AXT supplies germanium, a key element used to create triple junction solar cells. Triple junction cells are manufactured by the likes of Emcore (EMKR) and Spectrolab, two companies known for the highest efficiency, and their solar cells are sliced and diced by CPV solar panel companies, such as Amonix and Solfocus, and incorporated into CPV systems.
Other than CPV, the only other uses so far of germanium based solar panels are for space use. Terrestrial use of germanium CPV has still not caught on and is still refered to as the "zero billion dollar market".
There are of course other uses for germanium but the big demand would come from the emergence and adoption of CPV. The recent articles of the 30MW facility in Colorado by Amonix and the 10MW facility in Spain by Solfocus are a start. Keep track of their projects, and also of Emcore's performance (publicly available) to track whether AXT's germanium business is poised to take off.
The partnets they currently have use a Germanium subtrate to layer on. AXT has some customers in Europe, I think Emcore in the US and some chinese satellite manufacturer. Right now I believe almost all of their solar business is related to solar panels for satellites. Emcore is in a joint venture to build a production facility in China, so it is likely that AXT will be providing the substrate to that production. Emcore also sources some company from Nevada but they seem to be the ones supplying their plant in the US.