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Xiaoyang Zhu1

1, Columbia University, New York, New York, United States

Lead halide perovskites have been demonstrated as high performance materials in solar cells and light-emitting devices. These materials are characterized by coherent charge transport expected from crystalline semiconductors, but phonon dynamics typical of liquids. I will discuss how such “crystal-liquid” duality leads to the protection of charge carriers in a process which is called solvation in chemistry or large polaron formation in physics. As a result, the Coulomb potential is much screened on the picosecond time scale by a subset of longitudinal optical phonons, thus reducing the scattering of a charge carrier with charged defects, with other charge carriers, and with the remaining phonon bath. Such ultrafast charge carrier protection can account for the exceptional defect tolerance, moderate charge carrier mobility, and low radiative recombination rates. Large polaron formation, along with the liquid or phonon-glass character, may also explain the dramatic reduction in hot carrier cooling rates. Based on lessons we have learned from lead halide perovskites, I propose soft, polar, and dynamic disorder as design principles for defect tolerant semiconductors from nano, molecular, and hybrid materials.
H. Zhu, K. Miyata, Y. Fu, J. Wang, P. P. Joshi, D. Niesner, K. W. Williams, S. Jin, X.-Y. Zhu, “Screening in crystalline liquids protects energetic carriers in hybrid perovskites,” Science, 2016, 353, 1409-1413
K. Miyata, D. Meggiolaro, M. T. Trinh, P. P. Joshi, E. Mosconi, S. Jones, F. De Angelis, X.-Y. Zhu, “Large polarons in lead halide perovskites,” Science Adv. 2017, 3, e1701217.
K. Miyata, T. L. Atallah, X.-Y. Zhu, “Lead halide perovskites: crystal-liquid duality, phonon glass electron crystal, and large polaron formation,” Science Adv. 2017, 3, e1701469.

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