Design, fabrication and analysis of high efficiency multicrystalline silicon solar cells

Demand for energy has been increasing at a rate commensurate with the depletion of conventional energy sources. Among the various alternatives available today, solar energy is particularly attractive because it is not only free but inexhaustible and not localized in any part of the world. Photovoltaics (PV) is an ideal candidate to tap this enormous energy source because solar cells can convert sunlight directly into electricity without any undesirable impact on the environment. Low-cost and high-efficiency are the keys for large scale applicability of photovoltaic systems. Large grain multicrystalline silicon is a strong contender for cost-effective PV because of low material cost and potential for high efficiency. Prior techniques for cell fabrication suffered from substandard cell performance since multicrystalline silicon growth techniques, such as ingot casting, reduces the cost of multicrystalline silicon but at the same time introduces defects, impurities and grain boundaries, which tend to degrade solar cell performance. In this thesis, improvement of fundamental understanding of efficiency limiting defects and mechanisms along with the development and optimization of post-growth quality enhancement techniques, such as aluminum and phosphorus gettering, and hydrogen defect passivation by forming gas anneal led to a world record high efficiency of 17.8% on multicrystalline silicon substrate. These gettering and passivation techniques proved extremely beneficial in mitigating the impact of defects and impurities while being highly compatible with silicon cell processing. Without the addition of any extra processing steps, all of these techniques were integrated into an advanced process sequence which consistently produces record cells on low-cost defective materials. Constant feedback from in depth materials characterization and numerous model calculations proved essential for process optimization and for establishing guidelines for future research that will achieve 20% efficient multicrystalline silicon solar cells.