The Principle of Ionic Liquefaction of MALI by Introducing Methylamine Gas as a Structural Modifier Causing Lowering of the Ionic Strength between the Cation and the PbI6 Octahedra
Dmitry Bogachuk a b, Simone Mastroianni a, Lukas Wagner a, Michael Daub c, Andreas Hinsch a
a Fraunhofer-Institute for Solar Energy Systems ISE, Heidenhofstrasse 2, D–79110 Freiburg, Germany
b University of Freiburg, Department of Sustainable Systems Engineering (INATECH), Freiburg, 79110, Germany.
c . Institut für Anorganische und Analytische Chemie and Freiburger Materialforschungszentrum (FMF), Albert-Ludwigs-Universität Freiburg, Albertstraße 21, 79104 Freiburg, Germany
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
Poster, Dmitry Bogachuk, 170
Publication date: 11th February 2019

As we have shown recently, high efficient graphite based perovskite cells can be made by using the principle of recrystallization from the ionic liquefied MALI (methylammonium lead iodide) perovskite based on a gas assisted reaction between methylamine (CH3NH2) and MAPbI3, yielding a stabilized efficiency of 12.6% (certified by Fraunhofer ISE CalLab). [1] Here a solvent- and additive-free pure perovskite melt at room temperature is obtained. Raman spectroscopy and XRD show for the first time an experimental proof of the molecular structural change during formation of perovskite-methylamine ionic melt. Upon exposure of perovskite to various concentrations of CH3NH2, the ABX3 structure collapses, as can be seen by a shift in Raman peaks in low wavenumber region, corresponding to Pb-I valence vibrations. Thus, by controlling the CH3NH2 concentration the ionic strength in the perovskite melt can be tuned, and in turn the solution viscosity and the infiltration properties are influenced. Importantly, we have found that the stoichiometry also affects the crystallization kinetics and the charge carrier lifetime in the absorber layer.

The enhanced perovskite crystallization in the mesoporous cell structure results in reduced non-radiative recombination (as seen from time-resolved PL) leading to VOC over 1 V in an HTM-free architecture, already only 100 mV below that of champion cells. This new insight into the kinetics of CH3NH2-CH3NH3PbI3 ionic liquid formation and subsequent crystallization provides a significant progress towards better understanding of highest efficient HTL-free perovskite solar cells.

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