Stability of the mobility-lifetime product of holes in undoped a-Si:H under illumination

Stability of the mobility-lifetime product of holes has been studied by measuring the ambipolar diffusion length (Lamb) in undoped a-Si:H films. After 80 hours of AM1 light soaking L(amb) decreases from 217 to 152 nm, whereas photoconductivity excited by a HeNe laser with an intensity of 1.7 mW/cm(2) decreases from 7.12 X 10(-6) to 2 X 10(-7) Ohm(-1) cm(-1). Furthermore, L(amb) does not decrease in the first 10 minutes. These results are attributed to the asymmetric defect density (Nd) dependence of the mobility-lifetime products of electrons and holes due to the inherent asymmetry between the density-of-states distributions of the conduction and valence band tail states. Numerical simulation shows that there combination path via the band tail states dominates over that via the defects when N-d < 3 X 10(16) cm(-3).