Batteries: thermoelectric and photoelectric – Photoelectric – Cells
Reexamination Certificate
2000-04-03
2002-05-07
Diamond, Alan (Department: 1753)
Batteries: thermoelectric and photoelectric
Photoelectric
Cells
C136S244000, C136S255000, C257S461000, C257S465000, C257S466000, C257S437000, C438S080000, C438S098000, C438S072000, C438S081000
Reexamination Certificate
active
06384317
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to solar cells.
The present invention is also related to a cost-effective process of realization of solar cells.
TECHNOLOGICAL BACKGROUND
Most solar cells described in the prior art can be subdivided into several categories according to their general structure.
One of these categories is the group of the so-called back-contacted solar cells, meaning that both ohmic contacts to the two oppositely doped regions of the solar cells are placed on the back or non-illuminated surface of the solar cell. This concept avoids shadowing losses caused by the front metal contact grid on standard solar cells.
The most straightforward way to fabricate back contact solar cells is to place the carrier collecting junction between semiconductor regions of opposite doping close to the back surface of the cell (“back-junction” cell). The document “27.5-Percent Silicon Concentrator Solar Cells” (R. A. Sinton, Y. Kwark, J. Y. Gan, R. M. Swanson, IEEE Electron Device Letters, Vol. ED-7. No. 10, October 1986) describes such a device.
Since the majority of photons are always absorbed close to the front surface of the cell, the generated carriers in these regions have to diffuse through the entire base region of the cell towards the carrier collecting junction close to the back surface. For this concept, high quality material with minority carrier diffusion lengths longer than the cell thickness is needed, which makes this solution not applicable for most solar grade materials which generally have short diffusion lengths. Additionally, a perfect front surface passivation is required for cells having the carrier collecting junction close to the back surface.
The largest group of solar cells has the carrier collecting junction close to its front surface. The current from these solar cells is collected by a metal contact to the doped region on the front surface and by a second contact to the oppositely doped region on the back surface. Although this front grid structure can be optimized relatively easily in order to get high collection efficiencies, the trade off between resistance losses and shading losses necessitates a coverage of the front surface by 10-15% of the total area.
Another group of solar cells combines the two approaches. Such solar cells have both external contacts to the oppositely doped regions on the back surface and the collecting junction close to the front surface. The collected current from the front surface is lead through openings, which extend through the entire wafer, to the back surface. Using this structure, shading losses normally arising from the front metallization grid are greatly reduced.
Several patents make use of this approach.
Documents U.S. Pat. Nos. 4,227,942 and 4,427,839 disclose solar cell structures in which the metal contacts to both oppositely doped regions are placed on the back surface of the device. The connection to the front carrier collecting junction is realized by chemically etched vias which are arranged in an array. The metal grids and chemical etch mask are defined by photolithography. Photolithography, however, is an expensive processing step and difficult to implement into industrial solar cells production.
The document U.S. Pat. No. 4,838,952 discloses a similar structure wherein an array of holes is created with photolithographically defined areas using chemical etching. In this case, the holes do not extend from the top surface to the back surface of the device. They only extend from the back surface to the junction region. Due to the lower doping density at the junction region compared to the surface where the contacts are normally placed, the contact resistance is expected to be higher with this device if industrial metallization techniques such as screen printing are used. The disadvantages of photolithography also apply to this method.
The document U.S. Pat. No. 3,903,427 also describes a solar cell with an array of holes, machined by mechanical, electron beam or laser drilling, in order to lead the collected current from the front surface of the solar cell to the back surface. In this case, the metal contacts to the regions of opposite polarity are placed on the back surface, one above the other separated by a dielectric layer. This device makes it also necessary to have an insulating dielectric layer along the walls of the holes. This layer is difficult to combine with industrial metallization techniques such as screen printing metal paste and firing the metal paste which dissolves dielectric layers.
The document U.S. Pat. No. 4,626,613 discloses solar cells with both contacts on the back surface and an array of holes connecting front and back surface. The holes are used for conducting current from the front surface to the metal grid on the back surface of the appropriate polarity. The holes are machined by laser drilling or by scribing a set of parallel spaced grooves on the front and rear surface. The two sets of grooves on both surfaces are oriented perpendicularly so that after an appropriate etching process, holes are revealed at the points of intersection.
A similar structure is shown in U.S. Pat. No. 5,468,652, wherein the cell structure uses an array of laser drilled holes to conduct current collected on the front surface to the back surface where the metal contacts to the oppositely doped regions are placed. Although this latter case offers also some simplifications to cell processing compared to the ones suggested hereabove, there are still some common drawbacks of cell structures which make use of a large number of holes for electrically connecting the two surfaces of a cell.
In order to avoid resistive losses caused by current crowding effects within the heavier doped surface layer of the cell around the holes, the holes need to be spaced 1-1.5 mm to each other in both dimensions. On large area solar cells (10×10 cm
2
) a total number of more than 10000 holes would be necessary. Other difficulties arise from the metallization point of view. The close spacing of holes demands very narrow alignment tolerances for the two metal grids on the back surface. The large number of holes associated with the structures disclosed in the patents listed above makes these cell structures expensive and not well suited for mass production.
The document U.S. Pat No. 3,903,428 discloses a solar cell structure that uses one centrally located hole in combination with a metal grid on the cell's front surface to lead the collected current from the front to the back surface. The disclosed structure is best suited for round devices of small area due to increased resistive losses caused by current crowding round the centrally located hole. U.S. Pat. No. 3,903,428 also does not allow a second collecting junction to be placed on the back surface of the cells which would be possible with some of the structures discussed above.
The document JP-63-211773-A describes a solar cell structure where removing the external contact from the front surface increases the active area and makes both polarity contacts accessible from the back surface. Incident light generates electron-hole pairs in the bulk of the structure. Excess minority carriers diffuse towards the collecting junction formed by epitaxial growth at the front surface. Once they crossed the junction they can diffuse as majority carriers towards a metal contact, which is part of a conduction path towards external contacts at the back surface of the cell. The conduction path between the front and back surface is foreseen through a limited number of holes. The diffusion of the minority carriers through the whole wafer makes this approach difficult to use for lower quality materials. The distance a minority carrier can diffuse through the bulk region before it recombines is limited by the material quality. For high quality material, minority carriers can travel several times the base width before recombining. However, the diffusion length in low-grade material can be lower than the cell structure. In this case, carriers generated deep within t
Einhaus Roland
Kerschaver Emmanuel Van
Nijs Johan
Szlufcik Jozef
Diamond Alan
IMEC vzw
Knobbe Martens Olson & Bear LLP
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