In situ anion exchange with redox mediators influencing the photovoltaic parameters in vacancy-ordered halide perovskite-based photoelectrochemical solar cells†

Abstract

For building stable solar cells, it is essential to understand the interaction of constituent compounds in the devices. In a typical photoelectrochemical (PEC) solar cell, the absorber interacts with the electrolyte species, and it can affect the stability of the devices. This work reports the interaction of a vacancy-ordered halide perovskite absorber Cs₂Pt(Cl/Br/I)₆ with iodide/triiodide redox mediators in PEC solar cells. Our results showed that an in situ anion exchange occurred where Cs₂Pt(Cl/Br)₆ was converted to Cs₂PtI₆. Among the reduced (I⁻) and oxidized (I₃⁻) species, it was found that the former was responsible for the anion exchange, and this exchange followed a core–shell mechanism. Using a combination of microscopic and spectroscopic techniques, it was found that in the shell of the particle, the Cl/Br halides were replaced with iodide leaving the core with Cl/Br. However, with time, complete conversion was achieved leading to the formation of Cs₂PtI₆. This interaction, unlike in dye-sensitized solar cells or ABX₃-type PEC solar cells, showed an improvement in performance over time. The transition from Cs₂Pt(Cl/Br)₆ to Cs₂PtI₆ led to the formation of multiple heterostructures, where at the beginning of the reaction, a Type II heterojunction was formed within the absorber. With time, the core–shell structure transitioned to an inverse Type II heterostructure. These transitions led to differences in the photovoltaic performances and device stability.