High Efficiency solar cells based on Czochralski-grown upgraded metallurgical-grade silicon wafers

Abstract

With the expansion of the PV industry, the cost is coming down significantly. Still, polysilicon is one of the highest capital expenditure components of the PV module production. Solar grade silicon feedstock material purified from a metallurgical route, known as upgraded metallurgical grade (UMG) silicon feedstock, can be a low cost and a smaller carbon footprint alternative for standard electronic grade silicon (EG Si). However, metallurgical refinement techniques are less efficient at removing impurities than the conventional Siemens process, and hence UMG-Si usually contains more impurities than EG Si. These impurities can limit the bulk quality in the as-grown state and enhance degradation during high temperature processes, as a result reducing efficiency. The reduction in efficiency can directly increase the cost of the total PV systems. Thus any significant reduction in efficiency is not tolerable, even if the feedstock cost can be strongly reduced. Firstly, in this thesis, defect engineering of the n-type UMG Cz wafers was performed and demonstrated that the bulk quality of the UMG Cz wafers was primarily limited by the grown in oxygen precipitate nuclei as well as metallic impurities. Further, after a high temperature processing step such as boron diffusion, the bulk quality of the UMG Cz degraded significantly due to ring defects. As a result, the performance of solar cells fabricated on the as grown UMG Cz wafers will be limited by their bulk quality. Fortunately, several preventive and curative treatments, namely tabula rasa, phosphorus diffusion gettering, and hydrogenation in isolation and combination, were identified and demonstrated to improve the bulk quality of these wafers. Further, several experimental investigations were performed on the formation of ring defects in both EG Cz and UMG Cz silicon wafers. A faster onset of ring defects in UMG Cz samples was observed in comparison to EG Cz samples. Furthermore, the combination of FTIR line scans and micro PL and micro Raman maps showed that the ring defects which appear as a continuous band under standard PL image are in fact, a cumulative effect of individual recombination sites. Lastly, the thesis presents solar cell results to elucidate the benefit of applying pre fabrication treatments in these UMG wafers. With the pre-fabrication treatments, the silicon heterojunction solar cells based on the UMG Cz and EG Cz wafers showed a marked improvement in the efficiency from 18.0% to 21.2% and 21.2% to 22.7%, respectively. Comparison of the open circuit voltages of the as grown and pre-treated UMG Cz and EG Cz cells using Quokka simulations reveals that the bulk lifetime remains the primary limiting factor for the UMG Cz wafers. Also, in this thesis, a high efficiency solar cell process is modified by selecting a single boron diffusion step and applying phosphorus doped polycrystalline films as electron selective contacts with excellent impurity gettering properties to minimize the thermal budget. The application of this modified high efficiency solar cell process to n type UMG Cz wafers results in a solar cell with a record conversion efficiency of 22.6%. Show more