Terahertz emission from lead-halide perovskites: probing initial steps of charge separation and carrier - phonon coupling

In recent years, hybrid lead-halide perovskites achieved record-breaking power conversion efficiencies (>22%) in solar cells. This has been enabled by combination of long carrier lifetimes, moderately high carrier mobilities and high defect tolerance in these materials. However, these properties had never existed all together in any other solution-processed semiconductor before. This led to scientific debates on the nature of the charge carriers and their properties in hybrid perovskites.

Recently, we observed for the first time terahertz (THz) emission in lead-halide perovskites thin films following photo-excitation [1]. We show that the emitted THz fields encode information on the first steps of charge separation occuring these films used in solar cells. We find that charges (electrons and holes) separate from each otehr due to their different diffusivities, a process known as photo-Dember effect. We also shown that charge separation is not faciliated by ferroelectricity or surface fields. [1]

THz emission combined with optical pump/THz probe allows us to estimate individual carrier mobilities all optically for the first time. Carrier mobilities are quite close to each other while hole mobilities are slightly larger than the electrons. [1]

Finally, we observe coherent THz emission at the longitudinal optical (LO) phonon mode of the lead-halide octahedra. We show that emission dynamics of the LO phonons both indicate a strong coupling of the charge carriers to the soft perovskite lattice and dynamics of the polaron formation. [1]

Also, in a prior publication from the Lindenberg group we showed the first account of atomic structural response to light absorption in these lead-halide perovskites. Particularly, we found that above-gap excitation with photon energies larger than the band gap results in strong rotational disordering of the Pb-I octahedra. This means that Pb-Pb distance does not change much but I-I atomic correlations get strongly broadened. This observation indicates the responsible lattice vibrational modes associated with hot-carrier cooling in these materials. [2]


[1] B. Guzelturk, R. A. Belisle, M. D. Smith, K. Bruening, R. Prasanna, Y. Yuan, V. Gopalan, C. J. Tassone, H. I. Karunadasa, M. D. McGehee, A. M. Lindenberg,"Terahertz Emission from Hybrid Perovskites Driven by Ultrafast Charge Separation and Strong Electron-Phonon Coupling," Advanced Materials (2018).

[2] X. Wu, L. Z. Tan, X. Shen, T. Hu, K. Miyata, M. Tuan Trinh, R. Li, R. Coffee, S. Liu, D. A. Egger, I. Makasyuk, Q. Zheng, A. Fry, J. S. Robinson, M. D. Smith, B. Guzelturk, H. I. Karunadasa, X. Wang, X.-Y. Zhu, L. Kronik, A. M. Rappe, A. M. Lindenberg, “Light-induced picosecond rotational disordering of the inorganic sublattice in hybrid perovskites” Science Advances 3, e1602388 (2017). Link