Temperature-activated dielectric relaxation in lead-free halide perovskite single crystals
Rajashik Paul, Naveen Kumar Tailor, Apurba Mahaptra, Pankaj Yadav, Soumitra Satapathi
Abstract
Abstract Lead-free metal-halide perovskites have recently appeared as a promising candidate in optoelectronics and photovoltaics because of their non-toxicity, stability, and unique photophysical properties. Much scientific research has been done on optoelectronic characteristics and photovoltaic applications of lead-free perovskites, but the dielectric characteristics and insight into the relaxation phenomenon remain elusive. Here, we study the dielectric relaxation and conduction mechanism in the single crystalline (SC) A 3 Bi 2 X 9 (A = MA + /FA + ) perovskite using temperature-dependent electrochemical impedance spectroscopy in correlation with the modulus spectroscopy. With increasing temperature, the peak of − Z ″( ω ) shifts toward a high-frequency regime which specifies the thermally dependent relaxation mechanism in both crystals. The activation energy was estimated as 381 meV for MA 3 Bi 2 I 9 (MBI) crystal and 410 meV for the FA 3 Bi 2 I 9 (FBI) crystal suggesting hopping of mobile ions between lattice sites. The connected orientational polarization with the thermal motion of molecules leads to the enhancement in the dielectric constant ( ϵ ′) with temperature. The ϵ ″( ω ) in these crystals shows the significant ionic conductivity with a typical 1/ f γ type characteristics (in the low-frequency regime) where γ is found to be in the range of 0.93–1.0 for MBI crystal and 0.88–0.98 for FBI crystal. The correlated imaginary part of impedance (− Z ″) and modulus ( M ″) demonstrate the temperature-activated delocalized relaxation (non-Debye toward the Debye type) in these crystals. Stevels model suggests that the contribution of traps reduces with temperature rise and therefore conductivity enhances. Our study provides a comprehensive analysis and in-depth knowledge about the dielectric and conductivity relaxation mechanism in these lead-free perovskite SCs, which will help to implement efficient energy storage devices using these materials.