Evidence for Long-Range Hydrogen Motion in a-Si:H under Room-Temperature Illumination Using Raman Scattering of Amorphous Tungsten Oxide Overlayer

We demonstrate that one can detect minuscule amounts of hydrogen diffusion out of a-Si:H under illumination at room temperature, by monitoring the changes in the Raman spectrum of amorphous tungsten oxide as a function of illumination. The Staebler-Wronski effect, the light-induce creation of metastable defects in hydrogenated amorphous silicon (a-Si:H), has been one of the major problems that has limited the performance of such devices as solar cells. Recently, Branz suggested the 'hydrogen collision model' that can explain many aspects of the Staebler-Wronski effect. One of the main predictions of this model is that the photogenerated mobile hydrogen atoms can move a long distance at room temperature. However, light-induced hydrogen motion in a-Si:H has not been experimentally observed at room temperature. We utilized the high sensitivity of the Raman spectrum of electrochromic a-WO₃ to hydrogen insertion to probe the long-range motion of hydrogen 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 detected. By comparing the Raman signal changes with those for control experiments where hydrogen is electrochemically inserted into a-WO₃, we can estimate semiquantitatively the amount of hydrogen that diffuses out of the a-Si:H layer.