Michl, B: Material Limits of Multicrystalline Silicon

Multicrystalline silicon from directional solidification currently has the highest market share of all materials in silicon photovoltaics. The inferior electrical material quality compared to monocrystalline silicon drives the need for a detailed understanding of the material-caused efficiency limits. This question is tackled in thesis by a wide spectrum of characterization methods, cell processes, and material classes with the aim of gaining a complete picture of the material limits of multicrystalline silicon. The challenge of laterally varying and injection-dependent recombination in mc-Si is approached by a set of characterization techniques based on photoluminescence imaging that allow for a detailed, quantitative and spatially resolved characterization of efficiency losses. With these powerful characterization methods the impact of different improvements in the production process is quantitatively assessed: increased feedstock and crystallization crucible purity, doping with phosphorus instead of boron, and the influence of wafer thickness. The analysis is closely linked to advanced solar cell processing, including optimized phosphorus diffusion, boron diffusion, dielectric rear side passivation, honeycomb texturing, and silicon heterojunctions.