DE2405936A1 - SUN CELL - Google Patents

Description

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Bremen, ά & ι 02/07/1974 Applicant:

Gonmunications Satellite Corporation
Washington DC, USA

Solar cell

The invention "relates to a solar cell that is responsive to a light radiation which is the short wavelength range of the light spectrum between o, 3 and o _s 5 microns -umfasst _g and. By the penetrating an anti-reflective coating over the primary surface of the solar cell" the light _ΰ has "wherein the coating useful Liehtj, einseSiließlieh light having Kurser wavelength neither absorbed nor reflected using Siliciurasonnenzellen _c Lielit" energy into electrical energy are ,, both in scope to the earth .and in space known "the onto the silicon solar cell incident light is toe öe® semiconductor base material of the cell absorbs _p whereby llektronen-hole pairs are generated (ie, "carrier) ideal state, the charge carriers transition especially separated by the HalDled without bm transition ein® combination occurs. the charge carrier könaea & n dea slefe. opposite surfaces of the Sonaonaoll e with the help of ©

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metallic power connections are collected, creating a current flow.

The efficiency of the solar cell (i.e. the ratio between the electrical output power and the input power of the incident usable light) is directly related to the amount of usable incident light into the silicon cell Light. The usable light can be used in a certain solar cell as an electromagnetic energy in such Wavelengths are defined which, when absorbed by the solar cell, result in a generation of charge carriers Cell transition causes. The efficiency of the solar cell, however, is due to the reflection of the usable light from the upper surface of the solar cell. To reduce the problem of light reflection, anti-reflective is used Coating applied to the surface through which the light penetrates the solar cell.

As is already known, the environment in which a Solar cell should set the specific mechanical, chemical and optical properties that are used must have the anti-reflective coating. In the scope space travel, where reliability under extreme environmental conditions and during extraordinary Having these qualities is essential to a successful mission.

A fundamental optical property is that the anti-reflective coating is the reflection of the usable. Lightfts reduced. When used in the aerospace, c.ei a Abäeckscheibe preferably made of quartz over! Laser attireflektierenden coating is to protect the sun Selle vcr harmful radiation, the refractive index of the antireflective coating between learning that should Quarsabdeckscheibe and iex therebelow special

n cell, ie generally in the range between 2 ₉ o and 2.5. Another required optical property of an anti-reflective coating is permeability. The anti-reflective coating should not absorb any useful light, but should allow such light to pass through to the underlying solar cell. As the final analysis shows, the optical properties required for an anti-reflective coating are dependent on the refractive indices, both on the underlying solar cell and the cover panel and on the wavelength to which the solar cell responds.

From the German patent specification No. P 22 46 115 »filed on 2o. September 1972, a solar cell emerges which is particularly suitable for the entire visible spectrum and especially for light in the blue-violet region of the spectrum. This range corresponds to the short wave range of light from about 0.3 to 0.5 microns. So far, a solar cell with these properties is not known. In order to be able to successfully use a solar cell with a response that extends into the short wavelength range, it is necessary to apply an anti-reflective coating that does not absorb light over the entire visible spectrum, i.e. ₉ hours. from ® _@ 3 to 1.1 micron and which has a refractive index between the i © r cover plate and the solar cell.

The anti-reflective coating must also meet certain mechanical and chemical criteria. In view of the length and service life considerations, these properties are influenced by the physical properties of the solar cell. Such a solar cell, the area is responsive to light in the short Wellenlämg © a ° and in particular a ZeIIe wirdg _9, the sash ¹ of the · Patent Application mentioned above requires prepared olsio

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anti-reflective coating around certain specific criteria to meet. For example, the n-p transition described in the above patent application is the short one Waves responding solar cell only about 1,000 to 2,000 A from the top surface of the solar cell. Under under these conditions, the anti-reflective coating would damage the shallow junction if the coating were in the Invades solar cell. Eeraer must have any mechanical stress generated at the interface between the Anti-reflective coating and the semiconductor body be low, so that such a stress is not the Junction damaged.

Furthermore, the anti-reflective coating should not degrade when exposed to the ultra-violet in a vacuum Light is exposed. The effect of such deterioration could be a change in the refractive index of the anti-reflective Coating and the absorption of light in the short wavelength range. Regarding the silicon solar cells Also known is a phenomenon known as "dispersion" in which the index of refraction of silicon increases with decreasing wavelength. For these reasons, the anti-reflective coating should have a relationship with the Have wavelength which adapts to the variable refractive index of the silicon.

8 consider further criteria for an anti-reflective coating o refer to their stability, to the aihesional properties and the hardness. The anti-reflective material should be chemically stable, i, h. it shouldn't be its composition change during the process while dtm there one temperature, chemicals and humidity, and should not change during storage ensure constant optical properties. The adhesion of the anti-reflective coating to the solar cell should be excellent to make sure there is no degumming

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during the machining process or during the suspension occurs in moisture or a temperature cycle. Finally, the anti-reflective material should be hard enough to stand up during manufacture or use, and in particular are not damaged while the cover panel is being attached.

The invention is based on the object for the aforementioned Solar cell to propose an ant! Reflective layer, which meets the above conditions.

The solution to this problem is that niobium pentoxide (Fb ₂ ⁰ ^) is proposed as the material for an anti-reflective layer.

The Niobpento2iid to be used has a Breehungsindex of about 2.4, and the _t in accordance with the invention, "to be used Hiobpentoxid is amorphous. The proposed anti-reflective layer is preferably used for those solar cells that respond particularly to light in the short ¥ ellenläng @ a "bereicii (ie at 0.3 to 0.5 microns).

The invention is shown in more detail with reference to the drawing

FIG. 1 shows a three-dimensional view of a solar cell with an anti-reflective coating made of Miobpentoxidg

Figures 21 to 2G side view of the Sonn®az®ll © na ©! I at different stages atij? Manufacture of the sun rope © including dar antireflective ^ sadaa Beschich processing according to the invention and.

Figure 3 shows a photomask that during the manufacturing process the solar cell according to Figure 1 is used.

Before the invention is described in more detail, it is pointed out that that such dimensions as the size of the solar cell and the relative thickness of the various ones shown in FIG Layers, are not representative of an actual solar cell, but that they are used for illustrative purposes only. Furthermore 'the present invention relates to the use sourced from NbpOc as an anti-reflective coating, generally applicable to solar cells, however, a silicon solar cell which operates on light is used as a preferred embodiment for describing the invention responds in the short wavelength range and that in the above mentioned German patent application is described in more detail.

FIG. 1 shows a silicon solar cell 1 which has a layer 2 of a first conductivity type which is separated from a layer 3 of an opposite conductivity type by a junction 4. In the preferred embodiment, for example, has; the silicon cell 1 has an n-zone 2 which is separated from a p-zone 5 by an np junction 4 and which is removed from the upper surface of the zone 2 by 1,000 Å. As is known in the art of solar cells, a p-yp base material with an η-type diffused zone as described is preferred to an η-type base material with a p-type diffused zone; however, the invention can be applied in either case.

On the upper surface of an n-type zone 2 there is a metallic grid 5 for receiving charge carriers _f generated during the photoelectric process. As will be further described below, an anti-reflective coating 12 covers those surfaces of the upper ones

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Surface of zone 2 that are not covered by a metallic 5. The metallic collector grid 5 is of a fine geometric type and has metallic fingers 5Ά, a width of about 1 to 20 microns, as can be seen from the above-mentioned German patent application, Eb any other known metallic collector grid can also be used. According to the present invention, the reflective coating has 6 Nb ₂ Oc.

A conventional quartz cover plate 7 covers the anti-reflective one Coating 12 and the grid 5 with the exception of the busbar 5b of the grid 5. On the bottom of the p-zone 3 there is a conventional metallic contact 8 which can cover the entire base area of zone 3.

In a preferred embodiment of the invention, the Niobium pentoxide coating for a semiconductor die one appropriate size is used that has a shallow n-p junction under the surface coated by niobium pentoxide shall be. Reference is made to Figures 2A to 2G, from each of which a side view of the plate during subsequent successive work steps in a preferred th process for coating a solar cell with emerges.

The manufacture of this plate is not the subject of the ¹ l ^ charge, but only represents a typical process & @, τ _ΰ Sas only ssur explanation of the connections briefly described -fird "The starting point is a disk of silicon, <6LI © on" ine certain Size is sctaitten _# which one is sw?

as Sonnenztlle © ignet · The silicon eel is erased in a diffusion process, Tb © i the Έτβ® & ~ Cnormally phosphorus) is erased into a surface of the SIli®ii c CMüi it , in the % inen lä-p transition 4 sm © rs®ug © a ₀ ü & s

■ T'.iT- ' ^5f vs! © H tone of the upper öberfläsii © ü®t goae 2 sntfsrat iSü "Ji © Ii- fiipsT 2A is shown« lin

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for diffusion of a shallow transition into the solar cell, US patent application S.N. 331 74o, registered on February 13, 1973. described.

FIG. 2A shows a silicon wafer with an n-zone 2, which is separated from a p-zone 3 by a flat p-n Transition 4 is separated. With 9 a layer is designated, which consists of elemental niobium.

In the first stage of the present invention, the silicon wafer 1 has the required n-p junction 4 a layer 9 of elemental niobium is deposited on the entire upper surface 2 of the solar cell. In one preferred embodiment of the invention, the evaporation of the niobium with the help of an electron beam evaporation

Process. The niobium layer 9 is about 25o A thick (+ 25A), but the layer can also be 100 pounds or larger depending on the desired thickness of the resulting oxide. The oxide thickness should be 1/4 of the wavelength of light be in the middle of the spectrum of usable light for the particular solar cell. A 2oo A

thick layer of niobium can be oxidized onto a 55o A thick layer of niobium pentoxide. Although the technique of electron beam evaporation As is well known, certain points should be considered. To ensure a suitable deposit To provide a niobium source of great purity must be used, which is small enough to withstand an impermissible thermal To prevent radiation from the hot niobium metal, and which is bombarded by an electron beam in a high vacuum. Furthermore, the p-n silicon wafer should be protected from any electron damage arising from the electron beam which is directed towards the niobium. Al3 protection can be a positively charged metal electrode screen can be used, which attracts any electron beams. The niobium metal itself should be free of any impurities who, if they get to zone 2,

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During this or the subsequent stages of the coating process diffuse into the layer 2 and thereby the Damage the p-n junction 4 of the silicon solar cell.

The second stage is a layer of photo resist material 1o placed on the entire upper surface of the niobium layer 9. The photo cover 1o can be any commercially available available photo cover exist, which in the photolithographic process for the production of microcircuits can be used, such as those under the designation AZ-111 from the Shipley Company, Los Angeles, USA will be produced. Further photo covers are available, for example, from Kodak in Rochester, USA. the Photo cover AZ-111 is a "positive" cover that is after exposure and development becomes water soluble; however, a "negative" cover can also be used, which is after becomes insoluble after exposure and development. To prepare the desired pattern of the developed photo cover, becomes a photomask with a pattern that allows light to pass through and that is identical to the pattern used for the top metal grid 5, according to Figure 1, is desired (for a positive photo cover; a negative photo cover would be a negative have), brought over the photo cover 1o. Next is the top surface of the layer 9, which will be covered by the photo is exposed to ultra-violet light through the photomask. Then the photo mask is removed and the Layer of photo cover developed with the developer recommended by the AZ-111 photo cover manufacturer. The solar cell is washed off in water or in another solvent recommended by the manufacturer, whereby Removing the soluble developed photographic cover that has been exposed to light, creating a pattern of the developed Photo cover material 1oa remains on the upper surface of the layer 9, as can be seen in Figure 2C. At this Place is the pattern of the photo cover material 1oa the photo negative of the metallic grid pattern 5, which may be on

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the top surface of zone 2 is to be brought.

In the third stage, which can be seen from FIG. 2D, a metal contact layer is used 11, which consists for example of a mixture of chromium and gold with a thickness of about 2oooA, with the help vacuum evaporation over the entire top surface of the solar cell including the photo cover 1oa and the niobium pattern 9 applied. During this preferred application process, pure chromium is initially vaporized, with gold gradually mixed into the metal deposit. At the end of the evaporation process, chromium is removed from the phase and only pure gold completes the contact. The conditions of this process are known per se and a description this technique is not considered necessary.

The fourth stage is to remove the developed photo resist 1oa of dee surface of said elemental niobium. 9 Removal of the developed photo cover 10a is performed using the well-known "lift-off" technique in which the developed photo cover to be lifted off is dipped in acetone or some other chemical recommended by the manufacturer for removing the developed AZ-111. The acetone is placed in an ultrasonic bath in order to facilitate the removal of the developed photo-masking 1oa. The result of the lift-off process is that not only the photo cover 1oa, but also the metallic layer 11 over the photo cover is removed. This lifting process leaves the developed areas of elemental niobium and a pattern of chromium and gold on the top surface of the cell. The maximum thickness of the chrome and gold should be around 2oooA so that the developed photo cover underneath can be easily lifted off. The resulting cell structure can be seen from FIG. 2E.

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In the fifth stage, a conventional electroplating technique can be used _{5 to} apply a layer of silver 13 on top of the chrome and gold layer 11 to roughen the thickness of the metallic mesh to about 5 microns. At the same time, the rear silver contact 8 is plated onto the rear zone 3 with a diameter of 5 microns. The silver was found to fail to plate the elemental niobium during the process, leaving a silver coated pinger pattern over a niobium layer on one side of the cell and a rear silver contact on the other side.

In the last stage, the silicon solar cell with the silver-plated metal contact pattern and the surface made of elemental niobium is exposed to oxygen either in a thermal or anodic oxidation process. In the case of thermal oxidation, the silicon wafer is heated in air at a temperature of 400 ° C. for 3 minutes. The parameters can range from 35o ° to 45o ° C and 3-5 minutes and should be selected so that a uniform oxide without grain boundaries (non-crystal) is achieved. It should also be noted that higher temperatures initiate undesirable alloying of the chromium and gold with the semiconductor because of the eutectic temperature of these materials, while lower temperatures allow sufficient time for the chromium or gold atom to diffuse into the semiconductor interface. Under the appropriate conditions, niobium pentoxide is formed with a resulting index of refraction of 2.4. It should be noted that the other elements of the cell are not affected, since at this temperature and these times the silver does not oxidize and thus remains in its elemental state. Another oxidation technique known as anodic oxidation uses a platinum electrode as the cathode and the silicon wafer as the anode. Both electrodes are immersed in an electrolyte and a current is passed through them. The anodic one

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In one example, the oxidation process can last about 20 minutes and begin with an initial current of one mA. It is preferred to use an organic non-aqueous electrolyte such as tetrahydrofurfuryl alcohol this achieves a uniform riot pentoxide coating firmly adhering to the upper surface of zone 2. The result of either thermal or anodic oxidation is a layer 12 of niobium pentoxide on the top surface of zone 2 shown in Figure 2G.

The layer of niobium pentoxide is about 55oA thick around it 1/4 wavelength at 0.5 microns.

The niobium layer between the silicon surface 2 and the metallic Layer 11 creates excellent contact for the metal to take up the charge carriers Grid 5.

It should also be pointed out that cleaning steps are carried out at the various points in the process described are required to improve the purity of the resulting product. However, as these steps are known and do not affect the basic concept of the present invention, they have been included in the present description not included.

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Claims (3)

Claims J Solar cell which is responsive to light radiation which comprises the short wavelength range of the light spectrum between 0.3 and 0.5 microns and which has an anti-reflective coating over the primary surface of the solar cell through which the light penetrates, the coating being useful Light, including light of short wavelength, neither absorbed nor reflected, characterized in that the anti-reflective layer consists of niobium pentoxide (Nl ₂ Oc). 2. Solar cell according to claim 1, characterized in that that niobium pentoxide with a refractive index of 2.4 is used. 3. Solar cell according to claim 1 or 2, characterized in that that the niobium pentoxide used is amorphous. 409833/0821