Ok so right now the best solar cells get only about 20-30% of the energy available to it from the sun. This is because for each photon that knocks off an electron, any excess energy not needed to knock off the electron in the silicon is wasted. And I read in howstuffworks.com that really good solar cells do use multi-layer cells which absorve the light differently at different layers, and thus make efficient use of the light at each layer. So my question is, what's stopping the creation of a cell that has like lets say, 30 layers, each for a specific level of photon energy, thus extracting a much higher amount of energy from the sun? I also read an article in Discover/SCIAM I forget which which talked about how silicon, when made right, has the property to absorb light differently at different depths of the same piece of silicon. Now this article was how it could be used for better digital cameras, but could it be used for solar cells? Each layer absorbing photons with different energy levels. Thanks!