Effect of reducing agent SnF2 on Sn(IV) for the preparation of pure CH3NH3SnBr3 perovskite
Koji Yamada  a, Tatsurou Nakagama a, Youhei Yamane a, Satomi Hino  a
a Nihon University, 1-8-14 Kanda Surugadai, Tokyo, 101
Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics
Proceedings of International Conference Asia-Pacific Hybrid and Organic Photovoltaics (AP-HOPV17)
Yokohama-shi, Japan, 2017 February 2nd - 4th
Organizers: Tsutomu Miyasaka and Iván Mora-Seró
Poster, Satomi Hino , 038
Publication date: 7th November 2016

Perovskite solar cells using CH3NH3PbI3 or CH(NH2)2PbI3 as photovoltaic materials have been studied extensively because of the high efficiency and low production cost. In order to obtain environmentally friendly Pb-free solar cells, Sn(II) perovskite devices are expected. Perovskite Sn(II) halides, such as CH3NH3SnI3, CsSnI3 and CH3NH3SnBr3, however, are difficult to get as pure states, due to their instability in air. Since small amount of Sn(IV) impurities are incorporated into the perovskite lattice, the semiconducting property of the perovskite will be degenerated. Recently, semiconducting CsSnI3 perovskite has been reported using SnF2 as a reducing agent. [1] In this report, the following reaction was monitored by XRD and 119Sn NMR spectroscopy in order to evaluate reducing effect of SnF2 :

                          (CH3NH3)2SnBr6 + 2SnF2 → 2CH3NH3SnBr3 + SnF4        

Just after stoichiometric mixing of the starting materials, the color changed from pale yellow of (CH3NH3)2SnBr6 to orange, suggesting an instantaneous formation of the perovskite at the surface.  After the heating the mixture at 430 K, the formation of perovskite CH3NH3SnBr3 was confirmed from the 119Sn NMR and XRD. [2] Furthermore, the effect of SnF2 addition on the perovskite was discussed. The effect of SnF2 addition could not be observed on the 119Sn NMR spectra and XRD patterns. However, the 119Sn NMR spin-lattice relaxation times became longer, since Sn(IV) impurities were removed from the perovskite.   

[1] M. H. Kumar, S. Dharani, W. L. Leong, P. P. Boix, R. R. Prabhakar, T. Baikie, C. Shi, H. Ding, R. Ramesh, M. Asta, M. Graetzel, S.G. Mhaisalkar, N. Mathews, Adv. Mater.,2014, 26, 7122-7127.

[2] K. Yamada, K. Nakada, Y. Takeuchi, K. Nawa and Y. Yamane, Bull. Chem. Soc. Jap., 84 (9), 926-932 (2011).

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