Solid-state µ-MAS and thin-film NMR spectroscopy on hybrid organic/inorganic perovskite photovoltaic materials

• Applicant: Dr. Helen Grüninger
• Subject Area: Physical Chemistry of Solids and Surfaces, Material Characterisation ; Analytical Chemistry ; Solid State and Surface Chemistry, Material Synthesis
• Term: from 2019 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 426490040

Hybrid organic/inorganic lead halide perovskites (APbX3) have attracted great interest in the last decade due to their possible application as an alternative to conventional silicon solar cells, with excellent power conversion efficiencies of currently up to 23 %. Due to the flexible structure a wide bandwidth of chemical compositions including mixed cation and/or mixed halide systems are accessible. Furthermore, a series of different synthesis strategies and treatment of the resulting perovskites are possible, that besides chemical compositions impact on e.g. the morphology, particle size and crystallographic defects of the perovskite materials. These structural changes at varying levels often cause interlinked effects on the optoelectronic properties of the resulting thin film perovskites, the materials geometry that is used in solar cell applications. Systematic studies are rare compared to the enormous parameter space, and are essentially missing for in situ working conditions of solar cells, i.e. upon illumination. However, comprehensive studies at atomic resolution are essential to derive structure-property relations in order to guide future synthesis strategies and optimal chemical compositions for high efficiencies of lead halide perovskites. The research proposal, therefore, is dedicated to the closure of this gap for a suit of mixed hybrid organic/inorganic lead halide perovskites in thin film geometries. For this purpose we choose the NMR crystallographic approach as it provides complementary information about the local environment at atomic level in addition to long-range order, and thus represents the optimal methodology for systematic structural studies. The development and implementation of a novel thin film MAS NMR probehead based on microcoil designs will allow for direct measurements of thin film geometries. Furthermore, light sources will be installed in both, bulk and thin film MAS NMR probes, in order to allow for monitoring the structural behaviour of multiple perovskite compositions and morphologies in situ upon illumination. We expect that the key issues, how point defects, cation or halide order/disorder, as well as the structural response upon light irradiation impact on the optoelectronic properties will be answered for a suit of high-performance photovoltaic perovskite materials. The comparison of the results for both bulk samples and thin film geometries will provide insights in the influence of syntheses and thus crystallization conditions.Overall, the achieved structural insights about high-performance perovskite materials under actual device working conditions will eventually point to important structure-property relations to guide future tailoring of perovskite materials with respect to their application in solar cell devices.

• DFG Programme: Research Fellowships
• International Connection: Netherlands
• Host: Professor Dr. Arno Kentgens