Publications

2015

1. Infrared Spectroscopic Study of Vibrational Modes in Methylammonium Lead Halide Perovskites

   Glaser T., Mueller C., Sendner M., Krekeler C., Semonin O. E., Hull T. D., Yaffe O., Owen J. S., Kowalsky W., Pucci A., Lovrincic R. et al. (2015) Journal of Physical Chemistry Letters. 6, 15, p. 2913-2918  Abstract

   The organic cation and its interplay with the inorganic lattice underlie the exceptional optoelectronic properties of organo-metallic halide perovskites. Herein we report high-quality infrared spectroscopic measurements of methylammonium lead halide perovskite (CH3NH3Pb(I/Br/Cl)3) films and single crystals at room temperature, from which the dielectric function in the investigated spectral range is derived. Comparison with electronic structure calculations in vacuum of the free methylammonium cation allows for a detailed peak assignment. We analyze the shifts of the vibrational peak positions between the different halides and infer the extent of interaction between organic moiety and the surrounding inorganic cage. The positions of the NH3+ stretching vibrations point to significant hydrogen bonding between the methylammonium and the halides for all three perovskites.

   [All authors]
2. Excitons in ultrathin organic-inorganic perovskite crystals

   Yaffe O., Chernikov A., Norman Z. M., Zhong Y., Velauthapillai A., van der Zande A., Owen J. S. & Heinz T. F. (2015) Physical Review B. 92, 4, 045414.  Abstract

   We demonstrate the formation of large sheets of layered organic-inorganic perovskite (OIPC) crystals, as thin as a single unit cell, prepared by mechanical exfoliation. The resulting two-dimensional OIPC nanosheets of 2.4 nm thickness are direct semiconductors with an optical band gap of 2.4 eV. They exhibit unusually strong light-matter interaction with an optical absorption as high as 25% at the main excitonic resonance, as well as bright photoluminescence. We extract an exciton binding energy of 490 meV from measurement of the series of excited exciton states. The properties of the excitons are shown to be strongly influenced by the changes in the dielectric surroundings. The environmental sensitivity of these ultrathin OIPC sheets is further reflected in the strong suppression of a thermally driven phase transition present in the bulk crystals.

   Accepted version
3. Trap states in lead iodide perovskites

   Wu X., Trinh M. T., Niesner D., Zhu H., Norman Z., Owen J. S., Yaffe O., Kudisch B. J. & Zhu X. -. (2015) Journal of the American Chemical Society. 137, 5, p. 2089-2096  Abstract

   Recent discoveries of highly efficient solar cells based on lead iodide perovskites have led to a surge in research activity on understanding photo carrier generation in these materials, but little is known about trap states that may be detrimental to solar cell performance. Here we provide direct evidence for hole traps on the surfaces of three-dimensional (3D) CH₃NH₃PbI₃ perovskite thin films and excitonic traps below the optical gaps in these materials. The excitonic traps possess weak optical transition strengths, can be populated from the relaxation of above gap excitations, and become more significant as dimensionality decreases from 3D CH₃NH₃PbI₃ to two-dimensional (2D) (C₄H₉NH₃I)₂(CH₃NH₃I)_n-1(PbI₂)_n (n = 1, 2, 3) perovskites and, within the 2D family, as n decreases from 3 to 1. We also show that the density of excitonic traps in CH₃NH₃PbI₃ perovskite thin films grown in the presence of chloride is at least one-order of magnitude lower than that grown in the absence of chloride, thus explaining a widely known mystery on the much better solar cell performance of the former. The trap states are likely caused by electron-phonon coupling and are enhanced at surfaces/interfaces where the perovskite crystal structure is most susceptible to deformation.