Rapid Characterization of Organolead Halide Perovskites

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Rapid Characterization of Organolead Halide Perovskites
(A) Proton detected 1H-207Pb CP HETCOR spectrum of CH3NH3PbBr1.5Cl1.5. (B) 207Pb sensitivity comparison of direct excitation and CP HETCOR projection for CH3NH3PbBr1.5Cl1.5. (C) Picture, structure and 207Pb NMR spectrum of a model thin film of CH3NH3PbI3.

Organolead halide and mixed halide perovskites (CH3NH3PbX3, CH3NH3PbX3–nYn, X and Y = Cl–, Br–or I–), are promising materials for photovoltaics and optoelectronic devices but phase segregation halide ion speciation in mixed halide perovskites remain a challenge. 207Pb solid-state NMR spectroscopy can be a powerful tool to characterize these materials but current techniques lack sensitivity, requiring large sample volumes and long signal averaging periods. This is especially true for mixed halide perovskites, which give rise to extremely broad 207Pb solid-state NMR spectra. Here we show that both fast MAS and DNP-enhanced solid-state NMR spectroscopy can increase sensitivity while reducing data acquisition time. With fast MAS and proton detection, high signal-to-noise ratio two-dimensional (2D) 207Pb-1H heteronuclear correlation (HETCOR) NMR spectra can be acquired in less than half an hour from only ca. 5 μL of perovskite material. Lead halide perovskites with mixed formadinium and methylammonium organic cations are also highly efficient solar sell materials. We show that 2D 207Pb→1H CP-HETCOR experiments and simple 1H fast MAS experiments could be used to probe cation speciation and quantify cation stoichiometry. DNP provided sensitivity enhancements on the order of 1 to 20 for different perovskites. The cryogenic temperatures (110 K) used for DNP experiments also provide a significant boost in NMR sensitivity. Consequently, it was possible to obtain the 207Pb solid-state NMR spectrum of a 300 nm thick model thin film of CH3NH3PbI3 in 34 hours. This result demonstrates the possibility of using NMR spectroscopy for characterization of perovskite thin films in intact devices.

M.P. Hanrahan, L. Men, B.A. Rosales, J. Vela,* A.J. Rossini.* Sensitivity-Enhanced 207Pb Solid-State NMR Spectroscopy for the Rapid, Non-Destructive Characterization of Organolead Halide Perovskites, Chemistry of Materials, 30 (20) 7005-7015, 2018, DOI:10.1021/acs.chemmater.8b01899.