aSiH

Hydrogenated amorphous silicon, abbreviated a-Si:H and sometimes written as aSiH, is a non-crystalline form of silicon in which hydrogen atoms are incorporated into the silicon network. The hydrogen passivates dangling bonds, reducing defect states and enabling electronic and optical functionality in thin films. a-Si:H is widely used in thin-film photovoltaic cells and as the active layer in thin-film transistors for display backplanes.

Structure and properties: As an amorphous solid, it lacks long-range order, but the hydrogen content helps reduce mid-gap defect states. The material exhibits a relatively wide optical band gap of about 1.6 to 1.8 eV, and its electrical mobility is lower than that of crystalline silicon. Doping with boron or phosphorus can produce p-type or n-type material, enabling diodes, junctions, and transistor structures.

Production: a-Si:H is mainly deposited by chemical vapor deposition, most commonly plasma-enhanced CVD (PECVD). Silane (SiH4) and hydrogen are introduced at relatively low substrate temperatures, typically 150–300°C. The hydrogen concentration is tuned to passivate defects, often in the range of roughly 8–20 atomic percent. The resulting films can be doped during deposition or through subsequent diffusion.

Limitations: The Staebler–Wronski effect causes light-induced degradation of electronic and photovoltaic performance, particularly in solar cells, due to creation of metastable defect states. Degradation is mitigated by device architectures such as tandem junctions with microcrystalline silicon, optimized hydrogen content, and thermal annealing strategies.

Applications and impact: Since the 1980s, a-Si:H has been central to thin-film solar cells, providing economical photovoltaic options. It is also used in active-matrix thin-film transistor backplanes for LCD and some OLED displays, often in combination with other silicon alloys to extend infrared absorption. The technology continues to evolve with tandem and alloyed versions such as a-SiGe:H to improve efficiency and stability.