Influence of Surface Recombination on the Performance of SiNW Solar Cells and the Preparation of a Passivation Film
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Influence of Surface Recombination on the Performance of SiNW Solar Cells and the Preparation of a Passivation Film Shinya Kato1, Yuya Watanabe1, Yasuyoshi Kurokawa1,2, Akira Yamada1,3, Yoshimi Ohta4, Yusuke Niwa4 and Masaki Hirota4 1 Department of Physical Electronics, Tokyo Institute of Technology, 2-12-1-NE-16, O-okayama, Meguro-ku, Tokyo 152-8552, Japan 2 PRESTO, Japan Science and Technology Agency (JST), Japan 3 Photovoltaics Research Center (PVREC), Japan 4 Advanced Materials Laboratory, Nissan Research Center, Japan ABSTRACT Al2O3 was deposited on silicon nanowire (SiNW) arrays by atomic layer deposition (ALD) as a passivation layer to reduce surface recombination velocity. As a result, effective minority carrier lifetime was improved from 1.82 to 26.2 μs. From this result, the relative low-surface recombination rate of 2.73 cm/s was obtained from a calculation using one-dimensional device simulation (PC1D). The performance of SiNW solar cells was also simulated by considering the surface recombination velocity on the side of SiNWs using two-dimensional device simulation. It was found that Al2O3 deposited by ALD can improve open-circuit voltage of SiNW solar cells even if the structure has a high-aspect ratio and large surface area. Therefore, improvement in the performance of SiNW solar cells can be expected. INTRODUCTION Recently, a silicon nanowire (SiNW) has attracted great attention for its application in solar cells1-4 because it exhibits quantum size5 effect and optical confinement.2, 6 SiNWs have been prepared by several methods such as solution process7-9 and vacuum process.10-12 In previous work, we successfully fabricated 30-nm-diameter SiNW arrays by metal-assisted chemical etching using silica nanoparticles (MACES),8 which enables us to control the diameter and density of SiNWs. Over the past few years, many researchers have applied SiNWs to solar cells for the purpose of optical confinement. While the surface recombination of SiNW about field emission transistor (FET) has been investigated,13-15 detailed evaluation of surface recombination in SiNW solar cells has not yet been studied. To obtain high efficiency in solar cells, a passivation film should be applied to the SiNW array to reduce the surface recombination because of the large surface area. In this study, to suppress surface recombination, Al2O3 was deposited on p-type SiNW arrays of diameter 30 nm by atomic layer deposition (ALD), which enables us to cover the structure with a high-aspect ratio (such as a SiNW array). The crystalline silicon solar cell with Al2O3 has obtained high-effective lifetime and efficiency. To confirm the effect of a passivation layer, a minority carrier lifetime in SiNW arrays was measured by the microwave photo-conductivity decay (μ-PCD) method. Moreover, the side surface recombination velocity of SiNWs was calculated from measured lifetime using the onedimensional device simulator (PC1D).16-18 Many researchers have already reported the effect of surface recombination velocity at the interface on solar cell p
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