Evidence for light-induced long-range hydrogen motion in a-Si : H using Raman scattering of a-WO3

We demonstrate that one can detect minuscule amounts of hydrogen diffusion out of a-Si:H under illumination at room temperature by monitoring the chan ges in the Raman spectrum of a-WO3 as a function of illumination. The Staeb ler-Wronski effect, the light-induced creation of metastable defects in hyd rogenated amorphous silicon (a-Si:H), has been one of the major problems th at has limited the performance of solar cells based on this material. The r ecently suggested 'hydrogen collision model' can explain many aspects of th e Staebler-Wronski effect, but assumes that the photogenerated mobile hydro gen atoms can move a long distance at room temperature. However, light-indu ced hydrogen motion in a-Si:H has not been experimentally observed at room temperature. We utilized the high sensitivity of the Raman spectrum of elec trochromic a-WO3 to hydrogen insertion to probe the long-range motion of hy drogen at room temperature. We deposited a thin (200 nm) layer of a-WO3 on top of a-Si:H, and under illumination a change in the Raman spectrum was de tected. By comparing the Raman signal changes with those for control experi ments where hydrogen is electrochemically inserted into a-WO3, we can estim ate semi-quantitatively the amount of hydrogen that diffuses out of the a-S i:H layer. Published by Elsevier Science Ltd.