Optical studies and dielectric response of [DMA]2MCl4 (M = Zn and Co) and [DMA]2ZnBr4

Abstract

[DMA]₂ZnCl₄, [DMA]₂CoCl₄ and [DMA]₂ZnBr₄ crystallized in the monoclinic system, in the P2₁/n, P2₁/n and P2₁/c space groups, respectively. The optical properties of [DMA]₂MCl₄ (M = Zn and Co) and [DMA]₂ZnBr₄ were studied using ultraviolet-visible (UV-Vis) spectroscopy in the range of 200–800 nm. The Tauc model was used to determine the band gap energy of each hybrid compound. The calculated values of the direct and indirect band gaps (E_gd, E_gi) for all samples were found to be in the range of 1.91 eV to 4.29 eV for [DMA]₂ZnCl₄, 4.76 eV to 5.34 eV for [DMA]₂ZnBr₄ and 1.77 eV to 3.84 eV for [DMA]₂CoCl₄. The Urbach energy (E_u), extinction coefficient (k) and refractive index (n) of each compound was calculated. On the other hand, the dispersion of (n) is discussed in terms of the single oscillator Wemple–DiDomenico model. The single oscillator energy (E₀), the dispersion energy (E_d), and both the real ε_r and imaginary parts ε_i of the dielectric permittivity were estimated. The variation of optical conductivity with the incident photon energy has also been studied. We employed impedance spectroscopy to thoroughly investigate the dipolar dynamics in the prepared materials. The evolution of the dielectric loss, as a function of frequency, showed a distribution of relaxation times, which probably could be of a Maxwell–Wagner type interfacial polarization relaxation, possibly attributed to grain boundary effects or blocking at the contacts. In fact, the current work opens an efficient path to high quality organic–inorganic halide perovskites with good optical properties, which makes them suitable for application in nonlinear optoelectronic devices.