成果報告書詳細
管理番号20140000000672
タイトル*平成24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 (銅ペーストの研究開発)
公開日2015/2/19
報告書年度2012 - 2012
委託先名独立行政法人産業技術総合研究所
プロジェクト番号P07015
部署名新エネルギー部
和文要約
英文要約Title: High Performance PV Generation System for the Future. R and D on Ultimate Wafer-based Si Solar Cells. (Glass-fritless Cu Alloy Pastes for Low Temperature Sintering) (FY2010-FY2014)FY2012 Annual Report

Metallization pastes are expensive materials next to Si wafers in cell manufacturing, and are therefore often targeted to reduce cell costs under circumstances of drastic decrease in module prices. To tackle the problem, there is a trend to reduce Ag consumption per cell. International Technology Roadmap for Photovoltaics (ITRPV) 2012 predicts that Ag shall be even replaced with alternative metals on a large-scale basis starting in 2015 and Cu is intended to be the substitute. The reason why Cu has been extensively studied as the alternative can be ascribed to the fact that Cu is much cheaper and more abundant than Ag, and has a similar conductivity. However, it has two major issues to be solved for the replacement: its oxidation and diffusion of Cu into Si.Low temperature sintering pastes have been extensively investigated because they can lower the process cost and meet requirements for high efficient solar cells such as a hetero-junction solar cell. Ag resin pastes, which are generally formulated with polymeric resins and Ag particles are often used for this purpose. The mechanism of the thermal induced conductivity can be considered as follows. Heat curing causes shrinkage of a thermosetting polymer, which get the conductive particles so close as to establish the conductivity by mechanical contact with each other. However, the electrical conductivity could be limited since it relies on the partial physical contacts. Because Ag particle itself has a poor adhesive property to Si or ITO substrate, the insulating thermosetting polymer also serves as an adhesive layer. However, there is an inevitable trade-off: enhancing the adhesive property by increasing the polymer content results in rising the electrical resistivity and vice versa. We demonstrate a Cu paste for low temperature sintering (<200-C). Our developed Cu alloy paste features addition of a low melting point alloy (LMPA), which is expected to work as a conductive binder between Cu particles and a barrier layer to prevent Cu oxidation and migration of Cu into Si. After melted, LMPA can diffuse into the inside of Cu particles as well as the spaces between Cu particles so that it may cover the whole Cu surfaces with formation of a metal bonding with Cu. This report describes recent progress on the Cu paste in fiscal year 2012. RESULTS AND DISCUSSION Cross sectional analysis Cross section of the Cu paste after sintering was observed by SEM and analyzed by energy dispersive X-ray spectroscopy (EDX). The SEM image showed a kind of monolithic structure with no obvious particle structures. An elemental mapping on the image made the featureless structure clear, revealing that there are Cu particles, which do not change the spherical shape much, and LMPA components filling the space between the Cu particles.This demonstrates that the Cu metallization proceeded almost as our design concept shown. TEM analysis showed that a Cu alloy layer exists at the interface between Cu and LMPA. Again, the metal contact between Cu and LMPA was confirmed. Oxidation of Cu was investigated by XPS depth profile analysis. The samples were prepared by storing sintered Cu pastes on ITO coated glass substrates under an environment test condition (85 °C and 85% R.H.) for 750h. The depth profile procedure consists of four cycles of recording XPS spectra followed by Ar-etching the sample for 5 min.
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