2006 Fiscal Year Final Research Report Summary
microstructure Development of multicrystal silicon ingot for solar battery by the control of twinning
| Project/Area Number |
17560652
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| Research Category |
Grant-in-Aid for Scientific Research (C)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Metal making engineering
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| Research Institution | KYUSHU UNIVERSITY |
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Principal Investigator |
MIYAHARA Hirofumi Kyushu University, Faculty of Engineering, Associate Professor, 大学院工学研究院, 助教授 (90264069)
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| Project Period (FY) |
2005 – 2006
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| Keywords | twin boundary / multicrystal silicon / faceted growth / Solar battery / crystal growth / undercooling / photoelectric transformation / diffusion length of carrier |
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| Research Abstract |
The mechanisms of columnar structure growth of the multicrystal silicon and influence of twining on the microstructure formation are investigated to the development of the photoelectric transformation efficiency and the productivity of solar battery cell fabricated by the unidirectional solidification technique.
The solar battery-grade high-purity silicon crystals has been solidified in quartz crucibles, whose inside diameter is 20 mm and the height is 100 mm, at the velocity of 0.075 to 9.6 mm/s and a temperature gradient of 20K/cm by using the Bridgman type furnace at the Ar atmosphere.
The columnar structures are observed parallel to the heat flow direction in a velocity about 1.2 mm/min, and the larger size grains are obtained at the lower solidification speed. Twin boundaries, which have the crystal orientation relationship of sigma 3, grow parallel to the heat flow direction. Therefore, the undercoolings for the growth were measured by using seeds of <111> and <101> and twin introduced <211> crystals. The <111> specimens has larger undercooling of 2.5〜5.5K and <101> has smaller. However, twin introduced <211> specimens has lowest undercooling of 0.5〜3.5K. Since it is also revealed that the solidification velocity of twin introduced <211> crystal is higher by the molecular dynamics simulation, the reentrant corner of twin boundary becomes kink site and can give an advantage to the faceted growth and the development of grain size.
The silicon crystal grows to the orientation between <211> to <101> along to the crucible bottom. And the grain size was enlarged by increasing the undercooling as well as the faster solidification velocity of 0.6 to 2.4mm/min in the initial solidification region. However, in order to continue such large gain size, it is necessary to reduce the velocity to 0.12 to 0.3mm/min until middle and final solidification stage.
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