Separation of Impurity Iron from Polysilicon by Pulsed Electric Current
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https://doi.org/10.1007/s11837-020-04265-7 Ó 2020 The Minerals, Metals & Materials Society
SOLIDIFICATION BEHAVIOR IN THE PRESENCE OF EXTERNAL FIELDS
Separation of Impurity Iron from Polysilicon by Pulsed Electric Current SHAOFEI REN,1 LONGGE YAN,1 XINFANG ZHANG,1,4 HUI WANG,2 and YABO FU3,5 1.—State Key Laboratory of Advanced Metallurgy, School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China. 2.—Science and Technology on Reactor Fuel and Materials Laboratory, Nuclear Power Institute of China, Chengdu, Sichuan 610041, People’s Republic of China. 3.—Beijing Key Lab of Printing & Packaging Materials and Technology, Beijing Institute of Graphic Communication, Beijing 102600, People’s Republic of China. 4.—e-mail: [email protected]. 5.—e-mail: [email protected]
The effect of using a pulsed electric current to remove impurity iron from molten polysilicon was investigated. Using optical microscopy observation and area statistics of iron-rich content, it was found that iron tends to accumulate at the bottom of an ingot under the action of a pulsed electric current. A new separation mechanism is proposed, based on the decreased solubility of iron in polysilicon under conditions including a pulsed electric current. Thermodynamic calculations indicate the theoretical possibility of the formation of ironrich Si clusters. These clusters sink to the bottom of an ingot under the effect of gravity and form iron-rich precipitates with silicon, thereby achieving the iron removal. This technique provides a new method for purification of polysilicon.
INTRODUCTION With the continuous consumption of traditional fossil fuels and increasing energy requirements, humankind is facing great challenges in energy demand. One approach to meet this demand is the use of solar photovoltaic technology.1–3 As a new energy source, solar energy is cheap, safe, reliable, clean, and sustainable,4–6 and it eases pressure on other energy sources to some extent. As solar energy usage continues to grow rapidly, metallurgicalgrade polysilicon, which is the main raw material for solar cells and offers high performance and competitive cost, is becoming widely used.7,8 However, metallurgical-grade polysilicon contains a range of impurities that seriously affect the photoelectric conversion efficiency and restrict its development. Iron is the most common metallic impurity of polysilicon. Its presence deteriorates the performance of polysilicon material because it acts as a recombination center for minority carriers and
(Received February 24, 2020; accepted June 29, 2020)
reduces their lifetime.9,10 Studies have shown that, when the iron content increases from 0.01 ppm to 0.1 ppm, the conversion efficiency of solar-grade polysilicon decreases by 16.7%.11 Therefore, it is critical to separate impurity iron from polysilicon. Many studies have focused on the removal of impurity Fe, including the use of traditional chemical processes as well as recent metallurgical techniqu
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