Solar-grade silicon substrates by a powder-to-ribbon process

Abstract

A new technique for producing silicon ribbons for solar-cell substrates is described. The process begins with inexpensive, 98% pure silicon that is crushed and acid leached to raise the purity to 99.9%. This powder is spread on a graphite plate and electron-beam annealed to form a flat, self-supporting "preribbon." After removal of the graphite and unmelted powder, the preribbon is given a second electron-beam scan that recrystallizes the silicon into a smooth polycrystalline ribbon. This zone melting further improves the purity to over 99.99%. Ribbons 0.4 mm thick and up to 16 mm wide were produced in this initial work. The ribbons are p-type, 0.07 R. cm, and have long crystals about 1 mm wide. Electron-diffusion lengths of 20-30 p m were measured. Calculations indicate that solar-cell efficiencies up to 13% should be possible. If the process can be scaled up and automated, the cost of volume production could be as low as 43 cents/W. It is concluded that the process has the potential for achieving low-cost "solar-grade" substrates and has advantages over other processes. Further work is planned.On dCcrit une technique nouvelle pour la production de rubans de sil~ce utilisCs comme couches de support dans les piles solaires. Le procCdt utilise au depart du silicium h 98%, peu dispendieux, qui est Ccrast et attaqut a I'acide pour porter le degrt de puretC ? i 99,9%. La poudre obtenue est Ctendue sur une plaque de graphite et recuite par un faisceau d'Clectrons pour former un "prC-ruban" plat et capable de se tenir sans support. Aprks enlevement du graphite et de la poudre non fondue, ce prC-ruban est de nouveau balayC par un faisceau d'Clectrons qui recristallise le silicium en un ruban polycristallin rkgulier. Cette fusion par zones amCliore encore la purett, au dela de 99,99%. Des rubans ayant 0,4 mm dlCpaisseur et jusqu'h 16 mm de largeur ont ttC produits dans ce travail init~al. Ces rubans sont de type p , ont une rtsistivite de 0,07 Re cm et contiennent des cristaux allonges larges de 1 mm environ. Des longueurs de diffusion dlClectrons de 20 h 30 p m ont CtC mesurees. Les calculs indiquent qu'on pourrait obtenir des rendements de piles solaires allant jusqu'h 13%. Si le procCdC peut Stre exCcutC h plus grande Cchelle et automatisk, le coDt de production en grande quantitk pourrait &be aussi bas que 43 cents/W. On conclut que le procCdC peut fournir a faible coDt des couches de support de qualit6 "solaire" et qu'il prtsente des avantages sur d'autres procCdCs. On se propose de pousser plus loin ce travail. [Traduit par le journal] Can. J Phys 63, 859 (1985)

Hydrometallurgical purification of metallurgical grade silicon

“…Dietl reported that the best purification results were obtained by using hydrochloric acid and hydrofluoric acid mixtures and finely ground MG-Si (20 μm or less), with a particle size much below the grain size of MG-Si [11]. Norman et al obtained 99.9% Si by leaching in three successive steps with aqua regia, hydrofluoric acid, and hydrochloric acid [12]. Wang and Yin et al upgraded crushed MG-Si from 98 % to 99.96% through leaching by various acids [13][14].…”

Leaching behavior of impurities in Ca-alloyed metallurgical grade silicon

“…Dietl (1983) obtained the optimal purification result by utilizing the acid mixture of hydrochloric acid and hydrofluoric acid. Afterwards, Norman et al (1985) demonstrated that upgraded silicon with 99.9% purity can be achieved by leaching in three successive steps with aqua regia, hydrofluoric acid and hydrochloric acid, with an average particle size of 100 mesh.…”

Effect of oxidizing agents on the hydrometallurgical purification of metallurgical grade silicon

“…Dietl (1983) got Hydrometallurgy 121-124 (2012) the best purification result by using a mixture of HF and HCl and finely ground MGSi (b20 μm). Norman et al (1985) refined MGSi to 99.9% pure Si by successive leaching with aqua regia, HF and HCl. Voos (1961) has developed a process to produce silicon suitable for microwave diodes by the treatment of MGSi with aqua regia, H 2 SO 4 and HF.…”