Direct observation of surface polarization at hybrid perovskite/Au interfaces by dark transient experiments

Abstract

A distinctive feature of hybrid perovskite light-absorbing materials is the non-negligible ionic conductivity influencing photovoltaic performance and stability. Moving ions or vacancies can naturally accumulate at the outer interfaces (electrode polarization) upon biasing. Contrary to that approach, a modulation of conductive or recombination properties could manifest as an alteration in the low-frequency part of the impedance response, either producing inductive or large capacitive features. Under this last view, capacitances are not the response of polarized structures or charging mechanisms, but result from the modulation of currents. This work intends to provide pieces of evidence that assist us in distinguishing between these two dissimilar mechanisms, namely, real charge polarization and delayed current effects under bias in the dark. The analysis relays upon an experimental technique based on transient charging signals using the Sawyer-Tower circuit. Instead of applying an alternating small perturbation over a steady-state voltage (differential capacitance method), transient charging measures the resulting polarization upon a large bias step under the suppression of dc currents. Our findings reveal that real steady-state charge is indeed induced by the applied voltage in the dark, easily interpreted by means of charged real capacitors with values much larger than the geometrical capacitance of the film. The connection between that polarization and the charging of perovskite/contact interfaces is highlighted.

Understanding the Stability of MAPbBr3 versus MAPbI3: Suppression of Methylammonium Migration and Reduction of Halide Migration

“…It is worth mentioning that the concentrations found in the literature still differ by many orders of magnitude, mainly due to the differences in the models used to interpret experimental data (see section S1a). The results reported here are consistent with various experimental studies − that show ion concentrations between 10 14 and 10 16 cm –3 . Studies that report significantly higher ion concentrations typically assume a complete screening of the electric field in the perovskite …”

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

“…Obviously, that explanation should be evaluated with additional experiments, not only IS responses. Recent findings reveal, however, that real steady-state charge is indeed induced by the applied voltage in the dark, easily interpreted by means of charged real capacitors with values much larger than the film geometrical capacitance …”

Mixed Conductivity of Hybrid Halide Perovskites: Emerging Opportunities and Challenges

“…Moreover, most models assume only a single mobile ion species, which is not always the case. As a result, there is an ongoing debate about the mobile ion density and its influence on the operation of perovskite-based devices (Moia et al, 2019;Caram et al, 2020;McGovern et al, 2020).…”