Abstract
The interaction of high-energy electrons and X-ray photons with beam-sensitive semiconductors such as halide perovskites is essential for the characterization and understanding of these optoelectronic materials. Using nanoprobe diffraction techniques, which can investigate physical properties on the nanoscale, studies of the interaction of electron and X-ray radiation with state-of-the-art (FA0.79 MA0.16 Cs0.05 )Pb(I0.83 Br0.17 )3 hybrid halide perovskite films (FA, formamidinium; MA, methylammonium) are performed, tracking the changes in the local crystal structure as a function of fluence using scanning electron diffraction and synchrotron nano X-ray diffraction techniques. Perovskite grains are identified, from which additional reflections, corresponding to PbBr2 , appear as a crystalline degradation phase after fluences of 200 e- Å- 2 . These changes are concomitant with the formation of small PbI2 crystallites at the adjacent high-angle grain boundaries, with the formation of pinholes, and with a phase transition from tetragonal to cubic. A similar degradation pathway is caused by photon irradiation in nano-X-ray diffraction, suggesting common underlying mechanisms. This approach explores the radiation limits of these materials and provides a description of the degradation pathways on the nanoscale. Addressing high-angle grain boundaries will be critical for the further improvement of halide polycrystalline film stability, especially for applications vulnerable to high-energy radiation such as space photovoltaics.
Highlights
Halide perovskite materials exhibit promising characteristics for optoelectronics applications
Rothmann et al used transmission electron microscopy (TEM) to quantify structural changes in methylammonium lead iodide films (MAPbI3) at fluences as low as 100 e- Å-2,[11] in which loss of the organic moieties results in lattice contraction and the formation of a supercell, further described by Chen et al as MAPbI2.5, degrading into PbI2.[15,16] The appearance of an intermediate phase of degradation agrees with similar studies by scanning electron microscopy techniques.[17–19]
Moving beyond the workhorse hybrid MAPbI3 composition, Rothmann et al recently reported high-resolution scanning transmission electron microscopy (STEM) images of evaporated formamidinium lead iodide (FAPbI3) thin films, reporting the appearance of additional reflections at low fluences of 200 e- Å-2, the loss of the perovskite phase at 600 eÅ-2, and the eventual formation of PbI2 grown on the lattice-expanded degraded phase of FAPbI3 after 1,000 e- Å-2.[23]. The effect of scanning X-ray probes on halide perovskites, on the other hand, has been less studied than for electron microscopy.[5]
Summary
Halide perovskite materials exhibit promising characteristics for optoelectronics applications. Scanning nanoprobe characterisation techniques, such as scanning electron diffraction (SED) or nanoprobe X-ray diffraction (nXRD), are advantageous as they can access both the relevant nano- and microscales.[4,5] Both techniques use focused radiation, which interacts with the material and can impact its structure and chemistry, especially for materials with complex stoichiometries such as hybrid halide perovskites that mix organic and inorganic ions. High-angle grain boundaries in the polycrystalline structure trigger such changes to the nanostructure These studies establish critical radiation values and interaction mechanisms of electron and X-ray radiation for mixed-cation mixed-halide perovskites, allowing the elucidation of global mechanisms for degradation, as well as stability windows in both measurement and application of these radiation types
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