Revealing Nanomechanical Domains and Their Transient Behavior in Mixed‐Halide Perovskite Films

Abstract

Halide perovskites are a versatile class of semiconductors employed for high performance emerging optoelectronic devices, including flexoelectric systems, yet the influence of their ionic nature on their mechanical behavior is still to be understood. Here, a combination of atomic‐force, optical, and compositional X‐ray microscopy techniques is employed to shed light on the mechanical properties of halide perovskite films at the nanoscale. Mechanical domains within and between morphological grains, enclosed by mechanical boundaries of higher Young's Modulus (YM) than the bulk parent material, are revealed. These mechanical boundaries are associated with the presence of bromide‐rich clusters as visualized by nano‐X‐ray fluorescence mapping. Stiffer regions are specifically selectively modified upon light soaking the sample, resulting in an overall homogenization of the mechanical properties toward the bulk YM. This behavior is attributed to light‐induced ion migration processes that homogenize the local chemical distribution, which is accompanied by photobrightening of the photoluminescence within the same region. This work highlights critical links between mechanical, chemical, and optoelectronic characteristics in this family of perovskites, and demonstrates the potential of combinational imaging studies to understand and design halide perovskite films for emerging applications such as photoflexoelectricity.

Mechanical Properties of Organic Electronic Polymers on the Nanoscale

“…The role of humidity upon air exposure in these films is not known at this time, but detrimental effects have been observed previously in organic-inorganic semicrystalline films where the degradation mechanism begins at the grain boundaries. [36,37] All told, the fact that C16-IDTBT sports a high mobility despite its near-amorphous morphology in addition to a spatially homogeneous modulus that is comparatively low, makes this donor-acceptor co-polymer semiconductor a candidate for truly flexible organic microelectronics and electromechanical devices. [38]…”

Unveiling the Interaction Mechanisms of Electron and X‐ray Radiation with Halide Perovskite Semiconductors using Scanning Nanoprobe Diffraction

“…[ 23 ] However, we observe crucial differences given by the more complex perovskite composition used in this study, in which simultaneous formation of PbBr 2 is also observed at the grains, potentially aided by the higher volatility of I 2 compared to Br 2 , by the higher surface area present at the grain boundaries, and by small nanoscale chemical heterogeneity at the grain boundaries. [ 42,43 ] These findings can be extended to the radiation damage from a high‐energy X‐ray nanobeam, which also results in the emergence of lead halide species upon loss of the organic moieties across the whole scanned area (Figure S17, Supporting Information). In fact, the presence of the organic cation species at the A position has been proposed to trigger degradation processes toward PbI 2 as opposed to the Pb 0 , mostly only seen from inorganic compositions, as the organics can undergo radiolysis more easily than the Pb 2+ cation upon electron and X‐ray illumination.…”

“…Perovskite samples of similar compositions have been reported to show compositional heterogeneity across grains, with Br-rich clusters at the grain boundaries. [42,43] Based on the homogeneity of the diffraction pattern across the grain in the pristine state of the film, chemical heterogeneity must be small, yet present enough to help drive the reported local changes. Distinctively, degradation does not appear to originate from the twin boundaries, consistent with theoretical predictions, whereby the octahedron face-sharing present at the twin boundary stabilizes the twins.…”

Perovskite Solar Cells with Carbon‐Based Electrodes – Quantification of Losses and Strategies to Overcome Them

“…[27] The intuitive affiliation of GBs with differences in morphological properties of perovskite films (as can be seen from, for example, scanning electron microscopy SEM images) does not account for sub-grain crystallographic heterogeneity within the same morphological domain. [50][51][52] highlighting that for the exact identification of a GB in halide perovskite, diffraction-based techniques are required. [30,53] In this work, however, we use the term grain boundary to describe "morphological grains boundaries", observed from SEM measurements.…”