Light Management Using Periodic Textures for Enhancing Photocurrent and Conversion Efficiency in Thin-Film Silicon Solar
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Light Management Using Periodic Textures for Enhancing Photocurrent and Conversion Efficiency in Thin-Film Silicon Solar Cells Hitoshi Sai1, Takuya Matsui1, Adrien Bidiville1, Takashi Koida1, Yuji Yoshida1, Kimihiko Saito2, Michio Kondo1 1
Research Center for Photovoltaic Technologies, National Institute of Advanced Industrial Science and Technology, Central 2, Umezono 1-1-1, Tsukuba 305-8568, Japan. 2
Tsukuba Research Laboratory, Photovoltaic Power Generation Technology Research Association, Central 2, Umezono 1-1-1, Tsukuba 305-8568, Japan.
ABSTRACT Periodically textured back reflectors with hexagonal dimple arrays are applied to thin-film microcrystalline silicon (μc-Si:H) solar cells for enhancing light trapping. The period and aspect ratio of the honeycomb textures have a big impact on the photovoltaic performance. When the textures have a moderate aspect ratio, the optimum period for obtaining a high short circuit current density (JSC) is found to be equal to or slightly larger than the cell thickness. If the cell thickness exceeds the texture period, the cell surface tends to be flattened and texture-induced defects are generated, which constrain the improvement in JSC. Based on these findings, we have fabricated optimized μc-Si:H cells achieving a high active-area efficiency exceeding 11% and a JSC of 30 mA/cm2. INTRODUCTION Thin-film silicon solar cells (TFSSC) are a promising candidate for the future large-area photovoltaic systems operating in the gigawatt scale in high-temperature regions, because of the abundance and non-toxicity of the source materials and the superior temperature coefficient [12]. However, further conversion efficiency improvement is desirable to make TFSSC more costcompetitive. In TFSSC, so-called light trapping technology is crucial to absorb photons within thin Si films to compensate for their insufficient carrier transport properties and the high film deposition cost especially for microcrystalline silicon (μc-SI:H). For this purpose, textured substrates have been implemented to scatter the incident light and increase the optical path length inside the cells [1-22]. In recent years, periodically textured substrates or surface gratings have been actively studied as a more sophisticated platform with a potential for realizing a higher current density than achievable with the conventional random textures [9-22]. Because of their simplicity and uniformity in texture morphology, periodic structures have the possible advantage of much clearer correlations between texture parameters and the photovoltaic performance in solar cells. In addition, the periodicity allows us to use periodic boundary conditions in optical simulations, which reduces the calculation cost substantially. The high potential of periodic textures has been
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demonstrated in hydrogenated amorphous Si (a-Si:H) solar cells [9]. More recently, our group reported that a carefully chosen periodic texture with hexagonal dimple array (honeycomb texture) enhances the conversion efficiency as well as the current density in
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