Atomic-scale microstructure of metal halide perovskite

Abstract

Hybrid organic-inorganic perovskites have high potential as materials for solar energy applications, but their microscopic properties are still not well understood. Atomic-resolution scanning transmission electron microscopy has provided invaluable insights for many crystalline solar cell materials, and we used this method to successfully image formamidinium lead triiodide [CH(NH2)2PbI3] thin films with a low dose of electron irradiation. Such images reveal a highly ordered atomic arrangement of sharp grain boundaries and coherent perovskite/PbI2 interfaces, with a striking absence of long-range disorder in the crystal. We found that beam-induced degradation of the perovskite leads to an initial loss of formamidinium [CH(NH2)2+] ions, leaving behind a partially unoccupied perovskite lattice, which explains the unusual regenerative properties of these materials. We further observed aligned point defects and climb-dissociated dislocations. Our findings thus provide an atomic-level understanding of technologically important lead halide perovskites.

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

“…However, atomic‐scale electron microscopy has recently shown that strain occurring at the (morphological) grain boundaries extends for only a few unit cells. [ 53 ] We cannot discard that this may have an impact in our observations, but even with the high spatial resolution we achieve (≈10 nm) using AFM, we are not able to resolve heterogeneities at the atomic scale. We note that the diffusion coefficient (diffusion lengths) of the A‐site cations is at least three orders of magnitude (30 times) smaller than the one of halides, making their movement unlikely to be the origin of the transient phenomena we observe.…”

Response to Comment on “Eppur si Muove: Proton Diffusion in Halide Perovskite Single Crystals”: Measure What is Measurable, and Make Measurable What is Not So: Discrepancies between Proton Diffusion in Halide Perovskite Single Crystals and Thin Films

“…At the same time, Rothmann et al. [ 7 ] report finding by TEM intrinsic 0D, 1D, and 2D defects in thin polycrystalline films. Such defects could affect the properties of HaP thin films, even though, unless optimal surface passivation is performed, to a lesser extent than grain boundaries.…”

“…While agreeing that dislocations or other 1D defects (essentially the ones now considered relevant in ref. [3]) are “not normally detectable by optical or electron microscopy (SEM),” beam damage issues for TEM studies of HaPs are well‐documented, [ 7–10 ] and avoiding, beam‐induced defects remains a major challenge. Also, the preparation of few nm thick samples from single crystals for TEM studies is extremely complicated.…”

Small grains as recombination hot spots in perovskite solar cells

“…Grain boundaries may also exhibit other phenomena, such as complexation, 29 chemical excess of either PbI 2 30 or organic halides, 31 or the presence of amorphous phases, 32 especially in the case of non-stoichiometric compositions; however, recent work by Rothmann et al has shown that grain boundaries in high-quality polycrystalline perovskite films are atomically clean. 33 The situation may be further complicated by the fact that not all crystallographic grains are resolved by commonly used microscopic methods such as scanning electron microscopy (SEM) or atomic force microscopy (AFM). The identification of ''grains'' via these methods is based on the visible grooves that separate them, thus providing contrast.…”