Synthesis of Solar‐Grade Silicon from Rice Husk Ash – An Integrated Process

Abstract

Impurity optimized silicon is needed for the advancement of terrestrial photovoltaic power generation. In this study an approach to synthesis of solar grade silicon using rice husk ash has been pursued. Metallothermic reduction of the purified rice husk ash (RHA) was investigated within the temperature range of 500-950 °C using magnesium in varied amounts. The reduction product was purified by two stage acid leaching sequence. Analysis of the final silicon product by XRD, SEM, and ICP-OES showed crystalline silicon with boron to be less than 3 ppm, corresponding to a reduction by a factor greater than 10 while the phosphorus level was reduced by a factor of over 20 reaching less than 73 ppm. Transition metal impurities and other elements were generally reduced in the processing steps. INTRODUCTIONWorldwide concerns over energy related climate change coupled with spiralling cost and resource-scarcity of fossil based fuels in recent years has provoked interests in renewable and alternate energy technologies. Solar photovoltaic power is one of the main renewable energy alternatives being actively pursued worldwide. Although solar photovoltaic power production is a proven sustainable energy technology especially in the aerospace industry, development of this technology to meet terrestrial energy demands has been largely limited. One of the well known hindrances to widespread use of PV technology is the prohibitively high unit cost of solar PV generated electricity which is partly attributed to cost of substrate material used in fabrication of efficient solar cells. ' Presently, silicon in both monocrystalline and polycrystalline form is the dominant semiconductor material used in the production of most commercially available high efficient solar cells, commanding over 90% of the market share of existing PV technologies. 2 Much of the past and present research efforts to produce low-cost solar grade silicon have focussed on either upgrading metallurgical grade silicon by metallurgical refining processes or modification of the conventional Siemens process to produce silicon with purity in the range of 5-7N which is recognized to be suitable for fabrication of high efficient silicon solar cells 3 Up until now, the possibility of producing solar grade silicon from biomass resources such as rice husk ash which is known to have high purity silica content has only been explored by relatively few research groups.Singh and Dhindaw 4 reported producing polycrystalline silicon of 6N purity by magnesium reduction of rice husk ash followed by successive acid leaching refining. Subsequently, Böse et al ,

Influence of roasting-quenching pretreatment on the rice husk silica prepared by calcination method

“…However, the obtained silica will have different properties for different process or methods used. Thus, many RH silica-based products can be produced, such as silica gel [3], catalysts [4][5], biomedical applications [6], semi-conductor materials [7], dental composite filler [8], lithium battery anode [9], nanotube material [10], as well as solar grade silicon feedstock [11].…”