Impedance spectroscopic study of charge transport and relaxation mechanism in the lead-free hybrid perovskite CH3NH3CuCl3

Abstract

In the field of commercialization, organic–inorganic hybrid perovskite materials are becoming more popular these days in view of their prospective use in solar cells and also in other optoelectronic applications. In this paper, a non-toxic CH3NH3CuCl3 (MACuCl3) hybrid perovskite is successfully synthesized via the slow evaporation solution growth technique. The monoclinic phase of the material is checked using the X-ray diffraction measurement. The chemical composition of the prepared compound is discussed by a scanning transmission electron microscope coupled with the energy dispersive X-ray spectroscopy (STEM-EDS). Such systematic characterizations as differential scanning calorimetry (DSC) measurements and dielectric measurements indicate that MACuCl3 goes through a reversible phase transition at T1 ≈ 350/352 K MACuCl3 demonstrates a semiconducting property with a direct band gap value of approximately 2.54 eV. Over the 10−1-106 Hz frequency range, the dielectric constant, the loss factor, the electric modulus and also the electrical conductivity of MACuCl3 show strong temperature dependence. The Nyquist plot confirms the various contributions of grains and grain boundaries to the total impedance. In the high-frequency region, the dielectric constant tends to increase with temperature. The modified Cole–Cole plot asserts that while the relaxation time decreases with the rise in temperature, the space charge and free charge conductivity increase the moment the temperature climbs. In accordance with the modified Kohlrausch-Williams-Watts (KWW) equation, an asymmetrical nature corresponding to the non-Debye type of the perovskite is noticed in the electric modulus spectra at different temperatures. Moreover, the imaginary part of the electric modulus spectra is found to shift from the non-Debye toward the Debye type with the increase in temperature despite not getting the exact Debye response and emerging as a semi-conductor material. The study of the charge transfer mechanism of MACuCl3 is carried out based on Elliott's theory. The conduction mechanism in MACuCl3 is interpreted through the following two approaches: the overlapping large polaron tunneling (OLPT) model (phase 1) and the non-overlapping small polaron tunneling (NSPT) model (phase 2). Moreover, the high dielectric constant of MACuCl3 which is associated with a low dielectric loss makes it a possible candidate for energy harvesting devices.