Relationship of Giant Dielectric Constant and Ion Migration in CH₃NH₃PbI₃ Single Crystal Using Dielectric Spectroscopy

Dielectric investigations of MAPbI₃ single crystal were carried out in broad frequency and temperature ranges. Giant dielectric constant up to 10⁶ was found from these measurements at low frequencies and discussed as originating from the intrinsic polarization involved in the hybrid network and from the induced polarization of the electrodes by ion migration. Different temperature and frequency evolution regimes were pointed out and discussed for the dielectric functions (ε′(ω, T), ε″(ω, T)), as also shown for the electrical conductivity (σ′(ω, T), σ″ (ω, T)). Relevant phenomenological approaches based on the modified Cole–Cole and Debye-like relaxation models were applied to analyze the involved dielectric and conductivity behaviors. Thus, the migration of MA⁺ at high temperatures increases the electrode polarization in the low-frequency range. For intermediate frequencies, the hopping of ions leads to the Debye-type relaxation at low temperatures, while the modified Cole–Cole relaxation process is more relevant at high temperatures. The performed analysis points out the correlation of the ac conductivity ensured by the ion migration and the giant dielectric constant with activation energy dependent on the polar ions MA⁺.