The effects of impurities on the properties of Cz-grown silicon for solar cells
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The crystalline silicon solar cell has been dominating the PV market since it was introduced in the 1950´s. About 90% of solar modules currently installed are based on crystalline silicon, with cost-effective and high efficiency solar cells the long-term objective. To achieve cost- and energy-effective silicon materials, alternative feedstock such as metallurgically produced compensated solar grade silicon, and scrap from ingot growth may be used in the Cz process. Based on Cz material pulled from these types of feedstock and other, high carbon containing materials, systematic studies of the effects of impurities and defects on the bulk and solar cell properties, were carried out. Results of these studies may be summarized as: (1) The properties of p-type Cz-silicon produced with recycled top cuts and compensated solar grade (SoG) feedstock were initially investigated. Impurity levels in two experimental Cz ingots were characterized by glow dischage mass spectroscopy (GDMS) and Fourier Transformed Infra Red Spectroscopy (FTIR). The combined effects of oxygen, carbon, metallic- and dopant impurities on recombination properties were investigated by photoluminescence imaging (PL) and compared to bulk properties of a typical electronic grade (EG) silicon ingot. Ring pattern distribution of as-grown micro-defects (GMDs) in the ingots, related to elevated carbon and oxygen levels were delineated based on the results of two steps of dry oxidation. Moreover, the mechanism of defect formation in the experimental Cz-Si during solidification and oxidation was elaborated in light of thermodynamic theories. Meanwhile, the positive effect of phosphorus, accredited to P-Vacancy complexes in compensated feedstock, was discussed in relation to minority carrier lifetime, based on quasi steady state photoconductance (QSSPC) results, as well as on the formation of oxygen-related defects during solidification. (2) The effects of substitutional Carbon in the Si crystal lattice, originating from recycled materials and solidification processes were studied in detail. Three n-type Cz ingots with different carbon levels were used in the investigation. Copper decoration was used to quantify the number of as-grown defects, while a two step oxidation process (4 h at 750 ºC and 16 h at 1050 ºC) was used to study the evolution of as-grown defects and the formation and morphology of oxygen precipitates, stacking faults and smaller precipitates in the silicon during heat treatment. Carrier Density Imaging (CDI) revealed the defect distribution and distinguished their states. Results from the study show that substitutional carbon enhances the formation of a higher number density but smaller size as-grown oxygen defects, while the effective minority carrier lifetime in the all samples showed stronger dependence on the oxygen concentration than on the carbon content. (3) Temperature-dependent electrical properties were investigated with respect to compensation, via Hall Effect measurements in the temperature range 77-350K for the aforementioned p-type Cz-grown solar grade silicon ingots. The dependence of bulk properties including Hall mobility, carrier density, and resistivity on temperature were determined to steam from compensation and oxygen related micro-defects respectively. The electrical performances were demonstrated to be strongly dependent on the compensation level RC at low temperature (~77K). Top cut materials substituting poly-silicon to adjust the compensation level in Cz-silicon grown using compensated solar grade silicon gives rise to a more uniform resistivity along the silicon ingot, in addition to improving the Hall mobility. These effects were attributed to the presence of Al impurities, which result in similar carrier transport properties as Boron. Incompletely ionized dopants Boron and Phosphorous were observed through the measured carrier density, showing observable impact on Hall mobility around room temperature (RT) as phosphorus-vacancies complexes. (4) The experimental ingots produced from compensated feedstock and top-cut materials were also used to manufacture solar cells. In the processing of the solar cells, the phosphorous diffusion process was optimized to improve the bulk properties and thus to maximize the final solar cell characteristics. The solar cells produced from the investigated ingots showed efficiency values up to 18.5% and fill factor (FF) values up to 79%, comparable to conventional silicon produced from poly silicon. Appropriate compensation was finally demonstrated to be an efficient way to improve solar cells efficiency of Cz silicon produced from recycled silicon, even though higher dopant concentration incurred relatively faster light induced degradation.