Abstract
Advanced solar cells are moving to ever thinner formats in order to save mass and in some cases improve performance. As cells are thinned, the possibility that they may fracture or cleave due to mechanical stresses is increased. Fractures of the cell can degrade the overall device performance if the fracture propagates through the contact metallization, which frequently occurs. To address this problem, a novel semiconductor metallization system based on multi-walled carbon nanotube (CNT) reinforcement, termed metal matrix composite (MMC) metallization is under investigation. Electro-mechanical characterization of MMC films demonstrate their ability to provide electrical conductivity over >40 micron wide cracks in the underlying semiconductor, with the carbon nanotubes bridging the gap. In addition, these materials show a “self-healing” behaviour, electrically reconnecting at ~30 microns when strained past failure. Triple junction (TJ) space cells with MMC metallization demonstrated no loss in J sc after intentional fracture, whereas TJ cells with conventional metallization suffer up to 50% J sc loss.
Highlights
Advanced crystalline solar cells used on-earth and inspace are rapidly moving towards thinner devices
To mitigate the power loss and increase the lifetime of solar cells, we have developed and investigated silver (Ag) metal films imbedded with single and multi-walled carbon nanotubes (CNTs), known as metal matrix composites (MMCs), for the reinforced mechanical strength against stress-induced cracks [3, 4, 5]
MMC films have been deposited by either metal electroplating combined with CNT spray coating or by metal evaporation combined with CNT film transfer, some preliminary investigation of screen printing MMC films has been conducted [7]
Summary
Advanced crystalline solar cells used on-earth and inspace are rapidly moving towards thinner devices. Numerous techniques exist today to deposit bulk MMCs, including powder metallurgy, melting and solidification, and electrochemical routes [6] In this investigation, MMC films have been deposited by either metal electroplating combined with CNT spray coating or by metal evaporation combined with CNT film transfer, some preliminary investigation of screen printing MMC films has been conducted [7]. MMC electro-mechanical crack bridging capability was examined using a designed test apparatus, affectionately called the RACK (Resistance Across Cleaves and cracKs). For this testing, MMC grid lines were formed on a semi-insulating indium phosphide (InP) substrate. Unlike MMC films, the Ag gridlines did not withstand the initial crack generation (~4-μm-wide), and the electrical connection is immediately and irrecoverably lost
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