Development of Silver-Carbon-Nanotube Metal Matrix Composites for Metal Contacts on Space Photovoltaic Cells

The advanced solar cells used in space vehicles today are rapidly moving towards thin-film-based inverted metamorphic multijunction (IMM) solar cells mounted on flexible substrates.  However, the IMM cells are more prone to cracking than state-of-the-art triple junction cells.  The cell cracking can lead to metal contact failure on IMM cells, compromising the power generation.  To mitigate the power loss and increase the lifetime of IMM cells, silver metal films imbedded with carbon nanotubes (CNTs), otherwise known as metal matrix composites, have been developed and investigated for the reinforced mechanical strength against stress-induced cracking.  We have primarily focused on (1) surface functionalization of CNTs to make their surface more hydrophilic and wetting to metals, (2) optimization of a cyanide-free electrochemical deposition of silver, (3) electrochemical deposition, drop casting and nanospreader technique to control the composite microstructure, and (4) mechanical and electrical characterization of the composite films.  We observe that carboxylation of CNTs produces a stable, homogeneous suspension of negatively charged CNTs at pH > 6.  Lustrous-mirror-finish silver films are also successfully deposited, using a commercial cyanide-free silver-plating solution with precise control of current density.  Currently, one of the microstructures being explored is a silver-carbon-nanotube layer-by-layer structure, where the surface coverage of CNTs is an important parameter that directly affects the CNT packing fraction and metal intercalation through the CNT network.  We quantify the CNT surface coverage as a function of different deposition variables by digitally analyzing scanning electron microscopy images.  In this presentation, we will further discuss how this surface coverage correlates to the mechanical and electrical properties of the MMC films.  We characterize the mechanical properties, using nanoindentation and strain failure tests.  The initial nanoindentation analysis reveals that the composite film has a lower elastic modulus (10 GPa) than pure silver (73 GPa), which is contrary to our initial prediction given the high elastic modulus of CNTs (1000 GPa).  The lower elastic modulus is attributed to the electroplating process of silver, in which hydrogen is incorporated and trapped within the composite.  Our finite element analysis also corroborates this speculation, where the elastic modulus near 10 GPa is predicted with approximately 4% void fraction.  While the composite elastic modulus is lower than that of pure silver, the strain failure tests show that carbon nanotubes can bridge 20 to 50-μm-wide microcracks, maintaining electrical conductivity of the composite