Enhanced Light Trapping of Thin Film Si Solar Cell in Glass-Laminated Module
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Enhanced Light Trapping of Thin Film Si Solar Cell in Glass-Laminated Module Takashi Sugiyama, Kousuke Chonan, and Mika Kambe Asahi Glass Company Research Center, 1150 Hazawa-cho, Kanagawa-ku, Yokohama 221-8755 Japan ABSTRACT In efforts to achieve higher efficiency of thin film Si solar cell, light trapping is one of the most important strategies in designing the cell structure. From the past studies, it is well known that TCO with a higher haze can exhibit better light trapping characteristics for a superstrate type thin film Si solar cell, while such TCO could give lower Voc and FF values caused by steep valleys in the texture of TCO. Furthermore, TCO with too high haze in a longer wavelength region could make back reflectance at the Ag rear electrode/reflector lower due to plasmonic effect on rough surface of Ag and thus reduces the efficiency of light trapping. In order to solve this contradictory behavior of light trapping in a conventional thin film Si solar cell, we have newly developed a glass-laminated module of thin film Si solar cell which has enhanced light trapping without deteriorating Voc nor high reflectance of Ag rear reflector. INTRODUCTION Thin film Si solar cell has been expected as one of the solar cell technologies with the highest cost performance. However, in terms of cost including BOS, it is not good as expected, because its efficiency stays lower than the other technologies and long deposition time of thick microcrystalline Si (μc-Si) layer due to low deposition rate makes the manufacturing cost higher. In order to reduce the thickness of μc-Si layer, light trapping technique which makes use of a glass substrate with textured TCO has been most commonly used for a superstrate type thin film Si solar cell. The textured TCO makes the rear electrode/cell interface rougher, and the rough interface scatters reflected light partly at an angle with which the light is confined within the cell device. Besides the light trapping effect, the texture is also utilized for reduction of light reflection and for prevention of delamination at TCO/p-layer interface. A drawback of this technique is that highly-textured TCO deteriorates the cell properties of Voc and FF due to its steep valleys [1-3]. In order to obtain more efficient light trapping while maintaining higher Voc and FF, we take advantage of commonly-used technique of conventional glass lamination which has already been employed for fabrication of thin film Si solar cell module for a long time [4]. The glass-laminated modules we developed consists of a thin film Si solar cell on glass substrate with textured TCO, a V-grooved rear glass substrate with Ag mirror, and a PVB (Polyvinyl Butyral) interlayer between the two substrates. In the glass-laminated module, incident light travels through the TCO, the cell, and the PVB interlayer, and reflects at the V-grooved rear mirror. The reflected light travels back through the module with an angle of total internal reflection at the TCO substrate. The total internal reflection light travels through the m
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