Transient Exciton-Charge Interconversion in Single-Crystal\nHybrid Perovskites

Ultrafast\noscillatory transient absorption (TA) dynamics are observed\nin single crystals of hybrid organic–inorganic perovskite (CH<sub>3</sub>NH<sub>3</sub>)PbX<sub>3</sub> with X = I, Br, Cl. High-density\nphotoinduced charges, low binding energy of the excitons, efficient\ngeneration and high mobility of charges, and long diffusion length\nof both excitons and charges led to transient interconversion between\nexcitons and charges with high efficiency, which is responsible for\nthe oscillatory TA dynamics at re-excitation by the probe pulses.\nThe pump pulses initiated a quasi-equilibrium scheme of coexisting\nexcitons and charges with high densities, the probe pulse triggered\na perturbation through interconversion between these two kinds of\nexcited “particles,” which was overlapped on the intrinsic\nexciton relaxation dynamics, producing an oscillatory modulation with\ntime delay. This not only reveals important photoelectronic processes\nwith determined time scales, but also supplies physics for applications\nof this group of materials.