09d9bdb1_den__Solar_Cells_Versus_Leaves__The_Solar_Cell
[0] Price [The Leaf] Completely free. Millions are grown each day by trees around the world. A full grown tree can regrow thousands of leaves each spring. [The Solar Cell] 
Produced using amorphous silicone, which is expensive to isolate and work with. Although the cost of solar cells falls each year, the most efficient solar cells are still inconveniently expensive.
Experiment Garden: Solar Cells Versus Leaves
The Solar Cell