# Patent

## Title
Intermediate thermal expansion coefficient glass

## Abstract

CTE-matched silicate glasses and more particularly to low-alkali CTE-matched silicate glasses that are useful in semiconductor-based applications, such as photovoltaics are described along with methods of making such glasses.

## Background
This application is a Continuation-in-Part of U.S. application Ser. No. 12/573,213, which claims the benefit of priority to U.S. Prov. Appl. No. 61/103,126 filed on Oct. 6, 2008 and to U.S. Prov. Appl. No. 61/177,827 filed on May 13, 2009; a Continuation-in-Part of U.S. application Ser. No. 13/305,051, which claims the benefit of priority to U.S. Prov. Appl. No. 61/418,084 filed on Nov. 30, 2010, U.S. Prov. Appl. No. 61/503,248 filed on Jun. 30, 2011, and U.S. Prov. Appl. No. 61/562,651 filed on Nov. 22, 2011; a Continuation-in-Part of U.S. application Ser. No. 13/569,756, which claims the benefit of priority to U.S. Prov. Appl. No. 61/522,956 filed on Aug. 12, 2011; and a Continuation-in-Part of PCT/US12/49718 which claims benefit of priority to U.S. Prov. Appl. No. 61/515,042 filed on Aug. 4, 2011 and U.S. Prov. Appl. No. 61/565,050 filed on Nov. 30, 2011, all of which are hereby incorporated by reference in their entirety.


BACKGROUND
1. Field
Embodiments relate generally to CTE-matched glasses and more particularly to low- to no-alkali aluminosilicate, borosilicate, and aluminoborosilicate glasses which may be useful in semiconductor-based applications, such as photovoltaics, photochromics, electrochromics, or Organic Light Emitting Diode (OLED) applications.
2. Technical Background
The fusion forming process typically produces flat glass with optimal surface and geometric characteristics useful for many electronics applications, for instance, substrates used in electronics applications, for example, display glass for LCD televisions.
Over the last 10 years, Corning fusion glass products include 1737FTM, 1737GTM, Eagle2000FTM, EagleXGTM, JadeTM, and Gorilla GlassTM. Efficient melting is generally believed to occur at a temperature corresponding to a melt viscosity of about 200 poise (P). These glasses share in common 200 P temperatures in excess of 1600° C., which can translate to accelerated tank and electrode corrosion, greater challenges for fining due to still more elevated finer temperatures, and/or reduced platinum system life time, particularly around the finer. Many have temperatures at 3000 P in excess of about 1300° C., and since this is a typical viscosity for an optical stirrer, the high temperatures at this viscosity can translate to excessive stirrer wear and elevated levels of platinum defects in the body of the glass.
Many of the above described glasses have delivery temperatures in excess of 1200° C., and this can contribute to creep of isopipe refractory materials, particularly for large sheet sizes.
These attributes combine so as to limit flow (because of slow melt rates), to accelerate asset deterioration, to force rebuilds on timescales much shorter than product lifetimes, to force unacceptable (arsenic), expensive (capsule) or unwieldy (vacuum fining) solutions to defect elimination, and thus contribute in significant ways to the cost of manufacturing glass.
In applications in which rather thick, comparatively low-cost glass with less extreme properties is required, these glasses are not only overkill, but prohibitively expensive to manufacture. This is particularly true when the competitive materials are made by the float process, a very good process for producing low cost glass with rather conventional properties. In applications that are cost sensitive, such as large-area photovoltaic panels and OLED lighting, this cost differential has been large enough to make the price point of LCD-type glasses unacceptable.
Competing with the cost constraints for is the continuing drive to make new technologies, like PV, competitive with existing power production methods, e.g., hydro, coal, nuclear, wind, etc., in the power generation industry. To do so, in addition to cost, manufacturers are looking at conversion efficiency, device lifetime, and efficiency degradation—design challenges that need to be addressed to make PV a viable alternative. Soda lime glass has been a common substrate for most PV panels because of its low cost. However, soda lime glasses are not ideal for PV modules, especially CdTe-based thin film PV modules as sodium can cause problems with efficiency and device lifetime. Further, soda lime glasses can have sodium release issues that occur due to environmental conditions. These problems can lead to delamination issues and reduced efficiency. Clearly, there is still an unmet need to find glass compositions that provide optimal substrates for thin film PV devices.
SUMMARY
A first aspect comprises a glass comprising about, in mol %, 60 to 65 percent SiO2, 8 to 12 percent Al2O3, 7 to 15 percent B2O3, 0 to 8 percent M2O, and 9 to 15 percent RO, wherein, M is an alkali metal and wherein, R is an alkaline earth metal, and the glass has a coefficient of thermal expansion of from about 4.0 to about 7.5 ppm/° C. from 25 to 300° C. In some embodiments, the glass has a coefficient of thermal expansion of from about 4.5 to about 6.5 ppm/° C. from 25 to 300° C. In some embodiments, the glass has a coefficient of thermal expansion of from about 4.5 to about 6.0 ppm/° C. from 25 to 300° C. In some embodiments, the glass comprises about 0.1 to 8 mol % M2O. In some embodiments, the glass further comprises about 0.01 to 0.4 percent SnO2.
In some embodiments, the glass described above comprises about, in mol %, 61 to 64 percent SiO2, 8 to 12 percent Al2O3, 9 to 15 percent B2O3, greater than 0 to 4 percent M2O, and 12 to 15 percent RO.
In some embodiments, the glass described above comprises about, in mol %, 60 to 65 percent SiO2, 8 to less than 10 percent Al2O3, greater than 11 to 15 percent B2O3, greater than 0 to less than 1 percent M2O, and 9 to 15 percent RO.
In other embodiments, the glass described above comprises about, in mol %, 60 to 65 percent SiO2, 10 to 12 percent Al2O3, 7 to 11 percent B2O3, 1 to 8 percent M2O, and 9 to 15 percent RO.
In other embodiments, the glass described above comprises about, in mol %, 62 to 65 percent SiO2, 10 to 12 percent Al2O3, 7 to 11 percent B2O3, 3 to 8 percent M2O, 3 to 10 percent CaO, 3 to 8 percent SrO, and 1 to 8 percent M2O, wherein, CaO/(CaO+SrO) is from 0.4 and 1.
A second aspect comprises a glass comprising about, in mol %, 20 to 70 percent SiO2, 5 to 20 percent Al2O3, 0 to 15 percent B2O3, 0 to 1 percent M2O, 0 to 10 percent MgO, 8 to 35 percent CaO, 0 to 16 percent SrO, 0 to 9 percent BaO, and 20 to 45 percent RO, wherein, M is an alkali metal and wherein, R is an alkaline earth metal, and the glass has a coefficient of thermal expansion of from about 4.0 to about 7.5 ppm/° C. from 25 to 300° C. In some embodiments, the glass has a coefficient of thermal expansion of from about 4.5 to about 6.5 ppm/° C. from 25 to 300° C. In some embodiments, the glass has a coefficient of thermal expansion of from about 4.5 to about 6.0 ppm/° C. from 25 to 300° C. In some embodiments, the glass comprises about, in mol %, 57 to 68 percent SiO2, 5 to 12 percent Al2O3, 0 to 8 percent B2O3, 0 to 0.1 percent M2O, 0 to 10 percent MgO, 8 to 28 percent CaO, 0 to 10 percent SrO, 0 to 9 percent BaO, and 21 to 32 percent RO, wherein, M is an alkali metal and wherein, R is an alkaline earth metal, and the glass has a coefficient of thermal expansion of from about 4.0 to about 6.0 ppm/° C. from 25 to 300° C.
A third aspect comprises a glass comprising about, in mol %, 60 to 68 percent SiO2, 8 to 11 percent Al2O3, 6 to 11 percent B2O3, 0 to 1 percent M2O; 0 to 5 percent MgO, greater than 0 to 12 percent CaO, 1 to 12 percent SrO, 0 to 8 percent BaO, and 16 to 20 percent RO, wherein R is an alkaline earth metal, and M is an alkali metal, and the glass has a coefficient of thermal expansion of from about 4.0 to about 7.5 ppm/° C. from 25 to 300° C. In some embodiments, the glass has a coefficient of thermal expansion of from about 4.5 to about 6.5 ppm/° C. from 25 to 300° C. In some embodiments, the glass has a coefficient of thermal expansion of from about 4.5 to about 6.0 ppm/° C. from 25 to 300° C.
In some embodiments, the glasses described above are substantially free of BaO, Sb2O3, As2O3, or combinations thereof. In some embodiments, the glasses described above further comprises 2 mole percent or less of TiO2, MnO, ZnO, Nb2O5, MoO3, Ta2O5, WO3, ZrO2, Y2O3, La2O3, P2O5, or combinations thereof. In some embodiments, the glass is ion exchanged.
A fourth aspect comprises a photovoltaic (“PV”) module, comprising a CdTe solar cell material, one or more conductive oxides, and a glass substrate, wherein the glass substrate has a coefficient of thermal expansion of from about 4.0 to about 7.5 ppm/° C. from 25 to 300° C. In some embodiments, the glass substrate has a coefficient of thermal expansion of from about 4.5 to about 6.0 ppm/° C. from 25 to 300° C.
In some embodiments, the glass substrate in the PV module comprises about, in mol %, 60 to 65 percent SiO2, 8 to 12 percent Al2O3, 7 to 15 percent B2O3, 0 to 8 percent M2O, and 9 to 15 percent RO, wherein, M is an alkali metal and wherein, R is an alkaline earth metal. In some embodiments, in the glass substrate in the PV module comprises, in mol %, 60 to 65 percent SiO2, 8 to 12 percent Al2O3, 7 to 15 percent B2O3, 0.1 to 8 percent M2O, 9 to 15 percent RO, and 0.01 to 0.4 percent SnO2.
In some embodiments, the glass substrate in the PV module comprises about, in mol %, 20 to 70 percent SiO2, 5 to 20 percent Al2O3, 0 to 15 percent B2O3, 0 to 1 percent M2O, 0 to 10 percent MgO, 8 to 35 percent CaO, 0 to 16 percent SrO, 0 to 9 percent BaO, and 20 to 45 percent RO, wherein, M is an alkali metal and wherein, R is an alkaline earth metal.
In some embodiments, the glass substrate in the PV module comprises about, in mol %, 57 to 68 percent SiO2, 5 to 12 percent Al2O3, 0 to 8 percent B2O3, 0 to 0.1 percent M2O, 0 to 10 percent MgO, 8 to 28 percent CaO, 0 to 10 percent SrO, 0 to 9 percent BaO, and 21 to 32 percent RO, wherein, M is an alkali metal and wherein, R is an alkaline earth metal, and the glass has a coefficient of thermal expansion of from about 4.0 to about 6.0 ppm/° C. from 25 to 300° C.
In some embodiments, the glass substrate in the PV module comprises about, in mol %, 60 to 68 percent SiO2, 8 to 11 percent Al2O3, 6 to 11 percent B2O3, 0 to 1 percent M2O; 0 to 5 percent MgO, greater than 0 to 12 percent CaO, 1 to 12 percent SrO, 0 to 8 percent BaO, and 16 to 20 percent RO, wherein R is an alkaline earth metal, and M is an alkali metal.
A fifth aspect comprises a PV device, comprising a CdTe solar cell material having a coefficient of thermal expansion, one or more conductive oxides, and a glass substrate, wherein the glass substrate has a coefficient of thermal expansion that is within about ±2.0 ppm/° C. of the coefficient of thermal expansion of the CdTe solar cell material from 25 to 300° C. In some embodiments, the glass substrate in the PV module has a coefficient of thermal expansion of within about ±1.0 ppm/° C. of the coefficient of thermal expansion of the CdTe solar cell material from 25 to 300° C.
In some embodiments, the glass substrate in the PV device comprises about, in mol %, 60 to 65 percent SiO2, 8 to 12 percent Al2O3, 7 to 15 percent B2O3, 0 to 8 percent M2O, and 9 to 15 percent RO, wherein, M is an alkali metal and wherein, R is an alkaline earth metal. In some embodiments, in the glass substrate in the PV module comprises, in mol %, 60 to 65 percent SiO2, 8 to 12 percent Al2O3, 7 to 15 percent B2O3, 0.1 to 8 percent M2O, 9 to 15 percent RO, and 0.01 to 0.4 percent SnO2.
In some embodiments, the glass substrate in the PV device comprises about, in mol %, 20 to 70 percent SiO2, 5 to 20 percent Al2O3, 0 to 15 percent B2O3, 0 to 1 percent M2O, 0 to 10 percent MgO, 8 to 35 percent CaO, 0 to 16 percent SrO, 0 to 9 percent BaO, and 20 to 45 percent RO, wherein, M is an alkali metal and wherein, R is an alkaline earth metal.
In some embodiments, the glass substrate in the PV device comprises about, in mol %, 57 to 68 percent SiO2, 5 to 12 percent Al2O3, 0 to 8 percent B2O3, 0 to 0.1 percent M2O, 0 to 10 percent MgO, 8 to 28 percent CaO, 0 to 10 percent SrO, 0 to 9 percent BaO, and 21 to 32 percent RO, wherein, M is an alkali metal and wherein, R is an alkaline earth metal, and the glass has a coefficient of thermal expansion of from about 4.0 to about 6.0 ppm/° C. from 25 to 300° C.
In some embodiments, the glass substrate in the PV device comprises about, in mol %, 60 to 68 percent SiO2, 8 to 11 percent Al2O3, 6 to 11 percent B2O3, 0 to 1 percent M2O; 0 to 5 percent MgO, greater than 0 to 12 percent CaO, 1 to 12 percent SrO, 0 to 8 percent BaO, and 16 to 20 percent RO, wherein R is an alkaline earth metal, and M is an alkali metal.
Additional features and advantages will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the invention as described in the written description and claims hereof, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed.
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s) of the invention and together with the description serve to explain the principles and operation of the invention.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention can be understood from the following detailed description either alone or together with the accompanying drawing figures.
FIG. 1 is a graph of estimated liquidus viscosity.
FIG. 2 is an illustration of features of a photovoltaic device according to one embodiment.
FIG. 3 shows the measured residual in-plane bulk stress of CdTe film versus CTE of glass is represented with black markers and the black dashed line is the linear fit. The corresponding device efficiency is represented by the blue marker. The shaded area represents the CTE range of a CdTe film. The dash black line shows the residual stress in the CdTe film trends towards a minimum as the glass CTE approaches the CTE of the CdTe film in the shaded area. The corresponding efficiency data points show the best performance for the device film with the lowest residual stress magnitude. This suggests higher efficiencies can be obtained using a glass superstrate with a CTE that is matched to that of the CdTe film.
FIG. 4 is a graph of the CdTe film residual stress and corresponding efficiencies for devices fabricated using the close spaced sublimation process on glass substrates with varying CTE.
FIG. 5 is a graph of the efficiency and corresponding defect density as a function of the CdTe film residual stress for the same devices in FIG. 4.



DETAILED DESCRIPTION
Reference will now be made in detail to various embodiments of the invention. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like features.
In the following detailed description, numerous specific details may be set forth in order to provide a thorough understanding of embodiments of the invention. However, it will be clear to one skilled in the art when embodiments of the invention may be practiced without some or all of these specific details. In other instances, well-known features or processes may not be described in detail so as not to unnecessarily obscure the invention. In addition, like or identical reference numerals may be used to identify common or similar elements. Moreover, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In case of conflict, the present specification, including the definitions herein, will control.
Although other methods and can be used in the practice or testing of the invention, certain suitable methods and materials are described herein.
Disclosed are materials, compounds, compositions, and components that can be used for, can be used in conjunction with, can be used in preparation for, or are embodiments of the disclosed method and compositions. These and other materials are disclosed herein, and it is understood that when combinations, subsets, interactions, groups, etc. of these materials are disclosed that while specific reference of each various individual and collective combinations and permutation of these compounds may not be explicitly disclosed, each is specifically contemplated and described herein.
Thus, if a class of substituents A, B, and C are disclosed as well as a class of substituents D, E, and F, and an example of a combination embodiment, A-D is disclosed, then each is individually and collectively contemplated. Thus, in this example, each of the combinations A-E, A-F, B-D, B-E, B-F, C-D, C-E, and C-F are specifically contemplated and should be considered disclosed from disclosure of A, B, and/or C; D, E, and/or F; and the example combination A-D. Likewise, any subset or combination of these is also specifically contemplated and disclosed. Thus, for example, the sub-group of A-E, B-F, and C-E are specifically contemplated and should be considered disclosed from disclosure of A, B, and/or C; D, E, and/or F; and the example combination A-D. This concept applies to all aspects of this disclosure including, but not limited to any components of the compositions and steps in methods of making and using the disclosed compositions. Thus, if there are a variety of additional steps that can be performed it is understood that each of these additional steps can be performed with any specific embodiment or combination of embodiments of the disclosed methods, and that each such combination is specifically contemplated and should be considered disclosed.
Moreover, where a range of numerical values is recited herein, comprising upper and lower values, unless otherwise stated in specific circumstances, the range is intended to include the endpoints thereof, and all integers and fractions within the range. It is not intended that the scope of the invention be limited to the specific values recited when defining a range. Further, when an amount, concentration, or other value or parameter is given as a range, one or more preferred ranges or a list of upper preferable values and lower preferable values, this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit or preferred value and any lower range limit or preferred value, regardless of whether such pairs are separately disclosed. Finally, when the term “about” is used in describing a value or an end-point of a range, the disclosure should be understood to include the specific value or end-point referred to.
As used herein, the term “about” means that amounts, sizes, formulations, parameters, and other quantities and characteristics are not and need not be exact, but may be approximate and/or larger or smaller, as desired, reflecting tolerances, conversion factors, rounding off, measurement error and the like, and other factors known to those of skill in the art. In general, an amount, size, formulation, parameter or other quantity or characteristic is “about” or “approximate” whether or not expressly stated to be such.
The term “or”, as used herein, is inclusive; more specifically, the phrase “A or B” means “A, B, or both A and B”. Exclusive “or” is designated herein by terms such as “either A or B” and “one of A or B”, for example.
The indefinite articles “a” and “an” are employed to describe elements and components of the invention. The use of these articles means that one or at least one of these elements or components is present. Although these articles are conventionally employed to signify that the modified noun is a singular noun, as used herein the articles “a” and “an” also include the plural, unless otherwise stated in specific instances. Similarly, the definite article “the”, as used herein, also signifies that the modified noun may be singular or plural, again unless otherwise stated in specific instances.
For the purposes of describing the embodiments, it is noted that reference herein to a variable being a “function” of a parameter or another variable is not intended to denote that the variable is exclusively a function of the listed parameter or variable. Rather, reference herein to a variable that is a “function” of a listed parameter is intended to be open ended such that the variable may be a function of a single parameter or a plurality of parameters.
It is noted that terms like “preferably,” “commonly,” and “typically,” when utilized herein, are not utilized to limit the scope of the claimed invention or to imply that certain features are critical, essential, or even important to the structure or function of the claimed invention. Rather, these terms are merely intended to identify particular aspects of an embodiment of the present disclosure or to emphasize alternative or additional features that may or may not be utilized in a particular embodiment of the present disclosure.
For the purposes of describing and defining the claimed invention it is noted that the terms “substantially” and “approximately” are utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. The terms “substantially” and “approximately” are also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue.
It is noted that one or more of the claims may utilize the term “wherein” as a transitional phrase. For the purposes of defining the present invention, it is noted that this term is introduced in the claims as an open-ended transitional phrase that is used to introduce a recitation of a series of characteristics of the structure and should be interpreted in like manner as the more commonly used open-ended preamble term “comprising”.
As a result of the raw materials and/or equipment used to produce the glass composition of the present invention, certain impurities or components that are not intentionally added, can be present in the final glass composition. Such materials are present in the glass composition in minor amounts and are referred to herein as “tramp materials.”
As used herein, a glass composition having 0 wt % of a compound is defined as meaning that the compound, molecule, or element was not purposefully added to the composition, but the composition may still comprise the compound, typically in tramp or trace amounts. Similarly, “sodium-free,” “alkali-free,” “potassium-free” or the like are defined to mean that the compound, molecule, or element was not purposefully added to the composition, but the composition may still comprise sodium, alkali, or potassium, but in approximately tramp or trace amounts.
It has been found that the CTE of the super/substrate glass for CdTe thin-film photovoltaic modules should match, or be as closely matched as possible, to the CTE of the semiconductor film so as to minimize the level of stress in said film and consequently optimize the efficiency of the photovoltaic module. The compositions described herein specifies a glass super/substrate having a CTE in the range that is optimum for minimizing the resultant stress in the semiconductor layers of CdTe devices and provide improved conversion efficiency in the resulting devices.
The terms “solar cell,” “photovoltaic cell,” “PV cell,” “solar module,” “photovoltaic module,” “PV module,” “solar device,” “photovoltaic device,” “PV device,” or “device,” as used herein, refer to any article that can convert light into electrical energy. Suitable solar cells include wafer-based solar cells (e.g., solar cells comprising materials selected from crystalline-Si (c-Si), ribbon Si, or multi-crystalline-Si (mc-Si) (also called polycrystalline Si), and mixtures thereof). A solar cell assembly can comprise one or a plurality of solar cells. The plurality of solar cells can be electrically interconnected or arranged in a flat plane. In addition, the solar cell assembly can further comprise conductive pastes or electrical wirings deposited upon the solar cells.
As used herein, the term “substrate” can be used to describe either a substrate or a superstrate depending on the configuration of the photovoltaic cell. For example, the substrate is a superstrate, if when assembled into a photovoltaic cell, it is on the light incident side of a photovoltaic cell. The superstrate can provide protection for the photovoltaic materials from impact and environmental degradation while allowing transmission of the appropriate wavelengths of the solar spectrum. Further, multiple photovoltaic cells can be arranged into a photovoltaic module.
As used herein, the term “adjacent” can be defined as being in close proximity. Adjacent structures may or may not be in physical contact with each other. Adjacent structures can have other layers and/or structures disposed between them.
As used herein, the term “planar” can be defined as having a substantially topographically flat surface.
Although exemplary numerical ranges are described in the embodiments, each of the ranges can include any numerical value including decimal places within the range including each of the ranges endpoints.
As used herein, multivalent components of the exemplary compositions are represented, for example, as Fe2O3, SnO2, As2O2, Sb2O5. These materials are batched as said oxides but mixed valences or alternative valences could be used.
In the broadest terms, the glass compositions described here have a strain point of 600° C. or greater and a CTE that is matched to a CdTe thin film to be coated thereon. In some embodiments, “matched,” as used herein, mean the CTE of the glass substrate is equivalent to within ±2, ±1.5, ±1.0, ±0.75, ±0.5 or ±0.25 ppm/° C. of the CTE of a CdTe thin film to be coated thereon. In some embodiments, the CTE of the glass substrate is from about 4 ppm/° C. to about 8 ppm/° C., from about 4.5 ppm/° C. to about 8 ppm/° C., from about 4.5 ppm/° C. to about 7 ppm/° C., from about 4.5 ppm/° C. to about 6.5 ppm/° C., from about 5 ppm/° C. to about 7 ppm/° C., from about 5.5 ppm/° C. to about 6.5 ppm/° C., from about 4 ppm/° C. to about 7 ppm/° C., from about 4 ppm/° C. to about 6.5 ppm/° C., from about 5 ppm/° C. to about 8 ppm/° C., or from about 5.5 ppm/° C. to about 8 ppm/° C. In some embodiments, the CTE of the glass substrate is about 4, 4.5, 5, 5.5, 5.75, 6.0, 6.5, 7, 7.5, or 8 ppm/° C.
Accordingly, in one embodiment, the glass has a strain point of 600° C. or greater, for example, 620° C. or greater. In some embodiments, the glass has a coefficient of thermal expansion of 38×10−7 or greater, for example, 40×10−7 or greater, for example, 45×10−7 or greater.
The glass according to one embodiment can have a strain point of 620° C. or greater and/or a coefficient of thermal expansion of 45×10−7 or greater.
In some embodiments, the CTE of the CdTe thin film comprises an average value for a CdTe thin film. In some embodiments, the CTE of the CdTe thin film comprises a specific CTE value for the CdTe film which results from the composition, substrate, other components, manufacturing or deposition process.
A first aspect comprises a glass comprising, in mole percent, about:

    
    
        20 to 70 percent SiO2;
        5 to 20 percent Al2O3;
        0 to 15 percent B2O3;
        0 to 1 percent M2O;
        0 to 10 percent MgO;
        8 to 35 percent CaO;
        0 to 16 percent SrO;
        0 to 9 percent BaO; and
        20 to 45 percent RO;

wherein, M is an alkali metal and wherein, R is an alkaline earth metal.

    
    


In some embodiments, the glass comprises, in mole percent, about:

    
    
        57 to 68 percent SiO2;
        5 to 12 percent Al2O3;
        0 to 8 percent B2O3;
        0 to 0.1 percent M2O;
        0 to 10 percent MgO;
        8 to 28 percent CaO;
        0 to 10 percent SrO;
        0 to 9 percent BaO; and
        21 to 32 percent RO;

wherein, M is an alkali metal and wherein, R is an alkaline earth metal.

    
    


A second aspect comprises a glass comprising, in mole percent, about:

    
    
        60 to 65 percent SiO2;
        8 to 12 percent Al2O3;
        7 to 15 percent B2O3;
        greater than 0 to 8 percent M2O; and
        9 to 15 percent RO;

wherein, M is an alkali metal and wherein, R is an alkaline earth metal. According to another embodiment, the glass is substantially free of BaO. For example, the content of BaO can be 0.05 mole percent or less, for example, zero mole percent. In another embodiment, the glass comprises 0.01 to 0.4 mole percent SnO2.

    
    


In some embodiment, the glass comprises, in mole percent, about:

    
    
        61 to 64 percent SiO2;
        8 to 12 percent Al2O3;
        9 to 15 percent B2O3;
        greater than 0 to 4 percent M2O; and
        12 to 15 percent RO;

wherein, M is an alkali metal and wherein, R is an alkaline earth metal.

    
    


In some embodiment, the glass comprises, in mole percent, about:

    
    
        60 to 65 percent SiO2;
        8 to less than 10 percent Al2O3;
        greater than 11 to 15 percent B2O3;
        greater than 0 to less than 1 percent M2O; and
        9 to 15 percent RO;

wherein, M is an alkali metal and wherein, R is an alkaline earth metal.

    
    


In some embodiment, the glass comprises, in mole percent, about:

    
    
        60 to 65 percent SiO2;
        10 to 12 percent Al2O3;
        7 to 11 percent B2O3;
        1 to 8 percent M2O; and
        9 to 15 percent RO;

wherein, M is an alkali metal and wherein, R is an alkaline earth metal.

    
    


In some embodiment, the glass comprises, in mole percent, about:

    
    
        62 to 65 percent SiO2;
        10 to 12 percent Al2O3;
        7 to 11 percent B2O3;
        3 to 8 percent MgO;
        3 to 10 percent CaO;
        3 to 8 percent SrO; and
        1 to 8 percent M2O;

wherein, M is an alkali metal selected from K, Na, and combinations thereof and wherein, CaO/(CaO+SrO) is from 0.4 to 1.

    
    


A third aspect comprises a glass comprising, in mole percent, about:

    
    
        60 to 68 percent SiO2;
        8 to 11 percent Al2O3;
        6 to 11 percent B2O3;
        0 to 1 percent M2O;
        0 to 5 percent MgO;
        greater than 0 to 12 percent CaO;
    
    


1 to 12 percent SrO;

    
    
        0 to 8 percent BaO; and
        16 to 20 percent RO;

wherein R is an alkaline earth metal, and M is an alkali metal. In some embodiments, the M2O is from 0 to about 0.1 mole percent.

    
    


In some embodiments, M is an alkali metal selected from Li, Na, K, Rb, Cs, and a combination thereof. In some embodiments, M is selected from Li, K, Cs, and a combination thereof. In one embodiment, R is selected from Mg, Ca, Sr, Ba, and a combination thereof. In some embodiments, R is selected from Mg, Ca, Sr, and a combination thereof.
SiO2, an oxide involved in the formation of glass, functions to stabilize the networking structure of glass. In some embodiments, the glass composition comprises from about 20 to about 70 mol % SiO2. In some embodiments, the glass composition comprises from about 57 to about 68 mol % SiO2. In some embodiments, the glass composition can comprise from about 60 to about 68 mol % SiO2. In some embodiments, the glass composition can comprise from about 60 to about 65 mol % SiO2. In some embodiments, the glass composition can comprise from about 61 to about 64 mol % SiO2. In some embodiments, the glass composition can comprise from about 62 to about 65 mol % SiO2. In some embodiments, the glass composition can comprise from about 20 to about 70 mol %, about 20 to about 68 mol %, about 20 to about 65 mol %, about 20 to about 64 mol %, about 20 to 60 mol %, about 20 to 50 mol %, about 20 to about 40 mol %, about 20 mol % to about 30 mol %, about 30 to about 70 mol %, about 30 to about 68 mol %, about 30 to about 65 mol %, about 30 to about 64 mol %, about 30 to 60 mol %, about 30 to 50 mol %, about 30 to about 40 mol %, about 40 to about 70 mol %, about 40 to about 68 mol %, about 40 to about 65 mol %, about 40 to about 64 mol %, about 40 to 60 mol %, about 40 to 50 mol %, about 50 to about 70 mol %, about 50 to about 68 mol %, about 50 to about 65 mol %, about 50 to about 64 mol %, about 50 to 60 mol %, about 60 to about 70 mol %, about 60 to about 68 mol %, about 60 to about 65 mol %, about 60 to about 64 mol %, about 61 to about 70 mol %, about 61 to about 68 mol %, about 61 to about 65 mol %, or about 61 to about 64 mol %. In some embodiments, the glass composition comprises about 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, or 70 mol % SiO2.
Al2O3 may provide for a) maintaining the lowest possible liquidus temperature, b) lowering the expansion coefficient, or c) enhancing the strain point. In some embodiments, the glass composition can comprise from about 5 to about 20 mol % Al2O3. In some embodiments, the glass composition can comprise from 5 to about 12 mol % Al2O3. In some embodiments, the glass composition can comprise from about 8 to about 12 mol % Al2O3. In some embodiments, the glass composition can comprise from about 8 to about 11 mol % Al2O3. In some embodiments, the glass composition can comprise from about 8 to about 10 mol % Al2O3. In some embodiments, the glass composition can comprise from about 10 to about 12 mol % Al2O3. In some embodiments, the glass can comprise from about 5 to about 20 mol %, about 5 to about 15 mol %, about 5 to about 12 mol %, about 5 to about 11 mol %, about 5 to about 10 mol %, about 8 to about 20 mol %, about 8 to about 15 mol %, about 8 to about 15 mol %, about 8 to about 12 mol %, about 8 to about 11 mol %, about 8 to about 10 mol %, about 10 to about 20 mol %, about 10 to about 15 mol %, about 10 to about 12 mol %, about 10 to about 11 mol %, or about 15 to about 20 mol % Al2O3. In some embodiments, the glass composition can comprise about 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 mol % Al2O3.
B2O3 can be used as a flux to soften glasses, making them easier to melt. B2O3 may also react with non-bridging oxygen atoms (NBOs), converting the NBOs to bridging oxygen atoms through the formation of BO4 tetrahedra, which increases the toughness of the glass by minimizing the number of weak NBOs. B2O3 also lowers the hardness of the glass which, when coupled with the higher toughness, decreases the brittleness, thereby resulting in a mechanically durable glass, which can be advantageous for substrates used in photovoltaic applications. In some embodiments, the glass composition can comprise from 0 to about 15 mol % B2O3. In some embodiments, the glass composition can comprise from 0 to about 8 mol % B2O3. In some embodiments, the glass composition can comprise from about 7 to about 15 mol % B2O3. In some embodiments, the glass composition can comprise from about 9 to about 15 mol % B2O3. In some embodiments, the glass composition can comprise from about 11 to about 15 mol % B2O3. In some embodiments, the glass composition can comprise from about 7 to about 11 mol % B2O3. In some embodiments, the glass composition can comprise from about 6 to about 11 mol % B2O3. As mentioned above, the glasses, according some embodiments, comprise 7 to 15 mole percent, for example, 7 to 11 mole percent B2O3. In some embodiments, the glass composition can comprise from 0 to about 15 mol %, 0 to 12 mol %, 0 to 11 mol %, 0 to about 10 mol %, about 5 to about 15 mol %, about 5 to about 12 mol %, about 5 to about 11 mol %, about 5 to about 10 mol %, about 6 to about 15 mol %, about 6 to about 12 mol %, about 6 to about 11 mol %, about 6 to about 10 mol %, about 7 to about 15 mol %, about 7 to about 12 mol %, about 7 to about 11 mol %, about 7 to about 10 mol %, about 9 to about 15 mol %, about 9 to about 12 mol %, about 9 to about 11 mol %, about 9 to about 10 mol %, about 10 to about 15 mol %, about 10 to about 12 mol %, about 10 to about 11 mol %, about 11 to about 15 mol %, about 11 to about 12 mol %, or about 12 to about 15 mol %, B2O3. In some embodiments, the glass composition can comprise about 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15 mol % B2O3.
Also as mentioned above, the glasses, according to some embodiments, 9 to 45 mole percent RO wherein, R is an alkaline earth metal. Since MgO, CaO and BaO are effective in decreasing the viscosity of glass at a higher temperature and enhancing the viscosity of glass at a lower temperature, they may be used for the improvement of the melting property and enhancement of the strain point. However, if excessive amounts of both MgO and CaO are used, there is an increasing trend toward phase separation and devitrification of the glass. As defined herein, RO comprises the mol % of MgO, CaO, SrO, and BaO. In some embodiments, the glass composition can comprise from 20 to about 45 mol % RO. In some embodiments, the glass composition can comprise from 21 to about 32 mol % RO. In some embodiments, the glass composition can comprise from 9 to about 15 mol % RO. In some embodiments, the glass composition can comprise from about 12 to about 15 mol % RO. In some embodiments, the glass composition can comprise from about 16 to about 20 mol % RO. In some embodiments, the glass composition can comprise from about 9 to about 45 mol %, about 9 to about 40 mol %, about 9 to about 35 mol %, about 9 to about 32 mol %, about 9 to about 30 mol %, about 9 to 25 mol %, about 9 to 20 mol %, about 9 to about 15 mol %, about 12 to about 45 mol %, about 12 to about 40 mol %, about 12 to about 35 mol %, about 12 to about 32 mol %, about 12 to about 30 mol %, about 12 to about 25 mol %, about 12 to about 20 mol %, about 12 to about 15 mol %, about 16 to about 45 mol %, about 16 to about 40 mol %, about 16 to about 35 mol %, about 16 to about 32 mol %, about 16 to about 25 mol %, about 16 to about 20 mol %, about 20 to about 45 mol %, about 20 to about 40 mol %, about 20 to about 35 mol %, about 20 to about 32 mol %, about 20 to about 30 mol %, about 20 to about 25 mol %, about 21 to about 45 mol %, about 21 to about 40 mol %, about 21 to about 35 mol %, about 21 to about 32 mol %, about 21 to about 30 mol %, about 21 to about 25 mol %, about 25 to about 45 mol %, about 25 to about 40 mol %, about 25 to about 35 mol %, about 25 to about 32 mol %, about 25 to about 30 mol %, about 30 to about 45 mol %, about 30 to about 40 mol %, about 30 to about 35 mol %, about 30 to about 32 mol %, about 32 to about 45 mol %, about 32 to about 40 mol %, about 32 to about 35 mol %, about 35 to about 45 mol %, about 35 to about 40 mol %, or about 40 to about 45 mol % RO. In some embodiments, the glass composition can comprise about 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, or 45 mol % RO.
In some embodiments, MgO can be added to the glass to reduce melting temperature, increase strain point, or adjust CTE when used in combination with other alkaline earth compounds (e.g., CaO, SrO, and BaO). It can disadvantageously lower CTE relative to other alkaline earths (e.g., CaO, SrO, BaO), and so other adjustments may be made to keep the CTE within the desired range. Examples of suitable adjustments include increase SrO at the expense of CaO, increasing alkali oxide concentration, and replacing a smaller alkali oxide (e.g., Na2O) in whole or in part with a larger alkali oxide (e.g., K2O). In some embodiments, the glass can comprise about 0 to about 10 mol % MgO. The glass can comprise, for example, 1 to 8 mole percent MgO. In some embodiments, the glass composition can comprise greater than 0 to about 5 mol % MgO. In some embodiments, the glass composition can comprise about 3 to about 8 mol % MgO. In some embodiments, the glass composition can comprise 0 to about 10 mol %, 0 to about 8 mol %, 0 to about 5 mol %, 0 to about 4 mol %, 0 to about 3 mol %, 0 to about 2 mol %, 0 to about 1 mol %, about 3 to about 10 mol %, about 3 to about 8 mol %, about 3 to about 5 mol %, about 5 to about 10 mol %, about 5 to about 8 wt, or about 7 to about 10 mol % MgO. In some embodiments, the glass compositions can comprise about 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 mol % MgO.
In some embodiments, CaO can contribute to higher strain point, lower density, and lower melting temperature. More generally, it can be a component of certain possible devitrification phases, particularly anorthite (CaAl2Si2O8), and this phase has complete solid solution with an analogous sodium phase, albite (NaAlSi3O8). High Na and Ca contents taken alone can cause liquidus temperatures to be unacceptably high. However, CaO sources include limestone, an inexpensive material, so to the extent that volume and low cost are factors, in some embodiments it is can be useful to make the CaO content as high as can be reasonably achieved relative to other alkaline earth oxides. In some embodiments, the glass composition about from 0 to about 35 mol % CaO. In some embodiments, the glass composition about from 8 to about 35 mol % CaO. In some embodiments, the glass composition can comprise from 8 to about 28 mol % CaO. In some embodiments, the glass composition can comprise from 0 to about 12 mol % CaO. In some embodiments, the glass composition can comprise from >0 to about 12 mol % CaO. In some embodiments, the glass composition can comprise from about 3 to about 10 mol % CaO. In some embodiments, the glass composition can comprise from 0 to about 35 mol %, 0 to about 30 mol %, 0 to about 28 mol %, 0 to 25 mol %, 0 to 20 mol %, 0 to about 15 mol %, 0 to about 12 mol %, 0 to about 10 mol %, 0 to about 5 mol %, about 3 to about 35 mol %, about 3 to about 30 mol %, about 3 to about 28 mol %, about 3 to about 25 mol %, about 3 to about 20 mol %, about 3 to about 15 mol %, about 3 to about 12 mol %, about 3 to about 10 mol %, about 8 to about 35 mol %, about 8 to about 30 mol %, about 8 to about 28 mol %, about 8 to about 25 mol %, about 8 to about 20 mol %, about 8 to about 15 mol %, about 8 to about 12 mol %, about 8 to about 10 mol %, about 10 to about 35 mol %, about 10 to about 30 mol %, about 10 to about 25 mol %, about 10 to about 20 mol %, about 10 to about 15 mol %, about 10 to about 12 mol %, about 15 to about 35 mol %, about 15 to about 30 mol %, about 15 to about 28 mol %, about 15 to about 25 mol %, about 15 to about 20 mol %, about 20 to about 35 mol %, about 20 to about 30 mol %, about 20 to about 28 mol %, about 20 to about 28 mol %, about 25 to about 35 mol %, about 25 to about 30 mol %, about 25 to about 28 mol %, about 28 to about 35 mol %, about 28 to about 30 mol %, about 28 to about 30 mol %, or about 30 to about 35 mol % CaO. In some embodiments, the glass composition can comprise about 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, or 35 mol % CaO.
SrO can contribute to higher coefficient of thermal expansion, and the relative proportion of SrO and CaO can be manipulated to improve liquidus temperature, and thus liquidus viscosity. In some embodiments, the glass can comprise from 0 to about 16 mol % SrO. In some embodiments, the glass can comprise from 0 to about 10 mol % SrO. In some embodiments, the glass can comprise from 1 to about 12 mol % SrO. In other embodiments, the glass can comprise greater than 0 to about 16 mol % SrO. The glasses can comprise, in some embodiments, 0 to 5 mole percent SrO. In certain embodiments, the glass contains no deliberately batched SrO, though it may be present as a contaminant in other batch materials. In some embodiments, the glass composition can comprise from 0 to about 16 mol %, 0 to about 15 mol %, 0 to about 12 mol %, 0 to about 10 mol %, 0 to about 8 mol %, 0 to about 5 mol %, 0 to about 3 mol %, about 3 to about 16 mol %, about 3 to about 15 mol %, about 3 to about 12 mol %, about 3 to about 10 mol %, about 3 to about 8 mol %, about 3 to about 5 mol %, about 5 to about 16 mol %, about 5 to about 15 mol %, about 5 to about 12 mol %, about 5 to about 10 mol %, about 5 to about 8 mol %, about 8 to about 16 mol %, about 8 to about 15 mol %, about 8 to about 12 mol %, about 8 to about 10 mol %, about 10 to about 16 mol %, about 10 to about 15 mol %, about 10 to about 12 mol %, about 12 to about 16 mol %, about 12 to about 15 mol %, or about 15 to about 16 mol %, SrO. In some embodiments, the glass composition can comprise about 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, or 16 mol % SrO.
In some embodiments, the ratio CaO/(CaO+SrO) is advantageously kept between 0.4 and 1 to obtain a good balance between liquidus temperature (and hence liquidus viscosity) and melting temperature. For example, compositions with low alkali concentrations and high SrO concentrations have comparatively high melting temperatures, and if SrO is too high then liquidus temperatures may be elevated as well relative to glasses with more alkali oxide and lower SrO. However, for fixed concentrations of all other components, a local minimum in liquidus temperature is often obtained for CaO/(CaO+SrO) ratios between 0.4 and 1.
The glasses, according to some embodiments, comprise BaO. In certain embodiments, the glasses comprise less than 0.1 mole percent of BaO. In some embodiments, the glass composition can comprise from 0 to 10 mol % BaO. In some embodiments, the glass composition can comprise from 0 to 9 mol % BaO. In some embodiments, the glass composition can comprise from about greater than 0 to about 10 mol % BaO. In some embodiments, the glass composition can comprise from >0 to 8 mol % BaO. In some embodiments, the glass composition can comprise 0 to about 10 mol %, 0 to about 9 mol %, 0 to about 8 mol %, 0 to about 5 mol %, 0 to about 3 mol %, about 3 to about 10 mol %, about 3 to about 9 mol %, about 3 to about 8 mol %, about 3 to about 5 mol %, about 5 to about 10 mol %, about 5 to about 9 mol %, about 5 to about 8 mol %, about 8 to about 9 mol %, about 8 to about 10 mol %, or about 9 to about 10 mol % BaO. In some embodiments, the glass composition comprises about 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 mol % BaO
Generally, alkali cations can raise the CTE steeply, but also can lower the strain point and, depending upon how they are added, they can increase melting temperatures. The least effective alkali oxide for raising CTE is Li2O, and the most effective alkali oxide for raising CTE is Cs2O. In some embodiments, the glass composition can comprise from 0 to about 8 mol % M2O, wherein M is one or more of the alkali cations Na, Li, K, Rb, and Cs. In some embodiments, the glass composition can comprise from greater than 0 to about 8 mol % M2O. In some embodiments, the glass composition can comprise from greater than 0 to about 4 mol % M2O. In some embodiments, the glass composition can comprise from greater than 0 to about 1 mol % M2O. In some embodiments, M2O can comprise only trace amounts of Na2O. In some embodiments, M2O can comprise only trace amounts of Na2O and K2O. In certain embodiments, the alkalis in question can be Li, K and Cs or combinations thereof. In some embodiments, the glass composition is substantially alkali free, for example, the content of alkali metal can be about 1 weight percent or less, 0.5 weight percent or less, 0.25 mol % or less, 0.1 mol % or less or 0.05 mol % or less. In some embodiments, M2O is essentially 0 mol % or is only present in tramp amounts. The glass, according to some embodiments, can be substantially free of intentionally added alkali cations, compounds, or metals. In some embodiments, the glass composition can comprises from 0 to about 8 mol %, 0 to about 7 mol %, 0 to about 6 mol %, 0 to about 5 mol %, 0 to about 4 mol %, 0 to about 3 mol %, 0 to about 2 mol %, 0 to about 1 mol %, >0 to about 8 mol %, >0 to about 7 mol %, >0 to about 6 mol %, >0 to about 5 mol %, >0 to about 4 mol %, >0 to about 3 mol %, >0 to about 2 mol %, >0 to about 1 mol %, about 1 to about 8 mol %, about 1 to about 7 mol %, about 1 to about 6 mol %, about 1 to about 5 mol %, about 1 to about 4 mol %, about 1 to about 3 mol %, about 1 to about 2 mol %, about 2 to about 8 mol %, about 2 to about 7 mol %, about 2 to about 6 mol %, about 2 to about 5 mol %, about 2 to about 4 mol %, about 2 to about 3 mol %, about 3 to about 8 mol %, about 3 to about 7 mol %, about 3 to about 6 mol %, about 3 to about 5 mol %, about 3 to about 4 mol %, about 4 to about 8 mol %, about 4 to about 7 mol %, about 4 to about 6 mol %, about 4 to about 5 mol %, about 5 to about 8 mol %, about 5 to about 7 mol %, about 5 to about 6 mol %, about 6 to about 8 mol %, about 6 to about 7 mol %, or about 7 to about 8 mol % M2O. In some embodiments, the glass compositions can comprise about 0, 1, 2, 3, 4, 5, 6, 7, or 8 mol % M2O.
Sodium can be a mobile element or ion commonly used in standard window soda-lime glass compositions. The mobility of sodium can be problematic for the long-term reliability of PV modules since over the life of a PV module sodium can drift out of the glass under an applied field and migrate onto the active device layers of the module and degrade performance over time. In some embodiments, the glass is substantially sodium free. In some embodiments, the glass comprises 0.5 weight percent or less, 0.25 mol % or less, 0.1 mol % or less, about 0.05 mol % or less, or 0.001 mol % or less.
In alternative embodiments, sodium can be present from 0 to about 8 mol %. In some embodiments, the glass composition can comprises from 0 to about 8 mol %, 0 to about 7 mol %, 0 to about 6 mol %, 0 to about 5 mol %, 0 to about 4 mol %, 0 to about 3 mol %, 0 to about 2 mol %, 0 to about 1 mol %, >0 to about 8 mol %, >0 to about 7 mol %, >0 to about 6 mol %, >0 to about 5 mol %, >0 to about 4 mol %, >0 to about 3 mol %, >0 to about 2 mol %, >0 to about 1 mol %, about 1 to about 8 mol %, about 1 to about 7 mol %, about 1 to about 6 mol %, about 1 to about 5 mol %, about 1 to about 4 mol %, about 1 to about 3 mol %, about 1 to about 2 mol %, about 2 to about 8 mol %, about 2 to about 7 mol %, about 2 to about 6 mol %, about 2 to about 5 mol %, about 2 to about 4 mol %, about 2 to about 3 mol %, about 3 to about 8 mol %, about 3 to about 7 mol %, about 3 to about 6 mol %, about 3 to about 5 mol %, about 3 to about 4 mol %, about 4 to about 8 mol %, about 4 to about 7 mol %, about 4 to about 6 mol %, about 4 to about 5 mol %, about 5 to about 8 mol %, about 5 to about 7 mol %, about 5 to about 6 mol %, about 6 to about 8 mol %, about 6 to about 7 mol %, or about 7 to about 8 mol % Na2O. In some embodiments, the glass compositions can comprise about 0, 1, 2, 3, 4, 5, 6, 7, or 8 mol % Na2O.
As in the case of sodium, potassium is also an element or ion commonly found in standard window soda-lime glass compositions that has substantial mobility and may drift out of the glass. In some embodiments, the glass is substantially potassium free. In some embodiments, the glass comprises 0.5 weight percent or less, 0.25 mol % or less, 0.1 mol % or less, about 0.05 mol % or less, or 0.001 mol % or less.
In alternative embodiments, potassium can be present from 0 to about 8 mol %. In some embodiments, the glass can comprises from 0 to about 8 mol %, 0 to about 7 mol %, 0 to about 6 mol %, 0 to about 5 mol %, 0 to about 4 mol %, 0 to about 3 mol %, 0 to about 2 mol %, 0 to about 1 mol %, >0 to about 8 mol %, >0 to about 7 mol %, >0 to about 6 mol %, >0 to about 5 mol %, >0 to about 4 mol %, >0 to about 3 mol %, >0 to about 2 mol %, >0 to about 1 mol %, about 1 to about 8 mol %, about 1 to about 7 mol %, about 1 to about 6 mol %, about 1 to about 5 mol %, about 1 to about 4 mol %, about 1 to about 3 mol %, about 1 to about 2 mol %, about 2 to about 8 mol %, about 2 to about 7 mol %, about 2 to about 6 mol %, about 2 to about 5 mol %, about 2 to about 4 mol %, about 2 to about 3 mol %, about 3 to about 8 mol %, about 3 to about 7 mol %, about 3 to about 6 mol %, about 3 to about 5 mol %, about 3 to about 4 mol %, about 4 to about 8 mol %, about 4 to about 7 mol %, about 4 to about 6 mol %, about 4 to about 5 mol %, about 5 to about 8 mol %, about 5 to about 7 mol %, about 5 to about 6 mol %, about 6 to about 8 mol %, about 6 to about 7 mol %, or about 7 to about 8 mol % K2O. In some embodiments, the glass compositions can comprise about 0, 1, 2, 3, 4, 5, 6, 7, or 8 mol % K2O
In certain embodiments, the glass satisfies one or more of the following expressions:

1.0≦(M2O+RO)/Al2O3≦2; and

0.4≦CaO/(CaO+SrO)≦1.

The ratio (M2O+RO)/Al2O3 is advantageously greater than 1.0 to assist in removing bubbles from the glass during the initial melt step. This occurs because the alkali and alkaline earth metal oxides that are not involved in stabilizing Al2O3 are available to digest the silica source, typically a commercial sand. Surface area that might be sites for bubble nucleation and growth are therefore eliminated early in melting, and a comparatively bubble-free glass is obtained.
The glasses, according to some embodiments, can further comprise a variety of other components. For example, the glasses can comprise SnO2, Fe2O3, MnO, CeO2, As2O3, Sb2O3, Cl, Br, or combinations thereof. These materials can be added as fining agents (e.g., to facilitate removal of gaseous inclusions from melted batch materials used to produce the glass) and/or for other purposes. In certain embodiments, the glasses comprise SnO2 (e.g., as calculated in mole percent on an oxide basis, 0.02 to 0.3 SnO2, etc.) and Fe2O3 (e.g., as calculated in mole percent on an oxide basis, 0.005 to 0.08 Fe2O3, 0.01 to 0.08 Fe2O3, etc.). By way of illustration, in certain embodiments, the glasses comprise SnO2 and Fe2O3, wherein, in mole percent on an oxide basis:

0.02≦SnO2≦0.3; and

0.005≦Fe2O3≦0.08.

In certain embodiments, the glasses comprise less than 0.05% mole percent of Sb2O3, As2O3, or combinations thereof. In certain embodiments, the glasses comprise SnO2, Fe2O3, CeO2, Cl, Br, or combinations thereof and include less than 0.05% (e.g., less than 0.04%, less than 0.03%, less than 0.02%, less than 0.01%, etc.) mole percent of Sb2O3, As2O3, or combinations thereof. In certain embodiments, the glasses comprise SnO2 and Fe2O3 and include less than 0.05 mole percent (e.g., less than 0.04 mole percent, less than 0.03 mole percent, less than 0.02 mole percent, less than 0.01 mole percent, etc.) of Sb2O3, As2O3, or combinations thereof. In certain embodiments, the glasses comprise SnO2 and Fe2O3, wherein, in mole percent on an oxide basis:

0.02≦SnO2≦0.3; and

0.005≦Fe2O3≦0.08,

and include less than 0.05% mole percent of Sb2O3, As2O3, or combinations thereof.

The glasses, according to some embodiments, (e.g., any of the glasses discussed above) can include F, Cl, or Br, for example, as in the case where the glasses comprise Cl and/or Br as fining agents. For example, the glass can comprise fluorine, chlorine, and/or bromine, wherein, as calculated in mole percent: F+Cl+Br≦0.4, such as where F+Cl+Br≦0.3, F+Cl+Br≦0.2, F+Cl+Br≦0.1, 0.001F+Cl+Br≦0.4, and/or 0.005≦F+Cl+Br≦0.4. By way of illustration, in certain embodiments, the glass comprises SnO2 and Fe2O3 and, optionally, fluorine, chlorine, and/or bromine, such that, as calculated in mole percent on an oxide basis: 0.02≦SnO2≦0.3, 0.005≦Fe2O3≦0.08, and F+Cl+Br≦0.4; and, in certain embodiments, the glass comprises SnO2 and Fe2O3 and, optionally, Sb2O3, As2O3, fluorine, chlorine, and/or bromine, such that, as calculated in mole percent on an oxide basis, 0.02≦SnO2≦0.3, 0.005≦Fe2O3≦0.08, and F+Cl+Br≦0.4, and such that the glass includes less than 0.05 mole percent (e.g., less than 0.04, less than 0.03, less than 0.02, less than 0.01, etc.) mole percent of Sb2O3, As2O3, or combinations thereof.
In some embodiments, the glass is substantially free of Sb2O3, As2O3, or combinations thereof, for example, the glass comprises 0.05 mole percent or less of Sb2O3 or As2O3 or a combination thereof. For example, the glass can comprise zero mole percent of Sb2O3 or As2O3 or a combination thereof.
The glass can further comprise 2 mole percent or less of TiO2, MnO, ZnO, Nb2O5, MoO3, Ta2O5, WO3, ZrO2, Y2O3, La2O3, HfO2, CdO, SnO2, Fe2O3, CeO2, As2O3, Sb2O3, Cl, Br, P2O5, or combinations thereof.
The glasses, according to some embodiments, can further include contaminants as typically found in commercially prepared glass. In addition or alternatively, a variety of other oxides (e.g., TiO2, MnO, ZnO, Nb2O5, MoO3, Ta2O5, WO3, ZrO2, Y2O3, La2O3, P2O5, and the like) can be added, albeit with adjustments to other glass components, without compromising their melting or forming characteristics. In those cases where the glasses, according to some embodiments, further include such other oxide(s), each of such other oxides are typically present in an amount not exceeding 2 mole percent, and their total combined concentration is typically less than or equal to 5 mole percent, although higher amounts can be used so long as the amounts used do not place the composition outside of the ranges described above. The glasses, according to some embodiments, can also include various contaminants associated with batch materials and/or introduced into the glass by the melting, fining, and/or forming equipment used to produce the glass (e.g., ZrO2).
The glass, according to some embodiments, is down-drawable; that is, the glass is capable of being formed into sheets using down-draw methods such as, but not limited to, fusion draw and slot draw methods that are known to those skilled in the glass fabrication arts. Such down-draw processes are used in the large-scale manufacture of flat glass, for example, display glass or ion-exchangeable glass.
The fusion draw process uses an isopipe that has a channel for accepting molten glass raw material. The channel has weirs that are open at the top along the length of the channel on both sides of the channel. When the channel fills with molten material, the molten glass overflows the weirs. Due to gravity, the molten glass flows down the outside surfaces of the isopipe. These outside surfaces extend down and inwardly so that they join at an edge below the drawing tank. The two flowing glass surfaces join at this edge to fuse and form a single flowing sheet. The fusion draw method offers the advantage that, since the two glass films flowing over the channel fuse together, neither outside surface of the resulting glass sheet comes in contact with any part of the apparatus. Thus, the surface properties are not affected by such contact.
The slot draw method is distinct from the fusion draw method. Here the molten raw material glass is provided to a conduit. The bottom of the conduit has an open slot that is wider in one dimension than the other dimension with a nozzle that extends the length of the slot. The molten glass flows through the slot/nozzle and is drawn downward as a continuous sheet there through and into an annealing region. Compared to the fusion draw process, the slot draw process provides a thinner sheet, as only a single sheet is drawn through the slot, rather than two sheets being fused together, as in the fusion down-draw process.
In order to be compatible with down-draw processes, the aluminoborosilicate glass described herein has a high liquidus viscosity. In one embodiment, the glass has a liquidus viscosity of 50,000 poise or greater, for example, 150,000 poise or greater, for example, greater than or equal to 500,000 poise. The liquidus viscosities of the glasses are very closely correlated with the difference between the liquidus temperature and the softening point. This correlation is indicated by line 10 in FIG. 1. For down draw processes, the glasses preferably have liquidus—softening point less than about 230° C., more preferably less than 200° C.
Alternatively, the glass, may be formed by other processes, such as rolling, float, etc.
According to one embodiment, the glass is ion exchanged in a salt bath comprising one or more salts of alkali ions. The glass can be ion exchanged to change its mechanical properties. For example, smaller alkali ions, such as lithium or sodium, can be ion-exchanged in a molten salt containing one or more larger alkali ions, such as sodium, potassium, rubidium or cesium. If performed at a temperature well below the strain point for sufficient time, a diffusion profile will form in which the larger alkali moves into the glass surface from the salt bath, and the smaller ion is moved from the interior of the glass into the salt bath. When the sample is removed, the surface will go under compression, producing enhanced toughness against damage. Such toughness may be desirable in instances where the glass will be exposed to adverse environmental conditions, such as photovoltaic grids exposed to hail. A large alkali already in the glass can also be exchanged for a smaller alkali in a salt bath. If this is performed at temperatures close to the strain point, and if the glass is removed and its surface rapidly reheated to high temperature and rapidly cooled, the surface of the glass will show considerable compressive stress introduced by thermal tempering. This will also provide protection against adverse environmental conditions. It will be clear to one skilled in the art that any monovalent cation can be exchanged for alkalis already in the glass, including copper, silver, thallium, etc., and these also provide attributes of potential value to end uses, such as introducing color for lighting or a layer of elevated refractive index for light trapping.
In one embodiment, the glass is in the form of a sheet. The glass in the form of a sheet can be thermally tempered.
Another aspect comprises a photovoltaic thin-film solar device comprising the glass compositions described above as the starting super/substrate. In the case of CdTe-based modules, the design typically comprises the glass substrate, a transparent conductive oxide (“TCO”) such at fluorine doped tin oxide (“FTO”) or cadmium stannate (“CTO”), followed by resistive transparent layer such as tin oxide (“SnO2”) or zinc doped tin oxide (“ZTO”). Next to the resistive transparent layer is a n-type semiconductor such as CdS followed by a p-type semiconductor, such as CdTe.
In one embodiment, a photovoltaic device comprises the glass in the form of a sheet. The photovoltaic device can comprise more than one of the glass sheets, for example, as a substrate and/or as a superstrate. In one embodiment, the glass sheet is substantially planar. According to one embodiment, the glass sheet is transparent.
According to some embodiments, the glass sheet has a thickness of 4.0 mm or less, for example, 3.5 mm or less, for example, 3.2 mm or less, for example, 3.0 mm or less, for example, 2.5 mm or less, for example, 2.0 mm or less, for example, 1.9 mm or less, for example, 1.8 mm or less, for example, 1.5 mm or less, for example, 1.1 mm or less, for example, 0.5 mm to 2.0 mm, for example, 0.5 mm to 1.1 mm, for example, 0.7 mm to 1.1 mm. Although these are exemplary thicknesses, the glass sheet can have a thickness of any numerical value including decimal places in the range of from 0.1 mm up to and including 4.0 mm.
In another embodiment, the photovoltaic device comprising a glass sheet and an active photovoltaic medium adjacent to the glass sheet.
In one embodiment, the active photovoltaic medium comprises cadmium telluride.
Another embodiment, as shown in FIG. 2 features 200 of a photovoltaic device comprising a glass sheet 12 comprising any of the glass compositions previously described and an active photovoltaic medium 16 adjacent to the glass sheet, wherein the active photovoltaic medium comprises cadmium telluride. According to one embodiment, the glass sheet has a thickness as previously described. The photovoltaic device can further comprise a conductive layer 14, such as a transparent conductive oxide adjacent to or disposed on the glass sheet.
EXAMPLES
Example 1
The following is an example of how to fabricate a sample of an exemplary glass, according to one embodiment of the invention, as shown in Table 1. This composition corresponds to composition number 46 shown in Table 7.










TABLE 1







Oxide
mol %



















SiO2
63.5



Al2O3
10.7



B2O3
10.3



K2O
2.3



MgO
4.4



CaO
5.2



SrO
3.5



SnO2
0.1










In some embodiments, the total does not add up to 100%, since certain tramp elements are present at non-negligible concentrations.

Batch materials, as shown in Table 2 were weighed and added to a 4 liter plastic container:










TABLE 2








Batch



Batch Components
weight



















sand
1713.42



alumina
486.27



boric acid
570.42



Potassium carbonate
143.05



Magnesia
78.62



Limestone
240.73



Strontium carbonate
234.23



10% SnO2 and 90% sand
6.92










It should be appreciated that in the batch, limestone, depending on the source can contain tramp elements and/or vary amounts of one or more oxides, for example, MgO and/or BaO. The sand is advantageously beneficiated so that at least 80% by mass passes 60 mesh, for example 80 mesh, for example 100 mesh. The SnO2 added, in this example, was pre-mixed with sand at a level of 10% by weight so as to ensure homogeneous mixing with the other components. The bottle containing the batch materials was mounted to a tumbler and the batch materials were mixed so as to make a homogeneous batch and to break up soft agglomerates. The mixed batch was transferred to a 1800 cc platinum crucible and placed into a high-temperature ceramic backer. The platinum in its backer was loaded into a glo-bar furnace idling at a temperature of 1550° C. After 6 hours, the crucible+backer was removed and the glass melt was poured onto a cold surface, such as a steel plate, to form a patty, and then transferred to an annealer held at a temperature of 670° C. The glass patty was held at the annealer temperature for 2 hours, then cooled at a rate of 1° C. per minute to room temperature.
Tables 3-13 show exemplary glasses, according to embodiments of the invention, and made according to the above example. Properties data for some glasses are also shown in Tables 3-13.
In view of its low liquidus temperature of 940° C. and, hence, its extremely high liquidus viscosity in excess of 5,000,000 poise, glass 49, shown in Table 8 is an advantageous glass for applications, such as glass for photovoltaics. The exemplary glasses shown in Table 8 comprise, in mole percent:

    
    
        62 to 64 percent SiO2;
        8 to 12 percent Al2O3;
        9 to 15 percent B2O3;
        greater than 0 to 4 percent M2O; and
        12 to 15 percent RO;

wherein, M is an alkali metal and wherein, R is an alkaline earth metal.

    
    


Further, the glasses shown in Table 8 have anneal points ≧640° C., thermal expansion coefficients (CTE) of 40-50×10−7/° C., 200 poise temperatures of ≦1550° C., and liquidus viscosities of 500,000 poise. Liquidus viscosity may be dependent on the K2O content, for example, exemplary glass 49 has a maximum value in excess of 5,000,000 poise for an intermediate K2O content when compared to exemplary glasses 48, 50, and 51.













TABLE 3








Glass
1
2
3
4
5





Weight Percent
SiO2
58.11
59.14
59.01
58.88
58.35



Al2O3
17.76
18.07
18.03
17.99
17.82



B2O3
9.79
9.96
9.94
9.92
9.82



MgO
3.26
4.45
3.88
3.31
3.27



CaO
4.03
4.10
4.87
5.64
4.04



SrO
2.85
0
0
0
0



BaO
0
0
0
0
0



Na2O
3.98
4.05
4.04
4.03
3.99



K2O
0
0
0
0
2.60



SnO2
0.23
0.23
0.23
0.23
0.12



Fe2O3
0
0
0
0
0



total
100.01
100
100
100
100.01


Mole Percent
SiO2
63.3
63.3
63.3
63.3
63.35



Al2O3
11.4
11.4
11.4
11.4
11.4



B2O3
9.2
9.2
9.2
9.2
9.2



MgO
5.3
7.1
6.2
5.3
5.3



CaO
4.7
4.7
5.6
6.5
4.7



SrO
1.8
0
0
0
0



BaO
0
0
0
0
0



Na2O
4.2
4.2
4.2
4.2
4.2



K2O
0
0
0
0
1.8



SnO2
0.1
0.1
0.1
0.1
0.05



Fe2O3
0
0
0
0
0



total
100
100
100
100
100


Properties
strain
593
598
606
597
588



anneal
642
647
656
646
638



softening point
867
868
882
874
881



CTE
44.9
44
43.5
46.4
50.1



density
2.447
2.332
2.414
2.422
2.393


Viscosity
A



−2.2233
−2.5584



B



5556.58
6588.87



To



305.47
220.27



T @ 200 p



1533.625
1576.164



T @ 35 kP



1126.554
1147.957



T @ 250 kP



1034.561
1048.398



T(200 P) − T(35 kP)



407.071
428.206


Resistivity
A



3.0838
3.9616



B



1183.49
4266.54



To



2156.83
3061.95



R @ 200 p



15.30245
12.30353


Liquidus
air
1070
1090
1065
1060
995



internal
1060
1080
1060
1050
980



Pt
1040
1050
1030
1040
975



phase
Albite
Albite
Albite
Albite
Albite



Liquidus viscosity



173742.1
1300909



Int liq - soft
193
212
178
176
99



estimated
107951.5
64408.99
168454.9
179256.1
4244242



liquidus viscosity


















Glass
6
7
8
9






Weight Percent
SiO2
58.40
58.54
58.74
58.56




Al2O3
17.83
17.88
17.94
17.88




B2O3
9.83
9.85
9.88
9.85




MgO
3.28
3.91
4.54
3.62




CaO
4.90
4.05
4.07
4.46




SrO
0
0
0
0




BaO
0
0
0
0




Na2O
3.04
3.05
2.10
3.34




K2O
2.60
2.61
2.62
2.17




SnO2
0.12
0.12
0.12
0.12




Fe2O3
0
0
0
0




total
100
100.01
100.01
100



Mole Percent
SiO2
63.35
63.35
63.35
63.35




Al2O3
11.4
11.4
11.4
11.4




B2O3
9.2
9.2
9.2
9.2




MgO
5.3
6.3
7.3
5.83




CaO
5.7
4.7
4.7
5.17




SrO
0
0
0
0




BaO
0
0
0
0




Na2O
3.2
3.2
2.2
3.5




K2O
1.8
1.8
1.8
1.5




SnO2
0.05
0.05
0.05
0.05




Fe2O3
0
0
0
0




total
100
100
100
100



Properties
strain
592
601
613
594




anneal
643
652
663
646




softening point
885
890
901
883




CTE
47.8
47
43.7
47.2




density
2.402
2.401
2.399
2.401



Viscosity
A








B








To








T @ 200 p








T @ 35 kP








T @ 250 kP








T(200 P) − T(35 kP)







Resistivity
A








B








To








R @ 200 p







Liquidus
air
1050
1040
1115
1030




internal
1040
1030
1105
1020




Pt
1020
1015
1075
1000




phase
Albite
Albite
Albite
Albite




Liquidus viscosity








Int liq - soft
155
140
204
137




estimated
360553.2
631087.4
79584.37
710936.1




liquidus viscosity






















TABLE 4








Glass
10
11
12
13
14





Weight Percent
SiO2
58.78
57.16
57.49
57.83
58.17



Al2O3
17.95
17.46
17.56
17.66
17.76



B2O3
9.89
9.62
9.67
9.73
9.79



MgO
3.96
3.21
3.48
3.76
4.04



CaO
4.88
3.96
4.34
4.72
5.11



SrO
0
0
0
0
0



BaO
0
0
0
0
0



Na2O
2.68
0
0
0
0



K2O
1.75
8.49
7.34
6.18
5.01



SnO2
0.12
0.11
0.11
0.11
0.12



Fe2O3
0
0
0
0
0



total
100.01
100.01
99.99
99.99
100


Mole Percent
SiO2
63.35
63.35
63.35
63.35
63.35



Al2O3
11.40
11.40
11.40
11.40
11.40



B2O3
9.20
9.20
9.20
9.20
9.20



MgO
6.36
5.30
5.72
6.14
6.56



CaO
5.64
4.70
5.12
5.54
5.96



SrO
0
0
0
0
0



BaO
0
0
0
0
0



Na2O
2.80
0
0
0
0



K2O
1.20
6.00
5.16
4.32
3.48



SnO2
0.05
0.05
0.05
0.05
0.05



Fe2O3
0
0
0
0
0



total
100
100
100
100
100


Properties
strain
604
608
621
624
632



anneal
655
661
674
675
685



softening point
885
914
911
916
925



CTE
44.5
52.1
48.9
45.6
44.1



density
2.411
2.382
2.389
2.393
2.397


Viscosity
A



−2.7798
−2.7896



B



6788.52
6506.53



To



263.53
291.74



T @ 200 p



1599.635
1569.878



T @ 35 kP



1190.434
1178.954



T @ 250 kP



1093.652
1086.427



T(200 P) − T(35 kP)



409.2009
390.9249


Resistivity
A



−2.6974
−3.1433



B



7362.61
7835.22



To



−175.21
−98.68



R @ 200 p



28.24312
35.68633


Liquidus
air
1040
990
980
1030
1070



internal
1030
980
970
1015
1065



Pt
1005
965
960
1000
1050



phase
Albite
Orthoclase
Orthoclase
Cordierite
Cordierite



Liquidus viscosity



1794130.5
421516.23



Int liq - soft
145
66
59
99
140



estimated
520319.01
39473174.9
73132031.9
4244242
631087.4



liquidus viscosity



















Glass
15
16
17
18
19






Weight Percent
SiO2
58.52
58.87
58.17
58.33
58.51




Al2O3
17.87
17.98
17.76
17.81
17.87




B2O3
9.85
9.91
9.79
9.82
9.84




MgO
4.33
4.61
4.04
4.05
4.06




CaO
5.50
5.90
5.11
5.54
5.99




SrO
0
0
0
0
0




BaO
0
0
0
0
0




Na2O
0
0
0
0
0




K2O
3.82
2.62
5.01
4.33
3.62




SnO2
0.12
0.12
0.12
0.12
0.12




Fe2O3
0
0
0
0
0




total
100.01
100.01
100
100
100.01



Mole Percent
SiO2
63.35
63.35
63.35
63.35
63.35




Al2O3
11.40
11.40
11.40
11.40
11.40




B2O3
9.20
9.20
9.20
9.20
9.20




MgO
6.98
7.40
6.56
6.55
6.55




CaO
6.38
6.80
5.96
6.45
6.95




SrO
0
0
0
0
0




BaO
0
0
0
0
0




Na2O
0
0
0
0
0




K2O
2.64
1.80
3.48
3.00
2.50




SnO2
0.05
0.05
0.05
0.05
0.05




Fe2O3
0
0
0
0
0




total
100
100
100
100
100



Properties
strain
631
639
630
630
635




anneal
683
689
683
682
687




softening point
914
913
921
918
922




CTE
41.5
39.6
43.3
41.6
40.4




density
2.412
2.43
2.4
2.404
2.408



Viscosity
A









B









To









T @ 200 p









T @ 35 kP









T @ 250 kP









T(200 P) − T(35 kP)








Resistivity
A









B









To









R @ 200 p








Liquidus
air
1110
1135
1085
1070
1075




internal
1100
1125
1070
1065
1060




Pt
1080
1105
1060
1050
1045




phase
Cordierite
Cordierite
Cordierite
Cordierite
Cordierite




Liquidus viscosity









Int liq - soft
186
212
149
147
138




estimated
132273.398
64408.987
447990.94
482556.71
683059.804




liquidus viscosity






















TABLE 5








Glass
20
21
22
23
24





Weight Percent
SiO2
57.49
57.65
57.82
58.47
57.78



Al2O3
17.55
17.6
17.65
17.86
17.64



B2O3
10.85
10.88
10.91
9.84
10.9



MgO
4
4
4.02
4.06
4.02



CaO
5.04
5.48
5.92
5.99
5.92



SrO
0
0
0
0
0



BaO
0
0
0
0
0



Na2O
0
0
0
0
0



K2O
4.95
4.28
3.57
3.62
3.57



SnO2
0.12
0.12
0.12
0.16
0.16



Fe2O3
0
0
0
0
0



total
100
100.01
100.01
100
99.99


Mole Percent
SiO2
62.65
62.65
62.65
63.33
62.63



Al2O3
11.27
11.27
11.27
11.40
11.27



B2O3
10.20
10.20
10.20
9.20
10.20



MgO
6.50
6.48
6.49
6.55
6.49



CaO
5.89
6.38
6.87
6.95
6.87



SrO
0
0
0
0
0



BaO
0
0
0
0
0



Na2O
0
0
0
0
0



K2O
3.44
2.97
2.47
2.50
2.47



SnO2
0.05
0.05
0.05
0.07
0.07



Fe2O3
100
100
100
100
100



total







Properties
strain
623
630
632
637
630



anneal
673
680
682
688
680



softening point
916
913
914
922
918



CTE
43.7
42.2
40.3
41.1
41



density
2.394
2.395
2.401
2.411
2.405


Viscosity
A



−2.826
−2.7517



B



6611.93
6318.79



To



284.22
304.73



T @ 200 p



1573.842
1555.299



T @ 35 kP



1181.353
1170.820



T @ 250 kP



1088.206
1080.076



T(200 P) − T(35 kP)



392.489
384.480


Resistivity
A



−4.9927
−2.6521



B



13663.94
5980.3



To



−556.65
129.56



R @ 200 p



26.35204
34.86784


Liquidus
air
1040
1050
1065
1060
1040



internal
1035
1045
1050
1055
1030



Pt
1020
1025
1030
1040
1020



phase
Cordierite
Cordierite
Cordierite
Cordierite
Cordierite



Liquidus viscosity



565239.4
913330.2



Int liq - soft
119
132
136
133
112



estimated
1542714
872242.2
740167
836776.9
2153251



liquidus viscosity


















Glass
25
26
27
28






Weight Percent
SiO2
58.61
57.36
56.92
58.14




Al2O3
17.27
17.51
17.39
17.03




B2O3
9.86
10.82
10.74
10.45




MgO
4
3.98
3.96
3.85




CaO
6.3
6.61
7.31
5.67




SrO
0
0
0
0




BaO
0
0
0
0




Na2O
0
0
0
0




K2O
3.84
3.55
3.52
4.69




SnO2
0.12
0.16
0.16
0.16




Fe2O3
0
0
0
0




total
100
99.99
100
99.99



Mole Percent
SiO2
63.35
62.10
61.55
63.15




Al2O3
11.00
11.17
11.08
10.90




B2O3
9.20
10.11
10.02
9.8




MgO
6.45
6.43
6.38
6.23




CaO
7.30
7.67
8.47
6.60




SrO
0
0
0
0




BaO
0
0
0
0




Na2O
0
0
0
0




K2O
2.65
2.45
2.43
3.25




SnO2
0.05
0.07
0.07
0.07




Fe2O3
100
100
100
100




total







Properties
strain
633
628
630
624




anneal
683
677
677
674




softening point
918
907
901
911




CTE
41.9
42
43.2
44.1




density
2.412
2.414
2.424
2.404



Viscosity
A
−2.6477
−3.0308
−2.8977
−3.0702




B
6160.22
6642.81
6307
6906.16




To
314.38
271.68
291.74
253.21




T @ 200 p
1559.188
1517.558
1504.921
1538.979




T @ 35 kP
1170.945
1148.634
1139.254
1160.212




T @ 250 kP
1080.039
1059.794
1052.019
1068.756




T(200 P) − T(35 kP)
388.243
368.924
365.667
378.766



Resistivity
A
−2.6628
−2.9715
−2.7071
−3.4632




B
6172.99
6668.56
5845.08
8147.16




To
100.8
50.95
149.34
−101.76




R @ 200 p
37.14907
37.62072
40.24979
31.79388



Liquidus
air
1065
1060
1025
1010




internal
1050
1050
1015
1000




Pt
1040
1030
1000
990




phase
Anorthite
Anorthite
Anorthite
Anorthite




Liquidus viscosity
532706.2
319158
664567.2
1505199




Int liq - soft
132
143
114
89




estimated
872242.2
561623.8
1953516
7623399




liquidus viscosity



























TABLE 6






Glass
29
30
31
32
33
34
35
36
37
38


























Weight
SiO2
56.19
56.33
56.46
56.6
56.72
56.48
57.32
58.17
57.29
56.92


Percent
Al2O3
17.45
17.01
16.56
16.11
15.66
17.77
17.52
17.27
17.03
16.63



B2O3
11.27
10.63
9.98
9.33
8.68
11.17
9.70
8.21
9.26
9.38



MgO
2.78
2.61
2.43
2.26
2.08
2.76
2.39
2.01
2.27
2.29



CaO
4.62
4.36
4.10
3.84
3.58
4.57
3.94
3.31
3.77
3.84



SrO
5.83
5.52
5.20
4.89
4.57
5.76
5.00
4.23
4.79
4.88



BaO
0
0
0
0
0
0
0
0
0
0



Na2O
1.68
3.37
5.06
6.77
8.48
1.30
3.92
6.56
5.39
5.86



K2O
0
0
0
0
0
0
0
0
0
0



SnO2
0.18
0.18
0.21
0.21
0.23
0.18
0.20
0.23
0.21
0.21



Fe2O3
0
0
0
0
0
0
0
0
0
0



total
100
100.01
100
100.01
100
99.99
99.99
99.99
100.01
100.01


Mole
SiO2
62.18
62.16
62.14
62.12
62.10
62.56
63.28
64.00
63.08
62.60


Percent
Al2O3
11.38
11.06
10.74
10.42
10.10
11.60
11.40
11.20
11.05
10.78



B2O3
10.76
10.12
9.48
8.84
8.20
10.68
9.24
7.80
8.80
8.90



MgO
4.58
4.29
3.99
3.70
3.40
4.56
3.93
3.30
3.72
3.75



CaO
5.48
5.16
4.84
4.52
4.20
5.42
4.66
3.90
4.45
4.52



SrO
3.74
3.53
3.32
3.11
2.90
3.70
3.20
2.70
3.06
3.11



BaO
0
0
0
0
0
0
0
0
0
0



Na2O
1.80
3.60
5.40
7.20
9.00
1.40
4.20
7.00
5.75
6.25



K2O
0
0
0
0
0
0
0
0
0
0



SnO2
0.08
0.08
0.09
0.09
0.10
0.08
0.09
0.10
0.09
0.09



Fe2O3
0
0
0
0
0
0
0
0
0
0



total
100
100
100
100
100
100
100
100
100
100


Proper-
strain
611
590
578
565

620
589
577
577
576


ties
anneal
659
636
622
607

670
635
623
625
621



softening
887.1
856.5
831.1
807.2
780
899.5
866.3
850.5
843
836.1



point













CTE
41.6
47.7
53.9
59.6

40.4
48.1
56.6
52.8
55.4



density
2.478
2.48
2.492
2.494
2.501
2.472
2.467
2.463
2.472
2.478


Viscos-
A
−2.475
−2.209
−2.175
−2.086
−1.813

−2.817

−2.501
−1.845


ity
B
5764.2
5523.2
5563.1
5506.2
4988.5

6843.6

6402.1
5180.5



To
306
290.2
258.9
231
242.7

193.3

197.4
272



 200
1512.9
1514.85
1501.76
1486.11
1455.26

1530.46

1530.61
1521.51



 3000
1274.43
1261.55
1243.15
1220.77
1185.68

1280.6

1268.32
1245.39



30000
1135.13
1116.27
1095.19
1069.96
1035.77

1131.54

1114.85
1091.42



50000
1109.49
1089.74
1068.2
1042.53
1008.75

1103.84

1086.58
1063.64


Liquidus
internal
1010
1025
1040
1020
980
1030
1020
1070
1010
1040



Liquidus
516159.7
203050.1
88539.17
78110.11
89722.76



238527.3
79513.84



viscosity













Int liq -
122.9
168.5
208.9
212.8
200
130.5
153.7
219.5
167
203.9



soft


























TABLE 7






Glass
39
40
41
42
43
44
45
46
47

























Weight
SiO2
56.76
57.00
57.33
57.72
58.11
59.01
56.88
57.06
57.30


Percent
Al2O3
17.19
16.95
17.52
17.64
17.76
15.66
17.09
16.32
17.52



B2O3
10.19
9.6
9.66
9.72
9.79
9.84
10.20
10.72
9.70



MgO
2.50
2.35
2.37
2.81
3.26
2.45
2.56
2.65
2.39



CaO
4.17
3.92
3.97
4.00
4.03
4.01
4.15
4.36
3.94



SrO
5.24
4.98
5.00
3.93
2.85
5.04
5.16
5.42
5.00



BaO
0
0
0
0
0
0
0
0
0



Na2O
3.74
4.99
3.92
3.95
3.98
3.77
3.74
0
3.92



K2O
0
0
0
0
0
0
0
3.24
0



SnO2
0.20
0.20
0.23
0.23
0.23
0.23
0.23
0.23
0.23



Fe2O3
0
0
0
0
0
0
0
0
0



total
99.99
99.99
100
100
100.01
100.01
100.01
100
100


Mole
SiO2
62.63
62.78
63.3
63.3
63.30
64.6
62.7
63.50
63.28


Percent
Al2O3
11.18
11.00
11.40
11.40
11.40
10.10
11.10
10.70
11.40



B2O3
9.70
9.13
9.20
9.20
9.20
9.30
9.70
10.30
9.24



MgO
4.12
3.86
3.90
4.60
5.30
4.00
4.20
4.40
3.93



CaO
4.93
4.63
4.70
4.70
4.70
4.70
4.90
5.20
4.66



SrO
3.35
3.18
3.20
2.50
1.80
3.20
3.30
3.50
3.20



BaO
0
0
0
0
0
0
0
0
0



Na2O
4.00
5.33
4.20
4.20
4.20
4.00
4.00
0.00
4.20



K2O
0
0
0
0
0
0
0
2.3
0



SnO2
0.09
0.09
0.10
0.10
0.10
0.10
0.10
0.10
0.10



Fe2O3
0
0
0
0
0
0
0
0
0



total











Properties
strain
589
578
596
597
596
590
593
619
597



anneal
638
626
643
644
645
637
640
669
645



softening point
858.5
843.2
867.6
867
876.9
859.5
858.5
898
873



CTE
48.4
51.7
48.1
47.3
46.1
47.6
48
44.2
47.2



density
2.477
2.474
2.475
2.466
2.451
2.472
2.476
2.465
2.466


Viscosity
A
−2.549
−2.625



−2.625

−2.567




B
6218.5
6434.5



6434.5

6097.1




To
233.8
223.8



223.8

303.3




 200
1515.96
1530.02



1530.02

1555.78




 3000
1265.72
1278.27



1278.27

1312.07




30000
1118.85
1129.8



1129.8

1168.86




50000
1091.76
1102.35



1102.35

1142.43



Liquidus
internal
1045
1040
1095
1080
1060
1015
1060
1050
1065



Liquidus
130859
181336.2



321798.1

396638.1




viscosity












Int liq - soft
186.5
196.8
227.4
213
183.1
155.5
201.5
152
192






















TABLE 8







Glass
48
49
50
51





















Mole Percent
SiO2
63
62.95
62.9
62.85



Al2O3
11
10.3
9.65
9



B2O3
10.2
11.6
13.05
14.5



MgO
5.5
4.13
2.75
1.38



CaO
6.1
4.6
3.05
1.53



SrO
1.8
1.43
1.05
1.68



BaO
0
3.1
6.15
9.22



K2O
2.4
1.8
1.2
0.6



SnO2
0.1
0.1
0.1
0.1


Weight Percent
SiO2
57.2
54.9
52.9
51



Al2O3
16.9
15.3
13.8
12.4



B2O3
10.7
11.7
12.7
13.6



MgO
3.34
2.42
1.55
0.75



CaO
5.17
3.74
2.4
1.16



SrO
2.81
2.16
1.53
0.95



BaO
0
6.91
13.2
19.1



K2O
3.41
2.47
1.58
0.76



SnO2
0.23
0.22
0.21
0.2


Properties
strain
628
609
603
598



anneal
678
659
651
644



softening
909
890
876
858



CTE
42.8
43.9
45.4
46



Density
2.44
2.516
2.606
2.677


Viscosity
200
1529
1525
1528



Internal Liquidus
1020
940
980
980



Liquidus Viscosity
822,000
5,233,000
1,100,000

















TABLE 9








Example















Mol %
52
53
54
55
56
57
58
59


















MgO
2.1
2.1
2
2
2.2

2.35
1.9


CaO
21.2
20.6
20.1
19.85
21.65
13.2
25.6
20.8


SrO





8.7




BaO
2.1
2.1
2
2
2.2

2.35
1.9


RO
25.4
24.8
24.1
23.85
26.05
21.9
30.3
24.6


B2O3
2.5
5
7.5


8




Al2O3
8.8
8.6
8.4
8.25
6.05
10.1
11.7
9.4


SiO2
63.2
61.5
59.9
67.8
67.8
59.9
57.9
65.9


SnO2
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1


Tstr
676
~660
643
711
699
648
710
715


α
52.8
~52.5
52.2
50.4
54.6
50.4
57.5
50.3


ρ
2.674
~2.66
2.64
2.652
2.675
2.696
2.771
2.664


Tliq
1120
1150
1135
1190






ηliq (kP)


5.4







T200


1360

















TABLE 10








Example















Mol %
59
60
61
62
63
64
65
66


















MgO
5
5
10

2.9
8.7
5.2
8.5


CaO
24
21
18
21
32.1
8.7
10.45
8.5


SrO
16
14
12
14


5.2



BaO




2.9
8.65
5.2
8.4


ZnO



5






RO
45
40
40
40
37.9
26.05
26.05
25.4


B2O3
15
15
15
15



2.5


Al2O3
20
20
20
20
14.6
9.05
9.05
8.8


SiO2
20
25
25
25
47.4
64.8
64.8
63.2


SnO2




0.1
0.1
0.1
0.1


Tstr
619
620
621
606
~700
704
703
~680


α
73.7
70
67.3
69.9
~66
52.4
53.5
~52


ρ




2.885
2.865
2.853
~2.84


Tliq







1130


ηliq (kP)










T200

















TABLE 11








Example















Mol %
67
68
69
70
71
72
73
74


















MgO
8.3
8.05
5.1
4.95
4.8
2.7
2.6
4.1


CaO
8.3
8.05
10.15
9.9
9.6
2.9
2.0
9.9


SrO


5.1
4.95
4.8
11.8
9.6
3.3


BaO
8.2
8
5.1
4.95
4.8
0.7
3.6
2.2


ZnO










RO
24.7
24.1
25.4
24.7
24.1
18.0
17.7
19.4


B2O3
5
7.5
2.5
5
7.5
9.0
7.5
9.7


Al2O3
8.6
8.4
8.8
8.6
8.4
9.6
9.3
10.0


SiO2
61.6
59.9
63.2
61.6
59.9
63.3
65.4
60.8


SnO2
0.1
0.1
0.1
0.1
0.1
0.10
0.10
0.10


Tstr
~655
633
~680
~660
639
645
649
639


α
~51.5
51
~53
~53
53.1
46.2
46.5
46.5


ρ
~2.82
2.803
~2.82
~2.80
2.766
2.70
2.75
2.70


Tliq
1125
1080
1135
1080
1140
1130
1110
1045


ηliq (kP)

25.1

40.2






T200

1383

1420

















TABLE 12








Example















Mol %
75
76
77
78
79
80
81
82


















MgO
1.8
0
2.0
1.98
1.97
2.5
3.5
4.5


CaO
11.0
10.0
9.0
1.3
0.8
7.0
5.5
4.5


SrO
2.9
9.0
8.0
11.9
11.5
1.5
2.0
2.0


BaO
3.3
0
0
2.0
4.0
7.0
7.0
7.0


ZnO










RO
19.0
19.0
19.0
17.2
18.3
18.0
18.0
18.0


B2O3
10.7
8.0
8.0
6.4
6.4
9.0
9.0
9.0


Al2O3
8.5
9.0
9.0
8.7
8.6
9.0
9.0
9.0


SiO2
62.3
64.0
64.0
67.6
66.5
63.9
63.9
63.9


SnO2
0.07
0.10
0.10
0.17
0.17
0.10
0.10
0.10


Tstr
631
648
649
667
665
632
632
635


α
46.0
48.4
45.8
45.9
46.6
46.5
45.9
45.8


ρ

2.67
2.63
2.72
2.77
2.73
2.72
2.72


Tliq
1075
1150
1145
1100
1075
1050
1025
1020


ηliq (kP)



124
235
166
364
437


T200



1563
1545
1490
1490
1494

















TABLE 13








Example














Mol %
83
84
85
86
87
88
89

















MgO
3.8
3.9
1.8
0.0
2.0
2.0
2.0


CaO
6.0
5.7
11.0
10.0
9.0
1.3
0.8


SrO
2.2
2.5
2.9
9.0
8.0
11.9
11.5


BaO
7.6
6.2
3.3
0
0
2.0
4.0


ZnO









RO
19.5
18.3
19.0
19.0
19.0
17.2
18.3


B2O3
9.8
9.8
10.7
8.0
8.0
6.4
6.4


Al2O3
9.8
9.9
8.5
9.0
9.0
8.6
8.6


SiO2
60.9
62.0
62.3
64.0
64.0
67.6
66.5


SnO2
0.10
0.10
0.07
0.10
0.10
0.17
0.17


Tstr
630
631
613
648
649
667
665


α
48.2
46.0
46
48.4
45.8
45.9
46.6


ρ
2.77
2.71

2.67
2.63
2.72
2.77


Tliq
1045

1075
1150
1145
1100
1075


ηliq (kP)
106




124
235


T200
1443




1563
1545









Example 2
An experiment was performed to empirically demonstrate the impact of residual stress on CdTe device performance using glass substrates with varying CTE. The experiment consisted of fabricating CdTe devices using the vapor transport deposition process on three different glass substrates having a CTE of 3.1, 4.3 and 8.5 ppm/° C., respectively. FIG. 3 summarizes the residual stress and the corresponding efficiencies for the CdTe devices fabricated on each of the three glass substrates. The efficiencies were calculated using the following equation:

Eff=VOC×JSC×FF 

where the open circuit voltage VOC, the short circuit JSC and the fill factor FF are the device parameters that were extracted from the current-voltage behavior of each solar cell devices under AM1.5 irradiance at 25° C. The residual stress profile in the CdTe film was measured by glancing incidence x-ray diffraction over a range of (hkl) and at different incident beam angles. Uniform stress (σ) from a given direction (Ψ) results in uniform lattice strain (εΨ), which causes angular peak displacement, while non-uniform stress distorts peak shape. The measured strain is related to the applied stress by the elastic compliance coefficients. The in-plane strain-stress relationship for f.c.c. materials is:





  
    ɛ
    Ψ
    hkl
  
  =
  
    
      
        (
        
          
            
              d
              Ψ
            
            -
            
              d
              0
            
          
          
            d
            0
          
        
        )
      
      hkl
    
    =
    
      σ
      [
      
        
          
            (
            
              
                2
                ⁢
                
                  S
                  11
                
              
              +
              
                4
                ⁢
                
                  S
                  12
                
              
              -
              
                S
                44
              
            
            )
          
          3
        
        +
        
          
            (
            
              
                S
                44
              
              2
            
            )
          
          ⁢
          
            sin
            2
          
          ⁢
          Ψ
        
      
      ]
    
  




The CdTe compliance coefficients are S11=4.25×10-11 Pa, S12=−1.73×10-11 Pa, and S44=5.02×10-11 Pa. The measured (hkl) peak positions were reduced to the substrate reference frame and the residual stress associated with each peak was calculated.

To explain the results, we first we note that the CTE of the CdTe film can range from  ̃5.5 to  ̃6.5 ppm/° C. and is largely influenced by the CdTe deposition process which was noted earlier in this case to be by vapor transport. By convention, negative stress indicates in-plane compression (tension normal to substrate) and positive stress indicates in-plane tension (compression normal to substrate). The CdTe film deposited on the 8.5 ppm/° C. CTE glass is relatively relaxed at the surface and under significant in-plane compression within the film. The CdTe film deposited on the 4.3 ppm/° C. CTE substrate is under in-plane tension at the exposed surface and become more relaxed within the bulk, but with slight in-plane tension. The CdTe film deposited on the 3.1 ppm/° C. CTE substrate exhibit mixed stress at the surface and is under in-plane tension in the bulk. The mean residual bulk stress for this set of samples exhibits a linear correlation with glass CTE. The intercept CTE for which no strain is expected in this case is  ̃6 ppm/° C. The corresponding efficiency indicates a trend of increased efficiency as the residual stress magnitude decreases as illustrated by the grey dashed trace. This suggests a CdTe device with no residual stress in the film will exhibit optimum solar cell conversion efficiency.
To further illustrate the impact of the substrate CTE on CdTe solar cell device performance, another experiment was performed to empirically demonstrate the impact of residual stress on CdTe device performance using glass substrates with varying CTE. In this case, the experiment consisted of fabricating CdTe devices using the close spaced sublimation deposition process on glass substrates with CTE of 4.1, 5.8, 6.6 and 8.6 ppm/° C., respectively. Unlike the earlier described experiment, the glass substrates in this experiment were from the same alkali-free composition family so as to limit/mitigate any non-intended variables that could influence the experiment outcome. FIG. 5 summarizes the residual stress and the corresponding efficiencies for the CdTe devices fabricated on each of the glass substrates. As in FIG. 3, here we also observe the mean residual bulk stress for this set of samples exhibits a linear correlation with glass CTE. In this case, the intercept CTE for which no strain is expected is  ̃5.5 ppm/° C. And again, the corresponding efficiency indicates a trend of increased efficiency as the residual stress magnitude decreases as illustrated by the dashed trace.
Stress and strain are known to cause microstructural defects in semiconductor films that can degrade the electrical performance of devices made thereafter in the film. In the case of CdTe solar cells, it is suggested that the defect density in the CdTe film follows a direct trend with the residual stress magnitude in the film: higher residual stress leads to higher defect density. Higher defect density then leads to higher recombination losses in the bulk of the film which then leads to lower open circuit Voc. Lower Voc then leads to a lower efficiency. To this point, the devices summarized in FIG. 4 received additional characterization to demonstrate the impact of CdTe film defect density on the corresponding solar cell efficiency. The carrier concentration was calculated from the capacitance-voltage measurement that was performed on each solar cell device. The carrier concentration profile was then mapped into a device model that was constructed within the framework of a 2-D physics-based semiconductor device model from SILVACO TCAD software. The defect density was then extracted by fitting the measured current density-voltage curves. FIG. 5 shows the efficiency and the corresponding extracted defect density as a function of the CdTe film residual stress for the same devices in FIG. 4. Using the dashed lines as a guide, it can be shown that the defect density approaches minimum and the efficiency approaches optimum where the magnitude of the residual stress in the CdTe film is minimal.
The alkalis in the glass and low melting temperature combine to accelerate melting thus enabling high volume, low-cost melting and forming relative to alkali-free alternatives while retaining competitive properties, including in particular mechanical and dimensional stability when reheated to high temperature. These glasses are well suited for large-volume sheet glass applications, particularly OLED lighting and cadmium telluride (CdTe) photovoltaics, for which thermal stability, large volumes, and low cost are desirable substrate attributes.
It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

## Claims

1. A photovoltaic module, comprising:
a CdTe solar cell material,
one or more conductive oxides, and
a glass substrate, wherein the glass substrate has a coefficient of thermal expansion of from about 4.0 to about 7.5 ppm/° C. from 25 to 300° C., wherein the glass substrate comprises about, in mol %:
60 to 65 percent SiO2;
8 to 12 percent Al2O3;
7 to 15 percent B2O3;
0.1 to 8 percent M2O;
9 to 15 percent RO; and
0.01 to 0.4 percent SnO2.

2. The photovoltaic module of claim 1, wherein the glass substrate has a coefficient of thermal expansion of from about 4.5 to about 6.0 ppm/° C. from 25 to 300° C.

3. A photovoltaic module, comprising:
a CdTe solar cell material having a coefficient of thermal expansion,
one or more conductive oxides, and
a glass substrate, wherein the glass substrate has a coefficient of thermal expansion that is within about ±2.0 ppm/° C. of the coefficient of thermal expansion of the CdTe solar cell material from 25 to 300° C., wherein the glass substrate comprises about, in mol %:
60 to 65 percent SiO2;
8 to 12 percent Al2O3;
7 to 15 percent B2O3;
0.1 to 8 percent M2O;
9 to 15 percent RO; and
0.01 to 0.4 percent SnO2.

4. The photovoltaic module of claim 3, wherein the glass substrate has a coefficient of thermal expansion of within about ±1.0 ppm/° C. of the coefficient of thermal expansion of the CdTe solar cell material from 25 to 300° C.