coating is TiO2 and is in firm contact with the cell surface (its main function is to eliminate reflection losses). Typical cell areas are 2cm X 2cm or 2cm X 4cm. Space quality cells have to undergo a fairly rigorous environmental test which include vibration, thermal cycling and thermo-vacuum tests. Their life under radiation has also to be demonstrated by subjecting samples to electron radiation. Thermal cycling similar to conditions encountered in space are usual both at cell level and at panel level. Since a solar cell is essentially a semi-conductor device, it is fairly sensitive to damage. Proton and electron damage mechanisms are well known in space. These reduce power output by about 10% to 15% over the life of the spacecraft (typically between 3 and 7 years). The cover glass protection serves to restrict an otherwise high damage to the power output of the cells/panels. When a laser beam illuminates a typical solar cell or panel, it can do the following: • Heat the cell/panel. Within the limits that cells and panels are qualified (typically + 80°C to — 170°C), this would not have any effect. • Melt the cover glass and damage the anti-reflection coating. This would reduce the efficiency of the cell and also reduce the total power output by increasing long-term degradation effects. • In addition to the above, irradiation could heat up the n-p junction layer and either melt it or significantly damage the junction. This would drastically alter semi-conductor characteristics leading to failure. • Explode an area of the solar panel if heating is continued further. Between the stages of heating up and melting, the exact effects would depend on laser characteristics and the specific materials used in the cell. We can conclude with reasonable certainty that the melting of the cover glass would represent a situation where serious damage to the power output of the cell would have occurred because such a situation would also damage the n-p junction. Thus the minimum damage criteria that we could use for assessing the effects of laser radiation would be the melting of the cover glass protection layer. For the coverglass of a solar cell typically: Typical values of a for solar cells are between 0.6 and 0.7. (This is wavelength dependent and increases as laser irradiation continues). From equation (4) the minimum time required to heat up n-p junction layer Ar=39 milliseconds. Using equation (5(a)) we get
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