Cathodoluminescence and Electron Backscatter Diffraction to Correlate Optical and Structural Properties of Halide Perovskite Thin Films at the Nanoscale

Metal halide perovskites form a very exciting material class for various opto-electronic applications. These materials form multi-crystalline films of exceptionally high quality by methods as simple as spin-coating. The soft and ionic nature of the polycrystalline perovskite films leads to highly complex material properties that can vary over time and from grain to grain. To include perovskites in commercial applications, we need to understand their optoelectronic and structural characteristics on the scale of single grains (e.g. nanometers). Cathodoluminescence (CL) microscopy has been used successfully in the past to study grain-to-grain variation in luminescence efficiency and has shown that surface traps not only dominate carrier trapping, but also are distributed unevenly amongst various grains.¹ We want to connect the optical properties to the structure of the film to find the origin of this optical heterogeneity. To this day, only few studies have correlated opto-electronic properties to structure at the nanoscale.^2,3 We combine nanoscale characterization of optical and structural properties by measuring cathodoluminescence and electron backscatter diffraction analysis on the same location in perovskite thin films. Electron backscatter diffraction is an electron microscopy technique that gives information on the local grain orientation, grain boundaries and strain.^2,4 For the first time, we combine these electron microscopy-based techniques on CsPbBr₃ thin films to make a quantitative correlation between luminescence intensity and grain orientation. In these evaporated thin films we find that the CL intensity and spectral shape is not correlated to the orientation of a grain, but that CL intensity is significantly decreased at the grain boundaries. We use simulations and time-resolved CL to disentangle the influence of morphology from enhanced non-radiative recombination. We showcase a new method that creates a direct link between optical and structural properties of many grains at the same time. By locating the origin of processes such as electronic carrier trapping or halide segregation, we hope to further the understanding of perovskites at the nanoscale, and inform the efforts towards more effective film passivation.<br/><br/><br/><br/>References<br/><br/>(1) Bischak, C. G.; Sanehira, E. M.; Precht, J. T.; Luther, J. M.; Ginsberg, N. S. <i>Nano Lett.</i> <b>2015</b>, <i>15</i> (7), 4799–4807.<br/>(2) Jariwala, S.; Sun, H.; Adhyaksa, G. W. P.; Lof, A.; Muscarella, L. A.; Ehrler, B.; Garnett, E. C.; Ginger, D. S. <i>Joule</i> <b>2019</b>, <i>3</i> (12), 3048–3060.<br/>(3) Doherty, T. A. S.; Winchester, A. J.; Macpherson, S.; Johnstone, D. N.; Pareek, V.; Tennyson, E. M.; Kosar, S.; Kosasih, F. U.; Anaya, M.; Abdi-Jalebi, M.; Andaji-Garmaroudi, Z.; Wong, E. L.; Madéo, J.; Chiang, Y.-H.; Park, J.-S.; Jung, Y.-K.; Petoukhoff, C. E.; Divitini, G.; Man, M. K. L.; Ducati, C.; Walsh, A.; Midgley, P. A.; Dani, K. M.; Stranks, S. D. <i>Nature</i> <b>2020</b>, <i>580</i> (7803), 360–366.<br/>(4) Muscarella, L. A.; Hutter, E. M.; Sanchez, S.; Dieleman, C. D.; Savenije, T. J.; Hagfeldt, A.; Saliba, M.; Ehrler, B. <i>J. Phys. </i><i>Chem. Lett.</i> <b>2019</b>, <i>10</i> (20), 6010–6018.