Long-Lived Exciton Coherence in Mixed-Halide Perovskite Crystals

Compositionalengineering of the optical properties ofhybrid organic-inorganiclead halide perovskites is crucial for the realization of efficientsolar cells and light-emitting devices. We study the effect of bandgap fluctuations on coherent exciton dynamics in a mixed FA(0.9)Cs(0.1)PbI(2.8)Br(0.2) perovskite crystalby using photon echo spectroscopy. We reveal a narrow homogeneousexciton line width of 16 & mu;eV at a temperature of 1.5 K. Thecorresponding exciton coherence time T (2) = 83 ps is exceptionally long due to the localization of excitonsat the scale of tens to hundreds of nanometers. From spectral andtemperature dependences of the two- and three-pulse photon echo decay,we conclude that for low-energy excitons pure decoherence associatedwith elastic scattering on phonons is comparable with the excitonlifetime, while for excitons with higher energies, inelastic scatteringto lower energy states via phonon emission dominates.