Iodine Electrochemistry Dictates Voltage‐Induced Halide Segregation Thresholds in Mixed‐Halide Perovskite Devices

Abstract

Owing to straightforward stoichiometry-bandgap tunability, mixed-halide perovskites are ideal for many optoelectronic devices. However, unwanted halide segregation under operational conditions, including light illumination and voltage bias, restricts practical use. Additionally, the origin of voltageinduced halide segregation is still unclear. Herein, a systematic voltage threshold study in mixed bromide/iodide perovskite devices is performed and leads to observation of three distinct voltage thresholds corresponding to the doping of the hole transport material (0.7 ± 0.1 V), halide segregation (0.95 ± 0.05 V), and degradation (1.15 ± 0.05 V) for an optically stable mixed-halide perovskite composition with a low bromide content (10%). These empirical threshold voltages are minimally affected by composition until very Br-rich compositions, which reveals the dominant role of iodide/triiodide/iodine electrochemistry in voltage-induced Br/I phase separation and transport layer doping reactions in halide perovskite devices.

Phase Segregation and Sequential Expulsion of Iodide and Bromide in Photoirradiated Ruddlesden–Popper 2D Perovskite Films

“…The observed rate constant difference between the 405 and 490 nm traces likely stems from differences in iodide and bromide mobilities. A dominant role of iodide/​triiodide/​iodine electrochemistry in mixed-halide perovskite devices has been observed in voltage-induced phase segregation in mixed-halide perovskite films …”

“…A dominant role of iodide/triiodide/ iodine electrochemistry in mixed-halide perovskite devices has been observed in voltage-induced phase segregation in mixedhalide perovskite films. 47 Differences in halide ion mobility were further investigated by comparing the photoresponses of BA 2 PbBr 4 and BA 2 PbI 4 films. When films were immersed in DCM and subjected to steady-state (400 nm) irradiation, their excitonic absorption decreased (Figure 5A,B).…”

Halogen Redox Shuttle Explains Voltage-Induced Halide Redistribution in Mixed-Halide Perovskite Devices

“…Additional insight regarding the underlying mechanism behind this behavior is required to provide guidance for the development of robust devices. It was recently noted that voltage-induced halide segregation may plague any mixed-halide stoichiometry, including those below the bromide fraction threshold (∼20% bromide in mixed bromide/iodide perovskites) for stability against light-induced halide segregation, ,, meaning that voltage-induced halide segregation may be more widespread and problematic.…”

How Low Can You Go? Defect Quantification at the 1015 cm–3 Level in Mixed-Cation Perovskites Using Differential Pulse Voltammetry

“…Historically, electrochemical methodologies have provided critical insights into hole (electron) injection/extraction events from the valence (conduction) bands of semiconductor materials at a chosen interface of any device stack. − In these approaches, the semiconductor active layer acts as a working electrode in contact with inert electrolytes (i.e., noncoordinating and nonredox-active). For perovskite active layers, direct electrochemical quantification of redox-active defect states has been attempted using nonsolvents but in many cases suffered from dissolution and/or instabilities (i.e., film changes with analysis), phase segregation, and ultimately degradation. − Additionally, voltage biasing experiments have been used in devices in an attempt to study degradation based on halide migration and segregation. , Alternatively, our recently demonstrated voltametric approach employs a solid electrolyte/ionic liquid (SE/IL) “top contact” that equilibrates electrochemically with the perovskite to stabilize the film. Together with a reference and counter electrode combination, the perovskite is stressed under device-relevant bias and electric field distributions, inducing ion movement and defect reactivity .…”

“…58−61 Additionally, voltage biasing experiments have been used in devices in an attempt to study degradation based on halide migration and segregation. 62,63 Alternatively, our recently demonstrated voltametric approach employs a solid electrolyte/ionic liquid (SE/IL) "top contact" that equilibrates electrochemically with the perovskite to stabilize the film. Together with a reference and counter electrode combination, the perovskite is stressed under device-relevant bias and electric field distributions, inducing ion movement and defect reactivity.…”