Impact of thermally activated ionic dynamics on the trap-mediated current–voltage characteristics of a mixed-halide hybrid perovskite†

Abstract

Organic–inorganic metal halide perovskites (OIMHPs) are at the forefront of leading energy research. Therefore, it is pivotal to understand the effect of operating conditions like temperature, humidity, light exposure, etc. on these materials. The transient ionic dynamics and its effect on the steady-state J–V characteristics of an OIMHP, viz. FAPbBr₂I, having a mixed halide composition, were investigated by temperature-dependent dielectric spectroscopy and temperature-dependent space charge limited current (SCLC) measurements in the temperature range of 305–454 K. The contribution of the resistance and capacitance of grains and grain boundaries to the total impedance at different temperatures has been interpreted by analyzing the Bode plots using the Maxwell–Wagner equivalent circuit model. The AC conductivity spectra demonstrate different behaviors in two different temperature regimes. In the low-temperature (LT) regime (323–381 K), the temperature response of ionic conductivity is only dependent on hopping frequency (the ionic carrier concentration factor being temperature-independent), leading to almost similar activation energies of ionic conduction (E_a) and hopping migration (E_m), where E_a = E_m = 0.30 ± 0.05 eV. However, in the high temperature (HT) regime (395–454 K), we observed a difference in E_a (0.74 ± 0.05 eV) and E_m (0.50 ± 0.05 eV) values, which is attributed to the activation energy of mobile charge carrier formation (E_f = E_a − E_m = 0.24 ± 0.05 eV). We propose that the trapped ions in the LT regime are now released by overcoming the barrier E_f in the HT regime, leading to a substantial increase in the mobile ion concentration. Furthermore, we have unveiled the effect of these mobile ions and trapped carriers on the J–V characteristics in both temperature regimes by analyzing the temperature-dependent SCLC J–V characteristics in the Ag/FAPbBr₂I/Ag device configuration. The AC conductivity and electric modulus loss spectra scale to different master curves in the LT and HT regimes, further corroborating the observed thermally activated interplay of ionic conduction and hopping migration. The key findings of this work stimulate more such fundamental investigations of electrical transport in mixed halide OIMHPs and establish their potential in various energy storage applications like batteries, integrated PV-battery/supercapacitor systems, and others.