成果報告書詳細
管理番号20120000000541
タイトル*平成23年度中間年報 太陽エネルギー技術研究開発 太陽光発電システム次世代高性能技術の開発 極限シリコン結晶太陽電池の研究開発(浮遊キャスト成長法による高品質Si 多結晶インゴット結晶成長技術)
公開日2012/12/21
報告書年度2011-2011
委託先名国立大学法人東北大学
プロジェクト番号P07015
部署名新エネルギー部
和文要約和文要約等以下本編抜粋:
1. 研究開発の内容及び成果等
本テーマは、豊田工業大学を中心としたコンソーシアム体制にて、低コストで高効率な世界最高レベルの競争力を有する結晶シリコン太陽電池の実現を目指す研究のサブテーマとして実施されるものである。
英文要約Title:High Performance PV Generation System for the Future. R and D on Ultimate Wafer-based Si Solar Cells. (Growth technology of high-quality Si multicrystal ingots using the Floating Cast Method) (FY2010-FY2012) FY2011 Annual Report
We attempted to grow high-quality multicrystalline Si (mc-Si) ingot by the floating cast method. Growth from the top of the melt permits to control microstructures by utilizing dendrite crystals and/or seed crystal at the initial stage of the crystal growth. By intentional introduction of lateral temperature gradient of 1K/cm, growth direction of the dendrite crystal was found to be controlled, leading to control of grain boundary characters. By utilizing the seed crystal, the upper plane of the dendrite crystal can be controlled. As a result, the ingot contains large crystal grains originating from the dendrite crystals. However, microstructures were drastically changed around the bottom of the crucible, and a lot of small crystal grains with random orientations were found. The nucleation of small crystals is considered to be driven in order to relax the stress, which was introduced by the strong contact of the ingot with the bottom of the crucible due to the volume expansion. This was found to be accompanied by deterioration of electrical properties as evidenced by decrease of the minority carrier diffusion length. In order to reduce the contact of the ingot with the bottom of the crucible, we specially designed double crucibles so that the residual melt is spontaneously removed from the inner crucible to outer one at the final stage of solidification. When we used the specially designed crucibles, the bottom of the ingot was found to be almost flat, which suggests that the contact of the ingot with the bottom of the crucible was suppressed. To investigate the quality of the ingot, we attempted to improve the fabrication process of small-area solar cells. The process includes emitter formation by thermal diffusion of phosphorous, deposition of anti-reflection film, printing of metal pastes, and annealing. By optimizing emitter formation process, we achieved the conversion efficiency of 17.5% using a standard CZ Si substrate. By utilizing the process, we characterized performance of five solar cells fabricated by wafers formed from the bottom part of mc-Si ingot grown using the standard and special crucibles. When we grow the ingot with the standard crucible, large variation in filling factor was found due to the introduction of crystal defects. On the other hand, in-plane uniformity was drastically improved when we grow the ingot with the special crucible. This confirms that the contact of the ingot with the crucible must be suppressed when we growth the ingot from the top of the melt to realize high-quality mc-Si ingot.
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