ISRU solar cells
In-situ manufacture of solar cells requires possession of materials of sufficient purity and the ability to arrange them into thin layers. Although ideally each material is obtained and refined locally, significant interplanetary shipping mass can be saved just with local assembly because bulk materials do not require the support structures that fragile devices require to survive launch and landing.
- See Also: Solar panels
A solar cell is made up of multiple layers.
- The heart of the cell is two layers of semiconductor: a thin (e.g., 2 micron) n-type layer over a p-type layer. Certain frequencies of incident light cause electrons to flow across the n-p junction.
- The semiconductor layers are wrapped in conducting layers (usually metal) to collect the electrons. The top conductive layer must either be transparent (e.g., some oxides) or a fine mesh.
- An anti-reflective layer significantly increases efficiency.
- A final protective layer (e.g., glass) is usually added, although this may not be required for ambient vacuum environments such as asteroids and the Earth's moon.
- A substrate provides an electrically uniform environment as well as simple mechanical support for the above layers.
Silicon wafers can be cut from crystals grown using the Czochralski process.
Amorphous silicon yields lower efficiency cells (8% versus 14%) but vapor deposition techniques can be used during manufacturing and stringent clean-room conditions are not required.
Higher efficiency cells (20+%) require increasingly sophisticated manufacturing processes involving additional layers and exotic materials. Some additional efficiency can be obtained with concentrating optics.
Discussions of ISRU solar cell manufacture on the Moon commonly include one or more solar powered rovers that, together, comprise a more or less a self-contained solar cell factory. A typical scenario has a rover melt the lunar regolith to form a substrate onto which the power generation and wiring layers can be directly deposited using thermal evaporation in the ambient vacuum. However, such "paving" of the lunar regolith with solar cells is only effective near the equator. Further from the equator, solar cell panels must be set at an angle to the ground or else make do with less and less incident light.
Discussions of solar cell manufacturing rovers for Mars are fewer. Mars lacks ambient vacuum, and incident light is less because Mars is further from the Sun.
New Mars discussions
- A. Ignatiev and A. Freundlich, New Architecture for Space Solar Power Systems: Fabrication of Silicon Solar Cells Using In-Situ Resources (PDF), Space Vacuum Epitaxy Center, University of Houston, 2000.
- Horton et al., First demonstration of photovoltaic diodes on lunar regolith-based substrate, Astronautica, Volume 56, Issue 5, Pages 537-545 (March 2005).