Migration Behavior of Impurity Iron in Silicon Melt Under Pulsed Electric Current

doi:10.1007/s40195-024-01667-3

Abstract

Abstract:

The impurity iron in silicon material will seriously affect the photoelectric conversion efficiency of silicon solar cells. However, the traditional silicon purification method has the disadvantages of long cycle, high energy consumption and serious pollution. In this study, an efficient and green pulsed electric current purification technology is proposed. The electromigration effect of iron elements, the current density gradient driving of iron phase, and the gravity of iron phase all affect the migration behavior of iron phase in silicon melt under pulsed electric current. Regardless of the depth of electrode insertion into the silicon melt, the solubility of iron in silicon decreases under the pulsed electric current, which helps to form the iron phase. At the same time, the iron phase tends to sink toward the bottom under the influence of gravity. When the electrode is shallowly inserted, a non-uniform electric field is formed in the silicon melt, and the iron phase is mainly driven by the current density gradient to accelerate sink toward the bottom. When the electrode is fully inserted, an approximately uniform electric field is formed in the silicon melt, and iron elements are preferentially migrated to the cathode by electromigration, forming iron phase sinking at the cathode. The study of impurity iron migration behavior in silicon melt under pulsed electric current provides a new approach for the purification of polycrystalline silicon.

Key words: Metallurgical silicon, Pulsed electric current, Iron-rich phase, Current density gradient, Electromigration

Mengcheng Zhou, Yaxiong Dai, Changhao Liu, Shengli Ding, Xinfang Zhang. Migration Behavior of Impurity Iron in Silicon Melt Under Pulsed Electric Current[J]. Acta Metallurgica Sinica (English Letters), 2024, 37(5): 889-903.

Figures/Tables 17

Table 1 Chemical composition of metallurgical silicon (wt%)

Si	Fe	Ca	Al	V	Ti
Bal.	0.3381	0.0144	0.0685	0.0474	0.0407

Fig. 1 a Schematic diagram of experimental equipment; b different insertion methods of electrodes: shallow insertion (the electrode is inserted into 1/4 of the melt) and full insertion; c schematic diagram of sample cutting and sampling position

Table 2 Experimental parameters of EPT treatment

Electrode insertion method	Samples	Frequency (Hz)	Current (A)	Duty cycle ratio (%)
	Without EPT	-	-	-
Shallow insertion and full insertion	EPT-200 Hz	200	150	20
	EPT-1000 Hz	1000	150	20
	EPT-2500 Hz	2500	150	20
	EPT-3500 Hz	3500	150	20
Shallow insertion	EPT-20A	1000	20	20
	EPT-80A	1000	80	20
	EPT-150A	1000	150	20
Full insertion	EPT-20A	1500	20	20
	EPT-80A	1500	80	20
	EPT-150A	1500	150	20

Fig. 2 a Solidified silicon ingots; b micromorphology of rich iron phase; c, d EDS images of blocky iron phase and strip iron phase

Fig. 3 a SEM images of iron phase distribution in different areas of the sample without pulsed electric current; b schematic diagram of the distribution area of large-sized iron phases (the yellow area represents iron phase aggregation area with a length greater than 200 μm); c area percentage of iron phase from left area to right area of the sample without EPT; d area percentage of iron phase from top area to bottom area of the sample without EPT

Fig. 4 Distribution of iron phase at different pulsed frequency under shallow insertion: a sample EPT-200 Hz, b sample EPT-1000 Hz, c sample EPT-2500 Hz, d sample EPT-3500 Hz

Fig. 5 Histogram of iron phase area percentage from top to bottom at different pulsed frequencies under shallow insertion: a sample EPT-200 Hz, b sample EPT-1000 Hz, c sample EPT-2500 Hz, d sample EPT-3500 Hz

Fig. 6 Distribution of iron phase at different pulsed frequencies under full insertion: a sample EPT-200 Hz, b sample EPT-1000 Hz, c sample EPT-2500 Hz, d sample EPT-3500 Hz

Fig. 7 Histogram of iron phase area percentage from anode to cathode at different pulsed frequencies under full insertion: a sample EPT-200 Hz, b sample EPT-1000 Hz, c sample EPT-2500 Hz, d sample EPT-3500 Hz

Fig. 8 Distribution of iron phase in different regions under different electrode insertion methods: a from top to bottom, b from anode to cathode

Fig. 9 Equivalent size of iron phase at different pulsed frequencies under shallow insertion

Fig. 10 Equivalent size of iron phase at different pulsed frequencies under full insertion

Fig. 11 Variation of relative resistance of polycrystalline silicon over time

Fig. 12 a Distribution of current lines and b, c distribution of current density under shallow insertion

Fig. 13 Distribution of iron phase at different pulsed current intensities under shallow insertion: a sample EPT-20A, b sample EPT-80A, c sample EPT-150A

Fig. 14 a Distribution of current lines and b, c distribution of current density under full insertion

Fig. 15 Distribution of iron phase at different pulsed current intensity under full insertion: a sample EPT-20A, b sample EPT-80A, c sample EPT-150A

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