[0001] This application claims the benefit of priority

under 35 U.S.C. § 119 of U.S. Provisional Application

Serial No. 61/756623 filed on January 25, 2013 the content of which is relied upon and incorporated herein by

reference in its entirety.

FIELD

[0002] The disclosure relates to textured glass, and more particularly to textured glass useful for, for example, photovoltaic devices .

TECHNICAL BACKGROUND

[0003] One common method for improving the performance of thin film photovoltaic devices, for example, Si tandem devices, is to increase the path length ("light trapping") in the active layers as a result of scattering. This scattering can be induced by incorporating a fine scale texture (micron scale or smaller) at the TCO/a-Si interface. Alternatively, a large scale "patterning" on one side of the superstrate has also been used to increase performance by providing an increase in light transmitted by the air/glass interface (AR effect) or by increasing the recirculation of light reflected by the active layers of the cell.

[0004] There exists a need for superstrates which may

improve the performance of photovoltaic devices, for

example, thin film photovoltaic devices.

SUMMARY

[0005] The present disclosure involves patterning both

sides of a superstrate (thereby maximizing the AR effect as well as the recirculation of reflected light) and may improve the performance of thin-film solar cells. A dual textured superstrate may provide an improvement in the performance of thin-film photovoltaic solar cells by enhancing the amount of light absorbed by these devices.

[0006] This disclosure describes a dual textured glass that may be used in photovolataic devices such as thin film photovoltaic devices. Both the sun-side and cell- side of the glass are patterned. In some embodiments, the size of the features of the texture are on the order of tens of microns to millimeters in scale. The dual texture (i.e. on both sides of the glass) results in an increased anti-reflection (AR) effect along with a high degree of recirculation of light reflected from the active layers of the device. The sun-side and the cell-side of the glass have a periodic array of parallel grooves. The periods of the grooves on the two sides are the same, although, their base angles are different.

[0007] The dual textured glass used as a superstrate may improve the performance of thin-film photovoltaic cells with a textured cell-side interface (as is commonly used in Si tandem devices) or with nominally smooth interfaces (e. g. CdTe cells) .

[0008] 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 that description or recognized by practicing the embodiments as described herein, including the detailed description which follows, the claims, as well as the appended drawings .

[0009] It is to be understood that both the foregoing general description and the following detailed description are merely exemplary, and are intended to provide an overview or framework to understanding the nature and character of the claims . The accompanying drawings are included to provide a further understanding, and are incorporated in and constitute a part of this

specification. The drawings illustrate one or more embodiment ( s ) , and together with the description serve to explain principles and operation of the various

embodiments .

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 is a cross-sectional view of one

embodiment .

[0011] Figure 2 is a graph showing relative enhancement of a Si tandem device according to one embodiment.

DETAILED DESCRIPTION

[0012] Reference will now be made in detail to the present preferred embodiment ( s ) , examples of which are illustrated in the accompanying drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts.

[0013] As used herein, the term "parallel" means extending in the same direction without intersecting.

[0014] 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.

[ 0015 ] 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.

[ 0016] 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.

[ 0017 ] 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.

[ 0018 ] 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 .

[0019] 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.

[0020] 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."

[0021] One embodiment is shown in the cross-section of features of a photovoltaic device 100 in Figure 1. The textured glass 10 has a series of parallel grooves 12 and 14 on both sides of the textured glass. As can be seen in Figure 1, the relative orientation of the two sets of parallel grooves is such that the location of a groove peak on the sun-side 16 corresponds to a valley on the cell-side 18 (and vice-versa) .

[0022] In some embodiments, as shown in Figure 1, the base angle a of the grooves (measured from the horizontal) is in the range of from 30 to 60 degrees for the sun-side grooves and the base angle al of the cell-side grooves is in the range of from 3 to 10 degrees. In some

embodiments, the base angle a of the grooves (measured from the horizontal) is 45 degrees for the sun-side grooves and the base angle al of the cell-side grooves 8.5 degrees. Two ray paths originating from the sun are illustrated in Figure 1. In Figure 1, rays 20 show how the incident light reflected from the air/glass interface encounters the superstrate a second time (at an air/glass interface) . The result of this double reflection is that only a tiny fraction of the incident light is lost. For example, assuming the glass has an index of 1.5, the (polarization averaged) reflectance at an angle of 45 degrees is about 5%. So, the amount of light lost by this double reflection is only 0.25% (=5%*5%) . This grooved interface acts as an effective anti reflective interface since 99.75% of the incident light is coupled into the superstrate .

[0023] Rays 22 in Figure 1 show the path of the light that is transmitted by the air/glass interface. Here rays 22 encounter the grooved structure of the superstrate on the cell side. A fraction of this light is coupled to the active layers of the cell, whereas the rest is reflected. Although reflections could arise from the various layers of the cell, all reflections can be approximated as originating from essentially the same location because of the thinness of the layers (compared to the thickness of the superstrate) . This reflected light is represented by the ray traveling towards the glass/air interface of the superstrate. The angle of the grooves on the cell side is selected so that the reflected light travels vertically. In addition, it is advantageous to select the ratio of the period of the grooves, P, and the average thickness of the superstrate, h, such that the transmitted light strikes the cell-side of the superstrate at the proper location.

For a glass superstrate with an index of 1.51, the ratio of the average thickness to the period should be about

1.63. This upward traveling light undergoes two total internal reflections at the sun side of the superstrate and is redirected back towards the active layers of the cell for a second chance to be absorbed. Therefore, this dual grooved superstrate provides a very good AR effect while at the same time effectively recirculating the

internally reflected light back to the cell.

[0024] Figure 2 is a graph showing relative enhancement of a Si tandem device with flat interfaces 24 and of a Si tandem device with a cell side texture 26 as a function of ratio of the superstrate thickness (h) to the groove period (P) .

[0025] An exemplary dual textured glass was analyzed using a ray tracing program (Light Tools) to determine the enhancement in maximum achievable current density (MACD) . Two types of Si tandem cells were analyzed as a function of thickness/groove period (h/P) . The first cell had nominally flat interfaces and only had scattering from the fine structure of the TCO. In Figure 2, line 24 shows that the maximum enhancement agrees well with the predicted maximum occurring with h/P=1.63. The second cell analyzed (along with the TCO scattering) had a roughened surface in addition to the parallel grooves on the cell-side of the glass superstrate. The enhancement, as shown by line 26 in Figure 2, is even greater than the flat case (i. e. no additional roughened surface) showing that the two enhancement mechanisms (dual groove superstrate and roughness) are additive.

[0026] A CdTe cell was also investigated with this dual groove superatrate using the ideal h/p ratio of 1.63. In this case the relative enhancement was 6.9% compared with a CdTe cell with a flat, nongrooved, nonroughened superstrate. [0027] As stated above, the ratio of the average superstrate thickness, h, to the groove period, P, should be 1.63.

However, larger ratios are possible in which case the incident light will strike the proper place but shifted by an integer number of periods. In other words, the ratio could be expressed as h/P = (2n-l)*1.63, wherein n is an integer, for example, 1, 2, 3, .... The transmitted light will be incident on the groove with the same slope (as shown in Figure 1) but shifted over 1 period for n=2, 2 periods for n=3, etc.

However, the preferred ratio is h/P = 1.63 so as to minimize any degradation caused by time-of-day effects. In Figure 1, this is described as h=1.63P.

[0028] An advantage of the textured glass disclosed herein may be an improvement in photovoltaic device performance (via enhanced absorption of light) as compared to

conventional devices where either the sun-side or cell- side is textured, not both. With glass superstrates that only texture the sun-side, a tradeoff exists between any AR effect and the strength of the recirculation of the light reflected from the cell. For the case in which only the cell-side of the glass superstrate is textured

recirculation of the light reflected by the active layers is possible, however, these devices lack an AR effect at the flat air/superstrate interface.

[0029] It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the spirit or scope of the invention.