|MadSci Network: Engineering|
Sean, If you look at the solar spectrum, comparing intensity vs. wavelength, it would form an arc. In the middle, the highest intensity of light is green, with lower amounts of red and blue light on the ends. So a solar cell tuned to absorb primarily green light would be able to convert a fair portion of the total sunlight into energy. However, it's not that simple. Some of the higher energy blue light can also be utilized. You are probably familiar with atoms and their atomic orbitals of electrons. These orbitals overlap and electrons are shared between atoms to form molecules. The electrons shared between atoms are in constant motion. The most stable state would be when the electrons are dead center between the nuclei of two atoms. When the electrons are on the opposite sides of the nuclei, the two positively charged nuclei repel one another. Where as atoms have atomic orbitals, molecules have molecular orbitals. A large molecule could have many atoms with many overlapping atomic orbitals, but we simply the picture by looking at net molecular orbitals for the whole molecule. The molecular orbitals involved in bonding can be broken up into "bonding" and "anti-bonding" orbitals. When all the electrons are in bonding orbitals, the molecule is highly stable. If all the electrons were to move into anti-bonding orbitals, the molecule would blow apart into little pieces. In our daily lives we don't deal with atoms and molecules we deal with chunks of material composed of zillions and zillions of atoms. To generalize the "bulk" or net properties of a material we talk about bands of electron orbitals. The bonding electrons in a material like silicon can be grouped together into valence and conduction bands, analogous to the bonding and anti-bonding orbitals of a molecule. Electrons in the valence band hold the bulk material together. Electrons in the conduction band are free to jump from atom to atom, thus carrying an electric charge. This is how electrons "leap frog" through a conducting wire, sheet of metal, or semiconductor. Well the energy that it takes to "excite" an electron from the valence band up into the conduction band is very specific, and corresponds to a specific color of light. This is how electricity is generated in solar energy panels. Different compounds will absorb different colors of light, based on their "bandgap" (the energy difference between the valence and the conduction bands). If a material absorbs light of a shorter wavelength/higher energy (closer to the blue end of the spectrum), the energy does not exactly match the bandgap, therefore it can't excite an electron from the valence to the conduction band to produce electricity. However, an electron can absorb high energy light, then loose some of it's energy by giving off heat until it "relaxes" down to the energy level of the conduction band. So, this may have been much more detail than you asked for, but here is the take home message: Theoretically, a material designed to absorb green light, could convert all of that light into electricity, some of the "bluer" light, and none of the "reder" light. It turns out that the optimum balance is a little left of center, closer to the blue light. In this compromise, you loose a little of the green light, but you gain more of the light on the blue side of the spectrum.
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