A Closer Look

A Close Look
Amorphous silicon does not have the structural uniformity of single or multicrystalline silicon .Small deviations in this material result in defects such as"dangling bonds",where atoms lack a neighbor to which they can bond.These defects provide places for electrons to recombine with holes,rather than contributing to the electrical circuit.Ordinarily,this kind of material would be unacceptable for electronic devices,because defects limit the flow of current.But amorphous silicon can be deposied so that it contains a small amount of hydrogen,in a process called "hydrogenation." The result is that the hydrogen atoms combine chemicaly with many of the dangling bonds,essentially removing them and permitting electrons to move through the materials.
Staebler -Wronski Effect
Instability is the greatest stumbling block for amorphous silicon.These cells experience the stabler-wronski effect,where their electrical output decreases over a period of time when first exposed tho sunlight .Eventally ,however,the electrical output stabilizes.This effect can result in up to a 20% loss in output before the material stabilizes.Exactly why this effect occurs is not fully understood,but part of the reason is likely related to the amorphous hydrogenated nature of the material,one way to mitigate-though not eliminate -this effect is to make amorphous silicon cells that have a multijunction design(discussed in another section).


Cell design
Because of amorphous silicon unique properies,solar cells are designed to have an ultrathin(0.008 micrometer)p-type top layer,a thicker(0.5 to 4 micrometer)intrinsic middle layer,and a very thin (0.02 micrometer)n-type bottom layer .This design is called a "p-i-n"structer,being named for the typed of the three layers.The top layer is made so thin and relatively transparent that most light passes right through it,to generate free electrons is the intrinsic layer.The p-and n-layers produced by doping the amorphous silicon create an electric field across the entire intrinsic region,thus inducing electron movement in that i-layer.