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 is missing. Here, we provide clear evidence and quantitative estimation of photon recycling in CsPbBr₃ nanocrystal suspensions by combining measurements of steady-state and time-resolved photoluminescence (PL) and PL quantum yield with simulations of photon diffusion through the suspension. The steady-state PL shows clear spectral modifications including red shifts and quantum yield decrease, while the time-resolved measurements show prolonged PL decay and rise times. These effects grow as the nanocrystal concentration and distance traveled through the suspension increase. Monte Carlo simulations of photons diffusing through the medium and exhibiting absorption and re-emission account quantitatively for the observed trends and show that up to five 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 toward repeated, energy-directed radiative transfer between nanocrystals. These results highlight the importance of photon recycling for both optical properties and photovoltaic applications of inorganic perovskite nanocrystals.

中文翻译：

---

CsPbBr3 全无机钙钛矿纳米晶体中的光子回收

光子回收，即光子在吸收介质中重新吸收和重新发射的迭代过程，对光伏电池的功率转换效率起着重要作用。迄今为止，有几项研究提出，该过程可能发生在无机卤化铅钙钛矿的块状或薄膜中，但缺乏这种效应发生和大小的确凿证据。在这里，我们提供了 CsPbBr ₃中光子回收的明确证据和定量估计通过将稳态和时间分辨光致发光 (PL) 和 PL 量子产率的测量与通过悬浮液的光子扩散模拟相结合，来制备纳米晶体悬浮液。稳态 PL 显示出明显的光谱修改，包括红移和量子产率降低，而时间分辨测量显示 PL 衰减和上升时间延长。这些效应随着纳米晶体浓度和穿过悬浮液的距离的增加而增加。光子通过介质扩散并表现出吸收和再发射的蒙特卡罗模拟定量地说明了观察到的趋势，并表明最多涉及五个再发射周期。因此，我们确定了 4 个可量化的度量，PL 红移、PL QY、PL 衰减时间和 PL 上升时间，它们共同指向重复，纳米晶体之间的能量定向辐射转移。这些结果强调了光子回收对无机钙钛矿纳米晶体的光学特性和光伏应用的重要性。