arsenide (and other suitable III-V compound semiconductors), cadmium telluride (and other suitable II-VI compound semiconductors), silicon, silicon-germanium, germanium, and diamond (Group IV semiconductor compounds). It is well known to practitioners skilled in the art that the semiconductor band gap energy of these compounds can be tuned over the optical, near-infrared and far-infrared regions of the electromagnetic spectrum by blending different elements from columns II, III, IV, V and VI of the periodic table. The nanoparticle P-N junctions are dispersed as a colloidal suspension in a liquid precursor solution needed to form the transparent conducting medium as instructed by the de Rochemont &#39;715 and &#39;432 patents. The nanoparticle photovoltaic device  185  may be optionally attached to a mechanical reinforcing layer  187  or a high transparency amorphous silica layer  189  as discussed above. 
     Alternatively, the nanoparticle photovoltaic device may comprise multiple nanoparticle layers  191 A,  191 B,  191 C,  191 D ( FIG. 6B ), wherein each nanoparticle layer contains absorbing semiconductor nanoparticles having different semiconductor band gap energies to form a multi-color solar cell device. Conductive charged particle pairs (negatively charged electrons and positively charge holes) are created within a semiconductor when it absorbs light energy equal to or greater than the semiconductor band gap energy. Electron-hole pairs generated in a charge-depleted region of the semiconductor are swept by strong internal electric fields to generate the photovoltaic response. Multi-color solar cell devices would comprise a set of nanoparticle P-N junctions formed from semiconductor compounds having band gap energies corresponding to different regions of the electromagnetic spectrum. For instance, semiconductor nanoparticles absorbing light energy having electromagnetic wavelengths that are in the order of 400 nm would be absorbing predominantly violet colors, whereas nanoparticles absorbing light energy having electromagnetic wavelengths that are on the order of 800 nm and 1-2 microns would be absorbing predominantly red and near-infrared colors. In this instance, it is preferred to position the layer comprising absorbing semiconductor nanoparticles having the widest band gap (absorbing the shortest wavelength of light energy) corresponding to light energy with wavelengths ranging from 500 nm to 400 nm or more,  193 A as the nanoparticle layer upon which incident light energy  195  will enter the multi-layer structure, and to progressively position, as the case may be, nanoparticle layers  191 B,  191 C,  191 D, in order of decreasing nanoparticle  193 B,  193 C,  193 D band gap energy such that the nanoparticles with smallest band gap energy  193 D (absorbing the longest wavelength of light energy) corresponding to light energy with wavelengths ranging from 1.5 micron to 800 nm is located furthest away from the incident light energy source  195 . 
     Although the invention has been described with respect to various embodiments, it should be realized this invention is also capable of a wide variety of further and other embodiments within the spirit and scope of the appended claims.