The Impact of Mechanical Defects on the Reliability of Solar Cells in Aerospace Applications

A stable supply of electrical power is crucial for the success of space missions. Photovoltaic arrays are the most common means of in-orbit energy generation. Mechanical solar cell defects have the potential to impact their reliability considerably. In order to address this question, the environmental loads encountered in space were simulated experimentally. Five miniature solar arrays, featuring a total of ap200 cells with representative mechanical defects, were built and subjected up to 12 000 simulated eclipse phases. ap400 cell cracks on actual flight panels were monitored throughout sine vibration and acoustic noise testing to cover the mechanical loads during launch of a solar array. Electroluminescence imaging and pulsed solar simulator measurements were used to record crack patterns, to track their evolution and to quantify their electrical impact. The original size of the crack and the number of thermal cycles were identified as the main factors that determine crack growth. The number of crack growth events (CGE) per cycle was found to follow a power law dependence with an exponent of -0.85 on the number of thermal cycles and the total number of CGE accumulated over 2000 cycles showed a linear dependence on the size of the crack. A preferential crack growth direction along the short cell edge was found and attributed to the particular coupling between the cells and their carbon fiber support structure. Sine vibration and acoustic-noise testing did not result in any crack propagation. The electrical impact of mechanical defects originated predominantly from the interruption of the contact network on the cell front side. Cell area is electrically isolated either by a propagating crack that separates the singular electrical connection of a particular area, or along the original path of the crack. The quantitative laws derived allow to predict the propagation and the electrical impact of a given mechanical defect during 15 years in a geostationary-orbit environ- - ment