Measurements of dark conductivity, steady-state and transient photoconductivity in undoped and boron-doped microcrystalline silicon films exposed to room air are reported. Two aging processes are identified: (i) an increase in dark conductivity and mobility-lifetime product occurring over several days, that may be reversed by heating to 160 °C under vacuum, and subsequently re-cycled, (ii) an irreversible change in the density of states occurring over a period of several months. It is proposed that the reversible effect is associated with charge transport in a region of electron accumulation induced by weakly-adsorbed water, whereas the irreversible effect, identified from its transient photocurrent signature, is associated with a true change in the density of states within the transport path, possibly as a result of slow chemical reactions at silicon grain boundaries.
Smirnov, V., Reynolds, S., Main, C., Finger, F., & Carius, R. (2004). Aging effects in microcrystalline silicon films studied by transient photoconductivity. Journal of Non-Crystalline Solids, 338, 421-424. https://doi.org/10.1016/j.jnoncrysol.2004.03.010