Improved Stability of Hydrogenated Amorphous Silicon Solar Cells Fabricated by Triode-Plasma CVD

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A11.1.1

Improved Stability of Hydrogenated Amorphous Silicon Solar Cells Fabricated by Triode-Plasma CVD H. Sonobe1, 2, A. Sato2, T. Fujibayashi2, S. Shimizu2, T. Matsui2, A. Matsuda2 and M. Kondo2 1 Nagasaki Research and Development Center, Mitsubishi Heavy Industries, Japan 2 National Institute of Advanced Industrial Science and Technology, Japan ABSTRACT We have employed a triode-type plasma CVD system to fabricate highly stabilized hydrogenated amorphous silicon (a-Si:H) solar cells. The p-i-n type solar cells were fabricated on a textured SnO2/glass substrate (ASAHI VU type). By applying a triode system, the Si-H2 bond density in the film decreased to about one third (from 1.7 at.% for conventional parallel-plate-electrode to 0.6 at.% for a triode configuration), and correspondingly the degradation ratio decreased from 13 % to 10 %. We have achieved the degradation ratio of 5 % by optimizing the p layer deposition conditions. In case of a triode system, there were minor effects of higher hydrogen dilution in the stabilized efficiency. We have experimented the effects of the substrate temperature for a higher stabilized efficiency. Further improvement in solar efficiency has been made by applying antireflection layers to air/glass and TCO/p interfaces. As a result, we have achieved the stabilized efficiency of 9.22 % (Jsc = 15.9 mA/cm2, Voc = 0.863 V, FF = 0.672) with a degradation ratio of 7.8 %. We have also employed the triode-deposited a-Si:H solar cell to a tandem type solar cell with a structure of a-Si:H/hydrogenated microcrystalline silicon (µc-Si:H). We have achieved the stabilized efficiency of 10.9 % (Jsc = 12.0 mA/cm2, Voc = 1.31 V, FF = 0.691) with a degradation ratio of 7.3 %. INTRODUCTION In a-Si:H, light-induced degradation [1] correlates with the Si-H2 bond density in the film which dominates the structural flexibility of silicon network to cause the defect creation [2-6]. Plasma enhanced chemical vapor deposition using a silane source gas is most widely employed for fabricating a-Si:H, and the correlation between the Si-H2 bond density and the higher silane related radical generation in the plasma has been investigated. The Si-H2 bond density decreases at a higher deposition temperature, under hydrogen dilution of silane and using VHF plasma. However, higher deposition temperatures are disadvantages for the device application. It is also known that the Si-H2 bond density increases at higher deposition rates. Those results suggest that the determining steps of the Si-H2 bond density are not only the surface reaction but also the gaseous phase reaction. The higher silane radicals disrupt the surface and/or subsurface structural relaxation including the hydrogen elimination reaction and therefore require the higher deposition temperatures. It has been reported that higher deposition temperatures and hydrogen dilution using VHF plasma drastically improved the stabilized efficiency of a-Si:H solar cell at a high deposition rate of 2 nm/s and an efficiency of 8.2 % was obtained for the n-i-p type so

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