X-ray microscopy can provide unique chemical, electronic, and structural insights into perovskite materials and devices leveraging bright, tunable synchrotron X-ray sources. Over the last decade, fundamental understanding of halide perovskites and their impressive performance in optoelectronic devices has been furthered by rigorous research regarding their structural and chemical properties. Herein, studies of perovskites are reviewed that have used X-ray imaging, spectroscopy, and scattering microscopies that have proven valuable tools toward understanding the role of defects, impurities, and processing on perovskite material properties and device performance. Together these microscopic investigations have augmented our understanding of the This article is protected by copyright. All rights reserved. 2 internal workings of perovskites and help steer the perovskite community toward promising directions. In many ways, X-ray microscopy of perovskites is still in its infancy, which leaves many exciting paths unexplored including new super-resolution, multimodal, in situ, and operando experiments. To explore possibilities, pioneering X-ray microscopy along these lines is briefly highlighted from other semiconductor systems including silicon, CdTe, GaAs, CuIn x Ga 1-x Se 2 , and organic photovoltaics. An overview is provided on the progress made in utilizing X-ray microscopy for perovskites and present opportunities and challenges for future work.
Correlative X‐ray microscopy, including synchrotron X‐ray diffraction and fluorescence, is leveraged to understand the local role of europium as a B‐site additive in CsPbBr3 perovskite crystals. Europium addition reduces microstrain in the perovskite, despite the fact that the degree of europium incorporation into the perovskite varies locally, with a maximum loading over twice the nominal stoichiometry. The presence of europium improves photoluminescence yield and bandwidth, while shifting the emission to bluer wavelengths. Finally, europium‐containing crystals have greatly improved X‐ray hardness. The findings show promise for europium as an additive in perovskite optoelectronic devices.
Understanding the optoelectronic properties of optically active materials at the nanoscale often proves challenging due to the diffraction-limited resolution of visible light probes and the dose sensitivity of many optically active materials to high-energy electron probes. In this study, we demonstrate correlative synchrotron-based scanning X-ray Excited Optical Luminescence (XEOL) and X-ray Fluorescence (XRF) to simultaneously probe local composition and optoelectronic properties of halide perovskite thin films of interest for photovoltaic and optoelectronic devices. We find that perovskite XEOL stability, emission redshifting, and peak broadening under hard X-ray irradiation correlates with trends seen in photoluminescence measurements under continuous visible light laser irradiation. The XEOL stability is sufficient under the intense X-ray probe irradiation to permit proof-of-concept correlative mapping. Typical synchrotron X-ray fluorescence and nano-diffraction measurements use acquisition times 10-100x shorter than the 5-second acquisition employed for XEOL scans in this study, suggesting that improving luminescence detection should allow correlative XEOL measurements to be performed successfully with minimal material degradation. Analysis of the XEOL emission from the quartz substrate beneath the perovskite reveals its promise for use as a real-time in-situ X-ray dosimeter, which could provide quantitative metrics for future optimization of XEOL data collection for perovskites and other beam-sensitive materials. Overall, the data suggest that XEOL represents a promising route towards improved resolution in the characterization of nanoscale heterogeneities and defects in optically active materials that may be implemented into X-ray nanoprobes to complement existing X-ray modalities.
This paper describes the efficacy of barrier films coated with single- and multi-layer graphene in preventing degradation of perovskite films in air. Despite the impermeability of graphene to small species such as water and oxygen, the presence of numerous grain boundaries and defects in chemical vapor deposition (CVD)-grown graphene monolayer films can present pathways for permeation. However, the availability of these pathways can in principle be reduced by stacking multiple layers of graphene on top of each other. The barrier material considered here consists of the semi-permeable polymer parylene laminated with either 0, 1, 2, or 3 monolayers of graphene. These composite films are used to encapsulate triple cation perovskite films, which are then subjected to a degradation test under damp heat. We find that a monolayer of graphene confers a 15-fold reduction in degradation compared to the parylene films with no graphene and that three-layer graphene can yield a further 2× reduction in degradation. Although all of our films encapsulated with graphene/parylene exhibited substantial degradation compared to films encapsulated in glass with polyisobutylene edge seals, our results nonetheless reinforce the utility of graphene barriers for less demanding applications, including lightweight or flexible perovskite solar cells with shorter anticipated lifetimes.
Ferroelectric nanomaterials are of interest in catalysis, nonvolatile memory, and neuromorphic computing among other applications because of their switchable structure that can alter the electronic and interface properties of a single material. The investigation of the role of polarization on the surface structure and chemistry of ferroelectric nanomaterials is a longstanding challenge, as it ideally requires a combination of both nanoscale imaging and chemical spectroscopy. In this work, we study a model ferroelectric BaTiO 3 thin film by synchrotron Xray scanning tunneling microscopy (SX-STM), a unique method that integrates nanoscale surface imaging and chemically sensitive spectroscopy. We find that polarization switching from downward to upward in (001) single-crystalline BaTiO 3 thin films increases the intensity of X-ray absorption across Ba M, Ti L, and O K edges. Chemical mapping of nanometer-sized domains further demonstrates the modulation of surface structures upon polarization switching, as well as confirming the trends observed in singlepoint experiments across the surface. We complement these measurements with ab initio computational absorption spectroscopy to elucidate the effect of polarization switching on the core−hole excitations using the Bethe−Salpeter equation approach. Our experimental and theoretical results thus confirm a stronger binding strength for the upward-polarized surface with molecular O 2 as a model reactant, offering mechanistic evidence that supports previous reports. This work advances the understanding of the surface chemistry and electronic structure of ferroelectrics, which can ultimately aid strategies to design interfaces with tailored properties.
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