Recombination processes and optical bias in undoped a-Si:H

A quasi-monomolecular recombination model is shown to account for coplanar electrode transient photocurrent measurements in undoped amorphous, hydrogenated silicon (a-Si:H). Optical bias is found to modify both photocarrier dynamics and recombination, but with a response time which exceeds the recombination response time of the specimen. This model provides a new method to obtain the ratio of the microscopic trapping rate b₊ and extended-state mobility μ₀. Estimates were obtained using several specimens as a function of optical bias, temperature, and optical exposure history; the values range from 10^-12 to 10^-10 V-cm. These values are substantially lower than the previous estimate (10^-9 V-cm) based on extrapolated time-of-flight transient photocurrents, with possible implications for the magnitude of μ₀.