Almost 90% of solar cells currently on the PV market use crystalline Si as the main material [1]. Cutting loss of grown Si ingot to wafers during the conventional manufacturing process of crystalline Si solar cells is up to 60% of the initial weight of grown Si crystals [2], which is considered to be one of the main obstacles of the cost reduction of solar cells. Therefore, direct growth technique of shaped Si crystals without cutting process has been remarkably investigated to fabricate low-cost Si solar cells [3-5]. We have also been investigated to develop a novel growth technique of shaped Si crystals using the Si₃N₄ porous ceramic substrate [6]. The solar cells consist of spherical Si crystals are on the market recently. These spherical Si crystals have been fabricated by the drop tower method [7]. In this method, droplets of Si melt are crystallized during free-fall within a few seconds and most of the fabricated spherical Si becomes fine multicrystalline because of the high cooling rate of Si melt. These multicrystalline spherical Si need further re-melting and re-crystallizing process at slow cooling rate to improve crystalline quality for use as solar cells [8]. In this study, we report successful results of growth of high quality spherical Si crystals with slow cooling on the developed porous substrate.

Powders of Si₃N₄, SiO₂ and PMMA particles (5μm in diameter) were mixed and pressed into pellet. The porous ceramic film with same composition was also prepared on AlN board by spin coating method. These substrates were degreased under air at 873-1373 K and then fired under N₂ atmosphere at 1873 K. Porous structure was obtained by the thermal decomposition of mixed PMMA particles (Fig. 1). Spherical Si crystals were grown on the porous substrate in the horizontal tube furnace. Multiple pieces of semiconductor grade Si material (10 mg) was melted on the substrate at a temperature of 1753 K for 6 minutes with the heating rate set to 200 K/h and the cooling rate to 150 K/h.

Spherical Si crystals with diameters of up to 2 mm were successfully grown on the porous substrate with good reproducibility (Fig. 2a). The degree of undercooling of Si melt at crystallization of spherical Si was estimated to be less than 70 K, based on the difference between the heater temperature at melting of Si and that at crystallization of Si. Grown spherical Si crystals show metallic luster and most of them consist of single grain or only 2-3 grains with twinning planes observed from their etched cross sections (Fig. 2b). This confirmed that spherical Si crystals close to single crystals in their properties can be obtained stably by crystallizing Si on the porous substrate with a sufficiently low cooling rate without additional re-melting and re-crystallizing process of grown Si. The concentration of carbon and oxygen atoms in the grown Si crystals (C: 6 ppm, O: 5 ppm) analyzed by SIMS satisfies the specification of solar grade Si (C: 10 ppm, O: 20 ppm) [9]. Neither sticking nor infiltration of Si against the porous substrate was observed and the contact angle of Si against porous substrate was not changed through growth of spherical Si crystals on the same substrate with multiple times. Thus, the developed porous substrate is continuously reusable for the growth of high quality spherical Si crystals.

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. SEM micrograph of surface of the fabricated Si₃N₄ porous ceramic substrate. Holes formed by thermal decomposition of mixed PMMA particles were connected to each other.

Figure 2. (a) As-grown spherical Si crystals and (b) their cross sections after polishing and NaOHaq etching. They are composed of single grain or only 2-3 grains with twinning planes.

References

[1] W.G.J.H.M. van Sark et al., Energy Policy 35 (2007) 3121-3125. [2] D. Sarti and R. Einhaus, Sol. Energy Mater. Sol. Cells 72 (2002) 27-40. [3] I. Hide et al., J. Cryst. Growth 79 (1986) 583-589. [4] G. H. Lee and Z. H. Lee, J. Cryst. Growth 233 (2001) 45-51. [5] S. Sakuragi et al., Proc. 14th Workshop on Crystalline Silicon Solar Cells & Modules – NREL (2004) 188-191. [6] H. Itoh et al., J. Ceram. Soc. Jpn. (2013) in press. [7] K. Nagashio et al., Jpn. J Appl. Phys. 45 (2006) L623-626. [8] Z. Liu, A. Masuda and M. Kondo, J. Cryst. Growth 311(2009) 4116-4122. [9] M.G. Mauk et al., Proc. 3rd World Conf. Photovolt. Energy Convers. (2003) 939-942.

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