Moving Ions Vary Electronic Conductivity in Lead Bromide Perovskite Single Crystals through Dynamic Doping

Abstract

encountered in solar cell engineering. One of them is caused by the presence of mobile ions and how these species alter the internal electrical field, interact with the contact materials, or modulate electronic properties. [3-6] Upon biasing, charged moving ions accumulate in the vicinity of the outer interfaces causing electrical field partial shielding. [7-9] It has also been reported how intrinsic defects chemically react with the electrodes giving rise to losses in performance and device instabilities. [10,11] The occurrence of polarized interfaces in hybrid perovskite-based electronic devices was proposed [12] as an explaining mechanism for the measured excess capacitance at low frequencies. In dark conditions, mobile ions pile up at outer interfaces forming double layer-like structures in the vicinity of the perovskite/contact interface. [13,14] Excess dark capacitance of order 1-10 μF cm −2 can be readily explained in this way. In addition to purely electrostatic approaches for the interfacial phenomena, it is known that chemical reactions between mobile ions and contacting materials might give rise to the formation of dipole-like structures. [15,16] Also, deviations from stable electrical characteristics (i.e., hysteresis in current density-voltage J-V or non-ohmic response) have previously been correlated with the dynamics of migrating ions that interact with the contacts. [14,15,17] A survey about the chemical reactivity of the perovskite/contact materials can be found elsewhere. [14] In this sense, the kinetics of electrode charging may be understood not only Metal halide perovskite single crystals are being explored as functional materials for a variety of optoelectronic applications. Among others, solar cells, field-effect transistors, and X-and γ-ray detectors have shown improved performance and stability. However, a general uncertainty exists about the relevant mechanisms governing the electronic operation. This is caused by the presence of mobile ions and how these defect species alter the internal electrical field, interact with the contact materials, or modulate electronic properties. Here, a set of high-quality thick methylammonium lead tribromide single crystals contacted with low-reactivity chromium electrodes are analyzed by impedance spectroscopy. Through examination of the sample resistance evolution with bias and releasing time, it is revealed that an interplay exists between the perovskite electronic conductivity and the defect distribution within the crystal bulk. Ion diffusion after bias removing changes the local doping density then governing the electronic transport. These findings indicate that the coupling between ionic and electronic properties relies upon a dynamic doping effect caused by moving ions that act as mobile dopants. In addition to electronic features, the analysis extracts values for the ion diffusivity in the range of 10 −8 cm 2 s −1 in good agreement with other independent measurements.

Impedance Spectroscopy of Metal Halide Perovskite Solar Cells from the Perspective of Equivalent Circuits

“…The connection of ion distribution and electronic doping density has been recently corroborated using long-time impedance relaxation. 164 Zero-bias HF resistive features slowly recover steady-state values after poling, indicating that the overall ion concentration establishes the effective electronic conductivity through a dynamic doping mechanism. That approach allows for an indirect determination of the ionic diffusion coefficient, which also results in the range of 10 −8 cm 2 s −1 .…”

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

“…The high-frequency resistance ( R ) values were normalized and shown to follow a trend R ∝ t 1/2 (see Figure S6 ), in agreement with the IDD model (see section S2 of the Supporting Information). 17 , 21 …”

“… Time of flight (right-hand color bar) as a function of mobility and effective charge carrier density for the BVM regime of SCLC as in eq 1 . Parameters as for MAPbBr3 single crystal: V = 100 V, L = 2 mm, ϵ r = 76, 21 V 0 = 10 V. Note that the charge carrier density axis may refer to either ionic or electronic charge carriers in each case. …”

Impedance Spectroscopy for Metal Halide Perovskite Single Crystals: Recent Advances, Challenges, and Solutions

“…The linear fitting of C vs ε 0 A / L gives the dielectric constant. García-Batlle et al used IS on polished MAPbBr 3 SCs with chromium electrodes coated on the opposite faces and performed the measurements inside a metallic box that acted as a Faraday cage, under dark conditions . It was shown that moving ions cause dynamic doping, which affects the electronic conductivity in the MAPbBr 3 SCs, as these moving ions act as mobile dopants and locally vary the carrier density (Figure a).…”

“…(a) Variation of the impedance (i) as a function of the increasing bias with total measuring time ∼20 min at each voltage and (ii) after removing 5 V bias and evolution with time, and (iii) a detailed view of the variation of the impedance after removing 5 V bias and evolution with time for smaller impedances (last 5 h of the experiment). Reproduced with permission from ref . Distributed under a Creative Commons Attribution Non Commercial-No Derivatives 4.0 International (CC BY-NC-ND 4.0).…”