Amorphous solids, like common glass, are materials whose atoms are not arranged in any particular order. They don't form crystalline structures at all, and they contain large numbers of structural and bonding defects. But they have some economic advantages over other materials that make them appealing for use in solar electric, or photovoltaic (PV), systems.

In 1974, researchers began to realize that they could use amorphous silicon in PV devices by properly controlling the conditions under which it is deposited and by carefully modifying its composition. Today, amorphous silicon is common in solar-powered consumer devices that have low power requirements, such as wristwatches and calculators.

Amorphous silicon absorbs solar radiation 40 times more efficiently than does single-crystal silicon, so a film only about 1 micrometer—or one one-millionth of a meter—thick can absorb 90% of the usable light energy shining on it. This is one of the chief reasons that amorphous silicon could reduce the cost of photovoltaics. Other economic advantages are that it can be produced at lower temperatures and can be deposited on low-cost substrates such as plastic, glass, and metal. This makes amorphous silicon ideal for building-integrated PV products like the one shown in the photo. And these characteristics make amorphous silicon the leading thin-film PV material.

A Closer 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 deposited so that it contains a small amount of hydrogen, in a process called "hydrogenation." The result is that the hydrogen atoms combine chemically with many of the dangling bonds, essentially removing them and permitting electrons to move through the material.

Staebler-Wronski Effect

Instability is the greatest stumbling block for amorphous silicon. These cells experience the Staebler-Wronski effect, where their electrical output decreases over a period of time when first exposed to sunlight. Eventually, 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's unique properties, solar cells are designed to have an ultrathin (0.008 micrometer) p-type top layer, a thicker (0.5 to 1 micrometer) intrinsic middle layer, and a very thin (0.02 micrometer) n-type bottom layer. This design is called a "p-i-n" structure, being named for the types 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 in 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.