Photovoltaic cell with buffer zone

Abstract

Systems and methods that provide a barrier for protection of active layers associated with a vertical multi junction (VMJ) photovoltaic cell. Buffer zone(s) in form of an inactive layer(s) arrangement safe guard the active layers against induced stress or strain resulting from external forces/thermal factors (e.g., welding). The buffer zone can be in form of a rim on a surface of an end layer of a cell unit, to act as a protective boundary for such active layer, and to further partially frame the VMJ cell for ease of handling and transportation.

Description

CROSS REFERENCE TO RELATED APPLICATIONS [0001]This application is a continuation of U.S. patent application Ser. No. 12 / 536,987 filed on Aug. 6, 2009 entitled “PHOTOVOLTAIC CELL WITH BUFFER ZONE”, which claims the benefit of U.S. Provisional Application Ser. No. 61 / 088,936 filed on Aug. 14, 2008 entitled “SOLAR CELL WITH BUFFER ZONE” the entireties of which are hereby incorporated by reference.BACKGROUND [0002]Limited supply of fossil energy resources and their associated global environmental damage have compelled market forces to diversify energy resources and related technologies. One such resource that has received significant attention is solar energy, which employs photovoltaic systems to convert light into electricity. Typically, photovoltaic production has been doubling every two years, increasing by an average of 48 percent each year since year 2002, making it the world's fastest-growing energy technology. By midyear 2008, estimates for cumulative global solar energy product...

AI Technical Summary

Benefits of technology

[0008]The subject innovation supplies a buffer zone(s) at end layers of a high voltage silicon vertical multi junction (VMJ) photovoltaic cell, to provide a barrier that protects the active layers while providing an ohmic contact. Such buffer zone(s) can be in form of an inactive layer(s) arrangement that is additionally stacked upon and / or below end layers of the VMJ cell. The VMJ cell itself can include a plurality of cell units, wherein each cell unit employs several active layers (e.g., three) to form a PN junction and a “built-in” electrostatic drift field (which enhances minority carrier movement toward the PN junction.)

[0009]As such, various active layers such as nn+ and / or p+n junctions located at either ends of a VMJ cell (and as part of cell units thereof) can be safeguarded against adverse forms of stress and / or strain (e.g., thermal / mechanical compression, torsion, moment, shear and the like—which can be induced in the VMJ during fabrication and / or operation thereof.) Moreover, the buffer zone can be formed via materials that have substantially low resistivity ohmic contact, either metals or semiconductors, such that it will not contribute any substantial series resistance loss in the photovoltaic cell at operating conditions. For example, the buffer zone can be formed by employing low resistivity silicon wafers that are p-type doped, so that when using other p-type dopants such as aluminum alloys in manufacturing the VMJ photovoltaic cell, it will mitigate a risk of auto-doping (in contrast to employing n-type wafers that can create unwanted pn junctions—when an object is to create a substantially low resistivity ohmic contact. It is to be appreciated that the subject innovation can be implemented as part of any class of photovoltaic cells such as solar cells or thermophotovoltaic cells. Additionally, aspects of the subject innovation also can be implemented in other class(es) of energy-conversion cells such as betavoltaic cells.

[0010]In related aspects, the buffer zone can be in form of a rim on a surface of an end layer of a cell unit, which acts as a protective boundary for such active layer and further frames the VMJ cell for ease of handling and transportation. Likewise, by enabling a secure grip to the VMJ cell, such rim formation also eases operation related to the anti reflective coating (e.g., coating can be applied uniformly when the cell is securely maintained during operation, such as by mechanically clamping thereon.) Moreover, the buffer zones (e.g., the inactive layers positioned at ends of the VMJ) can be physically positioned adjacent to other buffer zones during the deposition—and hence any unwanted dielectric coating material that inadvertently penetrates down onto the contact surfaces can be readily removed without damaging active unit cells. The buffer zone can be formed from substantially low resistivity and highly doped silicon (e.g., a thickness of approximately 0.008″) Such buffer zone can subsequently contact conductive leads that partition or separate a VMJ cell from another VMJ cell in a photovoltaic cell array.

[0011]According to a further aspect, the buffer zone can be sandwiched between an electrical contact, and the active layers of the VMJ cells. Moreover, such buffer zones can have thermal expansion characteristics that substantially match those of the active layers, hence mitigating performance degradation (e.g., mitigation of stress / strain caused when leads are welded or soldered in manufacturing.) For example, highly doped low resistivity silicon layers can be employed, which match the thermal expansion coefficient (3×10−6 / ° C.) of all active unit cells. Accordingly, substantially strong ohmic contacts can be provided to the active unit cells, which additionally mitigate stress problems caused by welding / soldering and / or from mismatched thermal expansion coefficients in contact materials. Other examples include introducing metallic layers, such as tungsten (4.5×10−6 / ° C.), or molybdenum (5.3×10−6 / ° C.), which are chosen for thermal expansion coefficients substantially similar to the active silicon (3×10−6 / ° C.) p+nn+unit cells. The metallization applied to the outer layers of the low resistivity silicon layers of the buffer zone, or to the metallic layers of electrodes that are alloyed to the active unit cells, can be welded or soldered without introducing deleterious stress to the high intensity solar cell or photovoltaic cell—wherein such outer layers serve as ohmic contacts; rather than segments of unit cells in series with the other unit cells.

[0012]Various aspect of the subject innovation can be implemented as part of wafers having miller indices (111) for orientation of associated crystal planes of the buffer zones, which are considered mechanically stronger and slower etching than (100) crystal orientation silicon typically used in making active VMJ unit cells. Accordingly, low resistivity silicon layers can have a different crystal orientation than that of the active unit cells, wherein by employing such alternative orientation, a device with improved mechanical strength / end contacts is provided. Put differently, edges of (100) orientated unit cells typically etch faster and essentially round off corners of the active unit cells with such crystal orientation—as compared to the inactive (111) orientated end layers—hence resulting in a more stable device structure with higher mechanical strength for welding or otherwise connecting end contacts.

Problems solved by technology

Moreover, the buffer zone can be formed via materials that have substantially low resistivity ohmic contact, either metals or semiconductors, such that it will not contribute any substantial series resistance loss in the photovoltaic cell at operating conditions.

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