High Quality Perovskite Materials for X-Ray Detection: Effect of Mobile Ions on Dark Current Stability

Abstract

Long-term stability of perovskite-based X ray detectors under working condition remains as the bottle-neck for technological purpose. Several effects are attributed to the presence of mobile ions in these materials such as shielding of the internal electrical field upon biasing and chemical interaction between intrinsic moving defects and electrode materials. As a matter of fact, high and instable dark current levels found in these devices are considered to be connected to ion migration upon polarization. However, it is still unknown how ion displacement alters the electronic current level, and which ionic transport parameters or ionic species determine the time-scale of the current evolution. Therefore, new insights for carrier transport mechanisms, including contact effects in addition to bulk conduction, is still needed due to the complex ionic-electronic nature of this material. In this wok, different compositions of methylammonium lead bromide (MAPbBr3) and methylammonium lead iodine (MAPbI3) perovskites and structures (single- and micro-crystalline) are synthesized and tested by the analysis of the resistance evolution by impedance spectroscopy (IS) measurements and the investigation of current transient responses upon biasing. In addition to electronic features, our analysis of ion diffusion mechanism (short-circuit condition) and ion drift under an increasing electric field extracts values for the ion diffusivity in the range of ~10-8 cm2 s-1 and effective ionic mobilities in the order of ~10-6 V-1cm2 s-1, respectively. Additionally, dissimilar current responses are encountered for the bromide-based perovskite samples and the iodine ones presumably connected to the chemistry of the defect formation. Our findings corroborate the existence of a coupling between electronic transport and ion kinetics that ultimately establishes the time scale of electronic current. Since ion-originated modulations of electronic properties constitute an essential peace of knowledge to progress into the halide perovskite device physics, this thesis sheds light on the dark current issue by helping to clarify the ongoing debate about potential processes governing detector operation concerning long timescale functioning and current stabilization upon polarization.