We haven't hired any material science folks yet. If we were to double in size (15 people -> 30 people), I bet we'd need a material scientist and a chemical engineer. At present, we're mostly generalists, with a slight bias toward the mechanical.
It took us ~3 years to get to 15; in the last 2 years we've been roughly constant, plus or minus a few people. Maybe in 2012.
Has your education taught you how to engineer things? Did you take a lot of math classes?
We don't have any industrial engineers at present, but when we're growing, we wouldn't rule them out. Really, we want people who are very smart, work hard, communicate well, can solve problems, and actually care about cleantech.
Regarding the specifics of solar cells (before you edited your post above), the field we span is broad, and the problems are narrow-- you can't possibly hope to have expertise in more than a small fraction of the areas in which you work. Fortunately, nobody else can either.
