Rationalizing the light-induced phase separation of mixed halide organic–inorganic perovskites

Abstract

Mixed halide hybrid perovskites, CH3NH3Pb(I1−xBrx)3, represent good candidates for low-cost, high efficiency photovoltaic, and light-emitting devices. Their band gaps can be tuned from 1.6 to 2.3 eV, by changing the halide anion identity. Unfortunately, mixed halide perovskites undergo phase separation under illumination. This leads to iodide- and bromide-rich domains along with corresponding changes to the material’s optical/electrical response. Here, using combined spectroscopic measurements and theoretical modeling, we quantitatively rationalize all microscopic processes that occur during phase separation. Our model suggests that the driving force behind phase separation is the bandgap reduction of iodide-rich phases. It additionally explains observed non-linear intensity dependencies, as well as self-limited growth of iodide-rich domains. Most importantly, our model reveals that mixed halide perovskites can be stabilized against phase separation by deliberately engineering carrier diffusion lengths and injected carrier densities.

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 .…”

Illumination‐Induced Halide Segregation in Gradient Bandgap Mixed‐Halide Perovskite Nanoplatelets

“…Upon laser illumination, carriers on the top surface layers (FAPbBr 3 ) are photogenerated to the high excited state and then relax quickly into the edge of the conduction and valence bands by emitting phonons . Then, these charge carriers transfer toward the substrate side (I‐rich layers) by carrier diffusion due to the specific gradient bandgap, passing energy to the longitudinal optical (LO) phonon . The influence of transverse optical phonon and acoustic phonon can be ignored in this process .…”

“…The influence of transverse optical phonon and acoustic phonon can be ignored in this process . The LO phonon provides the driving force to deform the surrounding halide anion lattice through entropy, stochastic composition fluctuations, and lattice strain . Some of the halide ions are then activated as mobile ions.…”

Inorganic CsPbI2Br Perovskite Solar Cells: The Progress and Perspective

“…Another advantage is that the CsPbI 2 Br NC films present excellent photostability with suppressed light‐induced phase segregation . Colloidal NCs provide an efficient procedure to deposit high‐quality inorganic perovskite films under ambient air conditions.…”