Ions Matter: Description of the Anomalous Electronic Behavior in Methylammonium Lead Halide Perovskite Devices

Abstract

Carrier transport in methylammonium lead iodide (MAPbI 3 )-based hybrid organic-inorganic perovskites (HOIPs) is obscured by vacancy-mediated ion migration. Thus, the nature of migrating species (cation/anion) and their effect on electronic transport in MAPbI 3 has remained controversial. Temperature-dependent pulsed voltage-current measurements of MAPbI 3 thin films are performed under dark conditions, designed to decouple ion-migration/ accumulation and electronic transport. Measurement conditions (electric-field history and scan rate) are shown to affect the electronic transport in MAPbI 3 thin films, through a mechanism involving ion migration and accumulation at the electrode interfaces. The presence of thermally activated processes with distinct activation energies (E a ) of 0.1 ± 0.001 and 0.41 ± 0.02 eV is established, and are assigned to electromigration of iodine vacancies and methylammonium vacancies, respectively. Analysis of activation energies obtained from electronic conduction versus capacitive discharge shows that the electromigration of these ionic species is responsible for the modification of interfacial electronic properties of MAPbI 3 , and elaborates previously unaddressed issues of "fast" and "slow" ion migration. The results demonstrate that the intrinsic behavior of MAPbI 3 material is responsible for the hysteresis of the solar cells, but also have implications for other HOIP-based devices, such as memristors, detectors, and energy storage devices.

Perspective: Theory and simulation of hybrid halide perovskites

“…88 Based on a bulk activation energy of 0.58 eV 88 , a rate of 733 hops per second would be expected at T = 300 K, with an associated diffusion coefficient of 10 −12 cm 2 s −1 . Effective activation energies as low as 0.1 eV have been reported experimentally, 92,93 which likely correspond to diffusion along extended defects (dislocations, grain boundaries, surfaces) 94,95 . The corresponding diffusion rate of 10 −5 cm 2 s −1 is very fast, but comparable to surface diffusion of iodine observed in other compounds.…”

Operational stability of perovskite light emitting diodes

“…Given that ion migration is a thermally activated process, the common approach to determine the activation energy experimentally is to conduct temperature dependent measurements such as impedance spectroscopy [50], ionic conductivity [77] transient current [78] and transient capacitance [79]. However, the activation energies obtained by different groups show a large discrepancy ranging from 0.1 to 1.0 eV.…”

Origin of Hysteresis in CH3NH3PbI3 Perovskite Thin Films

“…This anomalous hysteresis influences the power conversion efficiency of a PV device and occurs as a result of the large discrepancy between the forward and reverse scans. Recent studies have demonstrated that such I – V hysteresis can originate from ion migration, ferroelectricity, a photoinduced capacitive effect, and charge trapping/detrapping . Among these, ion migration and ferroelectricity are considered plausible origins of the I – V hysteresis .…”