The microscopic mechanism of sodium doping in hybrid and all-inorganic halide perovskites

Dominik Kubicki^a^,^b, Daniel Prochowicz^d, Julia Wiktor^c, Clare Grey^b, Sam Stranks^a

^aCavendish Laboratory, Department of Physics, University of Cambridge, UK, JJ Thomson Avenue, Cambridge, United Kingdom

^bDepartment of Chemistry, University of Cambridge, UK

^cDepartment of Physics, Chalmers University of Technology, Sweden, Department of Physics, Gothenburg, Sweden

^dInstitute of Physical Chemistry, Polish Academy of Sciences, Warsaw, Poland, Kasprzaka, 44/52, Warszawa, Poland

International Conference on Hybrid and Organic Photovoltaics
Proceedings of 13th Conference on Hybrid and Organic Photovoltaics (HOPV21)

Online, Spain, 2021 May 24th - 28th

Organizers: Marina Freitag, Feng Gao and Sam Stranks

Oral, Dominik Kubicki, presentation 065
Publication date: 11th May 2021

Doping of hybrid and all-inorganic metal halide perovskites with alkali metal ions is a prominent means of enhancing their optoelectronic performance and stability. [1-3] In particular, sodium doping has been used to enhance the optoelectronic and solar cell metrics of CsPbBr₃ and MAPbI₃, [4-6] and the effect has been ascribed to the passivation of trap states associated with iodide vacancies. [7] However, the atomic-level mechanism of action of sodium in these compositions has been uncertain. Here, we use high-resolution solid-state sodium NMR to elucidate the speciation of sodium in MAPbI₃, the triple cation composition, and CsPbBr₃. We unambiguously show that sodium has no capacity to incorporate into the structure of these lead halide perovskites and remains in the materials as the unreacted sodium halide. However, owing to the exceptionally high affinity of sodium halides to ambient humidity, the unreacted halides facilitate the reaction of water vapour with the surface of the perovskites under ambient conditions, leading to complex sodium-doped hydrated surface layers. The results are corroborated by first principles calculations of ²³Na chemical shifts and are the first example of NMR crystallography applied to the highly disorder surfaces of metal halide perovskites.

References:

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Acknowledgements:

This work has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 841136. S.S. acknowledges the Royal Society and Tata Group (UF150033). The work has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (HYPERION, grant agreement No. 756962).

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