Diffusion length and lifetime of charge carriers

Diffusion length and lifetime of charge carriers are crucial characteristics of crystalline silicon solar cell and their diagnostics (Bullis and Huff, 1995; Guirgis et al., 2009; Pavelka et al., 2007b; Schofthaler and Brendel, 1995; Sinton and Cuevas, 1996; Trupke et al., 2011). Therefore, standardized test methods of lifetime are applied also to specify polysilicon quality (SEMI AUX017, n. d.; SEMI MF1535, n. d.; SEMI MF28, n. d.; SEMI MF391, n. d.; SEMI PV13, n. d.; SEMI PV9, n. d.). However, excellent lifetime and diffusion length values of the feedstock are necessary but not sufficient essentials to high solar efficiency. Loss analyses proved that even perfect crystals can end up at the low end when the solar cell process functions suboptimally (Borchert and Rinio, 2009; Cousins et al., 2006; Glunz, 2007) and, reciprocally, it is extremely unlikely that high – efficiency crystalline solar cells can be manufactured out of low quality polysilicon feedstock. For a review on yield analysis, please refer to Hesse (2009), Hesse et al. (2008b), and Industry Announcements (n. d.), Rogol et al. (2009).

[1]The reason for the focus on subsidies as the primary demand-pull decision variable in this model is that they can be designed to exclusively support organic PV, whereas carbon prices enhance demand for low – carbon technologies in general.

[2]Low R&D is $15 m year 1 for 10 years; high R&D is $80 m year 1 for 10 years. Low subsidy is 20c kWh-1 for 5 years. High subsidy is 25c kWh-1 declining to 5c kWh-1 over 20 years.

[3]Battelle and Westinghouse developed the chlorination of metallurgic silicon followed by zinc, and respectively, sodium reduction of STC in a closed loop process to yield silicon powder (Dietl et al., 1981). Westinghouse chlorinated a blend of coke and quartz to produce pure silicon (Dietl et al., 1981; Fey and Arcella, 1979). Please note that the cleaning of the produced silicon blend cannot be carried out in a sustainable process.

[4]The Union Carbide Corp. originally developed an FBR processfor solar grade silicon. When the gov­ernment had reduced the funding of solar R&D, Union Carbide started with the production of EG-Si rods and licensed the rod-technology from Komatsu Electronic Metals. Later, the Komatsu’s poly plant was acquired by Advanced Silicon Materials (ASiMi) in 1990 and eventually REC purchased ASiMi in August 2005 (Recgroup, 2013).

[5]Dismutation can be also considered to be a redistribution process ofsilicon substituents that leads to an equilibrium distribution of educts and products (Aylett, 1968; Moedritzer, 1966). Sometimes it is incorrectly named “disproportionation,” however, in disproportionation, identical molecules react and the central moiety of medium oxidation number becomes partly reduced and partly oxidized in the product molecules.

[6]Ethyl Corporation was purchased by Albermarle in 1962. In patents, Albermarle is sometimes mis­printed as Albemarle. Ethyl experimented with granular silicon in 1982 when MEMC bought

Albermarle (MEMC, 2013).

[7]Deposition of TCS leads to 1014—1015 Cl/cm3, Table 5 in reference Breneman and Dawson (1998), depending on the deposition rate (Shuichi, 1991).

[8]In the coincident site lattice (CSL) theory, the degree offit (S) between the structures ofthe two grains is describedby the reciprocal ofthe ratio ofcoincidence sites to the total number ofsites. Thus, a boundary with high S might be expected to have a higher energy than one with low S. Low-angle boundaries, where the distortion is entirely accommodated by dislocations, are S1. Some other low S boundaries have special properties especially when the boundary plane is one that contains a high density of coin­cident sites. Examples include coherent twin boundaries (S3) and high-mobility boundaries in face – centered cubic (FCC) materials (S7). Deviations from the ideal CSL orientation may be accommodated by local atomic relaxation or the inclusion of dislocations into the boundary.

[9]China’s national standard for SG-Si defines First Class, Second Class, and Third Class qualities with lower acceptor, donor, and carbon levels than the corresponding characteristics in SEMI PV17 (Linda, 2011). China’s national standard additionally specifies oxygen levels and lifetimes which are “to-be-defined” and respectively, unspecified characteristics in SEMI PV17(SEMI PV17—0611, 2013). The oxygen level must be specified when using remelt silicon feedstock.

[10]Thermal donors may bias resistivity data (Helmreich and Sirtl, 1977).

[11]Before FZ, pulling the homogeneity of polysilicon rods should be controlled (Schantz et al., 2010).

[12]Boron can be determined by closed vessel heating digestion using inductively coupled plasma atomic emission spectrometry (ICP-AES) (Tian et al., 2010).

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