all-inorganic perovskite nanocrystals*

Photon recycling, the iterative process of re-absorption and re-emission of photons in an absorbing medium, can play an important role in the power-conversion efficiency of photovoltaic cells. To date, several studies have proposed that this process may occur in bulk or thin films of inorganic lead-halide perovskites, but conclusive proof of the occurrence and magnitude of this effect are missing. Here, we provide clear evidence of photon recycling in CsPbBr3 nanocrystal suspensions by combining measurements of steady-state and time-resolved (TR) photoluminescence (PL) and PL quantum yield with simulations of photon diffusion through the suspension. We observe clear spectral modifications including red shifts and quantum yield decrease, while the TRPL measurements show prolonged PL decay and rise times. Monte Carlo simulations of photons diffusing through the medium account quantitatively for the observed trends and show that up to 5 re-emission cycles are involved. We thus identify 4 quantifiable measures, PL red-shift, PL QY, PL decay-time, and PL rise-time that together all point towards repeated, energy-directed radiative transfer between nanocrystals. These results further establish lead-halide perovskites as strong candidates for optoelectronic applications.