Vol.4 No.4 2012

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Research paper : Development of high efficiency flexible solar cells (A. Masuda )−196−Synthesiology - English edition Vol.4 No.4 (2012) 3 Elemental technology topics needed for the development of flexible solar cellsThe “Flexible Solar Cell Substrates Consortium” designated the basic elemental technology to be the technology for forming the texture suitable for light confinement in the thin-film silicon solar cells, onto the polymer base material itself. By doing this, the transparent conductive oxide does not have to have the texture added, and this will ease the conditions for film preparation of the transparent conductive oxide such as the substrate temperature, and the process will be applicable to the polymer base materials with low heat resistance. The idea of forming the texture onto the polymer base material itself was not obtained for the first time in this consortium, but several prior reports had been published[1][2]. For example, the transfer method by stamp with the texture, or the method using lithography had been suggested. However, the transfer by stamping is limited to thermoplastic polymer material only, and the printed texture does not have sufficient precision or reproducibility. In the method using lithography, it is difficult to obtain the ideal texture, and the lithography apparatus is expensive. In this research, the requirements were set that the technology should be universal where any polymer base material could be used and the texture suitable for light confinement could be transferred accurately. We investigated the new method for forming the texture onto the polymer base material. As a result, we concluded that the desirable results could be achieved with the substrate laminated with UV curable acrylic resin onto which the texture had been transferred from a mold. In this paper, we present the results of the comparison of the base material to which the texture of the Asahi-U had been transferred and the base material to which the texture of the moth-eye structure, which is known as a non-reflecting structure, had been transferred. The examples using the polyethylene naphthalate (PEN) film and polyimide (PI) film as the polymer base materials will be presented in this paper. However, the base materials that can be used in this method are not limited to those films, and the method has been shown to be effective in other polymer base materials such as polyethylene terephthalate (PET) and polycarbonate (PC). It has also been found to be usable on glass substrates. 4 Organization and management of the consortium4.1 Role division in the consortiumWhile the topics of elemental technologies necessary for the development of flexible solar cells became clear, it was necessary to combine the material technologies for polymer base material and UV curable acrylic resin, process technology for laminating the layers, simulation technology for optimal texture, roll-to-roll apparatus technology needed for mass-production, and process and device technologies for thin-film silicon solar cell fabrication. The RCPV did not have all such technologies, and as mentioned in chapter 2, it was determined that the research should be carried out as a joint research in the industry-academia-government collaboration consortium. In this chapter, we present the specific management policy applied to the consortium.In the “Flexible Solar Cell Substrates Consortium”, AIST organized the research and was in charge of the investigation of the conditions for fabricating the solar cells and the transparent conductive oxide on the polymer base material, while the participating companies were in charge of the development of the polymer base material. As shown in Fig. 3, the role division of the participating companies was as follows: Mitsubishi Gas Chemical Company, Inc. designed the texture by simulation, Teijin DuPont Films Japan Ltd. developed and supplied the PEN film (Teonex®), The Nippon Synthetic Chemical Industry Co., Ltd. developed and supplied the UV curable acrylic resin, Kimoto Co., Ltd. applied the hard coat to the polymer base material surface, and Toshiba Machine Co., Ltd. was in charge of the transfer of the texture to the acrylic resin by nano-imprinting technology using the single substrate method or the roll-to-roll method. The texture design is currently under development, and in this paper, only the results of the transfer of the moth-eye structure and the Asahi-U texture will be described. While Asahi-U uses the texture obtained by self-formation, the base material developed in this consortium has the texture formed by transfer, and the textures can be varied arbitrarily. Therefore, through the precise design of the texture, a solar cell with performance surpassing that using the Asahi-U substrate may be achieved. This implies that high efficiency solar cells can be achieved on polymer instead of the current thin-film silicon solar cells formed on a glass substrate. Therefore, the design of the texture is an extremely important research topic. Fig. 3 Scenario for the realization of flexible thin-film silicon solar cells in a consortium styleApparatus design optimized for polymer base materialNano-imprinting apparatus technologyRealization of high efficiency flexible thin-film solar cells using the base materials developed in consortiumMass-production demonstration testToshiba Machine (machine manufacturer)Hard coat technologyKimoto (polymer process manufacturer)UV curable resin technologyNippon Synthetic Chemical Industry (polymer manufacturer)Teijin DuPont Films (polymer manufacturer)Polymer base material technologyConsortiumAISTClarification of the requirements for base materials for flexible solar cellsClarification of the requirements for base materials for flexible solar cellsTexture design technologyTraining of young researchersAchievement of high efficiencyMitsubishi Gas Chemical (polymer manufacturer)Solar cell company (observer)Effective utilization of patentsTexture formation technology on polymer base materialPrior technologies for thin-film silicon solar cells

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