p-i-n diodes based on amorphous and microcrystalline silicon are used for different applications like sensors or solar cells. However, their long-term stability and dynamic behavior are still under discussion. Therefore, the optical properties of both types of diodes were investigated by a comparative study. The long-term stability of non-encapsulated devices was tested by means of light soaking, damp heat testing and high-temperature treatment of up to 2000 It. The dynamic properties of the thin-film silicon photodetectors were studied by measuring and analysing the admittance under different bias conditions. (c) 2007 Elsevier B.V. All rights reserved.