Organic–inorganic metal halide
perovskites (MHPs) exhibit
prominent electronic and optical properties benefiting the performance
of solar cells and light-emitting diodes. However, the dielectric
properties of these materials have remained poorly understood, despite
probably influencing delayed charge recombination and device capacitance.
Herein, we characterize the unprecedented dielectric behavior of MHPs
comprising methylammonium cations, Pb/Sn as metals, and Br/I as halides
using time-resolved microwave conductivity (TRMC) measurements. At
specific compositions, the above MHPs exhibit negative real and positive
imaginary photoconductivities, the polarities of which are opposite
those observed for conventional photogenerated charge carriers. Comparing
the observed TRMC kinetics with that of inorganic perovskites (SrTiO3 and BaTiO3) and characterizing its dependence
on temperature, frequency, and near-infrared second push pulse, we
conclude that the above behavior is due to the trapping of polaronic
holes/electrons by oriented dipoles of organic cations, which opens
a hitherto unexplored route to the dynamical control of dielectric
permittivity by photoirradiation.
Strontium titanate (SrTiO3) is a perovskite that is
important in water-splitting photocatalytic chemistry. Although excess
Sr is known to improve the photocatalytic activity, its effect on
charge dynamics remain largely unaddressed. Herein, we present a detailed
analyses of gigahertz complex transient photoconductivity (Δσ)
measured using time-resolved microwave conductivity (TRMC). We show
that charge carrier trapping associated with the emergence of an anomalous
positive imaginary part and the first-order rate constant of the normal
positive real part of Δσ dramatically decreased with increasing
Sr/Ti ratio. The second-order rate constant attributed to charge recombination
simultaneously decreased, and these rate constants were well correlated
with the improved hydrogen evolution rate of aqueous SrTiO3 suspensions with a Pt co-catalyst. These findings provide a fresh
perspective on the stoichiometry–carrier dynamics relationship
paramount for the optimization of composition-engineered photocatalysts
and reveal the broad implications for mechanistic studies based on
TRMC evaluation.
Organic-inorganic metal halide perovskite (MHP) has diversified into various fields, such as a solar cell and light-emitting diode. At the heart of the rich electronic behavior is the dielectric property associated with the permanent dipole of organic cation; however, knowledge on the charge-lattice interaction is still missing. We report an unusual complex photoconductive transient in MHPs composed of methylammonium cation (MA), mixture of lead-tin, and bromide (MA(Pb/Sn)Br3). This anomaly found in the time-resolved microwave conductivity (TRMC) evaluation is rationalized by the charge traps and consequent orientation of MA dipoles insensitive to the microwave. The energetics of photogenerated charges are examined by a near-infrared (IR) push pulse exposed at a certain delay after the ultraviolet (UV) pump pulse. Contributions from a two-photon process and liberation of trapped charges upon the IR irradiation are separately discussed. The identified mechanism points to the utmost importance of dielectric behavior in the gigahertz region, which is usually hindered by mobile charge carriers in the inherent electronic property of MHP.
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