Tensile strength of thin film silver-carbon nanotube metal matrix composites for IMM solar cell electrode applications

IMM solar cell materials often suffer from cracking during a traditional life cycle that includes thermal cycling, shock and vibration, as well as other mechanical stressors. These cracks can limit or destroy the effectiveness of the cell by breaking electrical connections to certain areas of the cell. This work investigates the mechanical properties of carbon nanotube metal matrix composite (MMC) thin films to be used as the grid fingers of the cells. The study fabricates free standing thin films to characterize the pure material properties. The study finds that the grain size of the top Ag surface decreases from 180 to 90 nm with increasing areal densities of SWCNTs from 0 to 10 μg/cm
²
, respectively The samples are characterized by tensile testing, which shows that as-deposited samples progressively decrease in strength with increasing SWCNT loading. Annealing the samples results in a reduction of tensile strength for all samples. However, the reduction is less significant when low areal density films are incorporated into the composite. This leads to a 10% increase in strength compared to the control sample containing only annealed Ag (no SWCNTs). The volume percentage of low areal density thin film structures was increased by thinning Ag layers and increasing the number of SWCNT layers. The results show that increasing the volume percentage of SWCNTs in the composite leads to improvements in mechanical strength for annealed samples.