NASA Tech Briefs: What are solar cells?
David Wilt: Solar cells are semiconductor devices that directly convert light into electricity. When light hits a solar cell, that photon of light excites an electron so that it is free to move away from its host atom – this is the first critical step. Inside a solar cell is an electric field that takes the loose electron and forces it to go in one direction. Once the electrons all get collected in this way, they are able to flow out of the metalization on the solar cell and provide power to an external load. Solar cells provide DC power much in the same way that a battery does. Large solar arrays tend to be comprised of many of these cells connected in either series or parallel electrical connections.
NTB: How does NASA utilize the technology?
Wilt: Almost all of the NASA spacecraft are solar powered. The International Space Station (ISS) and a great number of our satellites are solar powered. It’s the primary power system for all of our space missions. The shuttle does use other technology, but the new CEV (Crew Exploration Vehicle) actually will be totally solar powered.
NTB: What improvements were made to the new type of cells being developed at NASA?
Wilt: The cells we worked on will not necessarily degrade less in space, the advantage is that they will be larger in area, lower in mass, and still have very high efficiencies. Mass is a very important parameter for spacecraft; every pound that you have to send from Earth into low-Earth orbit costs roughly $1,000 and if you’re going to the Moon or Mars, it is significantly more expensive. So, making good cells that are low in weight is very important.
The cells we produced are called three-five devices. It is a type of semiconductor class of materials that were grown on silicon – the standard semiconductor used in most computers, watches, and other consumer electronics. Silicon is a very good material but it does not work with light very well; however, the three-five materials can convert and generate light quite nicely.
NTB: How are the cells being tested?
Wilt: We do a lot of ground-based tests where we can simulate what the Sun looks like and some of the on-orbit environmental things that the cells will experience on Earth. One thing that is very difficult to do on Earth is to do all of those kinds of simulations at the same time to see the synergistic effect. An important aspect of getting new technologies accepted by the community is getting them tested in a relevant space environment. This is what the Materials International Space Station Experiment (MISSE-5) is doing.
NASA Glenn developed the electronic system that actually characterizes the cells every orbit. Using a radio telemetry system that was provided by the Naval Academy, the data is transmitted back to Earth, so we get real-time feedback on the performance of our devices.
NTB: What are some of the terrestrial applications for these solar cells?
Wilt: Energy is a critical issue that everyone is aware of, and it is my understanding that the terrestrial solar-cell market has taken off dramatically. Part of the development for the devices I worked on was funded by NREL (National Renewable Energy Laboratory), which shows their interest in solar cells for terrestrial applications. The devices that we developed are probably going to be expensive enough that you’d have to put them in a concentrator system. This means that you’d have to have a lens-type device to focus the sunlight down on to the cells and by doing this – since concentrators have a tendency to be inexpensive – you would be able to afford higher-efficiency, better cells. These cells could run at high temperatures, which would enable them operate in a concentrator quite nicely.
There are other spin-off applications of this technology that our partners have been developing. For example, integrating these two types of materials (the silicon and three-five materials) opens up pathways for having standard electronic devices that could have a lot of optoelectronic capabilities built into them.