Si is the most widely used material for solar cells but the cost of photovoltaic power generation is higher than other power generating systems. Many studies now under way seek to reduce the cost of Si solar cells. Some of these studies focus on spherical Si solar cells due to its conservation of Si material and reduction of Si consumption per unit electric power generation to less than 1/5 that of Si wafers because cutting of Si is not needed in the manufacturing process [1,2]. Spherical Si is currently produced by free-fall in a drop tower with very fast cooling of Si melt. Fabricated spherical Si crystals by this method are often composed of fine crystal grains due to high undercooling of Si melt that cause the reduction of the conversion efficiency of spherical Si solar cells [3]. To prevent this fine crystallization of spherical Si, the novel crystal growth technique that enables slow cooling of spherical Si melt is necessary. We have been successfully prepared the porous Si₃N₄ ceramic substrate that repels Si melt for the direct growth of spherical Si crystals on it [4]. This study describes in-situ observation of the melting and crystallization behavior of Si on the porous substrate. The clear observation of Si with high magnification under low oxygen partial pressure environment was achieved by using our originally developed in-situ observation furnace (Fig. 1).

The porous Si₃N₄ substrate was prepared by mixing and press-forming Si₃N₄, amorphous SiO₂, and PMMA spherical micro-particles (2-20 μm in diameter) at a weight ratio of 4:1:5. The molded object was degreased in air atmosphere at 873-1373 K, then fired in N₂ atmosphere at 1873 K. The porous structure in the substrate was formed during the degreasing process by thermal decomposition of mixed PMMA particles. Si material with 1 mg weight was placed on the substrate which is located between two tantalum heaters. High-purity Ar gas (G2 grade) was flowed in the furnace to prevent the oxidation of Si. Melting and crystallization behavior of Si on the substrate was observed horizontally using a long focal microscope (KEYENCE, VH-Z50L).

The Si melt became spherical shape with about 1 mm in diameter on the fabricated porous ceramic substrate and the measured contact angle between them was 160° at the maximum, which is the largest value ever reported for Si [5-7]. When spherical Si began to crystallize by slowly cooling the Si melt (Fig. 2), the line-shaped pattern was observed at the melt surface, which is considered to be the indication of low undercooling of Si melt (ΔT < 100 K) [8]. Then crystal growth advanced in one direction with about 14 seconds, so the growth rate is less than 0.1 mm/s. It was confirmed that the grown spherical Si crystals are composed of single grain or twin from the etching result of their cross section. Therefore, growth of high quality spherical Si crystals is stably possible with slow cooling condition on the porous substrate that repels Si melt.

This study is supported by Regional Innovation Cluster Program (Global Type) by Ministry of Education, Culture, Sports, Science and Technology; Yamaguchi Green Materials Cluster (2009-2013).

Figure 1. Developed high temperature in-situ observation furnace (left) and its schematic diagram (right).

Figure 2. Crystal growth of spherical Si from low under cooling melt. Crystallization started at t = 1 s and finished at t = 15 s. A protuberance was formed at bottom left position due to volume expansion of Si accompanied by solidification.

References

[1] T. Minemoto et al., Jpn. J. Appl. Phys. 44 (2005) 4820-4824. [2] Z. Liu et al., Sol. Energy Mater. Sol. Cells 91 (2007) 1805-1810. [3] C. Okamoto et al., Jpn. J. Appl. Phys. 44 (2005) 7805-7808. [4] H. Itoh et al., J. Ceram. Soc. Jpn. (2013) in press. [5] K. Mukai and Z. Yuan, Mater. Trans. JIM 41(2) (2000) 338-345. [6] B. Drevet et al., J. Eur. Ceram. Soc. 29 (2009) 2363-2367. [7] I. Brynjulfsen et al., J.Cryst. Growth 312 (2010) 2404-2410. [8] K. Watanabe et al., J. Jpn. Soc. Microgravity Appl. 28(2) (2011) S64-S67.

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1. Growth of scorodite crystals from an aqueous solution bearing arsenic by photo catalysis method
2. Growth and characterization of KNbO₃ plate crystal from liquid with stoichiometric composition by the μ-PD method
3. Growth of Spherical Si Crystals on Porous Si₃N₄ Substrate that Repels Si Melt