JPH05267697A - Solar cell with front surface thin-film contacting part - Google Patents

Abstract

PURPOSE: To obtain a solar cell for which light energy loss caused by a front surface collector electrode can be reduced. CONSTITUTION: A solar cell is provided with a photoelectromotive junction part 26, a first conductor contacting pad 28 comprising a first electrical connection part located on a back surface 32 of the photoelectromotive junction part and a second electric connection part 30 comprising a photo-collecting electrode located on a front surface 24 of the photoelectromotive junction part. The second electrical connection part 30 includes a transparent conductor layer 40 on the front surface of the photoelectromotive junction part, second conductive contacting pads 46 and a conducting trace 48 which extends from the conductor layer 40 on the front surface of a photocell across the side surface 42 to the contacting pads 30, on the back surface 32 of the photoelectromotive junction part.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to solar cells, and more particularly to electrical connections for solar cells which improve their efficiency.

[0002]

Semiconductor solar cells are used to convert light energy into useful voltages and currents. In essence, a typical semiconductor solar cell includes a photovoltaic junction having an interface between a n-type and a p-type transparent semiconductor material. Irradiation of light onto the semiconductor material adjacent to the interface produces new hole-electron pairs other than those present therein, and a small number of charge carriers move in opposite directions across the interface. Minority carrier flow is not compensated, resulting in a net charge.

[0003]

Useful currents are obtained from the photovoltaic junction of the external electrical circuit by making ohmic contacts to the material on both sides of the photovoltaic junction. That is, the first contact portion is formed on the back surface of the photovoltaic junction (that is, in the direction opposite to the sun) and the second contact portion is formed on the front surface (in the direction of the sun). The backside first contact is easily formed by depositing a layer of conductive material on the backside of the photovoltaic junction.

The second contact on the front surface can be formed by depositing a grid pattern of conductors on the front surface of the photovoltaic junction. The grid pattern conducts charges from the front surface of the photovoltaic junction to an external circuit to complete the electrical circuit with the first contact, so that the solar cell can generate useful current.

The problem with this method is that the sun facing grid pattern of the solar cell shields one part of the photovoltaic junction and reduces its efficiency. In a typical solar cell formed as described above, the shading effect reduces the output current and efficiency of the solar cell by about 5%. Reduce, preferably eliminate, the shading of the front grating
There is a need for improved designs of solar cells that can use front-contact and back-contact methods.

[0006]

SUMMARY OF THE INVENTION The present invention provides front contact and back contact solar cells that eliminate the need for a grid pattern on the front surface of a photovoltaic junction. As a result, the efficiency of the solar cell is not reduced by the shading effect characteristics of the grid pattern on the front side of the photovoltaic junction. The electrical resistance of the cell is reduced by diffusing charge carriers through the semiconductor material into the front electrical connections.

According to the invention, the solar cell is
And a photovoltaic junction having a side surface. The first electrical connection of the photovoltaic junction has a first electrically conductive contact pad located on the back surface of the photovoltaic junction and a second electrical connection of the photovoltaic junction. The second electrical connection part is a transparent conductive layer on the front surface of the photovoltaic junction, and the second electrical connection part is located on the back surface of the photovoltaic junction.
And conductive traces extending from the conductive layer on the front side of the photovoltaic cell across the sides to the contact pads on the back side of the photovoltaic junction. The conductive traces can be composed of any conductive material, such as the same material as the conductive layer or metal. An insulating layer is preferably located between the conductive traces and the sides of the photovoltaic junction. At least two contact pads are preferably provided on the back side of the photovoltaic junction and two traces extending from the front side.

In another concept, a solar cell comprises a photovoltaic junction having a front surface and a back surface and a first electrical connection including a first conductive contact pad located on the back surface. A second electrical connection to the photovoltaic junction is a second transparent electrically conductive layer on the front surface of the photovoltaic junction and a second electrically connected to the transparent electrically conductive layer located on the front surface of the photovoltaic junction. Of conductive contact pads.

The present invention is operable with any type of photovoltaic junction morphology and material. Commonly available photovoltaic materials include silicon, gallium arsenide, and gallium arsenide germanium arsenide, although other materials can be used in connection with the present invention. The transparent conductive layer is preferably a thin layer of a transparent conductive oxide such as indium tin oxide or zinc oxide. Eliminates the need for front collector grid lines that shields one part of the photovoltaic junction and reduces its output and overall efficiency by using a transparent conductive layer on the sun-facing part of the photovoltaic junction To do.

The present invention provides an improved solar cell which reduces efficiency losses due to the front electrical connections. The efficiency loss due to the additional layers on the front side of the photovoltaic junction is minimal. The method of the present invention produces a net efficiency gain of about 5% at the output of the solar cell.

[0011]

1 shows a solar cell 20 of the present invention and its power generation mode. Rays 22 from the sun or the like illuminate a front surface 24 of a solar cell 20, which includes a photovoltaic junction 26 composed of a transparent semiconductive material. Electrical connections 28,30 are on the back 32 of the solar cell 20
Electrical leads 34 connect electrical connections 28, 30 to an external load 36. As a result of the photovoltaic action of the photovoltaic junction 26, charge carriers are generated at the photovoltaic junction 26, the front surface 24 and the back surface depending on the type and form of the photovoltaic junction 26.
Move to 32. These charge carriers are provided to the load 36 through leads 34 to provide useful current. The principles and operation of photovoltaic junction 26 used in solar cell 20 are well known in the art. Conventional photovoltaic cells are typically composed of doped silicon, gallium arsenide, or gallium germanium arsenide. The present invention is not limited to that used by a particular type of photovoltaic junction, but can be operated by any other type already known or discoverable.

FIG. 2 shows the solar cell 20 in detail. The first electrical connection 28 made of a conductive material such as aluminum, copper, an aluminum alloy, or a copper alloy has a photovoltaic junction.
It is arranged as a thin conductive contact pad 38 on the back 32 of the 26. Contact pad 38 is typically deposited by electrodeposition, sputtering or other convenient method and has a thickness of about 3 micrometers. The contact pad 38 is configured to cover a large portion of the back surface 32 to efficiently collect charge carriers migrating toward the back surface 32.

The second electrical connection 30 includes a plurality of elements. The transparent conductive layer 40 is deposited over substantially the entire front surface 24 of the photovoltaic junction 26. This transparent conductive layer 40 collects the charge carriers from the front surface 24 without the opaque metal grid or other collecting electrodes that tend to allow the light rays 22 to reach the photovoltaic junction 26 and partially block the photovoltaic junction. It is possible.

While any transparent conductive material can operate as layer 40, the presently preferred material for layer 40 is indium tin oxide (ITO) or zinc oxide, which is transparent and of suitable thickness. In the preferred embodiment, layer 40 is of a thickness that reduces the emitter sheet resistance to about 3-5 ohms / square and is composed of a transparent conductive oxide (TCO) such as ITO or zinc oxide. This thickness is typically about 1 to 2 micrometers. Sputtering of such thin layers of oxide is well known in the art.

By comparison, the sheet resistance of photovoltaic junctions is typically about 400 ohms / square. The resistance is reduced to about 10 ohms / square by applying an optimized pattern of front collector grid lines of conventional design. Coatings of ITO or zinc oxide reduce the sheet resistance to 3-5 ohms / square, with sufficient reduction to produce reduced resistance of photovoltaic cells and improved current flow.

It has been mentioned above that the photovoltaic junction 26 has a front surface 24 (in the direction of the sun) and a back surface 32 (in the direction away from the sun). In addition, photovoltaic junction 26 has at least one side surface 42 extending between front surface 24 and back surface 32. In a typical configuration, the photovoltaic junction 26 is rectangular when viewed from the rear (see FIG. 3) or the front (see FIG. 4), ie it has four sides 42.

In solar cell 20, insulating layer 44 is provided on at least one side surface 42, and is preferably provided on two opposing side surfaces 42 as shown. Insulation layer 44
Is also present in the portion of the back surface 32 of the area underlying the second conductive contact pad 46. Insulating layer 44 is any suitable insulator. For example, in the case of silicon photovoltaic junction 26, insulating layer 44 can be provided as SiO _x , usually by masking and oxidizing the appropriate regions of photovoltaic junction 26, as well as different insulating layers. Also applicable.

Conductive traces 48 are deposited over the insulating layer 44 on the sides 42 so as to extend from the transparent conductive layer 40 to the second contact pads 42. The charge collected from the front surface 24 of the photovoltaic junction 26 is transferred by the trace 48 to the back surface 32 of the photovoltaic junction 26.
To a second contact pad 46 located at. The traces 48 can be composed of any conductive material, for example, metal such as aluminum, copper, aluminum alloys, copper alloys. The traces 48 can also be composed of the same material as the transparent conductive layer 40, typically a transparent conductive oxide such as ITO or zinc oxide. Second contact pad 46
Can be composed of a conductive material such as aluminum, copper, aluminum alloys, copper alloys. Such metals are readily deposited as thin layers by vapor deposition or sputtering. The two contact pads 38,46 are preferably layers about 3 micrometers thick. 2 to 4
The solar cell 20 shown in FIG. 1 is connected to an external circuit as shown in FIG.

The method of the present invention has the important advantage of completely removing the opaque grid lines from the front surface of the solar cell. Thus, no cells are shaded by the photovoltaic junction, resulting in solar cells typically being about 5% more efficient than similar cells with conventional front grid collector electrodes. Since the electrical resistance of the conductive layer 40 is only 3-5 ohms / square in the preferred case, the cell is approximately 2 without a collector grid on its front surface.
Widths on the order of centimeters can be constructed. If a larger cell is constructed, thin middle collector grid lines can be provided in front of such a design with a spacing of about 2 centimeters.

FIG. 5 is a side view of another form similar to the configuration of FIG. 2 using these principles. The solar cell 60 has a photovoltaic junction 62 with a layer 64 of transparent conductive material on the front surface 66. The photovoltaic junction 62 and layer 64 are the same as previously described.
A second metal contact pad 68 that acts as a front contact pad 68.
(Similar to the function of the second contact pad in the embodiment of FIGS. 1-4) is located at the edge of the front face 66 and the lateral dimensions of the front face are less than about 2 cm in all directions with respect to the grid. large. The first contact pad 70 is disposed on the back surface 72 of the battery 60. Contact pads 68, 70 provide external electrical connections to solar cell 60.

A solar cell having the morphology shown in FIG. 5 was processed to show the possibility of using the transparent conductive layer 40 as a front collector. The photovoltaic junction is a 200 micrometer N-type gallium arsenide substrate, 1 micrometer N-type gallium arsenide buffer, 3 micrometers N-type gallium arsenide base, 0.5 micrometer N-type gallium arsenide base. Layers of emitters from the back to the front. Silica window about 0.05 μm thick and P ⁺ about 0.01 μm thick A layer of type gallium arsenide was placed over these active devices. A layer of zinc oxide or ITO of about 2 micrometers was placed on the cell as a conductive layer. The cell characteristics were measured and cells using both types of conductive oxide coatings were operational. Batteries with a conductive layer of zinc oxide achieved an efficiency of 14.5% in the first test.

Although specific embodiments of the invention have been described in detail for purposes of illustration, various modifications can be made without departing from the scope of the invention. Therefore, the present invention is limited only by the appended claims.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a solar cell and its electrical connection.

FIG. 2 is a side view of the solar cell of the present invention.

FIG. 3 is a plan view of the solar cell viewed from the rear.

FIG. 4 is a plan view of the solar cell viewed from the front.

FIG. 5 is a side view of another embodiment of the solar cell.

[Explanation of symbols]

20 ... Solar cells, 28,30 ... Electrical connections, 36 ... Loads, 38,46 ...
Contact pad, 40 ... Conductive layer, 42 ... Side, 48 ... Conductive trace.

Claims (2)

[Claims] 1. A first electrical connection of a photovoltaic junction comprising a photovoltaic junction having a front surface, a back surface, and a side surface, and a first conductive contact pad located on the back surface of the photovoltaic junction. A transparent conductive layer positioned on the front surface of the photovoltaic junction; and a transparent electrically conductive layer that is in contact with the photovoltaic junction. A solar cell comprising one or more second conductive contact pads in electrical contact with a conductive layer. 2. The second contact pad is located on the back surface of the solar cell, and the second electrical connection is further from the conductive layer on the front surface of the photovoltaic junction over one or more sides. The solar cell of claim 1, including one or more conductive traces extending to the second contact pad on the back surface of the junction.