Polarons, Polaritons, and the Lead Halide Perovskite Puzzle
Department of Chemistry, Columbia University, New York, NY 10027
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 (J. Phys. Chem. Lett. 2015, 6, 4758). 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 (Science, 2016, 353, 1409; J. Am. Chem. Soc. 2016, 2016, 138, 15717), which is three-orders of magnitude longer than those in conventional semiconductors. The protection of energetic carriers is directly correlated with the liquid-like motion of the lattice, as revealed by femtosecond Kerr-effect spectroscopy. We will discuss strong light-matter interaction and the observation of coherent light emission (Nature Mater. 2015, 14, 636) attributed to exciton-polariton condensation.