Spin-dependent photoconductivity as a function of wavelength: A test for the constant photocurrent method in a-Si:H

The central assumption of the constant photocurrent method (CPM), namely that the carrier lifetimes are independent of the photon energy if the photon flux is adjusted always to maintain a fixed photocurrent, is tested with the help of spin-dependent photoconductivity (SDPC). While the recombination of electrons at dangling bonds is indeed found to be independent of photon energy CPM conditions, we observe a variation of the dominant hole transport mechanism. At photon energies below band gap, hopping of holes in the valence band tail is observed as a rate limiting step. At higher energies, the corresponding resonance disappears, indicating a shift of the hole quasi-Fermi level towards the valence band mobility edge.