Polarons, Polaritons, and the Lead Halide Perovskite Puzzle

Seminar | January 31 | 4-5 p.m. |  Pitzer Auditorium, 120 Latimer Hall

 Prof. Xiaoyang Zhu, Department of Chemistry, Columbia University

 College of Chemistry

The feverish research activity on lead halide perovskites has been fueled by their exceptional optoelectronic properties, e.g., in solar cells and light-emitting devices. Hybrid lead halide perovskites exhibit carrier properties that resemble those of pristine nonpolar semiconductors despite static and dynamic disorder, but how carriers are protected from efficient scattering with charged defects and optical phonons is unknown. We have recently put forward the large polaron model to explain the carrier protection [1-3]. We find that nascent charge carriers are screened by “solvation” or large-polaron formation on time scales ≤ 250 fs, leading to protected carriers with dramatic suppression of electron- LO phonon scattering. This results in long-lived energetic electrons with excess energy ~ 0.25 eV above the conduction band minimum and with lifetime on the order of 100 ps, which is ≥103 time longer than those in conventional semiconductors. The exceptionally long-lived energetic carriers lead to hot fluorescence emission. The protection of energetic carriers is directly correlated with the liquid-like motion of the lattice, as revealed by femtosecond Kerr-effect spectroscopy. In contrast to that of energetic carriers, the protection of long-lived bandedge carriers do not require the presence of organic cations and is an intrinsic property of the soft perovskite lattice [4]. Finally, we will discuss strong light-matter interaction leading to efficient exciton-polariton formation and our most recent observation of coherent light emission without population inversion; this is distinctively different than our earlier findings of efficient lasing in these materials.

 Light refreshments at The Coffee Lab at 3:50pm

 seminarcoordinator-cchem@berkeley.edu, 510-643-0572