Cation engineering provides a route to control the structure and properties of hybrid halide perovskites, which has resulted in the highest performance solar cells based on mixtures of Cs, methylammonium, and formamidinium. Here, we present a multi-technique experimental and theoretical study of structural phase transitions, structural phases and dipolar dynamics in the mixed methylammonium/dimethylammonium MA1-xDMAxPbBr3 hybrid perovskites (0 ≤ x ≤ 1). Our results demonstrate a significant suppression of the structural phase transitions, enhanced disorder and stabilization of the cubic phase even for a small amount of dimethylammonium cations. As the dimethylammonium concentration approaches the solubility limit in MAPbBr3, we observe the disappearance of the structural phase transitions and indications of a glassy dipolar phase. We also reveal a significant tunability of the dielectric permittivity upon mixing of the molecular cations that arises from frustrated electric dipoles.
Mixing
of molecular cations enhances the optoelectronic properties
and stability of hybrid lead halide perovskites. Here, we use a multitechnique
approach to determine the phase diagram and molecular cation dynamics
of mixed methylammonium-formamidinium MA1–x
FA
x
PbBr3 (0 ≤ x ≤ 1) hybrid perovskites. The calorimetric, ultrasonic,
and X-ray diffraction experiments show a substantial suppression of
the structural phase transitions and stabilization of the cubic phase
upon mixing. We use the broad-band dielectric and Raman spectroscopies
to study the MA and FA cation dynamics in these compounds. The broad-band
dielectric spectroscopy indicates the absence of the MA cation ordering
and a gradual increase of the rotation barrier upon mixing. The room-temperature
dielectric permittivity substantially decreases as the fraction of
the FA cations is increased. No significant changes of the permittivity
are detected at temperatures, where the dielectric relaxations are
absent. We also observe weak signatures of a dipolar glass phase for
the intermediate mixing levels. The Raman spectroscopy supports the
dielectric results and reveals additional subtle information about
the FA cation dynamics.
Mixing molecular cations in hybrid lead halide perovskites is a highly effective approach to enhance the stability and performance of optoelectronic devices based on these compounds. In this work, we prepare and study novel mixed 3D methylammonium (MA)−ethylammonium (EA) MA 1−x EA x PbI 3 (x < 0.4) hybrid perovskites. We use a suite of different techniques to determine the structural phase diagram, cation dynamics, and photoluminescence properties of these compounds. Upon introduction of EA, we observe a gradual lowering of the phase-transition temperatures, indicating stabilization of the cubic phase. For mixing levels higher than 30%, we obtain a complete suppression of the low-temperature phase transition and formation of a new tetragonal phase with a different symmetry. We use broad-band dielectric spectroscopy to study the dielectric response of the mixed compounds in an extensive frequency range, which allows us to distinguish and characterize three distinct dipolar relaxation processes related to the molecular cation dynamics. We observe that mixing increases the rotation barrier of the MA cations and tunes the dielectric permittivity values. For the highest mixing levels, we observe the signatures of the dipolar glass phase formation. Our findings are supported by density functional theory calculations. Our photoluminescence measurements reveal a small change of the band gap upon mixing, indicating the suitability of these compounds for optoelectronic applications.
Mixing of molecular cations enhances the optoelectronic properties and stability of hybrid lead halide perovskites. Here, we use a multi-technique approach to determine the phase diagram and molecular cation dynamics of mixed methylammoniumformamidinium MA 1-x FA x PbBr 3 (0 ≤ x ≤ 1) hybrid perovskites. The calorimetric, ultrasonic and X-ray diffraction experiments show a substantial suppression of the structural phase transitions and stabilization of the cubic phase upon mixing. We use the broadband dielectric and Raman spectroscopies to study the MA and FA cations dynamics in these compounds. The broadband dielectric spectroscopy indicates absence of the MA cation ordering and a gradual increase of the rotation barrier upon mixing. The room-temperature dielectric permittivity substantially decreases as the fraction of the FA cations is increased. No significant changes of the permittivity are detected at temperatures where the dielectric relaxations are absent. We also observe weak signatures of a dipolar glass phase for the highest mixing level (x = 0.5). The Raman spectroscopy supports the dielectric results and reveals additional subtle information about the FA cation dynamics.
<p>We use a multi-technique approach
to determine the phase diagram and molecular cation dynamics of mixed
methylammonium-formamidinium MA1-xFAxPbBr3 (0 ≤ x ≤ 1) hybrid perovskites. The
calorimetric, ultrasonic and X-ray diffraction experiments show a substantial
suppression of the structural phase transitions and stabilization of the cubic
phase upon mixing. We use the broadband dielectric and Raman spectroscopies to
study the MA and FA cations dynamics in these compounds. The broadband dielectric
spectroscopy indicates absence of the MA cation ordering and a gradual increase
of the rotation barrier upon mixing. The room-temperature dielectric
permittivity substantially decreases as the fraction of the FA cations is
increased. No significant changes of the permittivity are detected at
temperatures where the dielectric relaxations are absent. We also observe weak
signatures of a dipolar glass phase for the highest mixing level (x = 0.5). The
Raman spectroscopy supports the dielectric results and reveals additional
subtle information about the FA cation dynamics.</p><br>
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