Dark sub-gap states in metal-halide perovskites revealed by coherent multidimensional spectroscopy

摘要： Metal-halide perovskites show excellent properties for photovoltaic and optoelectronic applications, with power conversion efficiencies of solar cell and LEDs exceeding 20%. Being solution processed, these polycrystalline materials likely contain a large density of defects compared to melt-grown semiconductors. Surprisingly, typical effects from defects (absorption below the bandgap, low fill factor and open circuit voltage in devices, strong non-radiative recombination) are not observed. In this work, we study thin films of metal-halide perovskites CH3NH3PbX3 (X = Br,I) with ultrafast multidimensional optical spectroscopy to resolve the dynamics of band and defect states. We observe a shared ground state between the band-edge transitions and a continuum of sub-bandgap states, which extends at least 350 meV below the band edge). We explain the comparatively large bleaching of the dark sub-bandgap states with oscillator strength borrowing from the band-edge transition. Our results show that upon valence to conduction band excitation such sub-gap states are instantaneously bleached for large parts of the carrier lifetime and conversely, that almost dark sub-bandgap states can be populated by light excitation. This observation helps unraveling the photophysical origin of the unexpected optoelectronic properties of these materials.