Weight of Solar Cells and Arrays Using Silicon Technology
The weight of the solar cell is dependent on the cell area and the silicon surface thickness. The larger the cell area and thickness the greater will be the weight of the solar cell. The larger the array dimensions the greater will be the array weight and size. The number of solar cells and solar arrays will increase with increase in electrical power consumption. Additional solar cells will be required to meet additional power consumption to compensate for the component losses, reflection losses, attitude stabilization, and control and reduction in cell voltage roughly by 0.5 percent per degree centigrade due to increased temperatures in space. It is important to point out that a solar array with a back surface that can radiate to space will be cooler and, therefore, will have a higher electrical output than an array whose back surface is blocked by the spacecraft or other equipment operating in the vicinity of the spacecraft.
Solar cells are protected by fused silica cover slides for thermal reasons and to enhance resistance to protons and electron radiation encountered in the orbits. The thickness of the fused silica cover slides ranges from 0.004 to 0.020 inches. The
cover thickness is selected consistent with radiation protection level required, allowable weight budget and within cell and array costs allowed. Fused silica material are preferred over sapphire and microsheet glass for the cover slides, because of its low cost and excellent stability in space environments. Increase in cost and weight due to cover slides must be taken into account during the weight and cost estimations.
Organic high-polymeric materials must be used as adhesives for bonding the cover slides to the solar cells. Potential adhesives best suited for space applications include epoxies and silicones because of their superior ability to withstand ionizing radiation over long intervals without structural change or change in optical transmittance. Photochemical decomposition results in changes in the optical properties of the adhesives, leading to reduction in the electrical output of the solar cells. By selecting an adhesive that has minimum absorption of the ultraviolet photons and distribution of the energy along the polymer chain, the ultraviolet degradation of optical properties of the adhesive can be minimized. Silicones have this property of minimum absorption and energy distribution and thus, are frequently used for space applications, where operating life exceeding 15 years in space is the principal design requirement.