Material properties of n-type silicon and n-type UMG solar cells (2016)

This thesis focuses on understanding the electrical properties of n-type mono-crystalline silicon. Such material has been widely used for high efficiency solar cells and has the potential to be used for low-cost high-efficiency devices and ultra-high efficiency devices. However, the fundamental properties of n-type silicon, such as the mobility of electrons and holes as well as the influence of growth and feedstock related defects on the minority carrier lifetime remains unclear. This thesis clarifies these issues and brings new understanding regarding the influence of excess carrier density and temperature on the mobility in n-type silicon, the influence of grown-in defects on the minority carrier lifetime in n-type silicon and the influence of feedstock related defects in n-type UMG silicon. In addition, this thesis also demonstrates the applicability of n-type upgraded metallurgical grade silicon as an alternative source of feedstock for high efficiency crystalline silicon solar cells. We experimentally measured the carrier mobility sum in n-type silicon as a function of doping density, injection level and temperature. Based on the measurements, an empirical model is derived over a temperature range from 150K to 450K. The empirical model shows good agreement with the existing mobility models, for instance, Klaassen’s mobility model. The model is then further extended to include data from p-type silicon and shows good agreement. This model provides experimental confidence for the use of existing mobility models in modelling highly injected bulk regions of high efficiency solar cells. We also investigate the impact of intrinsic-related lifetime-limiting defects in n-type as-grown Czochralski (Cz) silicon. The thermal stability and annihilation activation energy of the defects are measured based on minority carrier lifetime measurements. It is found that these defects in as-grown silicon can significantly reduce the minority carrier lifetime in n-type Cz silicon by several hundreds of microseconds and thus affect the performance of high efficiency solar cells. However, we demonstrate that these defects can be thermally annihilated at low temperature between 150 degree C to 300 degree C. Based on the annealing characteristics of the defects, it is found that these defects could be related to vacancies and are identified to be vacancy-phosphorus (VP) and vacancy-oxygen (VO), pairs by comparing to studies in irradiated silicon by Electron Paramagnetic Resonance (EPR) measurements. As n-type silicon can tolerate more impurities than p-type silicon, n-type solar grade silicon is a potential candidate for low cost high-efficiency solar cells. In this thesis, we used 100% upgraded metallurgical grade silicon (UMG) purified from a metallurgical route to fabricate silicon solar cells. We showed that the bulk lifetime is significantly affected by the boron diffusion and can be recovered by the phosphorus diffusion. An etch-back approach is applied to the cell fabrication process to maintain a relatively high bulk lifetime in the final state of the cells. A record efficiency of 21.6% is achieved using this process. However, UMG silicon is compensated, thus, the presence of boron still leads to the formation of boron-oxygen defects in n-type UMG silicon. The effect of light-induced degradation due to boron-oxygen defects is quantified and strategies for permanent deactivation of boron-oxygen defects in cell level are investigated. We demonstrate that the boron-oxygen defects in the cell level can be partially permanently deactivated through annealing under illumination. Moreover, simulation and loss analysis reveals that the lower mobility in UMG material due to material compensation does not lead to significant losses in the cell. With the current structure and material, UMG solar cells with efficiencies above 22% are achievable.