Plants are able to generate energy using photosynthesis. It is kind of the same general idea as what solar cells do except instead of generating electricity, plants generate chemical energy and store it for later use. Scientists have taken tobacco and by genetic engineering started to assemble the pieces that would be needed to make plants into solar energy generators.  Still a long way away from their ultimate goal, the idea of using plants is neat and could be a more ecofriendly way of producing solar power.  Some time you might be able to plant your own solar cells and power that iPod.
for more see
http://www.gizmodo.com.au/2010/01/tobacco-plants-used-to-grow-cheap-solar-cells/