Lead free perovskite CsSnX3 (X=Cl, Br, I) characterized by Xrd, Reflectance spectra, DTA and 119Sn NMR
Koji Yamada a, Satomi Hino a, Yohei Yamane a
a Nihon university, Koriyama,, Fukushima, 963, Japan
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, Koji Yamada, 049
Publication date: 7th November 2016

Perovskite solar cells using CH3NH3SnI3 or CsSnI3 as photovoltaic materials have been studied extensively. The conversion efficiency, however, are still 6% which is much lower than Pb devices. [1] This is due to the instability of Sn2+ state in air, since Sn4+ is more stable oxidation state. Therefore, small amount of Sn(IV) impurities are incorporated into the perovskite lattice and damage semiconducting property of the perovskite. However, chemical properties of Sn2+ are quite similar to those of Pb2+, Sn2+ perovskite analogue is expected to be a good candidate from the environmental problems.  Sn2+ and Pb2+ ions form similar valence electron configuration (5s2 or 6s2) and hence, MX2 (M=Sn, Pb, X=Cl, Br) form pyramidal MX3- anions in acidic solutions. Pyramidal anions are also recognized in crystals. In a suitable choice of cation, perovskite structure appears associated with strong color, such as orange, red or black. In these perovskite lattice an infinite linear chains -X-M-X-M- are formed three dimensionally. This infinite linear chain is formed mainly by p-orbitals of M and X. This bond is similar to that found in linear I3- anion and called “Three-center four-electron (3c-4e) bond”.  This concept is more generally accepted as a “Hypervalent bond”. [2] Characteristic band gap property and small effective mass of electron or hole are both originate from the hypervalent nature of these perovskite halides.  In this report, essential physical properties (structure, phase transition and band gap) for series of solid-solutions, CsSnCl3-xBrx and CsSnBr3-xIx, are summarized. Furthermore, reducing power of SnF2 on CsSnBr3 powder was observed by 119Sn NMR spectroscopy.

[1] W. Liao, D. Zhao, Y. Yu, C.R. Grice, C. Wang, A.J. Cimaroli, P. Schulz, W. Meng, K. Zhu, R.G Xiong, and Y. Yan, Adv. Mater. 2016, 28, 9333-9340.

[2] Characteristic Properties of Hypervalent Compounds: Static and Dynamic Structures of Sb(III), Sn(II), and Ge(II) Halides, K. Yamada and T. Okuda, in “Chemistry of Hypervalent Compounds,” ed. by K. Akiba, Wiley-VCH, New York (1999), Chap. 3, pp 49-80.

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