Using Cathodoluminescence Imaging on Perovskites

Perovskite materials have acquired considerable attention due to their potential to be used to develop efficient, cost-effective, and flexible optoelectronic devices such as light-emitting diodes and solar cells [1]. Perovskite is a common term for materials that have a perovskite crystal structure and an ABX3 composition, where A can either be an inorganic cation like Cs+, or an organic cation like CH3NH3 + (methylammonium, MA+) and H2NCHNH2 + (formamidinum, FA+). B is a metal di-cation like Pb2+, Sn2+, or Ge2+, and X is a halogen anion like Cl, Br, or I.

This article focuses on a completely inorganic perovskite material, CsPbIBr2, in which a combination of bromide and iodide is present [2]. The power conversion efficiencies can be enhanced by using a mixed halide composition. However, it is known that the Br and I halogen ions can move within the perovskite material under the influence of external stimuli such as light, which results in phase segregation. Such segregation can greatly affect the electrical properties of the material and its function as a light emitting device or solar cell [3,4].

Cathodoluminescence (CL) Microscopy

This effect takes place on microscopic length scales, which implies that high-resolution microscopy techniques should be used to examine it. Cathodoluminescence (CL) microscopy is an outstanding tool for this purpose. Usually, CL microscopy enables studying the material/optical properties of semiconductors by mapping the CL emission at sub-wavelength length scales. In direct band gap semiconductors like perovskites, the CL emission is typically dominated by band edge emission, while defect emission can also be observed depending on the (quality of the) material.

The Cs-based material is more robust when compared to partially organic perovskite materials, rendering it more suitable for microscopic optical and electron beam studies. It generally serves as a good model system for perovskite materials.

Intensity Map

For the CL imaging, the beam was raster-scanned over the surface and an intensity map was obtained. This map shows that the grain boundaries are brighter than the rest of the material. With hyperspectral CL imaging in which a complete emission spectrum is measured at every scanning pixel, it is possible to visualize the changes in the emission spectrum from point to point.

Comparison of the grain boundaries with the grain interiors revealed that the boundaries are richer in iodide, resulting in brighter emission and a red-shifted emission spectrum. This phase segregation was supported by high-resolution TEM studies on the same material. It is an interesting fact that these iodide-rich boundaries dominate the optical response when studying the system with photoluminescence where the material is irradiated with light [2].

(a) SEM image showing a top view of the perovskite layer. Different crystal grains are clearly visible. (b) Panchromatic CL intensity map revealing light emission efficiency at different points on the material. (c) False color CL image derived from a hyperspectral CL scan visualizing differences in emission spectrum from point to point. (d) CL spectra for the grain interiors (GI) versus the grain boundaries (GB). The grain boundaries are more red-shifted because of the higher iodide concentration. This is also apparent in the false color map in (c) where the boundaries are more red/orange compared to the green grain interiors. Images are from Ref. [2]. Experiments were performed on a SPARC system (Australian Research Council grant LE140100104) at the Monash Centre for Electron Microscopy (MCEM, Monash University, Australia).

Figure. (a) SEM image showing a top view of the perovskite layer. Different crystal grains are clearly visible. (b) Panchromatic CL intensity map revealing light emission efficiency at different points on the material. (c) False color CL image derived from a hyperspectral CL scan visualizing differences in emission spectrum from point to point. (d) CL spectra for the grain interiors (GI) versus the grain boundaries (GB). The grain boundaries are more red-shifted because of the higher iodide concentration. This is also apparent in the false color map in (c) where the boundaries are more red/orange compared to the green grain interiors. Images are from Ref. [2]. Experiments were performed on a SPARC system (Australian Research Council grant LE140100104) at the Monash Centre for Electron Microscopy (MCEM, Monash University, Australia).

The ability to visualize this phenomenon at these small length scales is vital in understanding and measuring it. This know-how can be applied to reduce the undesirable effects related to the segregation and perhaps even harness positive effects to enhance device performance.

References

[1] S. D. Stranks, H. J. Snaith, Metal-halide perovskites for photovoltaic and light-emitting devices, Nat. Nanotechnol. 10, 391–402 (2015).

[2] W. Li et al. Phase Segregation Enhanced Ion Movement in Efficient Inorganic CsPbIBr2 Solar Cells Adv. Energy Mater. 7, 1700946 (2017).

[3] D. J. Slotcavage et al. Light-Induced Phase Segregation in Halide-Perovskite Absorbers, ACS Energy Lett. 1, 1199-1205 (2016).

[4] Rachel E. Beal et al. Cesium Lead Halide Perovskites with Improved Stability for Tandem Solar Cells J. Phys. Chem. Lett. 7, 746-751 (2016).

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