Low-Dimensional Hybrid Lead-Free Azetidinium Metal Halides

In the past decade, hybrid halide perovskites have attracted attention in optoelectronics and photovoltaics due to their large absorption coefficient, appropriate bandgap, and decent electronic properties.^[1,2] However, the Pb-content in the crystal structure and instability issues have raised questions for industrialization of these materials. The synthesis of alternate lead-free hybrid halides is of high interest in the field. Inorganic metal halide, Cs₂AgBiBr₆ has been found to be one of the good alternatives, where the material does not include Pb, and is more stable than Pb-containing halides.^[3] Nevertheless, its wide and indirect bandgap does not yield a high power conversion efficiency in photovoltaics. As a hybrid structure of lead-free halides, (MA/FA)SnI₃ or (MA)₃Bi₂I₉ have been considered noteworthy. (MA/FA)SnI₃ has a proper bandgap, but poor stability under moisture. The Sn²⁺ easily oxidizes to Sn⁴⁺.^[4] (MA)₃Bi₂I₉, is not considered to be an ideal light absorber in photovoltaics due to its large bandgap and low dimensionality, but its structural and optical property induce interest in luminescent perovskite research.^[5]<br/>According to this background, we have synthesized several new lead-free azetidinium metal halides and investigated their fundamental material properties: [(CH₂)₃NH₂]₂AgBiBr₆, [(CH₂)₃NH₂]₃Bi₂I₉, [(CH₂)₃NH₂]₃Bi₂Br₉, [(CH₂)₃NH₂]₃Bi₂Cl₉, [(CH₂)₃NH₂]₃Sb₂I₉, [(CH₂)₃NH₂]₃Sb₂Br₉, and [(CH₂)₃NH₂]₃Sb₂Cl₉. Azetidinium, [(CH₂)₃NH₂]⁺ is one of the potential organic molecular cations for the A-site in hybrid halide perovskites since it has a similar effective radius as methylammonium and formamidnium.^[6] In spite of that, Azetidinium has been rarely studied in hybrid halide perovskites. Our materials have a low-dimensional perovskite-like structure from 0D to 2D where the replacement on the B-site is based on Ag⁺/Bi³⁺, Bi³⁺, and Sb³⁺. All materials have shown good stability in moisture. We found an excitonic band state. This is promising in tandem photovoltaics and luminescent perovskites. To understand the molecular dynamics of azetidinium in the crystal lattice, we used synchrotron X-ray diffraction measurements at low temperatures. We believe that our study contributes on an expanded understanding of halide perovskite materials for future research.<br/><br/><br/>[1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, <i>J. Am. Chem. Soc.</i>, <b>2009</b>, 131(17), 6050–6051.<br/>[2] A. K. Jena, A. Kulkarni, and T. Miyasaka, <i>Chem. Rev.</i>, <b>2019</b>, 119, 3036−3103.<br/>[3] M. Pantaler, K. T. Cho, V. I. E. Queloz, I. G. Benito, C. Fettkenhauer, I. Anusca, M. K. Nazeeruddin, D. C. Lupascu and G. Grancini, <i>ACS Energy Lett.</i>, <b>2018</b>, 3(8), 1781–1786.<br/>[4] A. Abate, <i>Joule</i>, <b>2017</b>, 1, 659–664.<br/>[5] R. L. Z. Hoye, R. E. Brandt, A. Osherov, V. Stevanovic, S. D. Stranks, M. W. B. Wilson, H. Kim, A. J. Akey, J. D. Perkins, R. C. Kurchin, J. R. Poindexter, E. N. Wang, M. G. Bawendi, V. Bulovic, and T. Buonassisi, <i>Chem. Eur. J.</i>, <b>2016</b>, 22, 2605–2610.<br/>[6] G. Kieslich, S. Sun, and A. K. Cheetham, <i>Chem. Sci</i>., <b>2014</b>, 5, 4712–4715.