Figure 1. Scheme of the photon recycling measurement method.

(a) Normalized photoluminescence (PL) spectrum of a 1 mm thick CH₃NH₃PbBr₃ (MAPbBr₃) single crystal (SC) measured with photomask (red curve), its normalized transmission spectrum (green curve), and the calculated transmitted PL spectrum (blue curve) by multiplying the PL spectrum by the transmission spectrum; (b) The schematic drawing of the photon recycling process in perovskite single crystals; The PL excited on the surface of the single crystal is absorbed during its transmission through the crystal and re-excite photons again, so that the emission from the bottom of the single crystal includes both the filtered photons (PL_F) and recycled photons (PL_R); (c) The schematic diagram of the measurement setup of photon recycling efficiency in perovskite single crystals; The PL_R and PL_F components can be differentiated based on their polarization difference, and the ratio of PL_R to incident PL (PL_I) defines the photon recycling efficiency; (d) The schematic diagram of the measurement setup to determine the ratio of optical birefringence induced unblocked PL_F after transmission through perovskite single crystal by illuminating it with a 650 nm light emitting diode (LED); The intensity ratio of birefringence effect induced unblocked light (I_B, when P1⊥P2) and transmitted light through the single crystal (I_T, when P1||P2) defines the contribution of the unblocked PL_F to the PL_R spectrum.