Ultrafast carrier cooling and thermalization in lead iodide perovskite probed with two-dimensional electronic spectroscopy

Summary form only given. Hybrid perovskite semiconductors have attracted strong scientific interest for optoelectronic applications. Despite the current drive for higher optoelectronic device efficiencies and stabilities, little information exists about the fundamental non-equilibrium interactions of photo-excited charge carriers in these materials. Time-resolved experiments with ultrashort laser pulses have been used to study the carrier dynamics in hybrid perovskites with transient absorption techniques. The cooling process was found to occur on 100s of femtosecond timescales [1-3], with a strong contribution from a hot phonon effect. The carrier thermalization process is expected to occur on much faster timescales [4-6] with seminal studies of thermalization dynamics in GaAs reported timescales ranging from 100 fs to 4 ps [5,7,8]. These results provided insights into dephasing times [7], band structure [5], and carrier-carrier scattering processes [9]. However, the initial build-up of the thermal distribution is typically difficult to resolve due to the requirement of high resolution, both in time and in energy.We perform two-dimensional electronic spectroscopy with sub-10fs resolution on lead-iodide hybrid perovskites, using the prototypical material CH3NH3PbI3, to observe the carrier interactions directly that lead to formation of a thermal carrier distribution. This technique offers the advantage of high spectral and temporal resolution by using a Fourier transform approach. We detect spectrally and temporally resolved information up to excited state energies ~500meV above the bandgap. We find a spectral hole from initially non-thermalized carriers when probing resonant with the excitation energies, which then energetically broadens during thermalization. At longer time delays, we observe the transition from thermalization to cooling behavior and extract carrier cooling times on the 100s of femtosecond scale, in agreement with previous reports.