Prof. ZHU XiaoyangColumbia University
Exhibition Hall, Hefei National Laboratory Building
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.
Hefei National Laboratory for Physical Sciences at the Microscale
Address: University of Science and Technology of China, No.96, JinZhai Road Baohe District, Hefei, Anhui, 230026, P.R.China.