Scanning Near-field Optical Microscopy
The investigation of optical near-field effects in thin-film solar cells is done by Scanning Near-field Optical Microscopy (SNOM, also: NSOM). The optical resolution of the SNOM is given by the probe geometry. A coated tapered fiber tip is used as the probe with an aperture smaller than 100 nm which allows the measurements of optical effects with a resolution better than 100 nm.
The tip is scanning line-by-line across the surface of the sample. The feedback loop ensures a constant tip-to-sample distance. Therefore, the local electric field intensities and the surface topography are measured simultaneously. Additionally, a three-dimensional measurement of the electric field intensity above the surface is possible. Light scattering and propagation beyond the surface is thereby visualized and analyzed. In the research group, two microscopes are available which are optimized for different working modes.
Research topics are the experimental determination of local light scattering properties of textured layer stacks as used in silicon-based thin-film solar cells, the investigation of light diffusion in a thin-film solar cell and the photocurrent generation from local illumination.