Following up on Monday's post, here's an excellent tutorial on PV provided by the US Dept of Energy. It discusses band gap, temperature effects, doping, etc. About 50% of sunlight that hits solar panels is outside the band gap and could perhaps be filtered by sheet films like these, though there are some off-axis issues with most filters.
In other news, Oriental Trading Company has 6" square mirror tiles for cheap. I'm still in.