THIN FILM SOLAR CELL

Abstract: A solar module of the type having at least two series connected solar cells which each include a transparent front-­ face electrode, a thin film photovoltaic device, and an aluminum back contact. A nickel film is provided over the aluminum back contact to provide improved resistance to degradation.

Description: This invention relates to a solar panel comprising thin film solar cells. The following U.S. patents, incorporated by reference: US-A-4,517,403 issued May 14, 1985 and US-A-4,584,427 issued April 22, 1986, provide a good background of the art of thin film solar cells and efforts made to develop commercially practical devices. For example, US-A-4,517,403 illustrates commercially practical series interconnection of thin film solar cells and US-A-4,584,427 provides an improvement which reduces internal resistance in individual cells. Commercially practical solar modules have been manufactured according to the teachings of these two patents. In the commercial devices, the stitch bars used to interconnect adjacent cells were formed by screen printing a paste including silver. The back contacts of cells were formed from sputtered aluminum. When this structure was scaled up to a large module, for example one foot wide by four feet long, a problem was encountered. When such large modules were tested according to Jet Propulsion Laboratory Block V test procedure, a significant reduction in module fill factor was experienced. As a result, it was apparent that some improvement to the method of manufacture or structure of the devices was needed to avoid this reduction of fill factor. US-A-4542578 concerns a thin film solar cell module, the back-side electrode of each solar cell comprising an aluminium film and a second film deposited thereon having a high moisture resistance e.g. titanium. According to the present invention, we have developed an improved solar cell module structure in which the back cell contact includes both a layer of aluminum deposited on the thin film photovoltaic device and a layer of nickel deposited on the aluminum film. The present invention may be better understood by reading the following detailed description of the preferred embodiments with reference to the accompanying single FIGURE which is a cross-sectional illustration of a portion of a thin film solar module according to the present invention. With reference now to the FIGURE, there is illustrated a basic solar module and cell structure according to the present invention. The structure is very similar to that described in US-A-4,517,403. The structure will be only briefly described herein and reference should be made to that patent for further detail. The primary structural member of the solar module illustrated in the FIGURE is a transparent glass substrate 10. On substrate 10 there is formed a transparent conductive oxide layer 12 which may consist primarily of tin oxide. As illustrated, the layer 12 is patterned to form separate front-face electrodes for each solar cell. For example, in the center of the FIGURE a complete front-face electrode 14 is illustrated. Along one edge of the front-face electrode 14 there is formed a cell interconnect bar or stitch bar 16. An active thin film photovoltaic device is formed as layer 18. In the preferred embodiment the device is a PIN silicon device as described in US-A-4,584,427 and a hydrogen glow step is used after the P layer deposition to reduce part of Contact 14. Back contacts include a first layer of aluminum 20. Layer 20 may be from 1000 to 5000 angstroms (10 angstroms = 1nm) thick and is preferably about 2000 angstroms thick. In accordance with the present invention the back contacts also include a layer 22 of nickel formed over film 20. Nickel layer 22 may be from 500 angstroms to 5000 angstroms thick and is preferably about 1200 angstroms thick. Layer 22 is preferably deposited by DC magnetron sputtering immediately after deposition of aluminum layer 20. The layer 22 is then patterned at the same time as layer 20. After patterning, the structure would be as illustrated at 24. Front-to-back electrode interconnection is preferably completed by application of a laser beam in the direction indicated by arrow 26. In practice the contact bar 34 is much thicker than illustrated and the laser beam is applied at essentially 90° to the substrate surface. This laser heating causes fusion of the region 28. The illustrated fused region 28 is highly conductive and provides an interconnection between the back contact 30 including both the aluminum and nickel films 20 and 22 and the front contact 32 of the adjacent cell through stitch bar 34. Solar panels constructed as illustrated in the FIGURE and a described above were tested according to the Jet Propulsion Laboratory Block V program and exhibited no degradation. The exact cause of the problem and the reason for the performance improvement are unknown. There are several possible explanations. The Jet Propulsion Laboratory Block V test program includes, among other tests, aging at 85°C and 85 percent relative humidity. It also includes 200 temperature cycles between -40°C and +80°C. We believe the degradation resulted from the temperature cycling and possibly the high humidity condition which may have affected the devices even though they were sealed between two glass sheets. The reduction in fill factor which occured during the test indicates that series resistance was increased at some point in the structure. It is possible that a combination of high temperature and humidity caused some type of interaction between the aluminum film 20 and some portion of the stitch bar material 34, possibly the silver. The precise mechanism for improvement of the structure by adding the nickel layer 22 is also unknown. It is possible that the nickel film 22 provides an improved seal to moisture over aluminum film 20. This explanation assumes, of course, that it is the presence of water vapor which caused increased resistance in the first place. It may also be possible that the presence of nickel in the fused region 28 produces a more stable alloy which, therefore, does not degrade as a result of the environmental testing program. While the layer 22 has been described as a nickel layer, the material used in the test devices included seven percent vanadium with the remainder being nickel. The material was deposited in a planar magnetron sputtering device such as that described in US-A-4,465,575 issued August 14, 1984. The inclusion of vanadium in a target eliminates magnetic characteristics of nickel which would otherwise interfere with such a sputtering process. We would prefer to use a less expensive nickel target, for example, pure nickel or an alloy of nickel and chromium. While the present invention has been illustrated and described with respect to particular structures and method of manufacture, it is apparent that various changes and modifications may be made therein within the scope of the appended claims.

Claims: A solar panel including at least two solar cells, said panel comprising a transparent insulating substrate (10), a patterned transparent conductive layer (12) forming separate front-face electrodes (14) for each solar cell, a thin film photovoltaic device (18) on said front-face electrode, a back electrode (30) comprising a layer of aluminium (20) on said photovoltaic device and a layer comprising nickel (22) on said aluminium layer, wherein said layer comprising nickel has the same pattern as the aluminium layer and forms therewith separate back electrodes for each solar cell and wherein an electrical interconnection (28, 34) is provided between the back electrode of one solar cell and the front-face electrode of an adjacent solar cell. A solar panel according to Claim 1, wherein said thin film photovoltaic device comprises one or more layers of amorphous hydrogenated silicon or alloys thereof. A solar panel according to any one of the preceding claims, wherein said back-face electrode is from about 100 nm to about 500 nm thick. A solar panel according to any one of the preceding Claims, wherein said layer comprising nickel is from about 50 nm to about 500 nm thick. A solar panel according to any of Claims 1 to 4, wherein a portion of said back electrode of each cell is overlapping a portion of the front-face electrode of an adjacent cell and wherein said interconnection comprises a conductive bar (16, 34) positioned on that portion of each front-face electrode which is overlapped by a portion of an adjacent cell back electrode, said conductive bar including silver. A solar panel according to Claim 5, wherein a portion of said back-electrode is fused with a portion of said conductive bar to form a low resistance interconnection between adjacent cells.

Inventors: JESTER, THERESA L., TANNER, DAVID P., YIN, MING-JAU
Applicants: SIEMENS SOLAR INDUSTRIES L.P.
Application year: 1988
Main classification: H01L31
IPC codes: H01L31, H01L27