Reversible Pb²⁺/Pb⁰ and I⁻/I₃⁻ Redox Chemistry Drives the Light‐Induced Phase Segregation in All‐Inorganic Mixed Halide Perovskites

Abstract

most successful to date were complex lead halides comprising simultaneously several univalent cations (Cs + , CH 3 NH 3 + or MA + , [H 2 NCHNH 2 ] + or FA +) and halide anions (typically Br − , I −) in their crystal lattice. [2] However, these materials suffer from low photostability. In particular, Hoke et al. first demonstrated that the mixed-halide MAPb(I 1−x Br x) 3 absorbers undergo rapid light-induced halide segregation with the formation of I-rich and Br-rich phases leading to both structural and energetic disorder resulting in a significant decrease in solar cell performance. [3,4] While the effect of short light exposure was found to be essentially reversible in the dark, long-term irradiation of the mixed halide perovskite films results in their complete degradation. [5] Therefore, light-induced halide phase segregation is considered as a severe limitation for achieving long-term operational stability of perovskite solar cells based on the absorbers incorporating more than a single halide anion. [6] Overcoming this problem is crucially important for the development of tandem devices with the upper cell based on the perovskite absorber with the tailored optical properties realized through halide mixing. Since the discovery of the light-induced halide phase segregation in complex lead halides, many research groups have investigated this phenomenon in detail in an attempt to reveal its mechanism. Multiple models varying in the origin

Photoinduced Halide Segregation in Ruddlesden–Popper 2D Mixed Halide Perovskite Films

“…Spectroscopic, [33,50] spectroelectrochemical, [51,52] and electronic and ionic conductivity [53] studies have identified hole capture by iodide as the primary step in inducing phase segregation. [54] There are several key differences between the excited state behavior of 2D and 3D perovskites that influence halide ion migration. 2D perovskites exhibit a larger degree of quantumand dielectric-confinement, as evidenced from larger exciton binding energies in the lower-dimensionality perovskites, for example, 320 meV for n = 1 PEA 2 MA n−1 Pb n I 3n+1 .…”

A Key 2D Intermediate Phase for Stable High‐Efficiency CsPbI2Br Perovskite Solar Cells

“…Hence, the significantly enhanced PL strength indicates that the defects in the LDIAG 0.5‐IMX perovskite film are reduced in number and the nonradiative recombination is greatly inhibited. [ 37 ] The corresponding TRPL decay is shown in Figure 4c and fitted with a double‐exponential decay function (Table S1, Supporting Information). The LDIAG 0.5‐IMX perovskite film exhibits long fluorescence liftetime, and the corresponding value of τ ave was 17.56 ns, which is increased tremendously compared with that of the control film (7.57 ns).…”

Beyond the Phase Segregation: Probing the Irreversible Phase Reconstruction of Mixed‐Halide Perovskites

“…demonstrate that the photoinduced phase segregation selectively takes place at grain boundaries rather than within grain centers. [ 42 , 43 ] Then, Kamat et al. report that the hole accumulation induces iodide to move from the lattice toward grain boundaries in perovskite thin films.…”

“…Particularly, Brabec et al demonstrate that the photoinduced phase segregation selectively takes place at grain boundaries rather than within grain centers. [42,43] Then, Kamat et al report that the hole accumulation induces iodide to move from the lattice toward grain boundaries in perovskite thin films. [44] By selectively injecting holes into the mixed-halide perovskite through electrochemical anodic bias, iodide is gradually expelled from the thin film leading to the reformation of MAPbBr 3 domains.…”