Thin-film silicon is a form of silicon commonly used in the design of photovoltaic cells used in technologies such as solar power generators and photodetectors. It is a very thin layer of silicon that is either amorphous, meaning it has no crystalline structure, or has one of several crystalline structures with special properties. Thin-film silicon is distinguished from bulk or wafer silicon, which is made of monocrystalline or polycrystalline silicon.
Thin-film silicon is a semiconductor and is useful in photovoltaic cells because of the way it is affected by light on the quantum level. When photons of light strike the silicon and are absorbed, this causes electrons in the silicon to increase in energy level, rising from their usual position in the densely packed valence energy band to the much sparser conduction energy band where they can move much more freely. At the same time, the absence of the negatively charged electron in the valence band creates a positive charge carrier called an electron hole in its place, which can move through the valence band as successive electrons move to fill the hole and so create a new one in their former position. This movement of charge carriers through the semiconductor produces an electrical current that can be used as a source of energy or provide signals to detection equipment.
Thin-film silicon can have one of several forms. Amorphous silicon has no crystalline structure, meaning that there is no long-range order or pattern in the arrangement of the silicon atoms. Nanocrystalline, sometimes called microcrystalline, silicon has small crystalline grains surrounded and separated by amorphous silicon. Protocrystalline silicon is silicon in an intermediate phase of the process of crystal formation in amorphous silicon. Black silicon is a form of silicon with a surface covered in microscopic monocrystalline needles, a structure that increases light absorption. Different forms of thin-film silicon are effective in catching different wavelengths of light, so thin-film solar cells often incorporate layers of more than one type to absorb as much sunlight as possible.
Thin-film silicon is used primarily in the solar power industry due to silicon's properties as a semiconductor. Thin-film cells have a number of advantages over bulk silicon solar cells, including lower material costs and smaller size, though this can come at the cost of lower power conversion efficiencies. The lower size and weight of thin-film cells also makes it more suitable for applications such as photovoltaic cells integrated into buildings and charging systems for vehicles. Thin-film silicon is also used in detection equipment, such as thermographic cameras and night-vision goggles.