I hear about silicon solar cells, but I have also heard mention of CdTe cells. I know that you work on CZTS and I've also heard mention of a crazy thing called CIGS. What does all this mean?

These different labels for cells refer to the material that makes up the absorber layer. CdTe, Si, Cu2ZnSn(S,Se)4 (CZTSSe), and Cu(In, Ga)Se2 (CIGS) are all materials that can be used in a specific layer of a solar cell (something used to convert energy from the sun into electricity) in which light is absorbed and the energy in that light converted into charge carriers (electrons and what are known as holes). There are a variety of solar technologies commercially available and a single technology may not immediately dominate. CdTe-based cells have been less expensive than other types recently, however there are scarcity concerns about this technology that will eventually limit the extent to which cost can be lowered. Si cells are pretty ubiquitous (and inexpensive), but are not quite as inexpensive as CdTe-based cells due to added processing requirements. CIGS cells are just recently coming to market. Technologies like CZTSSe have not been commercialized since they are still in the research stages (comparatively low, but rapidly improving efficiencies). A new technology on the block that may really shake things up are perovskite solar cells, which have had a meteoric rise in efficiency making them fairly comparable in terms of efficiency to CIGS and CdTe, but this technology has some major degradation issues (water, even the water vapor in air kills cells). Both CZTSSe and perovskite cells are earth-abundant and it is likely that they will be able to be made more inexpensively than competing technologies. Regardless, silicon-based cells will certainly remain in the overall mix. It is likely that, for a long time, solar cells based on several different materials systems will all compete, meanwhile enhancing our understanding of these systems.