Tech Briefs

Replacing traditional electrodes in IMM solar cells with those made from carbon nanotube metal matrix composites could improve reliability.

In order to address issues related to mechanical and vibrational stresses that are commonly experienced in mounting, launch, and deployment of spacecraft, metal matrix composite (MMC) electrodes were fabricated with carbon nanotubes (CNTs) as reinforcement. The research plans were centered on first developing and selecting appropriate processes for fabricating CNT-MMCs based on characterization of their material properties, followed by optimizing MMC designs based on electrical testing under stress. Efforts in the program were also directed toward implementing the CNT-MMCs into solar cell device processing in a process compatible with standard clean room procedures.

SEM images of (a) the interface of the 3 layers in the Ag/SWCNT matrix, and (b) SWCNTs bridging 3 µm gaps between the silver created by fracturing the substrate.

Layered style MMCs in which CNTs are embedded between Ag layers were rigorously characterized in terms of microstructure, mechanical, and electrical performance under stress. SEM (Scanning Electron Microscopy) analysis of fractures and the grain structure were used to understand the effects of CNT layer thickness on the over-coated Ag, and to observe CNTs mechanically bridging gaps in Ag upon fracture. In the process, an innovative method was used to fabricate free-standing Ag-single wall carbon nanotube (SWCNT) MMC thin films and the resulting films were characterized to understand their response to applied tensile stress through traditional tensile testing.

The results were used to understand the appropriate CNT layer thickness needed to achieve reinforcement and demonstrated improvements in the mechanical properties (i.e. toughness, strain-to-failure) as compared to pure Ag electrodes. Improvements in mechanical properties of the MMCs occurred as the SWCNT layer thickness was increased to 20 nm, exhibiting a 2.5x increase in toughness as compared to pure Ag electrodes. Higher SWCNT loadings led to degradation in performance.

SEM analysis and mechanical results suggest that the MMCs transition from a reinforced structure to discrete layers between 20 nm and 50 nm SWCNT layer thicknesses. The highest performing MMC with 20 nm SWCNT layer corresponds to ~0.7 vol. % SWCNTs in the composite. The SEM analysis shows evenly distributed SWCNTs protruding from the fracture edges, which would likely prevent catastrophic failure under less dramatic stresses. The free- standing film mechanical testing and grain structure analysis provided the important first step toward understanding the optimum SWCNT loading in the MMCs and provides a fundamental basis for which further studies can expand and develop additional improvements for advanced MMC electrodes.

Fabrication processes and electrical testing procedures were also developed for assessing the electrical performance of CNT-MMC grid finger structures under stress. The grid finger structures utilized SWCNTs, multi-walled carbon nanotube (MWCNTs), or combinations thereof, and the processing techniques were all compatible with standard microelectronic engineering processes. The results of electrical testing and SEM analysis of the MMCs under stress suggest that SWCNT-MMC electrodes give an advantage over pure Ag electrodes. The SWCNTs are limited in crack-bridging capabilities, but are able to maintain electrical performance up to ~6 μm gap widths due to SWCNT length. Incorporation of much longer MWCNTs into the SWCNT-MMCs results in ~4× to 5× increase in crack-bridging capabilities, which may be useful if it is found that some solar cell or flexible electronics applications suffer from gaps larger than 6 µm. Additionally, assessment of an alternative to the large-diameter chemical vapor deposition (CVD) MWCNTs used in the initial hybrid RACK electrical tests resulted in the selection of MWCNT materials from Nanocomp Technologies, Inc., which had small diameters, low defect content, and responded positively to traditional chemical and thermal purification techniques.

Finally, the CNT MMC fabrication process developed by RIT (Rochester Institute of Technology) was applied to commercial space solar cells, with demonstrations of both MWCNT and SWCNT MMC electrodes as top contacts. The as-fabricated cells all suffered from electrical shorting (between top and bottom cells) that was easily addressable by gently cleaning the edges with sandpaper, an issue that would not arise in standard production as the cells are not cleaved until after the top metallization is complete.

Cells performed relatively well, operating at ~90% of the Jsc specification and just below the Voc specification. The SWCNT MMC performed the best, reaching an efficiency of ~22%, compared to the typical ~29% factory specification. It is likely that the performance can be greatly improved by refining the lithography and other fabrication processes.

This work was done by Brian J. Landi and Nathanael Cox of the Rochester Institute of Technology for the Air Force Research Laboratory. For more information, download the Technical Support Package (free white paper) here under the Materials category. AFRL-0268