This is an interesting development in solar power technology:

Researchers at Michigan State University have created a fully transparent solar concentrator, which could turn any window or sheet of glass (like your smartphone?s screen) into a photovoltaic solar cell.

Currently each cell is week and supplies little energy compared to standard photovoltaic cell. However, with a little more efficiency and buildings like tall office blocks with lots of windows the potential to provide useful amounts of electricity is there.