Silicon Solar Cell and 350 Nanometer Cut-on Filter for Use Therein

Silicon solar cells have been utilized for some time in space satellites and in such applications are covered with a thin glass or fused silica sheet ranging in thickness from 150 to 1,000 micrometers or 6 to 60 mils in thickness. The cover is utilized to protect the silicon solar cell from charged particles, e.g. protons and electrons as might be found in the Van Allen belts and to increase the emissivity of the cell so that it radiates more heat and thereby reducing the cell temperature and improving its efficiency of conversion of light to electricity. Such covers have been cemented to the cell with an adhesive which is degraded by exposure to ultraviolet light. To prevent such degradation in the past, a thin film filter has been placed on the cover to block the harmful ultraviolet rays. Alternately, blockage has been accomplished by utilizing a glass cover which does not transmit in this portion of the spectrum. The disadvantage of such an approach is that although the ultraviolet light is prevented from reaching the cement, the ultraviolet energy is absorbed. This absorption raises the temperature of the silicon solar cell and thereby reduces its efficiency because the absorbed ultraviolet light is converted into heat. There is therefore a need for a new and improved solar cell construction which will overcome this disadvantage.

The solar cell construction comprises a body formed essentially of silicon and having a surface with a photovoltaic junction applied thereon. An anti-reflection coating is formed on the surface. A transparent protective cover is provided. A cut-on filter is carried on the underside of the cover for reflecting ultraviolet energy in the region of 350 nanometers and below. A layer of substantially transparent cement is provided for securing the protective cover to the body so that it overlies the junction and the anti-reflection coating. An additional anti-reflection coating is carried by the upper surface of the protective cover. The cut-on filter comprises a stack of high and low index layers for reflecting solar energy in the ultraviolet region of 350 nanometers and below. It also includes a layer of light absorbing material which overlies the cement and for absorbing any light energy below the region of 350 nanometers which passes through the stack of high and low index layers so that a minimum amount of energy in the region of 350 nanometers and below will reach the adhesive.

In general, it is an object of the present invention to provide a solar cell construction in which a substantial portion of the ultraviolet energy below 350 nanometers is reflected.

Another object of the invention is to provide a solar cell construction of the above character which absorbs less ultraviolet light and which has increased efficiency.

Another object of the invention is to provide a solar cell construction which is relatively insensitive to the angle of incidence of the solar radiation on the solar cell.

Another object of the invention is to provide a solar cell construction of the above character which is provided with at least one absorbent layer to absorb any ultraviolet light which may attempt to pass through the cut-off filter because of an angle of incidence different from normal.

Another object of the invention is to provide a solar cell construction of the above character in which the efficiency is improved by approximately four percent.

Additional objects and features of the invention will appear from the following description in which the preferred embodiments are set forth in detail in conjunction with the accompanying drawing.

FIG. 1 is a cross-sectional view of a typical solar cell construction incorporating the present invention.

FIG. 2 is a curve showing the transmittance of the anti-reflection coating utilizing the solar cell.

FIG. 3 is a graph showing the <E.sup.2 > calculation for the reflective 350 nanometers cut-on solar cell cover incorporating the present invention.

FIG. 4 is a graph showing the major cut-on shift of the reflective solar cell cover incorporating the present invention at angle of incidence of 0.degree., 15.degree., 30.degree., 45.degree. and 60.degree..

FIG. 5 is a graph showing the major reflectance of a standard 350 nanometer cut-on solar cell cover as compared to a solar cell cover incorporating the present invention.

A cross-sectional view of a silicon solar cell construction incorporating the present invention is shown in FIG. 1. As shown therein, it consists of a body 11 formed essentially of silicon having an index of refraction of approximately 3.5 to 4 and which is provided with a generally planar surface 12 that has a photovoltaic junction (not shown) formed thereon in a conventional manner. After the silicon solar cell has been fabricated, an anti-reflection coating 13 is formed thereon. This anti-reflection coating 13 can be of a conventional single layer type or alternatively can be a two layer antireflection coating of the type described in copending application Ser. No. 650,258 filed on Jan. 19, 1976 now U.S. Pat. No. 4,055,442.

A solar cell cover 16 is provided and is formed of a conventional ultraviolet transparent thin glass or fused silica sheet having a thickness of from 150 to 1,000 micrometers or from 6 to 60 mils and with an index of refraction of approximately 1.45 to 1.52. The cover 16 is provided with first and second planar parallel surfaces 17 and 18. A cut-on filter 19 of the present invention is provided on the first surface 17 and an anti-reflection coating 21 is provided in the second surface of the cover 16. The cut-on filter is provided for reflecting light energy in the ultra violet region between approximately 350 to 250 nanometers. A conventional adhesive or cement layer 22 is provided for securing the cover 16 with its first surface 17 with the cut-on filter 19 formed thereon facing the photovoltaic junction and for cementing the cover to the cell 11. The cement which is used for the layer 22 is substantially transparent and typically has an index of refraction of approximately 1.43 to 1.5. Thus it can be seen that the anti-reflection coating 13 is provided for matching the silicon body 11 which has an index of approximately 4 into the cement which has an index of approximately 1.4. The anti-reflection coating is formed of first and second layers with the first layer being formed of a material having a high index of refraction which is less than that of silicon, ranging from 2.35 to 2.4 and being closer to the silicon body than the second layer. The second layer is formed of a material having a low index of refraction which is greater than that of cement but less than that of the first layer, ranging from 1.6 to 1.7 as explained in said application Ser. No. 650,258 filed on Jan. 19, 1976.

Anti-reflection coating 21 is of a conventional type and can consist of a single layer quarter wave of a low index material such as magnesium fluoride or alternatively an anti-reflection coating of the type disclosed in U.S. Pat. No. 3,185,020. The anti-reflection coating 21 is used to match the index of refraction of the fused silica cover 16 and to that of the incident medium, which in outer space, may be a vacuum.

The cut-on filter 19 of the present invention has a high reflectance (non-absorbing) band in the ultraviolet region that is from approximately 350 nanometers and below. The long wavelength edge of this band is substantially angle insensitive. This is achieved by the use of a slightly modified quarter wave stack which is essentially non-absorbing at wavelengths longer than 250 nanometers and shorter than 1200 nanometers. The stack consists of a plurality of layers with alternate layers being formed of a material having a low index of refraction ranging from 1.4 to 1.7 and the remaining layers being formed of a material having a high index of refraction ranging from 2.00 to 2.4. For the low index material it has been found particularly desirable to utilize a fused silica having an index of refraction of 1.4686 and for the high index material to utilize tantalum pentoxide having an index of refraction of 2.23. It has been found that zirconium oxide can be utilized in place of tantalum pentoxide but it is less desirable than tantalum pentoxide because the lower index ratio means that the edge shift (hereinafter described) is more severe resulting in somewhat more absorption at an angle from normal as well as a need for a larger number of layers in a stack to achieve a given reflectance value. Another material which can be used for the non-absorbing high index material is hafnium oxide. Other materials which can be used for the non-absorbing low index material are silicon dioxide, silicon sesquioxide, magnesium fluoride and sapphire.

In the design of the quarter wave stack for the cut-on filter, the design wavelength of the center of the stack is selected such that the edge of the high reflection band is located at 350 nanometers to provide the desired high reflectance. It is possible to utilize only a single pair of high and low index layers for the filter although it is desirable to utilize a plurality of pairs.

When tantalum pentoxide is utilized, it has been found that it absorbs in the region of approximately 200 to 275 nanometers. However, there is a wavelength shift when the angle of incidence of the solar energy is substantially changed from normal or perpendicular to the surface which causes the edge of the cut-on filter to shift to a shorter wavelength or away from the 350 nanometers region as for example down to 325 nanometers. This in effect causes a window to be formed through which ultraviolet radiation can leak and pass through the cut-on filter unless additional means is provided as a part of the cut-on filter to absorb any ultraviolet energy which possibly can leak through the stack of high and low index layers. To eliminate the shift in the cut-on wave length with angle, at least one and preferably a few of the high index layers of the quarterwave stack on the side of the stack away from the incident light are changed from the high index material to a material which is capable of blocking or in other words absorbing ultraviolet energy such as titanium oxide. Other material which can be utilized for this purpose are titanium dioxide, cerium oxide and zinc sulphide. The effect of this substitution is to cause the energy below approximately 350 nanometers that is transmitted by a tilted cut-on filter to be absorbed in this layer thus permitting the energy from reaching the cement and degrading the same. Although this will cause some heating, this heating is far less than that encountered by a conventional design which utilizes only titanium oxide as the high index layer. In particular, it should be appreciated that generally solar panels are oriented so that the sunlight is incident normally on the cells rather than at an angle. When such is the case, very little ultraviolet light will penetrate through the cut-on filter and be absorbed by the titanium oxide layers and therefore the heating effect will be minimal.

A typical design incorporating the present invention is set forth below consisting of seventeen layers counting from the substrate.

From the above it can be seen that the stack begins with a high index layer and ends with a high index layer and that a number of pairs of high and low index layers are provided. Any number of pairs can be provided depending upon the desired result. The tantalum pentoxide/fused silica quarter wave stack is centered near 284 nanometers and has a reflection band that starts at 350 nanometers on long wavelength side and extends to the absorption edge of Ta.sub.2 O.sub.5 that occurs at approximately 275 nanometers. Absorption causes a rapid fall-off in the reflectance beyond 275 nanometers. Layers 3 and 5 are the ultraviolet absorbing layers. It should be appreciated that some variations to the typical design set forth above can be readily made with the exact values to be based upon trade offs between the index of refraction of the cover, the cost of materials and the amount of reflection desired.

When zirconium dioxide and fused silica are used to construct the ultraviolet high reflectors, two stacks are required to prevent short wavelength leaks since the absorption edge of zirconium dioxide is at too short a wavelength to preclude leaks by absorbing light.

FIG. 3 is the plot showing the electric field entering the stack. The layer codes are:

H=Ta.sub.2 O.sub.5

H.sup.1 =Ti.sub.2 O.sub.3

Unmarked-SiO.sub.2

The plot shows the strength of the light as a function of the depth in the filter. As can be seen as the light penetrates into the layers of the stack, its strength is decreased. By the time that the titanium oxide layers H.sup.1 are reached, the strength of the light has been reduced to almost zero so that there is no ultraviolet light to be absorbed by the titanium oxide layers which are the third and fifth layers counting from the substrate.

If the filter should happen to be tilted, then the light is no longer normally incident thereto and the edge will shift from 350 nanometers to shorter wavelengths to provide a small window. Any ultraviolet passing through this small window however will be absorbed by the third and fifth titanium oxide layers. These layers are back-up layers to prevent ultraviolet light from reaching the cement and degrading the same. The titanium oxide is a good material for this purpose because it is not angle sensitive and because intrinsically it absorbs ultraviolet wavelengths below 350 nanometers.

It should be appreciated that if desired, additional layers of titanium oxide can be added to obtain better blocking. However, the addition of more layers adds to the expense of the cut-on filter.

In FIG. 4 there is shown the cut-on shift of a filter incorporating the present invention. The angles of incidence are 0.degree., 15.degree., 30.degree., 45.degree. and 60.degree. as shown. The curves shown in FIG. 4 show that the angle shift is not very great.

In FIG. 5 there is shown a comparison between the measured reflectance of a standard 350 nanometer cut-on solar cell cover as compared to a solar cell cover incorporating the present invention which is identified as reflective 350. As can be seen from the curves, there is a great improvement in reflectivity. Any difference between 100% and the effective curve represents absorption. As can be seen, the reflective 350 solar cell cover absorbs much less ultraviolet energy than does the standard 350 nanometer cut-on solar cell cover. The reflectance of the 350 nanometer cut-on solar cell cover must be weighted against the curve of the solar energy in this region.

By providing such reflective 350 nanometer cut-on solar cell cover, approximately 4% and certainly over 3% of the total solar energy incident upon the solar cell is reflected in addition to that which is reflected by a conventional solar cell cover so that the total energy reflected is in the vicinity of 90%. By providing the reflective 350 nanometer cut-on solar cell cover and its consequent 4% increase in reflection of the total energy, 4% less heat is absorbed by the solar cell with a consequent increase in efficiency of the solar cell up to approximately 2%.

When a solar cell is operating in space, it is believed that the temperature reduction will be approximately 4.degree. C. In general, the performance of a solar cell changes by 1/2% per degree to give the approximately 2% more solar cell output.

It should be appreciated that the coating of the present invention can be also utilized in conjunction with absorbing glass covers which are used to block the ultraviolet. In such a case, the quarter wave stack is applied to the outer surface of the glass cover, thus reducing the absorption of ultraviolet energy by the cover. In the case of the ultraviolet transparent covers, such as fused silica, these coatings are placed on the inside (cement side) of the cover.

From the foregoing it can be seen by a particular choice of materials and layer thicknesses there has been provided a thin film interference coating which has a high reflectance in the ultraviolet portion of the spectrum (between 250 nanometers and 350 nanometers) which does not suffer from angle limitations. More particularly, the coating of the present invention consists of a stack of layers which has a high reflection band placed such that the sunlight strikes them first. Beneath these stacked layers there has been included additional layers which include a material which has an absorption band that starts at 350 nanometers and which extends beyond 250 nanometers.

It should be appreciated that although in connection with the present invention the disclosure has been directed primarily to the use of the invention in the ultraviolet region, the same principles can be utilized for other wavelength regions by the utilization of different combinations of materials.