Coupling between Ion Drift and Kinetics of Electronic Current Transients in MAPbBr₃ Single Crystals

Abstract

The optoelectronic properties of halide perovskite materials have fostered their utilization in many applications. Unravelling their working mechanisms remains challenging because of their mixed ionic–electronic conductive nature. By registering, with high reproducibility, the long-time current transients of a set of single-crystal methylammonium lead tribromide samples, the ion migration process was proved. Sample biasing experiments (ionic drift), with characteristic times exhibiting voltage dependence as ∝ V –3/2 , is interpreted with an ionic migration model obeying a ballistic-like voltage-dependent mobility (BVM) regime of space-charge-limited current. Ionic kinetics effectively modify the long-time electronic current, while the steady-state electronic currents’ behavior is nearly ohmic. Using the ionic dynamic doping model (IDD) for the recovering current at zero bias (ion diffusion), the ionic mobility is estimated to be ∼10 –6 cm 2 V –1 s –1 . Our findings suggest that ionic currents are negligible in comparison to the electronic currents; however, they influence them via changes in the charge density profile.

Observation of Long‐Term Stable Response in MAPbBr3 Single Crystals Monitored through Displacement Currents under Varying Illumination

“…To elucidate the origin of those currents measured by dielectric displacement at , we explore electronic currents plotted in Figure 2b, in which the long‐term current response upon the direct application of voltage steps to the sample in dark conditions is shown. The general protocol of measurement in Figure 2b is based on previous reports [ 10 ] : first the crystals are polarized for 3 h and then left to relax at 0 V for 2 h. As recently reported, direct currents originate from a drift of electronic species kinetically controlled by migrating ions. [ 10 ] However, a significant difference appears in comparison to the charging method: currents are no longer constant but increase with time.…”

“…The general protocol of measurement in Figure 2b is based on previous reports [ 10 ] : first the crystals are polarized for 3 h and then left to relax at 0 V for 2 h. As recently reported, direct currents originate from a drift of electronic species kinetically controlled by migrating ions. [ 10 ] However, a significant difference appears in comparison to the charging method: currents are no longer constant but increase with time. For times of ≈2000 s and 1 V bias, electronic dark current achieves values approximately equal to 10 nA, much greater than that registered through .…”

“…The ohmic character (slope ≈1) observed in the i–V curve is remarkable and agrees with some previous studies of symmetrical SCs perovskite‐based devices, [ 9,11,36,37 ] but not necessarily true for longer times. [ 10 ] Note that the typical measuring time of the i–V curve situates at ≈100 s. Figure 1a shows the configuration of the setup circuit for the measurement of charge at the reference capacitor. Here, a step voltage pulse () is applied to the sample (which is modeled by a parallel combination of and ) across a linear reference capacitor , with both elements in series connection.…”

“…This modulates the net built-in electric field, changing the injection barriers for electronic carriers. [10,11] The presence of moving ions modifies the current-voltage curves in perovskite solar cells, in addition to electronic mechanisms. [12][13][14] In particular, slow redistribution of ions are responsible for the hysteretic behavior.…”

Surface versus Bulk Currents and Ionic Space-Charge Effects in CsPbBr3 Single Crystals

“… 21 Recently, the BVM model has been found to describe the long-term kinetics of electronic transient currents in MAPbBr 3 single crystals. 27 …”