Vibrational Probe of the Structural Origins of Slow Recombination in Halide Perovskites

We use time-resolved vibrational spectroscopy to probe the structural origins of the remarkably long charge recombination lifetimes of halide perovskites. The N-H bend vibrational mode of CH₃NH₃⁺ organic cations is coupled to the inorganic perovskite lattice and provides a means to examine the structural fluctuations of the crystalline lattice of CH₃NH₃PbI₃ films. In the excited electronic state, the photogeneration of charge carriers causes the N-H bend vibrational dephasing dynamics to become very sensitive to temperature, revealing large changes in the amplitude of the structural motions of the lattice within a narrow 150-300 K temperature range of the tetragonal phase. The larger amplitude fluctuations of the lattice at elevated temperatures inhibit delocalization of photogenerated charges, causing them to self-trap into large polarons with delocalization lengths that decrease more than 30% as the temperature increases from 150 to 300 K. This self-trapping into large polarons introduces energetic barriers that decrease the capture cross section for electron/hole recombination by an order of magnitude at elevated temperatures. These findings indicate that the slow charge recombination kinetics of halide perovskites underpinning many of their remarkable properties are traced to the formation of energetic barriers that hinder wavefunction overlap of oppositely charged carriers in large polaron states. The findings also suggest that substitution of differently sized ions in halide perovskites can be used to tune the balance of charge transport versus charge recombination for photovoltaic or light-emitting applications because ions influence the structural flexibility and therefore the self-trapping of charge carriers into large polarons.