Performance limits of silicon solar cells due to structural defects

Abstract

To minimize the risk of catastrophic climate change, about ten terawatts of photovoltaics must be deployed in the next fifteen years. Reaching this target will require dramatic reductions in the cost and capital intensity of manufacturing photovoltaic modules coupled with a significant increase in module efficiency. The majority of the factory and equipment costs to produce the crystalline silicon modules that account for over 90% of modules sold today are for production of silicon wafers. While lower-cost wafers can be produced with cheaper equipment, the efficiency of modules incorporating these wafers is limited by the presence of structural defects, like grain boundaries and dislocations, that are absent from more expensive alternatives. This thesis presents a methodology to quantify the technology innovations necessary to reach climate-driven deployment targets for photovoltaics and shows an analysis based on current commercial technology incorporating monocrystalline silicon absorbers. Then, a model for the electrical activity of dislocations and grain boundaries and a methodology for incorporating this model into technology computer aided design (TCAD) simulations of high-efficiency solar cells are presented. The model and method are validated by comparison to analysis of the material properties and device performance of silicon solar cells containing structural defects. TCAD simulations across a wide range of defect concentrations and distributions are used to determine the material requirements for low-cost silicon containing structural defects to approach the performance of expensive, structural defect-free silicon in several high-efficiency solar cell architectures. Aspects of device design that mitigate the impact of these defects, notably higher injection-levels of electronic carriers, are identified.