Why Silicon for Space-Based Solar Cells
The author has investigated potential materials for fabrication of space-based solar cells, with particular attention to semiconductor energy gap (Eg). The materials investigated include silicon (1.1 and 2.3 eV), gallium arsenide (1.38 eV), indium phosphide (1.27 eV), cadmium telluride (1.50 eV), and gallium phosphide (2.24 eV). The semiconductor energy gap plays a critical role in determining maximum conversion efficiency. Solar cell conversion efficiencies as a function of material energy gap for various semiconductor materials are shown in Figure 6.1.
A market survey of the various semiconductor materials reveals that silicon is the cheapest material and is available in unlimited quantity without any restrictions. Material scientists indicate that silicon amounts for approximately 25 percent of the earth鈥檚 crust. In addition, silicon processing technology is well matured and the electrical, high frequency, thermal, and mechanical characteristics of silicon are fully known. Silicon cells can withstand operating temperatures as high as 125掳C
with no compromise in electrical performance and reliability. The main reason for selecting silicon is its rapid availability with minimum cost and with no delay in delivering the silicon chips to the manufacturing facility. It is important to mention that silicon n-p solar cells are preferred over p-n solar cells because they are more resistant to space radiation. Furthermore, the trend toward lighter solar arrays for communication satellites and surveillance and reconnaissance spacecraft and higher earth orbits results in wider temperature excursions from sunlight to eclipse periods. These wide temperature excursions and the requirements for long operating life ranging from 15 to 20 years, put great demand on the solar cell design, and the reliability of the intercell and intermodule connections on a solar array.
Material science studies indicate that silicon is the most ideal material for fabrication of space-based solar cells compared to materials such as gallium arsenide (GaAs), cadmium telluride (CdTe), cadmium indium selenide (CIS), cadmium sulfide (CdS), indium phosphide (InP), and cadmium indium gallium selenide (CIGS). Material cost, poor conversion efficiency, and higher fabrication complexity associated with CdS, CIS, and CIGS materials are the principal reasons for not using these materials for space-based solar devices. Note space-qualified CdS solar cells are very costly compared to conventional silicon solar cells. France, Germany, and England have been pursuing research and development activities on CdS, InP, and CdTe solar cells. The studies indicate that the cost per unit area of silicon solar cells decreases as the cell area increases, as illustrated in Figure 6.2. This figure also shows the relative solar cell cost as a function of cell or silicon wafer thickness. Although the conversion efficiencies of GaAs solar cells are higher than those of silicon solar cells, other factors such as density, thermal conductivity, thermal expansion coefficient, and intrinsic carrier concentration, as summarized in Table 6.1, make silicon preferred over the other materials.
The room temperature density of CdTe is 6.062, which is about 2.5 times higher than that of silicon. This means that solar cells and arrays made from silicon will be the lightest compared to other materials, which will not only significantly decrease the launch costs, but will provide enhanced thermal performance, higher mechanical integrity, and improved reliability over a long operating life.