Tetragonal-Cubic Phase Transition and Low-Field Dielectric Properties of CH3NH3PbI3 Crystals

Investigation of structural, morphological, and electrical conductivity study for understanding transport mechanisms of perovskite CH3NH3HgCl3

Along with morphological and structural studies, the temperature and frequency dependence of the electrical and dielectric properties of the CH₃NH₃HgCl₃ (MATM) compound was investigated and analyzed. SEM/EDS and XRPD analyses proved the purity, composition, and perovskite structure of the MATM. DSC analysis reveals the existence of an order–disorder phase transition of a first-order type at about 342 ± 2 K and 320 ± 1 K (heating and cooling, respectively), attributed to the disorder of [CH₃NH₃]⁺ ions. The overall results of the electrical study provide arguments for the ferroelectric nature of this compound and aim to broaden the current knowledge on the thermally activated conduction mechanisms of the studied compound via impedance spectroscopy. The electrical investigations have shown the dominant transport mechanisms in different frequency and temperature ranges, proposing the CBH model in the ferroelectric phase and the NSPT model in the paraelectric phase. The temperature dependence of the dielectric study reveals the classic ferroelectric nature of the MATM. As for the frequency dependence, it correlates the frequency-dispersive dielectric spectra with the conduction mechanisms and their relaxation processes.

Along with morphological and structural studies, the temperature and frequency dependence of the electrical and dielectric properties of the CH₃NH₃HgCl₃ (MATM) compound was investigated and analyzed.

DC bias electric field effects on ac electrical conductivity of MAPbI3suggesting intrinsic changes on structure and charge carrier dynamics

Methylammonium lead iodide (MAPbI₃) emerges as a promising halide perovskite material for the next generation of solar cells due to its high efficiency and flexibility in material growth. Despite intensive studies of their optical and electronic properties in the past ten years, there are no reports on dc bias electric field effects on conductivity in a wide temperature range. In this work, we report the combined effects of frequency, temperature, and dc bias electric field on the ac conductivity of MAPbI₃. We found that the results of dc bias electric fields are very contrasting in the tetragonal and cubic phases. In the tetragonal phase, sufficiently high dc bias electric fields induce a conductivity peak appearance ∼290 K well evidenced at frequencies higher than 100 kHz. Excluding possible degradation and extrinsic factors, we propose that this peak suggests a ferroelectric-like transition. In the absence of a dc bias electric field, the ac conductivity in the tetragonal phase increases with temperature while decreases with temperature in the cubic phase. Also, ac activation energies for tetragonal and cubic phases were found to be inversely and directly proportional to the dc bias electric field, respectively. This behavior was attributed to the ionic conduction, possibly of MA⁺and I^-ions, for the tetragonal phase. As for the cubic phase, the ac conduction dynamics appear to be metallic-like, which seems to change to a polaronic-controlled charge transport to increased dc bias electric fields.