aSi

a-Si, short for amorphous silicon, is a non-crystalline allotrope of silicon in which atoms are arranged in a disordered network rather than a periodic lattice. The material is commonly prepared with hydrogen incorporated (a-Si:H) to passivate dangling bonds and improve electronic properties.

Compared with crystalline silicon, a-Si has a higher density of localized defect states in the band gap and significantly lower carrier mobilities. The optical bandgap of intrinsic a-Si is about 1.7 eV, and hydrogenation can increase the effective gap and reduce mid-gap defects. The material exhibits strong optical absorption in the visible region, which enables very thin absorbing layers.

Production methods include chemical vapor deposition from silane (SiH4) or disilane, typically by plasma-enhanced CVD (PECVD) at low temperatures. Other deposition techniques such as sputtering and hot-wire CVD are also used. Films are deposited on substrates such as glass or flexible polymers and can be doped to create p- or n-type layers for electronic devices.

Key applications are thin-film solar cells and thin-film transistors for displays and sensors. a-Si:H solar cells are advantageous for large-area, low-cost devices, though their efficiency is typically lower than crystalline silicon and they are subject to performance degradation under light exposure.

Limitations include the Staebler–Wronski effect, a light-induced degradation of electrical properties. Ongoing research aims to improve stability by material modifications, including microcrystalline silicon (μc-Si) alloys and mixed-phase devices, as well as alternative deposition and passivation strategies.