CA2068668A1

CA2068668A1 - Front film contact solar cell

Info

Publication number: CA2068668A1
Application number: CA 2068668
Authority: CA
Inventors: Stanely J. Krause
Original assignee: Hughes Aircraft Co
Current assignee: Raytheon Co
Priority date: 1991-09-23
Filing date: 1992-05-14
Publication date: 1993-03-24
Also published as: JPH05267697A

Abstract

FRONT FILM CONTACT SOLAR CELL

ABSTRACT OF THE DISCLOSURE

A solar cell (20) includes a photovoltaic junction (26) having a front face (24), a back face (32), and a lateral side (42). A first electrical contact (28) for the photovoltaic junction (26) includes a first electrically conductive contact pad (38) located on the back face (32) of the photovoltaic junction (26). A second electrical contact (30) includes a transparent, electrically conductive layer (40) on the front face (24) of the photovoltaic junction (26), a second electrically conductive contact pad (46) located on the back face (32) of the photovoltaic junction (26), and an electrically conductive trace (48) extending from the electrically conductive layer (40) on the front face (24) of the photovoltaic junction (26), over the lateral side (42), and to the second contact pad (46) on the back face (32) of the photovoltaic junction (26). The transparent, electrically conductive layer (40) is preferably a transparent conducting oxide such as indium tin oxide or zinc oxide. The use of the transparent conductive layer (40) on the sun-facing portion of the photovoltaic junction avoids the need for collector grid lines on the front face, which if present shade a portion of the photovoltaic junction and reduce its output.

Description

~0~8668 FRONT FILM CONTACT SOLAR CELL

BACKGROUND_OF_THE INVENTION

Thls inventlon relates to solar cells 9 and, more par-ticularly, to an arrangement of the electrical connections to the solar cell that improves lts efficiency.

Semiconductor solar cells are utillzed to convert light energy -to usable electrical voltages and currents. Briefly, a typical semlconductor solar cell includes a photovoltaic Junction having an lnterface between n-type and p-type transparent semiconductor materials. Light shlning on the semiconductor materials ad~acent to the interface creates hole-electron pairs in additlon to those otherwise present, and the minority charge carriers migrate across the interface in opposite directions. There is no compensating flow of ma~ority carriers, so that a net electrical charge results.
A useful electrical current is obtained form the photovoltaic ~unction in an e~ternal electrical circuit by forming ohmic contacts to the materlals on either face of the photovoltaic ~unction. That is, a first contact is formed on the back face (i.e., away from the sun~ of the photovoltaic ~unction, and a second contact is formed on the front face (i.e., toward the sun). The first contact on the back face is readily formed by depositlng a layer of a conducting material on the back face of the photovoltaic ~unctlon.
The second contact on the front ~ace may be formed by depositing a grid pattern of electrical conductors onto the front face of the photovoltaic ~unction. The grid pattern conducts charges away ,: , , : .. . . ..
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from the front face of the photovoltalc Junctlon to an external circuit that completes the electrlcal circuit with the flrst contact, permittlng the solar cell to generate useful electrical currents.
A problem wlth this approach is that the grid pattern on the sun-facing front face of the solar cell shades a portion of the photovoltaic Junction, reducing its efficlency. In a typical solar cell formed in the manner discussed, there is about a five percent reduction in output current and efficiency of the solar cell as a result of the shading effect.
There ls a need for an lmproved deslgn of solar cell that reduces, and preferably ellminates, the shading effect of the front face grids, yet permits a front contact/back contact approach to be used. The present lnvention fulfills this need, and further provides related advantages.

SUMMARY ~F T~E INVENTION
The present invention provldes a front contact/back contact type solar cell whlch elimlnates the need for a grid pattern on t~e front face of the photovoltaic Junction. Consequently, the efficiency of the solar cell ls not reduced by the shading effect characteristic of the grid pattern on the front face of the photovoltaic Junctlon. The electrical resistance of the cell is reduced, because of shorter distances for diffusion of the charge carriers through the semiconductor material to the front face electrical contact.
In accordance with the invention, a solar cell includes a photovoltaic Junction having a front face, a back face, and a lateral side. A first electrical contact for the photovoltaic Junction has . :
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a flrst electrlcally conductlve contact pad located on the back face of the photovoltalc Junctlon, and a second electrical contact for the photovoltalc Junction. The second electrical contact comprises a transparent, electrlcally conductive layer on the front face of the photovoltaic Junctlon, a second electrlcally conductive contact pad located on the back face of the photovoltalc ~unctlon, and an electrically conduc-tlve trace extending from the electr~cally conductlve layer on the front face of the photovoltalc cell, over the lateral slde, and to the contact pad on the back face of the pho~ovoltalc Junctlon. The electrlcally conductlve trace may be made of any conductlve material, such as the same material as the conductlve layer or a metal. There ls prefera~ly a layer of lnsulatlon between the conductlve trace and the lateral slde of the photovoltaic ~unction. Deslrably, there are at least two second contact pads on the back face of the photovoltaic Junction, and two traces extending from the front face.
In another aspect, a solar cell comprlses a photovoltaic Junctlon havlng a front face and a back face, and a first electrical contact for the photovoltaic ~unctlon lncluding a first electrlcally conductlve contact pad located on the back face of the photovoltaic Junctlon. A second electrical contact for the photovoltalc ~unction comprises a transparent, electrically conductive layer on the front face of the photovoltalc Junction, and a second electrically conductive contact pad located on the front face of the photovoltalc ~unctlon 9 the second contact pad belng in electrlcal contact with the transparent, electrlcally conductive layer.
The invention ls operable wlth any type of photovoltaic ~unction configuration and material.
Conventional photovoltaic materlals now available , , : :

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2~$~8 include silicon, gallium arsenide, and gallium arsenide/germanium, but as others become available they can be used in con~unction with the present inven-tion. The transparent, electrlcally conduc-tive layer is preferably a thin layer of a transparent conducting oxide such as lndlum tln oxlde or zlnc 02ide. The use of the transparent conductlve layer on the sun-facing portion of the photovoltaic ~unction avoids the need for collector grid llnes on the front surface, which, where present, shade a portion of the photovoltaic Junction and reduce its output and overall efficiency.
The present invention provldes an improved solar cell which reduces the efficienc~ loss due to the front-face electrical contact. Efficiency loss due to the additlonal layer on the front of the photovoltaic ~unction is minimal. The present approach results in a net efficienc~ gain of about 5 percent in the output of the solar cell. Other features and advantages of the invention will be apparent from the following more detailed description of -the preferred embodiment, taken in con~unction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF TEE DRAWINGS

Figure 1 is a diagrammatic view of a solar cell and its electrical connectlon Figure 2 is a side elevational view of the solar cell of the invention;
Figure 3 is an elevational vlew of the solar cell of Figure 2, from its back face;
Figure 4 is an elevational view of the solar cell of Figure 2, from its front face; and .

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Figure 5 is a slde elevatlonal vlew of another embodiment of the solar cell.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 lllustrates a solar cell 20 ln accordance with the lnventlon and lts mode of generating power. Llght rays 22 from the sun or elsewhere shlne on a front face 24 of the solar cell 20, which contains a photoYoltalc Junctlon 26 made of transparent semiconducting materials. Electrlcal contacts 28 and 30 are made to a back face 32 of the solar cell 20, with electrical leads 34 e~tending from the electrical contacts 28 and ~0 to an external load 36. As a result of the photoelectric behavior of the photovoltaic ~unction 26, charge carriers are generated in the photovoltaic Junction 26 and migrate toward the front face 24 or the back face 32, dependlng upon the type and configuration of the photovoltaic ~unctlon 26. These charge carriers are conducted through the leads 34 and the load 36, to provlde a useful electrical current.
The principles and operation of photovoltalc ~unctions 26 such as used in ~olar cells 20 generally are well known in the art. Conventional photovoltaic solar cells are typically made of doped silicon, gallium arsenide, or gallium arsenide/germanium. The present invention is not limited for use with any particular type of photovoltaic Junction, and is operable with these and other types presently known or that may be discovered.
Figure 2 shows the solar cell 20 in greater detail. The flrst electrical contact 28, made of an electrically conductive material such as aluminum9 copper, an aluminum alloy, or a copper alloy, is .

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deposited as a -thln electrically conductlve contact pad ~8 on the back face 32 of the photovoltaic Junction 26. The contact pad 38 is typically deposited by electrodeposition or sputterlng, or other convenlent process, and is typicaLly about 3 micrometers thick. The contact pad 38 is made to cover a large fraction of the back face 32, to efficien-tly collect the charge carriers that migrate toward the back face 32.
The second electrical contact 30 includes several elements. A transparent, electrlcally conductive layer 40 ls deposited over substantlally all of the fron-t face 24 of the photovoltalc ~unction 26. Thls transparent, electrically conductive layer 40 permits light rays 22 to reach the photovoltalc Junction 26. It also permits electrical charge carriers to be collected from the front face 24 wlthout havlng an~ opaque metalllc grids or other collectors that tend to partially shade the photovoltaic Junction.
Any transparent, electrically conductive material is operable as the layer ~0, but a presently preferred material for the la~er 40 is indium tin oxide (ITO~ or zinc oxide, whlch are transparent and electrically conductive when present in the proper thickness. In the preferred embodiment, the layer 40 ls of a thickness that reduces the emitter sheet resistance to about 3-5 ohms per square and is made of a transparent conductlng oxide (TCO) such as ITO or zinc oxide.
This thickness is typically about 1-2 micrometers.
The spu-ttering of thin layers of such o~ides is well known in the art.
By comparison, the sheet resistance of the photovolta:Lc ~unction is typically about 400 ohms per square. The resistance is reduced to about 10 ohms per square through the application of an :~
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optimized pattern of front-face collector grid lines in the conven-tional deslgn. A coatlng of IT0 or zinc oxide reduces the sheet resistance to 3-5 ohms per square, a significant reduction that produces lowered resistance and improved current in the photovoltaic cell.
The photovoltaic ~unction 26 has been previously described as having a front face 24 (which faces toward the sun) and a back face 32 (whlch faces away from the sun). Additionally, the photovoltaic ~unction 26 has at least one lateral side 42 extending between -the front face 24 and the back face 32. In a typical con~iguration, the photovoltaic Junction 26 is rectangular when vlewed from the back (Figure 3) or the front (Figure 4), and therefore has four lateral sides 42.
In the solar cell 20, an insulation layer 44 is provided -on at least one lateral side 42, and preferably over two opposing lateral sides 42 as shown in the figures. The insulation layer 44 is also present on a portion of the back face ~2 in a region that underlies a second electrically conductive contact pad 46. The insulatlon layer 44 is any suitable electrical insulator. In the case of a silicon photovoltaic ~unction 26, for example, the insulation layer 44 may be conveni0ntly provided as SiOx by masking and then oxidlzing the appropriate regions of the photovoltaic junction 26. Equivalently, a different insulation layer could be applied.
An electrically conductive trace 48 is deposited overlying the insulation layer 44 on the lateral side 42 to extend from the transparent, electrically conductive layer 40 to the second 35 contact pad 46. Electrical charge collected from the front face 24 of the photovoltaic ~unct~on 26 are conducted by the trace 48 to the second contact : '' , `' `: ` ' ':`

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pad 46, which is located on the back face 32 of the photovoltaic ~unction. The trace 48 may be made of any electrically conductive material. The trace 48 may be made of a metal such as aluminum, copper, alumlnum alloy, or copper alloy. The trace 48 may also be made of the same materlal as the transparent, electrlcally conductive layer 40, typlcally a transparent conductive oxlde such as IT0 or zlnc oxide. The second contact pad 46 is made of an electrically conducting material such as aluminum, copper, alumlnum alloy, or copper alloy.
Such metals are readily deposited by evaporation or sputtering as thin layers. The -two contact pads 38 and 46 are preferably layers about 3 micrometers thlck.
The solar cell 20 of Figures 2-4 is externally connected in the manner shown in Figure 1. ..
The present approach has the important advantage that it entirely ellmlnates opaque grld lines from ~he front face of the solar cell, so that no portion of the photovoltaic ~unction is shaded.
The result is a higher efficiency for the solar cell, typically about 5 percent greater than 2S at-tained for a comparable cell having a conventional front-face grld line collector. Because the electrical resistance of the conductive layer 40 is only ~-5 ohms per square in the preferred case, the cell can be made as much as abou-t 2 centimeters wide wlthout the need for a collector grid on the front face of the cell. If larger cells are to be made, thin intermediate collector grid lines about 2 centimeters apart can be provided on the front face of such a design.
Figure 5 is an elevational view similar to that of F'igure 2 of an alternative design using these principles. A solar cell ~0 has a .

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photovoltaic ~unction 62 with a layer h4 of transparent conductive material on a front face 66.
The photovoltaic ~unction 62 and layer 64 are as previously described. A second metal contact pad 68, which serves as the front-face contact pad (analogous ln function to the second contact pad of the embodlment of Figures 1-4) ls deposited at the edges of the front face 66, or, lf the lat~ral dimensions of the front face are more than about 2 centlmeters in all directions, as a grld. A first contact pad 70 ls deposlted on a back face 72 of the cell 6a. The contact pads 68 and 70 provide the external electrical contact to the solar cell 60.
Solar cells having the configuration shown ln Flgure 5 were prepared to demonstrate feaslbllity of the use of a transparent conductlve layer 40 as the front-face collector. The photovoltaic ~unction had the following layers, proceedlng from the back face toward the front face: Substrate, 200 mlcrometers of N-type gallium arsenide; buffer, 1 micrometer of N-type galllum arsenlde; base, 3 micrometers of N-type gallium arsenide; emitter, 0.5 mlcrometers of P-type gallium arsenide. A silica wlndow about 0.05 micrometers thick, and a P+-type layer of gallium ~5 arsenlde about 0.~1 micrometers thick were placed over these active elements. A layer about 2 micrometers thick of either zinc oxide or IT0 was deposited over the cell as the conductive layer.
Characteristics of the cells were measured. The cells using both types of conductive oxide coatings were operable. The cell having the zinc oxide conductive layer achieved an e~ficiency of 14.5% in inltial testing.
Although particular embodiments of the invention have been descrlbed in detail for purposes of illustration, various modifications may be made without departing from the spirit and scope of the .
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lnvention. Accordlngly, the lnventlon ls not to be llmited except as by the appended clalms.

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Claims

1. A solar cell, comprising:
a photovoltaic junction having a front face, a back face, and a lateral side;
a first electrical contact for the photovoltaic junction comprising a first electrically conductive contact pad located on the back face of the photovoltaic Junction; and a second electrical contact for the photovoltaic junction, the second electrical contact comprising a transparent, electrically conductive layer on the front face of the photovoltaic junction, a second electrically conductive contact pad located on the back face of the photovoltaic junction, and an electrically conductive trace extending from the transparent, electrically conductive layer on the front face of the photovoltaic cell, over the lateral side, and to the contact pad on the back face o-E the photovoltaic junction.

2. The solar cell of claim 1, wherein the photovoltaic junction is selected from the group consisting of silicon, gallium arsenide, and gallium arsenide/germanium photovoltaic junctions.

3. The solar cell of claim 1, wherein the first contact pad and the second contract pad are made of a metal selected from the group consisting of aluminum, an aluminum alloy, copper, and a copper alloy.

4. The solar cell of claim 17 wherein the electrical trace is made of a metal selected from the group consisting of aluminum, an aluminum alloy, copper, and a copper alloy.

5. The solar cell of claim 1, wherein the electrical trace is made of a transparent conducting oxide.

6. The solar cell of claim 1, wherein the transparent, electrically conductive layer is made of a transparent conducting oxide.

7. The solar cell of claim 1, wherein the transparent, electrically conductive layer is made of a a material selected from the group consisting of indium tin oxide and zinc oxide.

8. The solar cell of claim 1, wherein the second electrical contact further includes a layer of insulation between the conductive trace and the lateral side of the photovoltaic Junction.

9. The solar cell of claim 1, wherein the first contact pad is a strip of electrically conductive material on the back face of the photovoltaic junction.

10. The solar cell of claim 1, wherein the second contact pad is a strip of electrically conductive material on the back face of the photovoltaic junction.

11. A solar cell, comprising:
a photovoltaic junction having a front face, a back face, and lateral sides;
a first electrical contact for the photovoltaic junction comprising a first electrically conductive contact pad located on the back face of the photovoltaic junction; and second electrical contact means for providing electrical contact to the photovoltaic junction, the second electrical contact means including a transparent, electrically conductive layer on the front face of the photovoltaic junction, and at least one electrically conductive second contact pad in electrical contact with the transparent, electrically conductive layer.

12. The solar cell of claim 11, wherein the second contact pad is on the back face of the solar cell, and the second electrical contact means further includes at least one electrically conductive trace extending from the electrically conductive layer on the front face of the photovoltaic junction, over at least one lateral side, and to the second contact pad on the back face of the photovoltaic junction, and a layer of insulation between the conductive trace and the at least one lateral side of the photovoltaic junction.

13. The solar cell of claim 11, wherein the second electrical contact means includes at least two electrically conductive second contact pads located on the back face of the photovoltaic junction, at least two electrically conductive traces extending from the electrically conductive layer on the front face of the photovoltaic junction, over at least two lateral sides, and to the second contact pads on the back face of the photovoltaic junction, and a layer of insulation between the conductive traces and the at least two lateral sides of the photovoltaic junction.

14. The solar cell of claim 11, wherein the second electrical contact means includes two electrically conductive second contact pads in the form of electrically conductive strips located at opposite sides of the back face of the photovoltaic junction, two electrically conductive traces extending from the electrically conductive layer on the front face of the photovoltaic junction, over oppositely disposed lateral sides, and to the two oppositely disposed second contact pads on the back face of the photovoltaic junction, and a layer of insulation between the conductive traces and the respective lateral sides of the photovoltaic junction over which they extend.

15. The solar cell of claim 11, wherein the transparent, electrically conductive layer is made of a transparent, electrically conducting oxide.

16. The solar cell of claim 11, wherein the transparent, electrically conductive layer is made of a material selected from the group consisting of indium tin oxide and zinc oxide.

17. A solar cell, comprising:
a photovoltaic junction having a front face and a back i`ace;
a first electrical contact for the photovoltaic junction comprising a first electrically conductive contact pad located on the back face of the photovoltaic junction; and a second electrical contact for the photovoltaic junction, the second electrical contact comprising a transparent, electrically conductive layer on the front face of the photovoltaic junction, and a second electrically conductive contact pad located on the front face of the photovoltaic junction, the second contact pad being in electrical contact with the transparent, electrically conductive layer.

18. The solar cell of claim 17, wherein the photovoltaic junction is selected from the group consisting of silicon, gallium arsenide, and gallium arsenide/germanium photovoltaic junctions.

19. The solar cell of claim 17, wherein the first contact pad and the second contract pad are made of a metal selected from the group consisting of aluminum, an aluminum alloy, copper, and a copper alloy.

20. The solar cell of claim 17, wherein the transparent, electrically conductive layer is made of a transparent conducting oxide.

21. The solar cell of claim 17, wherein the transparent, electrically conductive layer is from about 1 to about 2 micrometers thick.