Lockau, D.: Optical modeling of thin film silicon solar cells with random and periodic light management textures. , Technische Universität Berlin, 2013
Open Accesn Version
Better light trapping concepts are a prerequisite for the success of silicon thin film photovoltaics. This thesis presents optical simulations of silicon thin film solar cells with statistical and periodic absorption enhancing textures. For simulation of statistically textured solar cells a rough surface synthesization method is characterized and found applicable for generation of the morphology of commercial fluorine doped tin oxide surfaces. The extended rough interface scatterer is modeled by a Monte Carlo sampling of small interface representations. Model errors are induced by the small lateral computational domain extent and the choice of lateral boundary conditions. A quantification of these errors yields that a sampling of relatively small domain widths is sufficient for modeling extended rough surfaces in silicon thin film devices. Cell efficiencies resulting from the simulation of 2D rough surfaces and 1D surface cuts are compared. Finally, a commonly employed statistical ray tracing algorithm is evaluated against rigorous simulation for a test case. For light trapping texture design of polycrystalline thin film devices, the crystal growth characteristics need to be strongly considered. Simulations of a periodic light trapping texture are done in close connection to experimental development. A precise geometrical model is first reconstructed from cross–sectional images of the experimental structure. A comparison of optical absorptance measurements with the simulated absorptance of the model yields a very good quantitative agreement. The obtained model is completed to a full solar cell and further analyzed by scaling and by back reflector variation. Elevated light path improvement factors, however still below the statistical limit due to the parasitic absorption included in the simulated model, are found for specific texture periods. The results from the scaling analysis highlight the importance of achieving a few micrometers layer thickness of the deposited silicon to be able to attain high quantum efficiencies in solar cells. A considerable enhancement is reached by employing a detached flat back reflector with the studied silicon structures. The resulting simulated cells have a single–pass comparable absorptance of more than 37 µm of silicon. A different field of research, for which the periodic patterning and polycrystalline silicon growth methods - developed for microstructuring solar cell absorbers - might be applicable, is planar photonic crystal structures. In a first test, the general quality of a patterned and silicon coated substrate is assessed by a comparison of specular reflectivity measurements to simulated band structures. Good agreement is found between experiment and simulation.