Photon Recycling in CsPbBr₃ All-Inorganic\nPerovskite Nanocrystals

Abstract

Photon recycling,\nthe iterative process of re-absorption and re-emission\nof photons in an absorbing medium, can play an important role in the\npower-conversion efficiency of photovoltaic cells. To date, several\nstudies have proposed that this process may occur in bulk or thin\nfilms of inorganic lead-halide perovskites, but conclusive proof of\nthe occurrence and magnitude of this effect is missing. Here, we provide\nclear evidence and quantitative estimation of photon recycling in\nCsPbBr₃ nanocrystal suspensions by combining measurements\nof steady-state and time-resolved photoluminescence (PL) and PL quantum\nyield with simulations of photon diffusion through the suspension.\nThe steady-state PL shows clear spectral modifications including red\nshifts and quantum yield decrease, while the time-resolved measurements\nshow prolonged PL decay and rise times. These effects grow as the\nnanocrystal concentration and distance traveled through the suspension\nincrease. Monte Carlo simulations of photons diffusing through the\nmedium and exhibiting absorption and re-emission account quantitatively\nfor the observed trends and show that up to five re-emission cycles\nare involved. We thus identify 4 quantifiable measures, PL red shift,\nPL QY, PL decay time, and PL rise time that together all point toward\nrepeated, energy-directed radiative transfer between nanocrystals.\nThese results highlight the importance of photon recycling for both\noptical properties and photovoltaic applications of inorganic perovskite\nnanocrystals.