Ion Migration in Triple-Cation Mixed-Halide Perovskite Solar Cells with Potassium Passivation
Moritz Futscher a, Lucie McGovern a, Kangyu Ji b, Sandy Sanchez c, Sam Stranks b, Bruno Ehrler a
a Center for Nanophotonics, AMOLF, Science Park 104, 1098 XG Amsterdam, The Netherlands
b Cavendish Laboratory, Department of Physics, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, UK.
c Adolphe Merkle Institute, Chemins des Verdiers 4, CH-1700 Fribourg
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV19)
Roma, Italy, 2019 May 12th - 15th
Organizers: Prashant Kamat, Filippo De Angelis and Aldo Di Carlo
Oral, Moritz Futscher, presentation 033
Publication date: 11th February 2019

Solar cells based on halide perovskites show efficiencies close to highly-optimized silicon solar cells. However, ions migrating in these perovskites lead to device degradation and complicate the characterization of perovskite solar cells. We recently showed that transient ion-drift is a powerful method to quantify activation energy, concentration, and diffusion coefficient of mobile ions in perovskite solar cells. By studying methylammonium lead triiodide (MAPbI3) we found that both MA+ and I- ions migrate at room temperature, but with very different diffusion coefficients (10-9 and 10-12 cm2s-1 respectively).

Recently it was shown that introducing potassium into triple-cation mixed-halide perovskites passivates surfaces and stabilizes luminescence without compromising charge transport or extraction. [1] This has been attributed to the mitigation of both non-radiative losses and ion migration in perovskite films. Using transient ion-drift, we study ion migration in these triple-cation mixed-halide perovskites and find that the migration of mobile halide ions is comparable across different device geometries, but that the diffusion coefficient of mobile halide ions is two orders of magnitude lower than in MAPbI3 perovskites. We find no evidence of mobile cations, suggesting that cation migration is impeded in mixed-cation mixed-halide perovskites. We furthermore find that the activation energy of mobile halide ions in these triple-cation mixed-halide perovskites is not influenced by potassium passivation, but that the concentration decreases and the diffusion coefficient increases with increasing potassium passivation. This quantification of mobile ions in triple-cation mixed-halide perovskites will lead to a better understanding of ion migration and the influence of passivating agents on that migration.

This work is part of the research program of The Netherlands Organization for Scientific Research (NWO).

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