Metal halide perovskites exhibit exceptional optical and electrical properties, positioning them as promising materials for a range of optoelectronic applications, including solar cells and light-emitting diodes (LEDs). Their solution-processable nature enables the fabrication of high-quality polycrystalline films. However, their susceptibility to degradation under various stress conditions remains a significant challenge, highlighting the importance of understanding their structural and optoelectronic properties at the nano- and microscale, particularly at individual grains and grain boundaries.

In this study, we investigate multi-crystalline CsPbBr₃ films deposited on silicon substrates to explore the influence of grain orientation and boundaries on their optical properties. We utilise electron backscatter diffraction (EBSD) and cathodoluminescence (CL) spectral imaging to determine the crystal orientation and the optical properties of the same sample area, respectively. Both techniques raster-scan a high-energy electron beam of a few keV over the sample surface while collecting either EBSD patterns or CL spectra.

By combining EBSD and CL spectroscopy, we achieve, for the first time, a direct correlation between crystal orientation and optical signature at the nanoscale (CL) of perovskite films. Our findings reveal that (1) the CL intensity and spectral characteristics are independent of the crystal orientation within individual grains, and (2) the CL intensity decreases significantly at grain boundaries. Depth-resolved CL analysis provides further insights into reabsorption processes and light out-coupling mechanisms. Notably, no electron beam-induced degradation was observed, even after repeated scans at 15 keV for EBSD and 5 keV for CL.

We complement the experimental results with optical near-field simulations to gain deeper understanding of the dominating effect of light (CL) outcoupling as well as carrier diffusion upon electron exposure. Furthermore, we will utilise time-resolved photoluminescence and CL to correlate carrier lifetime and diffusion depending on the position of the electron beam, namely within a perovskite grain or boundary.

Additionally, we fabricated LEDs with spin-coated CsPbBr₃ films as the top layer, enabling in-situ CL and secondary electron (SE) mapping under electrical bias. This configuration leverages the high spatial resolution of SE imaging and the spectral precision of CL to examine the effects of electrical bias on device performance. Interestingly, we observe that the recorded electroluminescence (EL) spectrum is red-shifted and broadened in comparison to the CL spectrum.

In-situ and operando studies of perovskite films and devices will deepen the understanding of their degradation processes under electron excitation and electrical bias as well as the role of grain orientation and boundaries at the nanoscale.