在钙钛矿上使用阴极发光成像

钙钛矿材料由于其潜力用欧洲杯足球竞彩于开发有效，成本效益和柔性的光电设备（例如发光二极管和太阳能电池）而引起了广泛的关注[1]。钙钛矿是具有钙钛矿晶体结构和ABX的材料的常见术语欧洲杯足球竞彩₃组成，其中A可以是CS这样的无机阳离子⁺，或像CH这样的有机阳离子₃NH₃⁺（MA甲基铵⁺) and H₂nchnh₂⁺(formamidinum, FA⁺）。B是像PB这样的金属二燃料₂⁺, Sn₂⁺，或GE₂⁺, and X is a halogen anion like Cl^-, Br^-, or I^-。

图片来源：shutterstock/vvoe

This article focuses on a completely inorganic perovskite material, CsPbIBr₂，其中存在溴化物和碘化物的组合[2]。可以使用混合卤化物组成来提高功率转换效率。但是，众所周知，在外部刺激的影响下，BR和I卤素离子可以在钙钛矿材料（如光）的影响下移动，从而导致相分离。这种隔离会极大地影响材料的电性能及其作为发光装置或太阳能电池的功能[3,4]。

阴极发光（CL）显微镜

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.

强度图

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].

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）纳米技术。10，391–402（2015）。

[2] W. Li等。相分离增强了有效无机CSPBIBR2太阳能电池中的离子运动。能源母校。7，1700946（2017）。

[3] D. J. Slotcavage等。光诱导的卤化物 - 玻璃体吸收器中的相分离，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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