Area: Crystalline and Thin Film Silicon PV ELECTRODEPOSITION OF SI THIN FILMS IN IONIC LIQUID WITH GROWTH CONTROL FROM INITIAL STAGES Hidenori Takai 1 , Yasuhiro Tsuyuki 1 , Tatsuki Fujimura 1 , Masahiro Kunimoto 2 , Yasuhiro Fukunaka 2 , Piero Pianetta 3 , Takayuki Homma 1,2 1 Department of Applied Chemistry, Waseda University, Tokyo 169-8555, Japan, 2 Research Organization for Nano & Life Innovation, Waseda University, Tokyo 162-0041, Japan, 3 SLAC National Accelerator Laboratory, CA 94025, USA Corresponding e-mail: t.homma@waseda.jp Si is the major material for photovoltaic devices because of its earth-abundance, longer durability, and higher reliability, and so on. Thin-film silicon solar cells have been focused on not only to reduce the amount of Si used but also to promise low-cost processing on a large scale. Electrodeposition is an attractive method for simple fabrication of thin films as well as precise microstructural control. We previously reported the electrodeposited Si thin films, nanodots, and nanopillers in trimethylhexyl ammonium bis-(trifluorosulfonyl) imide (TMHATFSI) ionic liquid containing SiCl 4 as Si precursor. [1-4] The present study focuses nucleation and growth process of Si deposits in order to fabricate fine and compact films. The deposited films were characterized by scanning electron microscopy (SEM). Seebeck coefficient was then measured to evaluate the electronic property. In addition, we attempt to elucidate the reaction mechanism in order to control the microstructure more precisely by using electrochemical quartz crystal microbalance (EQCM), X-ray reflectivity (XRR), and density functional theory (DFT) calculation. EQCM and XRR methods provide in-situ information during electrodeposition from Si precursor (SiCl 4 ); mass change and reflectivity change dependent on film thickness, roughness, and density, respectively. Morphology of the films was successfully controlled and was considerably improved, and more compact Si thin films were obtained. Seebeck coefficient of as-deposited films was -0.38 ± 0.20 µV K -1 , which indicated they were n-type semiconductors. Furthermore, p-type Si thin films, whose Seebeck coefficient was 900 ± 534 µV K -1 , were also obtained from the solution containing 8.8 × 10 -4 mol L -1 AlCl 3 as a dopant. During the EQCM measurements, charge transfer stage without mass change at the electrode was observed, suggesting reduction reactions of SiCl 4 to elemental Si via intermediate states of Si species such as Si (III) and Si (II). Analysis of the reflectivity implied the formation of layered structure containing Si dimer such as Si 2 Cl 6 before deposition of elemental Si. The results of DFT calculation also showed that formation of Si-Si bond between two SiCl 4 was more favorable than that between SiCl 4 and electrode surface. This result supported EQCM and XRR results, which suggested an intermediate states formation during the electrodeposition of Si at initial stages. This study was financially supported in part by the Japan Science and Technology Agency (JST) CREST program. References [1] T. Homma, J. Komadina, Y. Nakano, T. Ouchi, T. Akiyoshi, Y. Ishibashi, Y. Nishimura, T. Nishida, and Y. Fukunaka, ECS Trans., 41 (2012) 9. [2] Y. Ishibashi, T. Akiyoshi, J. Komadina, Y. Fukunaka, T. Homma, ECS Trans., 50 (2012) 117. [3] J. Komadina, T. Akiyoshi, Y. Ishibashi, Y. Fukunaka, T. Homma, Electrochim. Acta, 100 (2013) 236. [4] Y. Tsuyuki, A. Pham, J. Komadina, Y. Fukunaka, T. Homma, Electrochim. Acta, 183 (2015) 49.