Lead halide perovskites have been demonstrated as high performance materials in solar cells and light-emitting devices. These materials are characterized by coherent band transport expected from crystalline semiconductors, but dielectric responses and phonon dynamics typical of liquids. Here we explain the essential physics in this class of materials based on their dielectric functions and dynamic symmetry breaking on nano scales. We show that the dielectric function in the THz region may lead to dynamic and local ordering of polar nano domains by an extra electron or hole, resulting a quasiparticle which we call a ferroelectric large polaron, a concept similar to solvation in chemistry. Compared to a conventional large polaron, the collective nature of polarization in a ferroelectric large polaron may give rise to order(s)-of-magnitude larger reduction in the Coulomb potential. We show that the shape of a ferroelectric polaron resemble that of a Belgian waffle. Using two-dimensional optical Kerr effect spectroscopy, we directly probe the energetics and local phonon responses of ferroelectric polarons. The ferroelectric polaron may explain the defect tolerance and low recombination rates of charge carriers in lead halide perovskites as well as providing a design principle of the “perfect” semiconductor for optoelectronics.
Biosketch： Xiaoyang Zhu is the Howard Family Professor of Nanoscience at Columbia University. He received a BS degree from Fudan University in 1984 and a PhD from the University of Texas at Austin in 1989. After postdoctoral research with Gerhard Ertl at the Fritz-Haber-Institute, he joined the faculty at Southern Illinois University as an Assistant Professor in 1993. In 1997, he moved to the University of Minnesota as a tenured Associate Professor, later a Full Professor, and a Merck endowed professor. In 2009, he returned to the University of Texas at Austin as the Vauquelin Regents Professor and served as directors of the DOE Energy Frontier Research Center (EFRC) and the Center for Materials Chemistry. In 2013, he moved to Columbia University where he remains. His honors include a Dreyfus New Faculty Award, a Cottrell Scholar Award, a Friedrich Wilhelm Bessel Award, an APS Fellowship, a Vannevar Bush Faculty Fellowship, and an ACS Ahmed Zewail Award. Among his professional activities, he serves as associate editors for Science Advances and Journal of Chemical Physics and on the editorial/advisory boards of Accounts of Chemical Research, Chemical Physics, and Progress in Surface Science. He is a scientific advisor to the Fritz-Haber-Institute of the Max-Planck Society in Germany and ShanghaiTech University in China. His research interests include photophysics in nano, molecular, and hybrid semiconductors and interfaces.