JM3A.8.pdf
Renewable Energy and the Environment Congress © 2013
Purifying metallurgical silicon to solar grade silicon by metal-assisted chemical etching Xiaopeng Li1,2*, Alexander Sprafke2, Stefan Schweizer2, Ralf B. Wehrspohn2,3 1. Max-Planck Institute of Microstructure Physics, Weinberg 2, 06120 Halle, Germany 2. Martin-Luther-Universität Halle-Wittenberg, Germany 3. Fraunhofer Institute for Mechanics of Materials (IWM), 06120 Halle, Germany * email address:
[email protected]
Abstract: Metal impurities have detrimental effects on the performance of Si solar cells. Through metal-assisted chemical etching, we fabricate Si nanowires from metallurgical Si while purifying it close to solar grade Si. OCIS codes: (000.2190) Experimental physics *email address:
[email protected]
1. Introduction Silicon is currently dominating the solar cell market. Since metal impurities act as recombination centers and have detrimental effects on Si solar cell performance [1], Si needs to be purified from metallurgical grade (MG-Si) to solar grade or even electronic grade to achieve reasonable efficiencies above 15%. The typical purification process (‘Siemens’ process)is very energy intensive, amounting up to hundreds of kWh/kg. Moreover, this process is not environmental friendly. The side products such as halogenated Si compounds are toxic and flammable [2]. High fabrication costs are imposed by the purification step of Si. Therefore, it would be an impactive technological breakthrough to invent a new cost-effective method to avoid the traditional ‘Siemens’ process. Here, we demonstrate a new method, so-called ‘metal-assisted chemical etching’ (MaCE) to create Si nanowires from MG-Si while purifying MG-Si close to SG-Si. MaCE is a simple room temperature solution based technique. It was firstly invented by Li et al. [3] in 2000, and further developed by Peng et al. [4]. MaCE consists of two steps. First, metal nanoparticle (NP) or a metal film is deposited onto a Si surface, then metal catalyzed Si etching in aqueous HF solution mixed with an oxidant is applied [5]. The formation of Si nanostructures is due to the anisotropic properties of MaCE: Si beneath the metal NPs or film are dissolved much faster than Si areas which are uncovered. We expect that during the creation of large surface areas of Si nanostructures by MaCE, metal impurities are exposed to the acidic solution, and thus removed from Si. 2. Si nanowire formation and purification Two different grades of Si were used in this study including upgraded metallurgical silicon (UMG-Si, multicrystalline wafer, p-type, 0.2-0.5 Ω cm, 99.999772% pure) and metallurgical silicon (multicrystalline wafer and powder, ~99.74% pure). Fig. 1 shows the schematic etching process. Dense Ag particles are deposited on the Si surface via galvanic displacement by immersing the wafer into 5 M HF + 10 mM AgNO3 solution. Afterwards, the Si wafers are etched in a solution containing 5 M + 0.3 M H2O2. HNO3 . A cleaning step was added to remove the Ag catalysts. MaCE on MG-Si powder was applied similarly. Ethanol was added to the etchant for wetting the Si particle surface.
JM3A.8.pdf
Renewable Energy and the Environment Congress © 2013
Figure 1. Schematic process of MaCE process and the subsequent HNO3 cleaning
Figure 2 SEM pictures of Si nanowires formed on (a) UMG-Si wafer, (c) MG-Si wafer and (e) MG-Si powder
Fig. 2 shows the SEM images of Si nanowires (SiNWs) formed on UMG-Si, MG-Si wafer and MG-Si powder. Large areas of SiNWs were obtained with diameters ranging from 20-200 nm. Interestingly, the MG-SiNWs are uniformly porous.. In contrast, the SiNWs fabricated from UMG-Si with much less metal impurities reveal a solid surface. We speculate that the porosity formation in MG-SiNWs is directly related to the removal of metal impurities [6]. In order to confirm the impurity level change before and after etching. We conducted an element analysis of the MG-Si powder by inductively coupled plasma optical emission spectrometry. All metal impurities met significant reduction after MaCE + HNO3 shown in Table 1. Especially for Fe having the highest amount in
JM3A.8.pdf
Renewable Energy and the Environment Congress © 2013
MG-Si, its concentration decreased by 98%. MG-Si was successfully upgraded from 99.74% to 99.9884% close to solar grade Si.
3. Conclusion We have fabricated SiNWs from UMG-Si and MG-Si by MaCE. The obtained SiNWs from ‘dirty’ Si would have a great potential for applications not only in solar energy conversion devices, but also in thermoelectrics, lithium ion batteries and sensors while keeping fabrication cost extremely low. Moreover, this room temperature MaCE technique can be easily scaled up for mass production, and would possibly emerge as a new route to upgrade MG-Si to solar grade in the near future.
ng / g
B
P
Ba
Ca
Mg
Ti
Zn
4.70E3
6.57E3
1.01E4
1.74E5
2.79E4
1.80E4
6.93E3
6.57E3
3.73E3
2.32E3
1.38E4
2.69E3
5.50E3
3.36E2
Reduction
No
43.2%
77.1%
92.0%
90.4%
69.4%
95.1%
ng / g
Cr
Fe
Ni
Pb
Mo
Cu
MG-Si
1.21E4
2.28E6
2.31E3
5.58E3
6.80E2
1.27E4
4.20E3
4.43E4
1.18E2
5.78E2
1.12E2
1.02E4
65.4%
98.1%
94.9%
89.7%
83.6%
19.5%
MG-Si MaCE+ HNO3
MaCE+ HNO3 Reduction
Table 1. The metal impurity level in raw MG-Si powder and MG-Si powder processed by MaCE and HNO3. The reduction percentage indicates the metal removal ratio. 4. References [1] S. Pizzini, “Towards solar grade silicon: Challenges and benefits for low cost photovoltaics. ” Solar Energy Materials Solar Cells 94, 15281533 (2010). [2] A. Luque, S. Hegedus, Handbook of photovoltaic science and engineering. 2nd Edition. [3] X. Li, P. W. Bohn, “Metal-assisted chemical etching in HF/H2O2 produces porous silicon.” Appl. Phys. Lett. 77 2572-2574 (2000). [4] K.-Q. Peng, Y. Wu, H. Fang, X. Zhong, Y. Xu, J. Zhu, “Uniform, axial-orientation alignment of one-dimensional single-crystal silicon nanostructure arrays.” Angew. Chem. Int. Ed. 44, 2737-2742 (2005). [5] Z. Huang, N. Geyer, P. Werner, J. d. Boor, U. Goesele, Metal-assited chemical etching of silicon: a review. Adv. Mater. 23, 285-308, (2011). [6] X. Li, Y. Xiao, J. H. Bang, D. Laush, S. Meyer, P.-T. Miclea, J.-Y. Jung, S. L. Schweizer, J.-H. Lee, R. B. Wehrspohn, “Upgraded silicon nanowires by metal-assisted etching of metallurgical silicon: a new route to nanostructured solar-grade silicon”, Adv. Mater. 25, 3187-3191 (2013).
