Local Observation of Phase Segregation in Mixed-Halide Perovskite

Abstract

Mixed-halide perovskites have emerged as promising materials for optoelectronics due to their tunable band gap in the entire visible region. A challenge remains, however, in the photoinduced phase segregation, narrowing the band gap of mixed-halide perovskites under illumination thus restricting applications. Here, we use a combination of spatially resolved and bulk measurements to give an in-depth insight into this important yet unclear phenomenon. We demonstrate that photoinduced phase segregation in mixed-halide perovskites selectively occurs at the grain boundaries rather than within the grain centers by using shear-force scanning probe microscopy in combination with confocal optical spectroscopy. Such difference is further evidenced by light-biased bulk Fourier-transform photocurrent spectroscopy, which shows the iodine-rich domain as a minority phase coexisting with the homogeneously mixed phase during illumination. By mapping the surface potential of mixed-halide perovskites, we evidence the higher concentration of positive space charge near the grain boundary possibly provides the initial driving force for phase segregation, while entropic mixing dominates the reverse process. Our work offers detailed insight into the microscopic processes occurring at the boundary of crystalline perovskite grains and will support the development of better passivation strategies, ultimately allowing the processing of more environmentally stable perovskite films.

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

“…This extraction of charge‐carriers from the low‐bandgap regions happens despite the fact that the iodide‐rich regions are known to comprise only a small fraction of the volume of the segregated perovskite layer, and the energy required to escape these low‐bandgap regions into the fully mixed perovskite phase (≈200 meV) is greater than the thermal energy available to the charge‐carriers at room temperature (≈30 meV). In order to explain how the low‐energy charge‐carriers transferred into the extraction layers we note that the iodide‐rich regions of perovskite are reported to preferentially form around the grain boundaries in the perovskite material . While the effect of grain boundaries on charge‐carrier lifetimes is still under debate, charge‐carrier transport along perovskite grain boundaries has previously been reported .…”

“…The responsible trap states therefore are likely to have an intrinsic neutral charge, as a negative charge would produce a low capture cross‐section for electrons, and a positive charge would be partly or entirely neutralized by a captured electron, resulting in smaller electric fields within the perovskite and therefore a lower driving force for halide segregation upon illumination. We suggest that these segregation‐responsible trap states are comprised of crystal lattice distortions located near the grain boundaries in the perovskite material, congruous with the evidence in the literature that these regions are strongly correlated with halide segregation …”

Perovskite Solar Cells: Can We Go Organic‐Free, Lead‐Free, and Dopant‐Free?

“…In comparison, perovskites of mixed composition (mixed cation and anion) show better structural stability but they still fall short in long-term stability because of phase segregation and photoinstability. [19][20][21] Intrinsic stability of perovskite is important but from the device performance stability point of view, the interfacial exchange occurring between perovskite and spiro-OMeTAD is even more important. [22] It has been reported that MA + and I − can migrate into spiro-OMeTAD, which deteriorates the performance because of enhanced interfacial resistance and recombination.…”

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

“…It is speculated that the formation of I-phase at the edge is favorable to minimize the total strain energy of the perovskite microplatelets under continuous illumination. [25,[41][42][43] Please note that under one-photon excitation (488 nm) in Figure 1, the penetration depth of the incident laser is within the first 100 nm, [44] where the carrier dynamics are highly impacted by the surface state of the perovskite nanoplatelets. To investigate the impacts of surface state on photoinduced phase segregation, we conducted a comparison of one-photon and two-photon PL of the same microplatelet, as shown in Figure 2.…”