How does pressure effect materials and systems? What does this have to do with solar energy?

Photo: NASA, Plumbrook Facility (where they tested the Saturn V rocket and have the largest vacuum chambers on the planet). If you're ever near Sandusky, OH, check to see if they are giving tours!

We talk about how temperature affects performance, but not much about pressure. Pressure can cause similarly drastic changes in how materials perform. To illustrate this point, check out this video

As you can see, the water started to boil at a temperature well below 212 degrees F. It then froze. A changing pressure can greatly affect how materials behave. This is a major issue in the vacuum of space. Electronic equipment may release trapped gas, which can then condense on camera lenses, solar cells, CCD sensors, and other critical materials. NASA has had to develop rigorous tests to make sure that materials they put in space do not offgas significantly. These tests are outlined in engineering standards. Organizations like NASA or ASTM International release standard test methods they and other groups can use to quantify this and other effects. Offgasing can also affect the structural integrity of materials if a certain phase becomes unstable at certain pressures under which the material is expected to perform and a phase transformation is allowed to take place. You are probably most familiar with this happening when you change temperature, but it can also happen when pressure is changed.

When you think of a solar cell in operation, unless we are talking about a space application, this occurs at atmospheric pressure. However, many of the techniques used to make solar cells require a vacuum (very low pressures). For example, sputtering and thermal evaporation are typically performed under vacuum conditions (at very low pressures). At higher pressures, it is more difficult to control these processes. Gases conduct heat (and convection currents can move heat), causing samples to be non-uniform. Furthermore, gas atoms can disrupt and change the chemistry at the surface of a growing film. Although solution-based processes can also be used to fabricate solar cells (and these tend to be inexpensive and easily scale-able), many of the high-efficiency devices have been fabricated using vacuum-based techniques since it is easier to control processes in vacuum at fairly low cost.