Optical and Electrical Effect of Texture Morphology on the Performance of Thin-Film Solar Cells
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Optical and Electrical Effect of Texture Morphology on the Performance of Thin-Film Solar Cells Daiji Kanematsu, Mitsuhiro Matsumoto, Shigeo Yata, Yoichiro Aya, Akira Terakawa and Masahiro Iseki Next-Generation Energy Device Development Center, Panasonic Corporation, 180, Ohmori, Anpachi-Cho, Anpachi-Gun, Gifu 503-0195, Japan ABSTRACT We correlated the texture morphology and the solar cell properties by measuring the distribution in the texture morphology. As a result, the short-circuit current ISC was approximated across various types of substrates by the standard texture height. Furthermore, we investigated the texture morphology from the point of view of the electrical effects. With regard to this point, the open-circuit voltage VOC was correlated to the steepest texture angle. Therefore, we consider that the both of the ISC and the VOC can be improved by controlling the distribution in the texture morphology. INTRODUCTION The high-efficiency amorphous silicon / microcrystalline silicon (a-Si/μc-Si) tandem solar cells and modules have been developed in our center since 2005. We already achieved stabilized efficiencies of 10.7% for a large module (Gen.5 size) and 12.2% for a small cell (1 cm2) [1-3]. Optical confinement technologies are important in order to improve the efficiency of thin-film silicon solar cells as well as Si film quality, solar cell structure etc. Transparent conductive oxides (TCO) with a randomly textured surface are widely used to enhance optical confinement. Conventionally, the haze ratio representing the light diffusion is used in order to estimate the optical effect of the textured TCO. However, the haze ratio is not so closely correlated to the solar cell properties, because the texture has some optical effects other than the light diffusion, such as the anti-reflection[2]. On the other hand, the surface texture affects the quality of the Si layers and, in turn, the electrical properties of the solar cells. Therefore, these relationships have been correlated to the texture morphology in order to determine the optimum texture morphology [4,5]. However, there is generally a trade-off between the optical effects and the electrical effects, for that reason it has been difficult to improve the texture morphology. In this study, we found the novel indexes of the texture morphology which are closely correlated to the short-circuit current ISC and the open-circuit voltage VOC instead of the haze ratio. Furthermore, we suggest a texture morphology that is compatible with the optical effects and the electrical effects based on these novel indexes. EXPERIMENT We measured the texture morphology of various TCO substrates and the properties of aSi single junction solar cells fabricated on these substrates. In order to correlate the texture morphology and the cell properties, we focused on the width and angle of texture. We measured
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the texture morphology by Atomic Force Microscope (AFM) over an area of 5 μm × 5 μm (512 × 512 pixels). Figure 1 shows a part of a height data on rectangular coordina
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