Figure 2. Photon recycling efficiency in MAPbBr₃ single crystals (SCs).

(a) PL_I (blue curve) and PL_T (black curve) spectra of a 1.3 mm thick MAPbBr₃ single crystal showing its PL self-absorption ratio, where PL_I represents the incident PL from SC1 shown in Fig. 1(c), and PL_T is measured by adjusting the polarization direction of P1 and P2 in Fig. 1(c) to be parallel with each other; (b) PL_T and PL_R spectra showing the PL re-emission ratio, where PL_R is measured by adjusting the polarization direction of P1 and P2 in Fig. 1(c) to be perpendicular with each other; (c) The spectra of the 1.3 mm thick MAPbBr₃ SCs excited by a 650 nm LED whose photon energy is far below the band gap of MAPbBr₃, and measured with the polarization direction of P1 and P2 in Fig. 1(d) to be P1⊥P2 (blue curve, I_B) or P1||P2 (orange curve, I_T) in order to determine the ratio of optical birefringence induced unblocked PL after transmission through perovskite single crystal; (d) The summarized photon recycling efficiency of MAPbBr₃ single crystals with different thicknesses.