July 30, 2013
Dr. Sheila Bailey is a senior scientist in the Space Environment and Experiments Branch at NASA Glenn Research Center, where she has worked since 1985. She received her Bachelor’s and Master’s degrees in Physics and her Ph.D. in Solid State Physics and currently works with photovoltaics. She taught at Baldwin Wallace College for 27 years and has been an associate faculty member of the International Space University, which she attended in 1989, since 1992.
Her work at NASA GRC focuses on building “a better space solar cell,” and she investigates nanostructures and materials to do this. In her time at Glenn she has built the photoluminescence lab, a scanning electron microscope lab, and the nanocharacterization lab. In her talk, Sheila discussed the current technology and status of commercial and space solar cells as well as the various ways of improving them for future use:
Most solar cells are single silicon crystal or thin-film amorphous crystal but there is much ongoing research into how these cells can be improved. There are several approaches to “improving” the current solar cell; efficiency, longevity, and cost are a few key components. For space solar cells, weight, mass, and volume are much bigger issues than on the ground, since the cell must fit into the rocket and on the side of a satellite or lander. Solar cells in space have a long history – they are reliable, have no moving parts, have no consumables, are a proven technology, and are generally lightweight (100+ W/kg). In the Earth environment, we are mostly concerned with protecting solar cells from dust and humidity; in space, however, the cells have to deal with the pre-launch pad environment as well as radiation.
There are many approaches to increasing the efficiency of solar cells. Current commercial efforts are focused on the triple junction cell, which captures more of the solar spectrum than a single or double junction but is also more complex as the layers have to be integrated and most solar cells fail at the joints between layers. Other high efficiency approaches than simply increasing the number of junctions include metamorphic growth, inverted metamorphic growth (now >34% efficiency), dilute nitride devices, mechanical stacking (again the integration adds complexity and potential for failure), optical spectrum splitting, concentrator designs (above 12:1 ratio generally needs its own cooling system), and quantum confinement. Of course, efficiency is not the only approach to bettering a solar cell. Lower cost approaches make solar cells more disposable. Epitaxial lift off, in which the substrate is reused, and cheap Poly III-V cells, thin-film cells on flexible substrates, are approaches that utilize parts of cells already built or enable coverage over larger areas.
Dr. Bailey’s work has centered around epitaxial lift off and more recently on quantum dot cells, which is a highly advanced technology that has a lot of potential, since the cells generally handle radiation and heat better, making them an ideal candidate for concentrator cells. This technology could put solar cells into the ultra-high efficiency range of 40–50%.
Finally, Dr. Bailey discussed past missions to Mars and the advantages and disadvantages the Mars environment presents to solar cells as a primary power source. While Mars has a lower temperature than Earth, which is good for solar cells, it is also very dusty and has lower solar intensity than the Earth. The dust accretion on cells leads to a decrease in energy output of about 0.28% per solar day. In general, however, solar cells are an excellent power source for space missions.