40% efficiency barrier on solar cells achieved..

http://www.sciencedaily.c.../2006/12/061206123954.htm

sounds good... until ...

http://en.wikipedia.org/w...abundance_of_elements.png


one remembers that As, Ge, Ga, In, and P  are all pretty rare (though not uber-rare),  but also very difficult to process/extract .  oh, and they're lovely health hazards