Search

The influence of charge components on emergence of a homogeneous phase of silicon in the process of silicon dioxide reduction by aluminium has been studied. Optimal process parameters affecting the quality of the end product have been identified. These include the ratio of components of the synthetic charge (CaO, SiO2 , CaF2 ), the optimal amount of SiO2 , the optimal amount of a reducing agent (Al) and the optimal Si/slag ratio. The homogeneous phase of silicon is easily separated from the slag, which contributes to the reduction of waste silicon during further technological operations.

Nowadays, compensated silicon (Si) is used in photovoltaic (PV) processes, whether it is through intentional co-doping of resistivity-adjusted Czochralski ingots for high efficiency n-type Si solar cells, as a result of alternative Si purification processes for the production of low-cost Si feedstock, or as a result of recycling end-of-life materials. Whatever the origin of the compensated Si, the doping concentrations need to be accurately and quickly characterized in order to control such processes. In this work, a rapid and highly sensitive characterization technique based on low temperature Hall Effect measurements is described in scientific details and compared to three well-established chemical methods: Glow Discharge Mass Spectrometry (GDMS), Inductively-Coupled Plasma Mass Spectrometry (ICP-MS), and Secondary Ion Mass Spectrometry (SIMS). The characterized samples were extracted from the n-type top part of a casted solar grade Si ingot. A very good agreement is observed between the dopants densities extracted from the electrical method and from the standard methods. With the advantage of a very low detection limit combined with a short measurement time, the advanced Hall Effect technique is promising for the rapid and accurate characterization of dopant concentrations in compensated Si. [ABSTRACT FROM AUTHOR]

Abstract: In this study we have investigated two multicrystalline silicon ingots made from a compensated solar grade silicon (SoG-Si) test feedstock. Both ingots were solidified under the same conditions in a pilot-scale directional solidification furnace. In order to study the distribution and the impact of Cr in the compensated SoG-Si feedstock, 50ppmw Cr (27ppma) was added to one of the ingots. Electrical properties, such as resistivity and minority carrier lifetime, have been compared for the two ingots. Solar cells performance is also presented. The minority carrier lifetime is significantly reduced by the addition of Cr, being up to 19μs for the undoped reference ingot, and <1μs for the Cr-doped material. The maximum solar cell conversion efficiency of the solar cells made from the Cr-doped compensated solar grade silicon in this study is 14.4%, which is similar to the cells made from the ingot with no Cr addition. This work shows that Cr contamination significantly impairs electrical properties but they can, to some degree, be improved by gettering during solar cell processing. [Copyright &y& Elsevier]

Abstract: In this study, it was aimed to remove boron, one of the major impurities of MG-Si (metallic grade silicon), in molten silicon by a steam plasma method utilizing EMC (electro magnetic casting) process. In order to effectively remove boron, less volatile than silicon, plasma injection gas was systematically varied. As a result, the most effective way to remove boron was additionally injecting hydrogen gas and steam with argon gas, which is a reactive gas of a general plasma injection routine. Although mass loss of silicon increased with increase in plasma injection time, the refining effect of boron and several metallic impurities like aluminium and titanium was improved significantly. [ABSTRACT FROM AUTHOR]

An estimate has been made of the feasibility of a metallurgical purification process, the NEDO (New Energy and Industrial Technology Development Organization) melt-purification process, for manufacturing solar-grade silicon from metallurgical-grade silicon. Equipment has been developed to pilot manufacturing plant scale. The system comprises an electron-beam furnace for phosphorus removal and a plasma furnace for boron removal. Each furnace has a mold for directional solidification to remove metallic impurities. The concentration of each impurity in the silicon ingot purified through the whole process satisfied the solar-grade level. The Solar-grade silicon produced showed p-type polarity and resistivity within the range 075-175 omega cm. [ABSTRACT FROM AUTHOR]