Background

Crystalline solar cells for all applications (terrestrial, space, unmanned aerial vehicles, etc.) are becoming thinner and thinner in order to save cost, reduce weight and in some cases improve performance. In addition, solar modules (assemblies of cells) are moving to thinner and lower weight designs. These changes mean that solar cells will be subjected to higher stresses, due loads from wind, snow, handling, temperature extremes, etc., and the cells, being thinner, are more likely to crack. The performance of cracked cells degrades when the crack in the semiconductor propagates through the metallization on the cell. When this happens, the cell can suffer from partial or complete electrical loss as seen in Figure A. 

FIGURE A.   
  
   
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    Electroluminescent image of a silicon solar cell with electrically bridged cracks (A), partially bridged cracks (B) and fully isolating cracks (C). Cracks that start as type “A” can progress to type “B” and “C” cracks as a result of additional stress. From Kontages, Solar Energy Materials 2011.

FIGURE A. 

Electroluminescent image of a silicon solar cell with electrically bridged cracks (A), partially bridged cracks (B) and fully isolating cracks (C). Cracks that start as type “A” can progress to type “B” and “C” cracks as a result of additional stress. From Kontages, Solar Energy Materials 2011.

A study of terrestrial solar panel degradation over time shows that cell cracking and delamination are the main degradation mechanisms (see blue shaded area in figure below from IEA PVPS Review of Failures of Photovoltaic Modules 2014). Reducing the Levelized Cost of Electricity (LCOE) from solar power is of critical importance to improving the adoption of solar energy. The LCOE can be reduced by various methods, including lowering solar panel costs and improving efficiency, but one of the most important methods is to increase the panel life from the current 25 years to 40 or even 50 years. To enable this increase in panel life, addressing degradation from cell cracking is extremely important.

FIGURE B.   
  
   
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  Solar panel performance degradation mechanisms as a function of time.

FIGURE B. Solar panel performance degradation mechanisms as a function of time.


Osazda Energy Technology

To address cell crack performance degradation, Osazda Energy has developed a novel Metal Matrix Composite (MMC) metallization in which low-cost, multi-walled Carbon Nano-Tubes (CNT) are added to conventional solar cell metallization. These CNT have demonstrated the ability to electrically bridge cracks in the cell 10x larger than conventional metallization. And if the CNT disconnect, they have shown the ability to self-heal. The micrographs (shown in Figure C) display CNT electrically bridging fractured solar cell metallization.

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  Scanning Electron Microscope images of CNT bridging cracks in solar cell metallization

FIGURE C. Scanning Electron Microscope images of CNT bridging cracks in solar cell metallization

To demonstrate the effectiveness of the Osazda Technology, solar cells with conventional metallization and Osazda’s MMC metallization were fabricated and then intentionally cracked. The cell with conventional metallization (Figure D) lost ~50% of its initial performance after cracking (note large dark area in post crack electroluminescence image). The cell with Osazda MMC metallization, on the other hand, showed essentially no loss in performance (Figure E). 

FIGURE D. Cell with conventional metallization            

FIGURE D. Cell with conventional metallization            

FIGURE E. Cell with Osazda Energy MMC metallization

FIGURE E. Cell with Osazda Energy MMC metallization

Osazda Technologies MMC metallization can be applied to virtually any solar cell using a variety of low-cost, high-throughput processes, including screen printing, electroplating as well as evaporation. Contact us to discuss how our MMC metallization can extend the life of your solar cells.


Read Our White Papers

http://ieeexplore.ieee.org/document/7286719/

http://ieeexplore.ieee.org/document/7749724/

Abstract: In this work, we present the use of a simple, cost-effective, and manufacturable method of depositing carbon nanotubes onto metal films to create metal matrix composite gridlines for photovoltaic cells. Carbon nanotubes are deposited using a spray coating method to create layer-by-layer microstructure composites. Initial strain failure tests show the ability of composite lines to remain electrically connected with fractures up to 35μm-wide, where carbon-nanotubes electrically bridge the gap. The metal-carbon-nanotube composites are electrically characterized though I-V sweeps. The composite lines can carry current densities ranging from 500 to 2500 A/cm2.

Published in: Photovoltaic Specialists Conference (PVSC), 2016 IEEE 43rd

Issued Patent