We will discuss new approaches to studying highly technologically relevant materials that have so far been difficult to address using high-resolution multi-dimensional NMR methods. One example will be the study of hybrid (organic-inorganic) multi-cation lead halide perovskites which hold promise for a new generation of easily processible solar cells. We show how low temperature (~100K) NMR allows us to obtain the first quantitative, cation-specific data on cation reorientation dynamics in hybrid mixed-cation formamidinium/methylammonium lead halide perovskites. We use 14N, 2H, 13C and 1H solid-state MAS NMR to elucidate cation reorientation dynamics, microscopic phase composition, and the MA/FA ratio, in (MA)x(FA)1-xPbI3 between 100 and 330 K. The reorientation rates correlate in a striking manner with the carrier lifetimes previously reported for these materials and provide evidence of the polaronic nature of charge carriers in PV perovskites. The methods also allow us to obtain rapidly spectra with high sensitivity to elucidate the atomic level nature of Cs and Rb incorporation into the perovskite lattice of formamidinium-based materials. We use 133Cs, 87Rb, 39K, 13C and 14N solid-state MAS NMR to probe microscopic composition of Cs-, Rb-, K-, MA- and FA-containing phases in double-, triple- and quadruple-cation lead halides in bulk and in a thin film. Contrary to previous reports, we find no proof of Rb or K incorporation into the 3D perovskite lattice in these systems. Another example will address the challenge of how to obtain enhanced sensitivity in bulk inorganic materials, which are often difficult to study because of prohibitively long T1s. We will describe an approach to significantly increase NMR sensitivity in bulk oxides, and we demonstrate it with examples of gallium phosphide.

Solid-state NMR approaches to inorganic materials: From lead-halide perovskites to gallium phosphide

255th National Meeting (2018)

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