Low-cost solar grade silicon purification process with Al–Si system using a powder metallurgy technique

Separation and Purification Technology 77 (2011) 33–39

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Low-cost solar grade silicon purification process with Al–Si system using a powder metallurgy technique Xin Gu, Xuegong Yu, Deren Yang ∗ State Key Lab of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, PR China

a r t i c l e

i n f o

Article history: Received 7 August 2010 Received in revised form 11 November 2010 Accepted 14 November 2010 Keywords: Al–Si Solar grade Si Purification Powder metallurgy

a b s t r a c t Silicon solar cell is one of the cleanest and most potential renewable resources. However, the high cost of raw material is impeding the development of silicon solar cell. In this paper, we have investigated a purification process designed for low-cost solar grade silicon with Al–Si system, using a powder metallurgy technique. It is found that by modulating the external pressure and/or protection ambient, the alloying of Al–Si powder mixture is easily accomplished at a relatively lower temperature. The mechanism of Si products purified from the Al–Si melt has been discussed based on their morphological characterizations. The impurity contents of the purified Si products can be controlled at a very low level (∼3 ppmw). Meanwhile, the yield of the purified Si products using the powder metallurgy technique is clarified to be higher than that of the conventional Al–Si purification technique without external pressure or protecting ambient. The process is quite potential of providing low-cost solar grade silicon feedstock for photovoltaic industry. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Since conventional fossil fuels are drastically decreasing, there is a strong demand for sustainable energy sources, among which photovoltaics (PV) based on solar cells is undoubtedly one of the cleanest ways to produce electricity. In PV industry, Si materials have been widely employed for the fabrication of commercial solar cells for decades, since it is the second most abundance in the Earth’s crust and meanwhile has excellent electrical and mechanical performances [1]. But, nowadays, most of Si raw materials used for solar cells are high-cost electronic-grade, which act as barriers for the cost reduction of cell fabrication. So, to date, various methods of refining metallurgical grade (MG) Si to solar grade (SOG) have been proposed [2–9]. Fractional crystallization is a usual metallurgical route to remove impurities from MG Si, in which impurities are apt to be repelled from the solid/liquid interface and therefore be collected in the last solidified part [6]. After cycling several times, the main part of purified Si products will contain low concentrations of impurities. This approach is very effective to remove metallic impurities such as Cu, Al and Fe due to their small segregation coefficients (ratio of the impurity content in solid Si to that in liquid Si), which could be used for PV industry. However, it is less useful to remove impurities with relatively large segregation coefficients, like B and

∗ Corresponding author. Tel.: +86 571 87951667; fax: +86 571 87952322. E-mail addresses: [email protected], [email protected] (D. Yang). 1383-5866/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2010.11.016

P. Therefore other techniques such as plasma torch or electron beam treatment under oxidation ambient or vacuum conditions have to be employed to remove impurities like B and P [10–12]. But, these methods necessarily require large electricity consumption or specially designed equipments, which are against the requirement of reducing the cost. The process that refines MG Si at a temperature lower than Si melting point is developed to save energy consumption. It is based on the formation of Si-metal alloy, wherein metal acts as the solvent. The Al–Si eutectic system has been intensively investigated [13–19]. It has been known that the impurities in the liquid Al–Si alloy have a higher solubility than those in the liquid Si. The ratio of the impurity content in solid Si to that in liquid Al–Si alloy is much smaller, compared to ratio of impurity in solid Si to liquid Si, especially for that of B or P [15,17,18]. Therefore, the Al–Si alloying technique can be used for removing the impurities in MG Si. However, it is recently recognized that the purification of MG Si based on usual Al–Si alloying process still consumes considerable energy especially for powder Si purification. The heating temperature is practically higher than the melting point of Si (>1421 ◦ C) [20,21], requiring large energy consumption. The worse factor is that the silicon material is more easily contaminated at high temperature, which leads to higher content in the purified products. All these make Al–Si system purification less practicable for the PV application. The objective of this paper is to present a powder metallurgy technique [22] for the purification of MG Si, which can significantly reduce the thermal budget. The qualities of the upgraded MG Si

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products are also characterized by combining different measuring methods. This technique is a potential alternative to produce lowcost SOG feedstock for PV industry.

of 800–1250 ◦ C for 2–10 h, with a ramp-up rate of ∼100 ◦ C/min, and then cool down the melt to 600 ◦ C, with a ramp-down rate of less than 0.5 ◦ C/min; (4) at 600 ◦ C, separate the Al–Si alloy melt and collect the purified Si products by making use of the difference in the density between solid Si and Al–Si liquid (the details can be found in Refs. [13,14,23]); (5) clean the products by hydrochloric (HCl) solution leaching to remove the residual Al attachment on the surface of Si; (6) secondary cleaning of the products in a solution (HF:HNO3 :CH3 COOH = 1:3:15) to remove the near-surface region of products containing a higher concentration of Al. Note that the compositions of Al–Si alloys studied in this work are marked in the binary diagram, see Fig. 2 [24,25]. Next, the purified Si products were characterized by an optical microscope (OM), a scanning electron microscope (SEM) with energy dispersive X-ray fluorescence spectrometer (EDX), and Xray diffraction (XRD) technique. Meanwhile, the impurity contents in the Si products were determined by combining a spreading resistance profiling (SR) and ICP-MS technique together.

2. Experimental

3. Results and discussion

The starting materials are powder MG-Si and Al, both having a purity of 99% in weight. The contents of various impurities in the MG-Si are listed in Table 1, determined by inductively coupled plasma mass spectrometry (ICP-MS). Fig. 1 shows the schematic process of purification of MG-Si, which consists of the following steps: (1) mix MG-Si and Al in a steel-made hollow cylinder mould; (2) press the mixture into the solid bulk with external pressures; (3) heat to form alloy melt in the temperature range

3.1. Optimization of Al–Si purification process using powder metallurgy technique

Table 1 Impurity contents of MG Si and refined Si (ppmw). Impurities

MG Si

Refined Si

B Mg Al P Ca Fe Ni Cu Ti V Mn Zn

8 108 2500 13 250 3200 223 210 550 97 54 120

1.55 0.01 41.61 0.41 0.25 0.21 0.15 0.2 0.12 0.05 0.14 0.53

Fig. 3(a) shows the lowest melting temperatures for the formation of Al–Si alloy melt (Tm ) after simply heating the Si and Al powders in different weight ratios. Note that the lowest melting temperature means the actual temperature that the Si–Al mixtures melted, determined by a series of experiments in which heating

Fig. 1. Schematic process of purifying MG-Si using the powder metallurgical technique.

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35

a 1300

Lowest melting temperature Liquidus temperature

Temperature(°C)

1200

1100

1000

900

800 30

35

40

Si content(wt%) b 1300

30wt% 1200

35wt% 40wt%

temperature is the only variable. It can be seen that the theoretical liquidus temperatures (Te ) for Al–Si mixture derived from their binary phase diagram (Fig. 2), raise with an increase of Si content that is larger than 12.2 mol%. Tm basically follows the similar tendency with the increase of Si–Al ratio as Te . However, the Tm for the practical alloying process is much higher than the Te for the same Al–Si composition. This should be resultant from the formation of an oxidation layer on the surface of MG Si and Al powders. The oxide films can not only be native, but also be generated by the oxidation of Si and Al powders exposed in air at high temperatures. The oxide coating on the Al and Si powders will prevent the contact between Si and Al during alloying, and meanwhile cause a barrier for heat conduction in the alloying system. Hence, this purification process always needs a high Tm and therefore considerable thermal consumption to form Al–Si alloy melt. Fig. 3(b) shows the variation of lowest melting temperatures for the formation of Al–Si alloy melt with various Si/Al ratios under external pressure. Note that the heating is in air ambient. It can be seen that, with the same ratio of Si/Al in weight, the Tm decreases with an increase of the external pressure. Compared to simply mixing Al/Si powders without the external pressure, a reduction of Tm by 110–140 ◦ C can be obtained by applying a moderate (40 MPa) pressure and 130–170 ◦ C for a high pressure (80 MPa). The reduction of Tm is more pronounced in the case of the Si/Al mixture containing higher Si content. It is believed that the external pressure can reduce the gaps in-between Al and Si powders, and therefore reduce the air for the following oxidation of powders during the heating. Moreover, the pressure application can also enhance the direct contact of Al and Si powders by crashing the native oxide layers on their surfaces. Thus, a higher external pressure definitely results in a lower Tm for the formation of Al–Si alloy melt. Fig. 3(c) shows the lowest melting temperatures for the formation of Al–Si alloy melt versus various Si/Al ratios with a high external pressure and different protection ambient. It can be seen that compared to the air protection, the implement of Ar ambient can significantly reduce Tm by a value of 20–80 ◦ C. This should be associated with the fact that inert gas can prevent the oxidation of Al and Si powders during heating. More interestingly, with 10% hydrogen gas mixed into Ar ambient, Tm for Al–Si alloying can be further reduced by a value of 10–30 ◦ C. It is believed that not only can the mixture of Ar and H2 avoid the oxidation, but also the H2 component can reduce the native oxide grown on the Al and Si

Tm(°C)

1100

1000

900

800 0

40

80

External pressure (Mpa)

c

1200

Air Ar 1100

Tm(°C)

Fig. 2. The compositions of Al–Si alloys studied in this work, marked by gray. The binary phase diagram is taken from Ref. [25] but originally from Ref. [24].

90% Ar+10% H2

1000

900

800 30

35

40

Si content(wt%) Fig. 3. Lowest melting temperatures for the formation of Al–Si alloy (Tm ) by heating Si and Al powders in various ratios in weight under different conditions. (a) In air ambient, without external pressure. The theoretical liquidus temperatures (Te ) are also shown. (b) In air ambient, with different external pressures. (c) Under a high external pressure (80 MPa), in different protection ambient.

powders. This consequently benefits the reduction of Tm for the formation of Al–Si alloy melt. 3.2. Morphological characterization of purified Si products Fig. 4(a) shows an OM image of the cross-section of the Al–Si alloy bulk which was obtained by fast cooling the Al–Si melt to room temperature in 10% NaOH solution (>2000 ◦ C/s) [26] without collecting Si products. It can be easily recognized that a high density of Si crystals with a thickness of 50–60 ␮m are randomly distributed in the Al–Si alloy. More interestingly, most of Si products exist as pairs, between which there is an impurity inclusion layer, as the enlarged micrograph shown in Fig. 4(b). Then Fig. 4(c)

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Fig. 4. (a) Cross-sectional micrograph of an intermediate product measured by an OM after cooling the Al–Si alloy melt to room temperature. (b) Enlarged OM micrograph of a Si product embedded in the Al–Si alloy in (a). A red dashed circle denotes the impurity inclusion layer. (c) SEM micrograph of the cross-section of a Si product before HCl leaching. An EDX line scan is along the yellow line. (d) EDX line profile of Al. (e) SEM micrograph of the cross-section of a Si product after HCl leaching. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of the article.)

shows the SEM image of the cross-section of the inclusion layer. Because the sample was mechanically polished before SEM measurement, the bright phase in the image was probably caused by the lower resistivity of the inclusion layer due to the potential contrast [27]. To determine what composes the inclusion layer, an EDX line scan was carried out across yellow line denoted in the image

and the result is depicted in Fig. 4(d). An Al step occurs in the profile and has an approximate width of 2 ␮m, the coincidence of Al width suggesting that Al is the main impurity existing between the Si pairs. After leaching in HCl solution, a gap between Si pair appears, as shown in Fig. 4(e). This indicates that the Al inclusion layer can be easily removed by HCl solution leaching.

Concentration (ppmw)

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37

Upper limit[32]

10000

This paper before DS This paper after one DS 100

1

0.01

B

Al

P

Fe

Ni

CuTi,V,Cr

Fig. 7. Impurity contents of the final refined Si products and the ones after one time directional solidification, as well as their endurable upper limits (Ref. [32]) of concentration acceptable for SOG Si feedstocks. Since the content of Fe, Ni, Cu, Ti, V and Cr is very small, the exact values are not presented here. Fig. 5. Digital camera photograph of a typical Si product after HCl leaching. The inset is the corresponding XRD spectrum.

For practical application, the purified Si products can be collected after discharging the Al–Si alloy melt from the crucible at 600 ◦ C. After acid leaching and deionized water cleaning, the Si products generally exhibit sheet-like shape with a diameter of ∼1.5 cm. Fig. 5 shows an example of such Si products. The inset in Fig. 5 is the XRD profile of the Si product with a normal incidence of X-ray to its plane. Note that only one peak associated with 1 1 1 Si appears in the spectrum, which indicates that the product is single crystalline Si with 1 1 1 crystallographic orientation. It is well known that during the cooling down of Al–Si melt, the supersaturated Si is inclined to segregate. According to Wulff construction that helps determine the equilibrium shape of a crystal [28], the segregated Si should be apt to form the 1 1 1 orientated crystals in the melt. At a low cooling rate (<0.5 ◦ C/min) that is adopted for purification process, the twin plane re-entrant edge growth mechanism [29] is dominant and the Si crystal tends to form sheet-like morphology [30]. Moreover, the Si sheets can change its growth direction by multiple twinning [31], which results in an increase of the inter-flake spacing for further growth. Thus, the gap between each two Si sheets in a pair occurs first and then is filled with Al-rich melt. 3.3. Impurities in purified Si products Fig. 6 shows the distribution of electrically active dopant concentration along the cross-section of a Si product after HCl solution 18

3

Dopant con.(atoms/cm )

8.0x10

18

Step

Step

6.0x10

18

4.0x10

18

2.0x10

0.0

0

3

6

9

12

15

Depth (μm) Fig. 6. SR analysis profile along the cross-section of a Si product before the high Al concentration region is etched.

leaching, which is measured by SRP technique. Note that the concentration of electrically active dopants is equal to the carrier concentration. Near the surface of the purified Si product, the concentration of carriers is up to about 5 × 1018 atoms/cm3 , while in deeper region, the carrier concentration declines to the value less than half of that near the surface. At a depth of 11 ␮m, the carrier concentration further decreases to 1 × 1018 atoms/cm3 . Since the Al atoms in Si mostly stay at the substitutional sites in Si lattice, acting as acceptors, the carrier concentrations actually reflect the distribution of Al atoms. Theoretically, the concentrations of Al in the purified Si product must be equal to the solid solubility of Al in Si during solidification from the liquid phase. Hence, it is inferred that the highest concentration of Al near the surface of Si product should correspond to its solubility at the temperature of forming alloy melt (950 ◦ C), about 5 × 1018 atoms/cm3 [24]. Similarly, since the crystallization temperature is 600 ◦ C, the Al concentration in the bulk of Si product approximately equals to the solubility of Al in Si at 600 ◦ C. However, the formation of the two relatively abrupt steps in the carrier profile near the product surface is not yet clear by now. It is probably related to a fast cooling process from 950 ◦ C to 600 ◦ C, and also a short-time crystallization process. Based on the SR profile, it is suggested that the near surface region containing a higher concentration of Al should be removed for further purifying the Si products. After removing the near-surface region of the products with high Al content by the solution described in Step (6) of the purification process, the impurity contents of final Si products were determined by an ICP-MS, as shown in Table 1. It can be seen that most of impurities, except Al, have been effectively reduced in the final products, and an impurity content of 3.62 ppmw is obtained if Al is not taken into account. Generally, for each impurity, there exists an endurable upper limit of concentration acceptable for SOG Si feedstock, which has been demonstrated in Ref [32]. Here, we compare the impurity contents in our final products with their upper limits, as shown in Fig. 7. It can be clearly seen that the contents of all impurities are below their upper limits, including metals, B and P. This indicates that our Si products theoretically can be used for SOG Si feedstock in PV industry. Nevertheless, considering the detrimental influence of metals in Si on solar cell performances [33–35], we propose that a following directional solidification (DS) would be better implemented for further reducing the content level of metals in our products, since their segregation coefficients mostly are quite small. We also present the impurity contents of Si products after one time DS, comparing with their upper limits mentioned in Ref. [32], see Fig. 7. It can be seen that the contents of all impurities are further reduced and

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of purified Si products forming in the Al–Si system has also been discussed. All the impurities in the MG-Si can be reduced to a low level by this purification process, and a following DS can be employed to further improve the purity of Si product. The yield of purified Si products using the powder metallurgy technique with external pressure is higher than that of the conventional Al–Si purification process without external pressure or protecting ambient. The present results are interesting for the production of low-cost SOG Si feedstock in photovoltaic industry.

100 Theoretical Powder metallurgy method

Yield(%)

80

Conventional process

60

40

Acknowledgements The authors thank National Natural Science Foundation of China (Nos. 60906002 and 50832006), ‘973 Program’ (No. 2007CB613403) and the Fundamental Research Funds for the Key Universities (2009QNA4007).

20 20

25

30

35

40

45

Si content(wt%) Fig. 8. Theoretically maximum yield obtained from the Al–Si phase diagram and the yields of purified Si products by Al–Si alloy melt using optimized powder metallurgy technique (external pressure: 80 MPa; protection ambient: H2 and Ar mixture) and using no external pressure with air protection, as in a conventional Al–Si system purification technique.

moreover, the contents of metallic impurities are almost negligible compared to their upper limits. Furthermore, compared to the products of the enclosed Al–Si purification routes without powder metallurgy technique [13,19–21], our Si products have higher purity. This is probably because we have achieved the low temperature purification and thus the products are less contaminated at a lower purification temperature. So the purification process studied in this paper is more advanced and promising in purifying MG silicon for the PV application.

References [1] [2] [3] [4] [5]

[6]

[7] [8] [9]

[10]

3.4. Yield

[11]

According to Al–Si binary phase diagram, the theoretical maximum yield (thoery ) of purified Si products can be expressed as follows: thoery =

1 − ((1/w) − 1) f (T0 ) 1 − f (T0 )

[12] [13]

(1)

where w is the Si content in the initial Al–Si mixture, and f(T0 ) is the Si weight percentage in the Al–Si alloy at a crystallization temperature T0 . Fig. 8 shows the theoretically maximum yield obtained from the Al–Si phase diagram and the yields of purified Si products by Al–Si alloy melt using optimized powder metallurgy technique (external pressure: 80 MPa; protection ambient: H2 and Ar mixture) and using no external pressure with air protection, as in a conventional Al–Si system purification technique. The heating temperature, cooling condition and crystallization time are all the same. Note that under the same Al/Si ratio, the yield of process using the powder metallurgy technique under external pressure is higher than that of the process without external pressure or protecting ambient, which indicates that quite a larger portion of initial Si have been collected as the final products. It can be concluded that purification of Si with the powder metallurgy technique with external pressure is more efficient than the powder metallurgy technique without external pressure. Additionally, it also should be mentioned that the by-product of Al–Si alloy can be recycled and repeatedly used in the large scale production of SOG Si. 4. Conclusion In this paper, we have proposed an MG-Si purification process with Al–Si melt using a powder metallurgy technique, which effectively reduce the melting temperature. Meanwhile, the mechanism

[14] [15] [16] [17] [18]

[19] [20] [21] [22] [23] [24] [25] [26]

[27] [28]

[29] [30]

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