Defect-Assisted Photoinduced Halide Segregation in Mixed-Halide Perovskite Thin Films

Abstract

Solution-processable metal halide perovskites show immense promise for use in photovoltaics and other optoelectronic applications. The ability to tune their bandgap by alloying various halide anions (for example, in CH3NH3Pb­(I1–x Br x )3, 0 < x < 1) is however hampered by the reversible photoinduced formation of sub-bandgap emissive states. We find that ion segregation takes place via halide defects, resulting in iodide-rich low-bandgap regions close to the illuminated surface of the film. This segregation may be driven by the strong gradient in carrier generation rate through the thickness of these strongly absorbing materials. Once returned to the dark, entropically driven intermixing of halides returns the system to a homogeneous condition. We present approaches to suppress this process by controlling either the internal light distribution or the defect density within the film. These results are relevant to stability in both single- and mixed-halide perovskites, leading the way toward tunable and stable perovskite thin films for photovoltaic and light-emitting applications.

Trap States, Electric Fields, and Phase Segregation in Mixed‐Halide Perovskite Photovoltaic Devices

“…These three recombination pathways persist as the perovskite layer segregates, and the hatched pathways in Figure show the additional pathways—involving the iodide‐rich regions of perovskite—that open up as halide segregation occurs. The rise in EQE at long wavelengths shown in Figure 3a—and absorption measurements in the literature—prove that a small but not insignificant number of charge‐carriers are directly photogenerated in the iodide‐rich regions of the perovskite, which is reflected in Figure as the corresponding pathway into the red iodide‐rich phase box. Additionally, the drop in mixed perovskite PL signal intensity indicated in the bottom panel of Figure 2a is attributed to a flow of charge‐carriers from the mixed perovskite phase into the lower‐bandgap regions of perovskite, which is depicted as a pathway between the two phases in Figure .…”

FA‐Assistant Iodide Coordination in Organic–Inorganic Wide‐Bandgap Perovskite with Mixed Halides

“…The phase‐segregation is reduced with 5% FA + introduced into the perovskite film (Figure S4b, Supporting Information), and it is completely inhibited with 10% FA + (Figure S4c, Supporting Information). The inhibited phase‐segregation could be due to the reduced defect density of the perovskite film 49,50. However, when further increasing the FA + content to 20%, the phase‐segregation issue starts to emerge (Figure S4d, Supporting Information), which could be caused by the deteriorated phase impurity (δ‐phase and PbI 2 ) 51…”

Tracking Dynamic Phase Segregation in Mixed‐Halide Perovskite Single Crystals under Two‐Photon Scanning Laser Illumination

“…Intensity and duration of illumination are the critical factors to induce phase segregation of mixed halide perovskite . Herein, we performed power dependent measurement on the perovskite microplatelets (200 to 800 mW cm −2 ), as shown in Figure a.…”