Phase Diagram and Dielectric Properties of MA(1-x)FA(x)PbI(3)

With the help of 10 different compositions (x) between 0.0 and 0.4 for MA(1-x)FA(x)PbI(3) and about 20 temperature points between 15 and 310 K for each composition, we establish a rich temperature composition phase diagram in terms of structural variations determined by powder and single-crystal X-ray diffraction. Four crystallographic phases are found to exist for this solid solution series, namely, cubic (Pm-3m), tetragonal (I4/mcm), orthorhombic (Pnma), and large-cell cubic (Im-3). Variable-temperature dielectric measurement reveals qualitative changes in dielectric properties of these materials driven by the observed structural phase transitions; it establishes that with an increasing FA content the high-temperature-dominant Curie-like behavior in dielectric constant of MAPbI(3) gets suppressed and an unusually prominent glassy behavior evolves in direct correlation with the structural evolution. This reveals a strong structure-property correlation existing in these solid solutions, qualitatively modifying properties of MAPbI(3) with FA substitution

Behavior of Methylammonium Dipoles in MAPbX(3) (X = Br and I)

Dielectric constants of MAPbX(3) (X = Br, I) in the 1 kHz-1 MHz range show strong temperature dependence near room temperature, in contrast to the nearly temperature -independent dielectric constant of CsPbBr3. This strong temperature dependence for MAPbX(3) in the tetragonal phase is attributed to the MA+ dipoles rotating freely within the probing time scale. This interpretation is supported by ab initio molecular dynamics simulations on MAPbI(3) that establish these dipoles as randomly oriented with a rotational relaxation time scale of similar to 7 ps at 300 K. Further, we probe the intriguing possibility of transient polarization of these dipoles following a photo excitation process with important consequences on the photovoltaic efficiency, using a photoexcitation pump and second harmonic generation efficiency as a probe with delay times spanning 100 fs-1.8 ns. The absence of a second harmonic signal at any delay time rules out the possibility of any transient ferroelectric state under photoexcitation

Critical Comparison of FAPbX₃ and MAPbX₃ (X = Br and Cl): How Do They Differ?

Dielectric measurements on formamidinium lead halide perovskites, FAPbCl₃ and FAPbBr₃, compared to those of MAPbCl₃ and previously reported MAPbBr₃, reveal the strongly suppressed temperature dependence of dielectric constants in FA compounds in the temperature range of approximately 140–300 K. Although the behavior of dielectric constants of FA compounds for temperatures <140 K resembles that of the MAPbX₃ system, the absence of any strong temperature dependence in sharp contrast to MA analogues in the higher temperature range up to room temperature suggests that the formamidinium (FA) dipoles are in a deep-frozen glassy state unlike the MA dipoles that rotate nearly freely in the temperature range relevant for any photovoltaic application. This observation is further supported by the temperature-dependent single-crystal X-ray diffraction (XRD) results

Critical Comparison of FAPbX₃ and MAPbX₃ (X = Br and Cl): How Do They Differ?

Dielectric measurements on formamidinium lead halide perovskites, FAPbCl₃ and FAPbBr₃, compared to those of MAPbCl₃ and previously reported MAPbBr₃, reveal the strongly suppressed temperature dependence of dielectric constants in FA compounds in the temperature range of approximately 140–300 K. Although the behavior of dielectric constants of FA compounds for temperatures <140 K resembles that of the MAPbX₃ system, the absence of any strong temperature dependence in sharp contrast to MA analogues in the higher temperature range up to room temperature suggests that the formamidinium (FA) dipoles are in a deep-frozen glassy state unlike the MA dipoles that rotate nearly freely in the temperature range relevant for any photovoltaic application. This observation is further supported by the temperature-dependent single-crystal X-ray diffraction (XRD) results

Critical Comparison of FAPbX₃ and MAPbX₃ (X = Br and Cl): How Do They Differ?

Dielectric measurements on formamidinium lead halide perovskites, FAPbCl₃ and FAPbBr₃, compared to those of MAPbCl₃ and previously reported MAPbBr₃, reveal the strongly suppressed temperature dependence of dielectric constants in FA compounds in the temperature range of approximately 140–300 K. Although the behavior of dielectric constants of FA compounds for temperatures <140 K resembles that of the MAPbX₃ system, the absence of any strong temperature dependence in sharp contrast to MA analogues in the higher temperature range up to room temperature suggests that the formamidinium (FA) dipoles are in a deep-frozen glassy state unlike the MA dipoles that rotate nearly freely in the temperature range relevant for any photovoltaic application. This observation is further supported by the temperature-dependent single-crystal X-ray diffraction (XRD) results

Critical Comparison of FAPbX₃ and MAPbX₃ (X = Br and Cl): How Do They Differ?

Dielectric measurements on formamidinium lead halide perovskites, FAPbCl₃ and FAPbBr₃, compared to those of MAPbCl₃ and previously reported MAPbBr₃, reveal the strongly suppressed temperature dependence of dielectric constants in FA compounds in the temperature range of approximately 140–300 K. Although the behavior of dielectric constants of FA compounds for temperatures <140 K resembles that of the MAPbX₃ system, the absence of any strong temperature dependence in sharp contrast to MA analogues in the higher temperature range up to room temperature suggests that the formamidinium (FA) dipoles are in a deep-frozen glassy state unlike the MA dipoles that rotate nearly freely in the temperature range relevant for any photovoltaic application. This observation is further supported by the temperature-dependent single-crystal X-ray diffraction (XRD) results

Critical Comparison of FAPbX₃ and MAPbX₃ (X = Br and Cl): How Do They Differ?

Dielectric measurements on formamidinium lead halide perovskites, FAPbCl₃ and FAPbBr₃, compared to those of MAPbCl₃ and previously reported MAPbBr₃, reveal the strongly suppressed temperature dependence of dielectric constants in FA compounds in the temperature range of approximately 140–300 K. Although the behavior of dielectric constants of FA compounds for temperatures <140 K resembles that of the MAPbX₃ system, the absence of any strong temperature dependence in sharp contrast to MA analogues in the higher temperature range up to room temperature suggests that the formamidinium (FA) dipoles are in a deep-frozen glassy state unlike the MA dipoles that rotate nearly freely in the temperature range relevant for any photovoltaic application. This observation is further supported by the temperature-dependent single-crystal X-ray diffraction (XRD) results

Behavior of Methylammonium Dipoles in MAPbX₃ (X = Br and I)

Dielectric constants of MAPbX₃ (X = Br, I) in the 1 kHz–1 MHz range show strong temperature dependence near room temperature, in contrast to the nearly temperature-independent dielectric constant of CsPbBr₃. This strong temperature dependence for MAPbX₃ in the tetragonal phase is attributed to the MA⁺ dipoles rotating freely within the probing time scale. This interpretation is supported by ab initio molecular dynamics simulations on MAPbI₃ that establish these dipoles as randomly oriented with a rotational relaxation time scale of ∼7 ps at 300 K. Further, we probe the intriguing possibility of transient polarization of these dipoles following a photoexcitation process with important consequences on the photovoltaic efficiency, using a photoexcitation pump and second harmonic generation efficiency as a probe with delay times spanning 100 fs–1.8 ns. The absence of a second harmonic signal at any delay time rules out the possibility of any transient ferroelectric state under photoexcitation