Structure and Optical Characterization of Nanocrystalline Silicon Thin Films for Solar Cells
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1211-R03-12
Structure and optical characterization of nanocrystalline silicon thin films for solar cells Ryo Morisawa1, Akira Shirakura1, Chen-Chung Du2, Jen-Rong Huang2, Muh-Wang Liang2, David Ch Wu3, Tetsuya Suzuki1 1 Center for Science of Environment, Resources and Energy, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan 2 Mechanical and Systems Research Laboratories, Industrial Technology Research Institute Building 11, 195, Section.4, Chung Hsing Road, Chutung, Hsinchu, Taiwan
ABSTRACT Effects of very high frequency- plasma enhanced chemical vapor deposition (VHFPECVD) using diluted ultrapure silane at higher dilution ratio (R>30) on microstructures and optical characteristics of hydrogenated nanocrystalline silicon (nc-Si:H) film were studied. Nanocrystalline silicon films were prepared by at RF power ranging from 50 to 300 W. It was found that the transition from amorphous phase to nanocrystalline phase occurred between 100 W and 150 W. The nucleation mechanism toward nc-Si:H near the transition point of amorphous phase was discussed based on transmission electron microscopy with atomic scale. Further, it is suggested from UV-visible spectroscopy that nc-Si:H films with the best optical properties would be obtained near the transition point from the amorphous phase to the crystalline phase.
INTRODUCTION In the last few decades, amorphous silicon (a-Si:H) and hydrogenated nanocrystalline silicon (nc-Si:H) have attracted much attention as promising materials for infrared sensors, photovoltaic (PV) solar cells and many other applications [1]. Particular attention has focused on their application for thinner films to produce solar cells. Due to the high absorption coefficient in the visible part of solar radiation, even 300 nm of the film thickness is sufficient for a reasonably efficient solar cell [2]. However, a-Si solar cells suffer from significant photodegradation in their efficiency [3]. To solve this problem, nc-Si:H, also called microcrystalline silicon or nanocrystalline silicon, was developed in 1983 [4]. There are no constraints on using various substrates, because nc-Si:H can be deposited at low temperature. Therefore, either glass or plastic sheet can be used for its substrate [5]. Thus, nc-Si:H is expected to exhibit a wide range of applications and lower manufacturing cost. The deposition of a-Si:H or nc-Si:H thin films, by using plasma deposition techniques, for example, VHF-plasma enhanced chemical vapor deposition (VHF-PECVD) is highly system dependent and influenced by process parameters [6]. The optimization of these parameters and their impact on optical properties has been investigated by many researchers [7]. The structure and optical properties of Si:H are determined mainly by the substrate temperature and hydrogen concentration [8]. However, the influence of RF-power on the properties of nc-Si:H deposited by VHF-PECVD with pure silane has not been sufficiently studied.
This study aims to investigate the characteristic features of the nc-Si:H prepared at
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