Multicrystalline (mc-) Si is the most widely used material for photovoltaic application. It should be pointed out that the efficiency of high-grade mc-Si solar cell is only 2 % lower than that of single crystal Si although the former contains a certain amount of grain boundaries (GBs) and dislocations. This small negative contribution of GBs on the solar cell efficiency may be attributed to the characteristic geometry of GBs in mc-Si. Most of the mc-Si ingots are grown by the unidirectional solidification method in the cast furnace. The ingots are cut in certain pieces and then sliced into wafers. Since the wafers are cut perpendicular to the unidirectional orientation, the GBs lie perpendicular to the wafer surface. In this study, we carefully considered geometrical effect of grain structure in unidirectional mc-Si ingots. The test ingots of 100 mm diameter were grown on the microcrystal templates [1]. After the typical growth, the ingot was vertically cut and the grain evolution behaviour was elucidated carefully. Fig. 1 shows the vertical cut image (a) and electron backscattered diffraction (EBSD) pattern of mc-Si ingot (b). The grains at the initial stage are a few hundred μm in size and start to grow until several mm in size. The grain shapes at first are rather spherical but becomes rod-like with growth. Such a change is characterized by the preference of grain growth factor. The preferential grain orientations and GB characters are also elucidated by EBSD analysis. According to the growth, the grains tend to align to lower Miller indices like [100] or [111]. The GB networks also become simpler except for a few small angle grain boundaries. Such a simplified configuration of grains gives a significant reduction of electrical activities of mc-Si.

Fig. 1 (a) vertical cut image and (b) EBSD pattern of mc-Si ingot.

Acknowledgement

This work was partly supported by the New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade and Industry (METI).

Reference

[1] K. Arafune et al., J. Crystal Growth, 308 (2007) 5.

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