Patent Application
20080072956
Referring now more particularly to the accompanying drawings in which like reference numerals indicate like parts throughout the several views.
Photovoltaic devices such as solar cells convert solar radiation and other light into usable electrical energy. The energy conversion occurs typically as the result of the photovoltaic effect. Solar radiation (e.g., sunlight) impinging on a photovoltaic device and absorbed by an active region of semiconductor material (e.g., a semiconductor film including one or more semiconductor layers such as a-Si layers) generates electron-hole pairs in the active region. The electrons and holes may be separated by an electric field of a junction in the photovoltaic device. The separation of the electrons and holes by the junction results in the generation of an electric current and voltage. In certain example embodiments, the electrons flow toward the region of the semiconductor material having n-type conductivity, and holes flow toward the region of the semiconductor having p-type conductivity. Current can flow through an external circuit connecting the n-type region to the p-type region as light continues to generate electron-hole pairs in the photovoltaic device. In certain example embodiments, single junction amorphous silicon (a-Si) photovoltaic devices include at least three semiconductor layers making up an absorbing semiconductor film. In particular, a p-layer, an n-layer and an i-layer which is intrinsic can make up the absorbing semiconductor film in certain example instances. The amorphous silicon film (which may include one or more layers such as p, n and i type layers) may be of hydrogenated amorphous silicon in certain instances, but may also be of or include hydrogenated amorphous silicon carbon or hydrogenated amorphous silicon germanium, or the like, in certain example embodiments of this invention. For example and without limitation, when a photon of light is absorbed in the i-layer it gives rise to a unit of electrical current (an electron-hole pair). The p and n-layers, which contain charged dopant ions, set up an electric field across the i-layer which draws the electric charge out of the i-layer and sends it to an optional external circuit where it can provide power for electrical components.
It is noted that while certain example embodiments of this invention are directed toward amorphous-silicon based photovoltaic devices, this invention is not so limited and may be used in conjunction with other types of photovoltaic devices in certain instances including but not limited to devices including other types of semiconductor material, tandem thin-film solar cells, CdS/CdTe based solar cells, crystalline solar cells, and the like.
Front glass substrate 1 and/or rear superstrate (not shown) may be made of soda-lime-silica based glass in certain example embodiments of this invention. While these substrates may be of glass in certain example embodiments of this invention, other materials such as quartz or the like may instead be used. Moreover, the superstrate (not shown) at the rear of the photovoltaic device is optional in certain instances. Glass of substrate 1 and/or of the superstrate may or may not be thermally tempered and/or patterned in certain example embodiments of this invention. Additionally, it will be appreciated that the word “on” as used herein covers both a layer/film being directly on and indirectly on something, with other layers possibly being located therebetween.
While the AR coating is provided on a light incident glass substrate of a photovoltaic device in
Referring to
The metal fluoride portion of the composite low-index AR coating 3 is advantageous in that it permits the AR coating to realize a low refractive index (n). In certain example embodiments of this invention, the glass substrate 1 may have a refractive index (n) of from about 1.48 to 1.60 (e.g., about 1.52), whereas the AR coating 3 (which may be of a single layer in certain example embodiments) may have a refractive index (n) of from about 1.20 to 1.45, more preferably from about 1.23 to 1.40, and most preferably from about 1.25 to 1.35 (at 450 nm).
The low refractive index (n) of AR coating 3 is advantageous in that it allows less reflection so that more light can pass through the glass substrate 1 and reach the active semiconductor layer(s) of the photovoltaic device. Meanwhile, the silica portion of the AR coating 3 is advantageous in that it increases the solar transmission of the coating 3 so that less light is absorbed by the coating; again, this increases the amount of light which can pass through the glass substrate 1 and reach the active semiconductor layer(s) of the photovoltaic device whereby power of the device can be increased. Moreover, the silica portion of the AR coating 3 is also advantageous in that it increases the mechanical durability of the coating 3, and permits it to be used in many different environments with less risk of damage.
In certain example embodiments of this invention, the metal fluoride (e.g., MgF2 and/or CaF2) portion of the AR coating 3 may make up from about 0.5 to 50% of the coating (weight percentage), more preferably from about 1-45%, even more preferably from about 1-25%, and most preferably from about 1-15% of the coating 3; the remainder of the coating may be made up of silica and/or other element(s). In certain example embodiments of this invention, the AR coating 3 includes at least about 50% silica, more preferably at least about 60% silica, even more preferably at least about 70% silica, and possibly at least about 85% silica (weight percentage).
Yet another example advantage of coating 3 is that it may consist of only a single layer in certain example embodiments of this invention, thereby reducing the number of steps needed to form the AR coating. While the AR coating 3 may be of only a single layer in certain example embodiments, it is possible that a multi-layer coating may be used for coating 3 in other example embodiments of this invention.
AR coating 3 may be deposited on the glass substrate 1 in any suitable manner, including but not limited to using spin coating, dip coating, or flow coating techniques. The AR coating 3 may be formed as follows in certain example instances. A Mg inclusive salt such as magnesium acetate, or Mg inclusive carboxylate salt, or any other Mg inclusive salt, may be dissolved in a solvent such as alcohol, propanol, ethylene glycol, other glycol, or the like so as to be in liquid form and form a solution. As another example, the solution may be formed by causing magnesium ethoxide or the like to be dissolved in a solvent such as propanol (e.g., propanol-2). The solution may then be mixed with an acid or the like including F. Then, the resulting sol or the like may be mixed with a silica inclusive sol and then applied via spin coating onto, directly or indirectly, a glass substrate 1. Then, the coated substrate may be heat treated for thermal tempering, curing, and/or the like (e.g., for from about 1-15 minutes, more preferably from about 2-10 minutes). The heat treated coated article, including the tempered glass substrate 1 with AR coating 3 thereon, may then be used in making a photovoltaic device as the light incident substrate of such a device.
For purposes of example and without limitation, coating 3 may be from about 0.5 to 15 μm thick in certain example embodiments of this invention, more preferably from about 1-10 μm thick.
In certain example embodiments of this invention, high transmission low-iron glass may be used for glass substrate 1 in order to further increase the transmission of radiation (e.g., photons) to the active layer(s) 50 of the solar cell or the like. For example and without limitation, the glass substrate 1 may be of any of the glasses described in any of U.S. patent application Ser. Nos. 11/049,292 and/or 11/122,218, the disclosures of which are hereby incorporated herein by reference.
Certain glasses for glass substrate 1 (which or may not be patterned in different instances) according to example embodiments of this invention utilize soda-lime-silica flat glass as their base composition/glass. In addition to base composition/glass, a colorant portion may be provided in order to achieve a glass that is fairly clear in color and/or has a high visible transmission. An exemplary soda-lime-silica base glass according to certain embodiments of this invention, on a weight percentage basis, includes the following basic ingredients:
Other minor ingredients, including various conventional refining aids, such as SO3, carbon, and the like may also be included in the base glass. In certain embodiments, for example, glass herein may be made from batch raw materials silica sand, soda ash, dolomite, limestone, with the use of sulfate salts such as salt cake (Na2SO4) and/or Epsom salt (MgSO4×7H2O) and/or gypsum (e.g., about a 1:1 combination of any) as refining agents. In certain example embodiments, soda-lime-silica based glasses herein include by weight from about 10-15% Na2O and from about 6-12% CaO.
In addition to the base glass above, in making glass according to certain example embodiments of the instant invention the glass batch includes materials (including colorants and/or oxidizers) which cause the resulting glass to be fairly neutral in color (slightly yellow in certain example embodiments, indicated by a positive b* value) and/or have a high visible light transmission. These materials may either be present in the raw materials (e.g., small amounts of iron), or may be added to the base glass materials in the batch (e.g., cerium, erbium and/or the like). In certain example embodiments of this invention, the resulting glass has visible transmission of at least 75%, more preferably at least 80%, even more preferably of at least 85%, and most preferably of at least about 90% (sometimes at least 91%) (Lt D65). In certain example non-limiting instances, such high transmissions may be achieved at a reference glass thickness of about 3 to 4 mm. In certain embodiments of this invention, in addition to the base glass, the glass and/or glass batch comprises or consists essentially of materials as set forth in Table 2 below (in terms of weight percentage of the total glass composition):
In certain example embodiments, the total iron content of the glass is more preferably from 0.01 to 0.06%, more preferably from 0.01 to 0.04%, and most preferably from 0.01 to 0.03%. In certain example embodiments of this invention, the colorant portion is substantially free of other colorants (other than potentially trace amounts). However, it should be appreciated that amounts of other materials (e.g., refining aids, melting aids, colorants and/or impurities) may be present in the glass in certain other embodiments of this invention without taking away from the purpose(s) and/or goal(s) of the instant invention. For instance, in certain example embodiments of this invention, the glass composition is substantially free of, or free of, one, two, three, four or all of: erbium oxide, nickel oxide, cobalt oxide, neodymium oxide, chromium oxide, and selenium. The phrase “substantially free” means no more than 2 ppm and possibly as low as 0 ppm of the element or material. It is noted that while the presence of cerium oxide is preferred in many embodiments of this invention, it is not required in all embodiments and indeed is intentionally omitted in many instances. However, in certain example embodiments of this invention, small amounts of erbium oxide may be added to the glass in the colorant portion (e.g., from about 0.1 to 0.5% erbium oxide).
The total amount of iron present in the glass batch and in the resulting glass, i.e., in the colorant portion thereof, is expressed herein in terms of Fe2O3 in accordance with standard practice. This, however, does not imply that all iron is actually in the form of Fe2O3 (see discussion above in this regard). Likewise, the amount of iron in the ferrous state (Fe+2) is reported herein as FeO, even though all ferrous state iron in the glass batch or glass may not be in the form of FeO. As mentioned above, iron in the ferrous state (Fe2+; FeO) is a blue-green colorant, while iron in the ferric state (Fe3+) is a yellow-green colorant; and the blue-green colorant of ferrous iron is of particular concern, since as a strong colorant it introduces significant color into the glass which can sometimes be undesirable when seeking to achieve a neutral or clear color. It is noted that the light-incident surface of the glass substrate 1 may be flat or patterned in different example embodiments of this invention. Moreover, it is noted that other types of glass, other than that discussed above, may be used for substrate 1 in certain other embodiments of this invention.
The following examples are provided for purposes of example only. The following examples are examples of different example embodiments of this invention.
In Example 1, 2.14 grams of magnesium acetate (Mg(CH3COO)2), an example Mg inclusive salt, was dissolved in 15 ml of propanol-2. Then 4 ml of trifluoro acid (TFA) and 4 ml of deionized water were added. The solution was stirred for 2 hrs. The experiment was done with ExtraClear low iron glass available from Guardian Industries Corp. The MgF2 film was fabricated by spin coating this solution onto the ExtraClear glass substrate 1 using an example spin coating technique of 2650 rpm for 30 seconds. Curing is optional. This resulted in a glass substrate 1 with an AR coating 3 thereon, the AR coating 3 being made of purely MgF2. The glass substrate 1 with the resulting AR coating 3 thereon was then heat treated in furnace at 625° C. for 3 and a half minutes for thermal tempering. The optical spectra of this coating 3 on glass substrate 1 is shown as line A in
In Example 2, 3.16 grams of calcium acetate was dissolved in 15 ml of propanol-2. Then, 4 ml of trifluoro acid (TFA) and 4 ml of deionized water were added. The solution was stirred for 2 hrs. The experiment was done with ExtraClear low iron glass available from Guardian Industries Corp., as was the case with all other Examples herein. The CaF2 film was fabricated by spin coating this solution onto the ExtraClear glass substrate 1 using an example spin coating technique of 2650 rpm for 30 seconds. This resulted in a glass substrate 1 with coating 3 thereon, the AR coating 3 being made of purely CaF2. The glass substrate 1 with the resulting coating 3 thereon was then heat treated in furnace at 625° C. for 3 and a half minutes. The optical spectra of this coating 3 on glass substrate 1 is shown as line B in
In Example 3, a magnesium fluoride-silica composite AR coating 3 was prepared from the sols of magnesium fluoride sol and silica sol. Magnesium fluoride sol was prepared as described above in Example 1. The silica sol was prepared as follows. A polymeric component of silica was prepared by using 64% wt of n-propanol, 24% wt of Glymo, 7% wt of water and 5% wt of hydrochloric acid. These ingredients were used and mixed for 24 hrs. The coating solution was prepared by using 21% wt of polymeric solution, 7% wt colloidal silica in methyl ethyl ketone supplied by Nissan Chemicals Inc, and 72% wt n-propanol. This was stirred for 2 hrs to give silica sol. The magnesium fluoride sol and silica sol were mixed in 50:50 percent weight ratio for 30 minutes. The coating method onto the glass substrate 1 and subsequent heat treatment were the same as mentioned above in Example 1. The result was a coated article including glass substrate 1 and an AR coating thereon made of a composite of MgF2 and silica. The optical spectra of this coated article is shown by line A in
In Example 4, a magnesium fluoride-silica composite AR coating 3 was prepared from the sols of magnesium fluoride sol and silica sol. Magnesium fluoride sol was prepared as described above in Example 1. The silica sol was prepared as follows. A polymeric component of silica was prepared by using 64% wt of n-propanol, 24% wt of Glymo, 7% wt of water and 5% wt of hydrochloric acid. These ingredients were used and mixed for 24 hrs. The coating solution was prepared by using 21% wt of polymeric solution, 7% wt colloidal silica in methyl ethyl ketone supplied by Nissan Chemicals Inc, and 72% wt n-propanol. This was stirred for 2 hrs to give silica sol. The magnesium fluoride sol and silica sol were mixed in 20:80 percent weight ratio for 30 minutes. The coating method onto the glass substrate 1 and subsequent heat treatment were the same as mentioned above in Example 1. The result was a coated article including glass substrate 1 and an AR coating thereon made of a composite of MgF2 and silica. The optical spectra of this coated article is shown by line B in
Example 5 was the same as Examples 3 and 4, except that the magnesium fluoride and silica sols were used in a 10:90 percent weight ratio, respectively. The optical spectra of this coated article is shown by line C in
Example 6 was the same as Examples 3-5, except that the magnesium fluoride and silica sols were used in a 5:95 percent weight ratio, respectively. The optical spectra of this coated article is shown by line D in
In Example 7, a calcium fluoride-silica composite coating was prepared from the sols of calcium fluoride sol and silica sol. Calcium fluoride sol was prepared as described above in Example 2. The silica sol was prepared as described above in Example 3. The calcium fluoride sol and silica sol were mixed in 2:98 percent weight ratio, respectively, for 30 minutes. The coating and heating method were the same as described above in Examples 1-6. The optical spectra of this coated article is shown by line E in
In Example 8, the AR coating 3 was a composite of MgF2, CaF2, and silica. In other words, the AR coating 3 of this example was a composite of silica and bimetallic fluoride. The magnesium fluoride sol, calcium fluoride sol, and silica sol were prepared as described above in Examples 1-7. Then, the magnesium fluoride sol, the calcium fluoride sol, and the silica sol were mixed in a 1:1:98 percent weight ratio, respectively, and stirred for thirty minutes. The coating technique and subsequent heat treatment were the same as in Examples 1-7. The optical spectra of this coated article is shown by the line in
In Example 9, 0.57 grams of magnesium ethoxide was dissolved in 15 ml of propanol-2. Then 4 ml of trifluoro acid (TFA) was added. The solution was stirred for 2 hrs. The experiment was done with Guardian's ExtraClear low iron soda lime silica glass substrate 1. The MgF2 AR film 3 was deposited on the glass substrate 1 using a spin coating technique with 2650 rpm for 30 secs. The glass 1 with the AR coating 3 thereon was then heat treated in furnace at 625° C. for 3 and half minutes for film curing and/or tempering. This MgF2 AR coating 3 was measured to increase the light transmission through the glass substrate 1 by 1.6%, thereby increasing the power (theoretical energy output) of the photovoltaic device by 1.7% (W/m2), when used in a solar cell as shown in
In Example 10, the magnesium fluoride sol was prepared as mentioned in Example 9 above. The magnesium fluoride-silica composite coating was prepared from the sols of magnesium fluoride sol and silica sol. The silica sol was prepared by the method described in Example 4. The 10% wt of metal fluoride sol and 90% wt of silica sol were mixed and stirred for 30 minutes. The coating deposition technique onto the glass substrate 1 and the subsequent heat treatment were the same as mentioned above in Example 9. Moreover, this MgF2-silica composite AR coating 3 of Example 10 was measured to increase the light transmission through the glass substrate 1 by 2.4%, thereby increasing the power (theoretical energy output) of the photovoltaic device by 2.9% (W/m2), if used in a solar cell as shown in
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
