Experimental evidence for proton tunneling in methylammonium lead triiodide perovskites
Lionel Hirsch a, Yan-Fang Chen a, Yu-Tang Tsai a, Dario M. Bassani a
a CNRS/Univ. Bordeaux
nanoGe Perovskite Conferences
Proceedings of International Conference on Perovskite Thin Film Photovoltaics, Photonics and Optoelectronics (ABXPV18PEROPTO)
Perovskite Thin Film Photovoltaics (ABXPV18). 27-28 Feb
Rennes, France, 2018 February 27th - March 1st
Organizer: Jacky Even
Oral, Lionel Hirsch, presentation 044
DOI: https://doi.org/10.29363/nanoge.abxpvperopto.2018.044
Publication date: 11th December 2017

Hybrid organic−inorganic perovskite (HOIP) solar cells are a promising technology that combines the advantage of solution process technology with high power conversion efficiency (PCE), still in constant improvement. This success story started in 2009 with the report by Kojima et al. of a HOIP solar cell with 3.8% PCE. The highest certified PCE is currently at 22.1% in a single-junction device. While the toxicity of lead and long-term device stability are still major obstacles to be overcome for their widespread deployment in solar cells and electroluminescent devices, HOIPs represent a class of materials that combine promising electronic properties with solution processing technology.

Perovskites in general are known to possess ionic conductivity and ion movement has been postulated to be directly or indirectly responsible for their lack of long-term stability and unusual behavior, including hysteresis. Therefore, understanding ion migration in perovskites is of major importance in improving their performance. Ion movement in HOIPs may occur extrinsically, i.e. from one of the contacts, or take the form of ion migration, where an ion is displaced from an occupied to a vacant site. Numerous studies confirmed the rotational movement of the MA ions within the inorganic cage formed by the PbI2 octahedra, which has been associated to the temperature and frequency dependence of the complex permittivity.[1]

To test the relationship between MA rotation and the frequency dispersion in the dielectric response in HOIP materials, we conducted a series of experiments in which the hydrogen atoms in MAI were selectively substituted with deuterium. Our results unambiguously demonstrate that MA migration or rotation is not directly responsible for the frequency dependence of the dielectric constant at intermediate frequencies at 1 – 3 kHz.[2] Instead, we observe that deuteration of the ammonium group leads to a large inverse kinetic isotope effect and show that proton tunneling is responsible for the mid-range frequency-dependence of the dielectric constant in HOIP materials. Deuteration of the methyl group in MA induces a normal secondary KIE that is consistent with these findings

 

[1] Frost, J. M.; Walsh, A. Acc. Chem. Res. 2016, 49, 528-535

[2] Y-F Chen, Y-T Tsai, L. Hirsch and D. M. Bassani, J. Am. Chem. Soc., Article ASAP DOI: 10.1021/jacs.7b09526

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