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

Abstract

Voltage-induced halide segregation greatly limits the optoelectronic applications of mixed-halide perovskite devices, but a mechanistic explanation behind this phenomenon remains unclear. In this work, we use electron microscopy and elemental mapping to directly measure the halide redistribution in mixed-halide perovskite solar cells with quasi-ion-impermeable contact layers under different bias polarities to find iodide and bromide accumulation at the cathode and anode, respectively. This is consistent with a mechanism based on preferential iodide oxidation at the anode, leading to unbalanced + I i , I X , and Br X fluxes. Importantly, switching the anode from "inert" Au to "active" Ag prevents segregation because Ag oxidation precludes the oxidation of lattice iodide, which suggests employing redox-active additives as a general strategy to suppress halide segregation. Overall, these results show that halide perovskite devices operate as solid-state electrochemical cells when threshold voltages are exceeded, providing fresh insight to understand the impacts of voltage bias on halide perovskite devices.

Effect of Trap States, Ion Migration, and Interfaces on Carrier Transport in Single-Crystal, Polycrystalline, and Thick Film Devices of Halide Perovskites CH3NH3PbX3 (X = I, Br, Cl)

“…This could be due to a number of factors discussed as follows: (i) the larger I – ion possesses a much greater polarizability, which produces a large internal field to counter the external applied bias, and leads to lower charge carrier mobility . (ii) The use of an inert (FTO) anode along with Ag cathode leads to the formation of a redox shuttle of the halide ion across the absorber (Figure a), which leads to charge carrier (electron/hole) capture and reduced mobility . This effect would be much more pronounced for the larger I ions moving through 5 μm thick films.…”

“…This could be due to a number of factors as stated below: (i) the use of Ag for both cathode and anode, instead of the FTO cathode with a charge transporting layer, forms an electrode–absorber interface with a different injection barrier as compared to the transporting layer (Figure S13). (ii) The Ag anode acts to suppress halide oxidation and therefore the mobile halide ion redox shuttle observed in the thick film devices (Figure b) . This reduces the level of charge carrier capture by mobile halide ions and enhances their mobility.…”

“…(ii) The Ag anode acts to suppress halide oxidation and therefore the mobile halide ion redox shuttle observed in the thick film devices (Figure 14b). 54 This reduces the level of charge carrier capture by mobile halide ions and enhances their mobility. The redox shuttle suppression could explain the increase in the mobility of polycrystalline and single crystalline I devices as compared to thick films (Tables S5 and S6).…”

Origins of Photoluminescence Instabilities at Halide Perovskite/Organic Hole Transport Layer Interfaces

“…In our previous work, we studied voltage-induced halide segregation probed by PL peak shifts and cross-sectional scanning transmission electron microscopy–energy-dispersive X-ray spectroscopy (STEM–EDX). We found that iodine photo-electrochemistry and iodine transport pathways play important roles in halide segregation for mixed iodide/bromide perovskites, with an organic hole transport layer (HTL) providing a fast external iodine transport pathway, which depends strongly on ionization energy (IE). − Since HTLs with different highest occupied molecular orbital (HOMO) levels show drastically different iodine diffusion rates, it is expected that different HTLs may have different effects on light-induced halide segregation. Therefore, an in-depth study on how different HTLs affect light-induced halide segregation is necessary for the design of stable mixed-halide perovskite-based optoelectronic devices.…”

Mixed-Halide Perovskites with Halogen Bond Induced Interlayer Locking Structure for Stable Pure-Red PeLEDs

“…Accordingly, the pure-red perovskite emitters can be conveniently approached by halide substitution to generate Br–I mixed perovskites . However, the mixed halide perovskites suffer from severe ion migration during device operation, leading to spectral instability that significantly limits their further commercialization. − …”