Commercially available solar cells are dominated by silicon based photovoltaic cells. The silicon used in photovoltaic industry are 99.9999 percent purity called solar grade silicon. However, the problem with solar grade silicon is its expensiveness and the lack of dedicated process for the manufacture of solar grade silicon. The current manufacturing of solar grade silicon is done by purification of metallurgical grade silicon, which is a complex process, highly energy intensive and involves emission of corrosive and toxic gases. Alternative approach would be to use plant biomass for the extraction of amorphous silica to be processed further to obtain silicon.
Extraction of amorphous silica from plant biomass has been of focus for research to address the problems of solar cell manufacturing. Metallic impurities in silica obtained from plant biomass are lesser than the conventional sources like quartz and diatomaceous earth. Rice husk is reported to have the highest content of silica among all the plant materials. In this study, rice husk was used to extract high-purity silicon. This was done by extracting silica of high purity through pre-treatment of rice husk and optimization of time and temperature for the process. This was followed by magnesiothermic reduction process to obtain silicon.
The higher cost of raw material for photovoltaic cell manufacturing is due to the escalated demand for solar grade silicon compared to the supply. The reasons for the gap between the demand and supply of solar grade silicon are lack of direct process method for its production, and the requirement of large quantity of raw material unlike the electronic grade which requires lesser quantity of higher purity silicon. This research would be beneficial as a background work for up scaling of the process for obtaining high purity silicon from rice husk, which would be the raw material for making of photovoltaic cells.
In this project, silica of high purity of 99.96 percent was obtained through pretreatments using three acids followed by combustion. Use of ethylene-diamine-tetraacetic acid was found to be crucial for achieving higher purity which was not reported in any previous work done in this area. Time and temperature for obtaining silica without carbon presence in the residue was optimized by kinetic modelling. High purity silica was used for reduction process to obtain silicon. Stoichiometric ratio of silica: magnesium accounting to highest recovery of silicon through reduction process was found. Silicon was successfully separated from other by-products and was found to be 99.93 percent pure.
India's Sustainable Development
One of the technological gap faced by India are the lack of commercially feasible route for manufacturing of high purity silicon. India is the second highest producer of rice in the world. The rice husk obtained as by-product in milling industry is seen as a waste and burned. This leads to pollution of the atmosphere. The value addition of rice husk for extracting silica and hence obtaining silicon would serve converting waste to wealth. Further, the lag in meeting the demand for raw material required for photovoltaic industry can be met with. This also helps reduce the initial cost of solar cells.
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