Role of Photon Recycling and Band Filling in Halide Perovskite Photoluminescence under Focussed Excitation Conditions

We study the temporal, spatial, and spectral evolution of charge carriers in hybrid perovskite thin films after optical excitation with a small laser spot. These are conditions typical of microscopy-based studies. After excitation, the spreading of charge carriers in space occurs due to a combination of diffusion, recombination, and photon recycling. To quantitatively understand their relative importance, a numerical model is used to simulate the anticipated photoluminescence (PL) while accounting for these processes. We also experimentally study the evolution of the PL spectrum at different radial positions from the center of the excitation spot. This allows us to extend our previous work on carrier diffusion performed using time-resolved PL microscopy. Our numerical model predicts a position-dependent spectral shift of the waveguided photon energy in agreement with previous reports. However, we show that the spectral shift under our experimental conditions is dominated by band filling. Finally, we study the interplay between pump spot size, diffusion constant, and fluence on carrier transport in the presence of photon recycling. Our results show that in typical micro-PL experiments and modest carrier densities, photon recycling plays a negligible role in enhancing the lateral spreading of carriers.