Direct Observation of Bandgap Oscillations Induced by Optical Phonons in Hybrid Lead Iodide Perovskites

• Published in: Advanced functional materials 2020-05, Vol.30 (22), p.n/a
• Main Authors: Guo, Peijun, Xia, Yi, Gong, Jue, Cao, Duyen H., Li, Xiaotong, Li, Xun, Zhang, Qi, Stoumpos, Constantinos C., Kirschner, Matthew S., Wen, Haidan, Prakapenka, Vitali B., Ketterson, John B., Martinson, Alex B. F., Xu, Tao, Kanatzidis, Mercouri G., Chan, Maria K. Y., Schaller, Richard D.
• Format: Article
• Language: English
• Subjects: coherent optical phonons; Current carriers; Electrons; electron–phonon coupling; Energy gap; hybrid perovskites; Lattice vibration; Materials science; Optical properties; Optical pumping; Optoelectronics; Oscillations; Perovskites; Phonons; Raman scattering; transient optical spectroscopy
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.201907982

Hybrid organic–inorganic perovskites such as methylammonium lead iodide have emerged as promising semiconductors for energy‐relevant applications. The interactions between charge carriers and lattice vibrations, giving rise to polarons, have been invoked to explain some of their extraordinary optoelectronic properties. Here, time‐resolved optical spectroscopy is performed, with off‐resonant pumping and electronic probing, to examine several representative lead iodide perovskites. The temporal oscillations of electronic bandgaps induced by coherent lattice vibrations are reported, which is attributed to antiphase octahedral rotations that dominate in the examined 3D and 2D hybrid perovskites. The off‐resonant pumping scheme permits a simplified observation of changes in the bandgap owing to the Ag vibrational mode, which is qualitatively different from vibrational modes of other symmetries and without increased complexity of photogenerated electronic charges. The work demonstrates a strong correlation between the lead–iodide octahedral framework and electronic transitions, and provides further insights into the manipulation of coherent optical phonons and related properties in hybrid perovskites on ultrafast timescales. Purely vibrationally excited lead–iodide perovskites are prepared using off‐resonance, infrared optical excitation far below the bandgap. The transient optical response manifested as bandgap oscillations below and above the static bandgap is attributed to the Ag optical phonon mode at 25 cm−1. This mode, arising from antiphase octahedral rotations, is observed in both 3D perovskite CH3NH3PbI3 and layered 2D perovskite [CH3(CH2)3NH3]2PbI4.