A.R. Lim, J-H Chang
Keywords: solar cell, perovskite, NMR, crystal growth
Summary:Organic-inorganic hybrid compounds have both organic and inorganic properties. Such compounds have diverse structures and properties and therefore have many potential applications. Organic-inorganic hybrid lead perovskites find applications in solar cells; however, their toxicity and stability remain major drawbacks. Thus, studies are actively developing nontoxic and stable organic-inorganic hybrid compounds. Organic-inorganic hybrid lead-free perovskite compounds show greater promise for applications in solar cells.1, 2 The structures of (MA)2MCl4 compounds, where MA is methyl ammonium (CH3NH3), an organic ion, and M is a divalent metal (M=Mn, Co, Cu, Zn, and Cd), consist of a sequence of alternating organic-inorganic layers. (MA)2Zn1-xMnxCl4 (x=0, 0.1, 0.3, 0.5, 0.7, and 1) lead-free zinc-based perovskite mixed crystals are grown using the slow evaporation method. The crystal structures are defined through X-ray diffraction experiments. This study discusses the effects of the partial replacement of Zn2+ ions with paramagnetic Mn2+ ions in ecofriendly (MA)2Zn1-xMnxCl4 lead-free perovskite crystals through magic angle spinning nuclear magnetic resonance relaxation times. Further, the spin-lattice relaxation time, T1ρ, in the rotating frame is measured for 1H and 13C nuclei of (MA)+ cations to understand the local environments with respect to the Mn2+ ion content, as shown in Figure. The structures and chemical shifts for CH3 and NH3 with x=0.1 and 0.3 were highly similar to those with x=0, whereas those for CH3 and NH3 with x=0.5 and 0.7 were highly similar to those with x=1. Nonetheless, the 1H T1ρ depending on the paramagnetic ion content became shorter because (MA)+ cations were bound to the paramagnetic Mn2+ layer through N-H•••Cl bonds. The main effects were the differences in the metal Zn2+ and Mn2+ ions. The temperature dependence of the T1ρ values for 1H reflected the modulation of the inter-NH3 and inter-CH3 dipolar interactions by (MA)+ cations. The differences in chemical shifts, line widths, and T1ρ for 1H and 13C in (MA)+ cations could potentially be due to differences in the electron structures of the metal ions. Acknowledgment: This research was supported by the Basic Science Research program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (2018R1D1A1B07041593). References  J. A. Aramburu, P. G.-Fernandez, N. R. Mathiesen, J. M. G.-Lastra, and M. Moreno, J. Phys. Chem. C 122, 5071 (2018).  A. M. Elseman, A. E. Shalan, S. Sajid, M. M. Rashad, A. M. Hassan, and M. Li, ACS Appl. Mater. Interfaces 10, 11699 (2018).