Dipolar Disorder, Localised States and their Effect on Charge Transport in Perovskites
Ferdinand Grozema a
a Department of Chemical Engineering, Delft University of Technology, The Netherlands, Van der Maasweg, 9, Delft, Netherlands
nanoGe Perovskite Conferences
Proceedings of nanoGe International Conference on Perovskite Solar Cells, Photonics and Optoelectronics (NIPHO19)
International Conference on Perovskite Thin Film Photovoltaics
Jerusalem, Israel, 2019 February 24th - 27th
Organizers: Lioz Etgar and Kai Zhu
Oral, Ferdinand Grozema, presentation 062
DOI: https://doi.org/10.29363/nanoge.nipho.2019.062
Publication date: 21st November 2018

A unique aspect of hybrid halide perovskite materials is the presence of an organic cation that occupies the cages formed by lead (Pb) and Iodide (I). In the basic perovskite CH3NH3PbI3, the methylamonium (MA) cation can rotate relatively freely, while slightly large cations can be restricted in their motion. Interestingly, the MA cation is dipolar and the organization of the dipoles in the material depends on the mutual interaction of the dipoles. It has been suggested that the formation of ordered domains can lead to electronic localized states that are different for electrons and holes, and therefore the dipolar disorder should influence the dynamics of charges formed on photo-excitation.

In this work we have performed a combined computational and experiemental study to unravel the relation between dipolar disorder and charge dynamics. Using a combination of Monte Carlo simulations and classical molecular dynamics we have studied the dynamics of the MA dipole at different temperature and show that large ordered domains are formed at low temperature, while above a certain temperature the domains are very small and the dipole are rotation relatively freely. The temperature at which this transition occurs suggests that the phase transitions in the perovskite are induced by dipole alignment.

Subsequently, we have studied the effect of dipole orientation on the delocalization of electronic states in the material. It is shown that for organic cations with a relatively high dipole moment, such as MA, localized states are formed that are different for electrons and holes. For a low-dipole moment cation, formamidinium (FA) such localized states are not observed.

Finally, in order to gain insight in the effect of dipole motion on charge dynamics, we have performed time-resolved microwave conductivity measurements combined with generation of charges by irradiation with a short, high-energy electron pulse. We observe substantial changes in mobility and lifetime of charge carriers in CH3NH3PbI3 after the low temperature tetragonal (β) to orthorhombic (γ) phase transition. We observed that the mobility and lifetime of charge carriers increase as the temperature decreases and a sudden increment is seen after the β/γ phase transition. For CH3NH3PbI3 the mobility and the half-lifetime increase by a factor of three to six compared with the values before the β/γ phase transition. We attribute the considerable change in the dynamics at low temperature to the decrease of the inherent dynamic disorder of the organic cation (CH3NH3+) inside the perovskite crystal structure. When replacing the dipolar MA cation by FA such sudden changes in the charge dynamics are not observed. This indicates that the dipolar motion of the organic cation does influence the dynamics of charges, particularly at lower temperature.
 

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