Near-field effects on cathodoluminescence outcoupling in perovskite thin films
Halide perovskite semiconductors are a promising material for high-efficiency solar cells. Their optical properties can vary within and between crystallographic grains. We present spatially-resolved cathodoluminescence (CL) spectroscopy at 2 keV and 5 keV on polycrystalline CsPbBr3 perovskite films to study these variations at the nanoscale. The CL maps show a strongly reduced intensity near the polycrystalline grain boundaries. We perform numerical simulations of the far-field emission of the electron beam-generated optical near fields using the surface profiles from AFM as input. We find that near grain boundaries the light outcoupling is strongly reduced due to enhanced internal reflection and light trapping at the curved surfaces. Lateral variations in CL intensity inside grains are due to Fabry-Perot-like resonances in the film, with the substrate acting as a back reflector. Our results show that near-field coupling and interference effects can dominate nanoscale luminescence maps of halide perovskite films. The results are broadly relevant for the analysis of cathodoluminescence and photoluminescence of corrugated thin films.