Charge Carriers Are Not Affected by the Relatively Slow-Rotating Methylammonium Cations in Lead Halide Perovskite Thin Films

Abstract

Recently, several studies have investigated dielectric properties as a possible origin of the exceptional optoelectronic properties of metal halide perovskites (MHPs). In this study we investigated the temperature-dependent dielectric behavior of different MHP films at different frequencies. In the gigahertz regime, dielectric losses in methylammonium-based samples are dominated by the rotational dynamics of the organic cation. Upon increasing the temperature from 160 to 300 K, the rotational relaxation time, τ, decreases from 400 (200) to 6 (1) ps for MAPb-I3 (-Br3). By contrast, we found negligible temperature-dependent variations in τ for a mixed cation/mixed halide FA0.85MA0.15Pb(I0.85Br0.15)3. From temperature-dependent time-resolved microwave conductance measurements we conclude that the dipolar reorientation of the MA cation does not affect charge carrier mobility and lifetime in MHPs. Therefore, charge carriers do not feel the relatively slow-moving MA cations, despite their great impact on the dielectric constants.

Relationship between the Nature of Monovalent Cations and Charge Recombination in Metal Halide Perovskites

“…Our study employed an approach that enabled a direct probe of the impact of monovalent cation compositions on charge trapping rate without significantly changing the charge trap densities as a function of monovalent cation compositions, and this lends support to the mechanism of charge trap screening. 30 Studies indicating that the rotating dipole of the organic cations do not significantly impact charge carriers reinforce our conclusions that the cations do not affect the carriers themselves, as seen by the lack of a trend in our bimolecular recombination results. Rather, the significant trend in our monomolecular recombination results indicate that it is the charge traps that are impacted, and potentially screened by, the dipoles of the organic cations.…”

“…Ultimately, they concluded that addition of MA results in the trap states shifting to shallower energy levels which ultimately lowers the monomolecular recombination rate. Our study employed an approach that enabled a direct probe of the impact of monovalent cation compositions on charge trapping rate without significantly changing the charge trap densities as a function of monovalent cation compositions, and this lends support to the mechanism of charge trap screening . Studies indicating that the rotating dipole of the organic cations do not significantly impact charge carriers reinforce our conclusions that the cations do not affect the carriers themselves, as seen by the lack of a trend in our bimolecular recombination results.…”

Shining Light on the Photoluminescence Properties of Metal Halide Perovskites

“…The authors also connected such delocalization to the dynamic rotation of the organic MA + cation, and speculate that the lower PLQE observed in all‐inorganic perovskites, such as CsPbI 3 , might be due to the lack of this rotating cation. [ 185 ] However, at least one study suggests that, while the local dielectric environment changes due to the cation rotation, the charge carrier is unaffected, [ 186 ] suggesting that there is more work needed to bring experiment and theory in agreement on the impact of defects on optoelectronic properties.…”

Polymorphism in metal halide perovskites

“…216 Because the MA + cations can rotate in all directions in both the tetragonal and cubic phases of MAPbI 3 , their role in modulating photophysical processes across the tetragonal-cubic transition may be less pronounced compared to the orthorhombic-tetragonal transition. Examining temperature-dependent dielectric properties of MAPbI 3 using microwave conductance experiments found that cation rotation was too slow to influence charge carrier mobilities and lifetimes in the range of 160 -300 K. 217 Recent temperaturedependent photoconductivity experiments on FAPbBr 3 , on the other hand, revealed multiple discontinuities in the peak intensities and peak locations, only two of which corresponded to crystallographic phase changes from the cubic to tetragonal phase at 266 K and the tetragonal to orthorhombic phase at 153 K. 218 Figure 15 displays the photoconductivity characteristics, heat capacity, and mean square displacement (MSD) of hydrogen atoms as a function of temperature. Five distinct discontinuities can be observed in the temperature-dependent excitonic peak center, represented by gray circles in Figure 15a.…”