Speaker
Description
Over the last decade, lead halide perovskite (LHP) semiconductors have emerged as a promising material for optoelectronic devices such as solar cells or LEDs. These materials show a range of outstanding optoelectronic properties including long charge carrier diffusion lengths and apparent defect tolerance. However, the microscopic origin of these properties remains under debate. It has been suggested that their highly polarizable and anharmonic lattice beneficially affects their optoelectronic properties in the form of dynamic charge carrier screening. To understand this effect, comprehension of the lattice polarizability and its dynamics are required. The lattice response following femtosecond charge generation and thermalization might be conceptually mimicked by applying an ultrashort transient electric field without generating charge carriers. Yet, previous studies utilizing optical fields in the dilute charge carrier injection regime have only shown a dominating electronic polarizability and have fallen short from providing insights on the ultrafast lattice response [1].
To study the ultrafast lattice response to a transient electric field, here we move the excitation to the terahertz (THz) spectral region, resonant to the optical modes of the lead halide lattice, by employing intense, close to single-cycle, THz fields. By probing the THz induced Kerr effect, we observe a strong THz polarizability in both inorganic CsPbBr3 and in the hybrid MAPbBr3 materials. Performing four-wave mixing simulations, we find that it is crucial to account for dispersion and optical anisotropy in interpreting the transient responses. By studying different single crystal thicknesses and polycrystalline thin films, we rule out effects arising from light propagation. At room temperature, we the signal features are explained by an overwhelming electronic polarizability. Cooling down to lower temperatures, we finally observe a contribution from the lattice polarizability by witnessing the nonlinear excitation of coherent phonons. We find these phonon modes to agree with previous static Raman spectroscopy experiments and to correspond to octahedral twists of the inorganic cage. In conclusion, we pinpoint a single phonon mode which dominates the lattice polarizability, providing another stepping stone towards a better understanding of how lattice dynamics enable charge carrier protection in this class of defect tolerant semiconductors.
References
[1] S. F. Maehrlein et al., PNAS, 118,7, e2022268118 (2021).
