Optical near-field investigations of photonic structures for application in silicon-based thin-film solar cells

Abstract

In this thesis, light scattering and propagation inside a silicon-based thin-film solar cell is investigated using optical simulations based on the finite-difference time-domain method. The special focus in this thesis lies in the analysis of the influence of randomly textured surfaces on cell performance. Due to the random nature of these structures and their varying sizes, simulation domains have to be sufficiently large to have a statistically significant distribution of features. The investigations focus on three different areas: The first area is light scattering at different interfaces in transmission as well as reflection. These simulations are compared to results from an improved scalar scattering model proposed by Domin´e et al. [J. Appl. Phys. 107, p. 044504, 2010]. The agreementof both methods is very good, with the limits of the scalar model lyingin multiple interfaces and layers with a thickness below the peak-to-peak roughness of the surface. Secondly, the absorptance inside different hydrogenated amorphous and microcrystalline silicon layers is investigated for different structures; these include comparisons between conformal surfaces and surfaces as obtained in real devices by silicon growth. Further investigations in this area included simple stretching of the surfaces along different axes, as well as more complex modifications based on the scalar scattering theory; additionally, an amorphous/microcrystalline silicon solar cell is simulated and compared to experimental results to find limitations in the simulation approach. All of these simulations show a better performance for steeper features with a lateral size of about 500 nm. Additionally, the changes in topograhpy introduced by the silicon growth has a significant impact on cell performance. The last part of this thesis compares optical simulations to measurements of a scanning near-field optical microscope (SNOM). When comparing simulated intensities directly above a rough surface to measurements, it is found that the offset of the tip due to its finite physical size is the strongest influence, while light scattering at the tip has very little impact on (relative) intensity measurements