Temperature-activated dielectric relaxation in lead-free halide perovskite single crystals

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) A3Bi2X9 (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 MA3Bi2I9 (MBI) crystal and 410 meV for the FA3Bi2I9 (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/fγ 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.

Elucidating Polaron Dynamics in Cs2AgBiBr6 Double Perovskite

“…This sort of behavior shows that the rise in conductivity may be a result of both a decrease in the trapped charge density inside crystals and an increase in the mobility of charge carriers. Furthermore, to reveal the conduction mechanism in Cs 2 AgBiBr 6 , we have derived the ac conductivity from the impedance data using the following eqs and . σ normalA normalC = t A Z ′ false( Z ′ 2 + Z ″ 2 false) Total conductivity is given by σ normalt normalo normalt normala normall ( ω ) = σ normald normalc + σ ( ω ) where σ dc and σ(ω) are the dc and ac conductivity, respectively. ,− The calculated ac conductivity with frequency in the temperature range 0–90 °C is presented in Figure a. The conductivity enhances with the temperature rise due to enhanced ion migration and charge carrier conduction with the field.…”

Structural Elucidation of Electrical and Optical Properties in Lead‐free Organic‐Inorganic Copper Bromide [(CH3)4N]2CuBr4 Single Crystals

“…To study the conductivity and the conduction mechanism in the [(CH 3 ) 4 N] 2 CuBr 4 SCs, we used the impedance data to calculate the AC conductivity according to equation 6 given below: [54] $$$\vcenter{\openup.5em\halign{$\displaystyle{#}$\cr \sigma =\ {{e}\over{A}}{{Z{ {^\prime}}}\over{{Z}^{"2}+{Z}^{{ {^\prime}}2}}}\hfill\cr}}$$$ …”

“…To study the conductivity and the conduction mechanism in the [(CH 3 ) 4 N] 2 CuBr 4 SCs, we used the impedance data to calculate the AC conductivity according to equation 6 given below: [54] s…”

Investigation of Temperature‐Activated Charge‐Carrier Dynamics and Dielectric Relaxation in Fe, Cu–Doped CeO2

“…[ 47 ] It suggests that at higher temperatures, the likelihood of trapping charge carriers decreases. [ 48 ] As a result, the contribution of traps to the material's conductivity decreases, and conductivity is enhanced. This increase in conductivity is directly related to the TD enhancement of dielectric loss ( ε ″) at low frequencies.…”