title: Role of Polar Phonons in the Photo Excited State of Metal Halide Perovskites.
abstract: The development of high efficiency perovskite solar cells has sparked a multitude of measurements on the optical properties of these materials. For the most studied methylammonium(MA)PbI3 perovskite, a large range (6-55 meV) of exciton binding energies has been reported by various experiments. The existence of excitons at room temperature is unclear. For the MAPbX3 perovskites we report on relativistic Bethe-Salpeter Equation calculations (GW-BSE). This method is capable to directly calculate excitonic properties from first-principles. At low temperatures it predicts exciton binding energies in agreement with the reported 'large' values. For MAPbI3, phonon modes present in this frequency range have a negligible contribution to the ionic screening. By calculating the polarization in time from finite temperature molecular dynamics, we show that at room temperature this does not change. We therefore exclude ionic screening as an explanation for the experimentally observed reduction of the exciton binding energy at room temperature and argue in favor of the formation of polarons.
authors.affiliation: University of Vienna, Faculty of Physics and Center for Computational Materials Science, Sensengasse 8/12, 1090 Vienna, Austria.
authors.forename: Menno
authors.initials: M
authors.lastname: Bokdam
authors.affiliation: University of Vienna, Faculty of Physics and Center for Computational Materials Science, Sensengasse 8/12, 1090 Vienna, Austria.
authors.forename: Tobias
authors.initials: T
authors.lastname: Sander
authors.affiliation: Consiglio Nazionale delle Ricerche - CNR-SPIN, I-67100 L' Aquila, Italy.
authors.forename: Alessandro
authors.initials: A
authors.lastname: Stroppa
authors.affiliation: Consiglio Nazionale delle Ricerche - CNR-SPIN, I-67100 L' Aquila, Italy.
authors.forename: Silvia
authors.initials: S
authors.lastname: Picozzi
authors.affiliation: Solid State and Structural Chemistry Unit, Indian Institute of Science, 560012 Bengaluru, India.
authors.forename: D D
authors.initials: DD
authors.lastname: Sarma
authors.affiliation: University of Vienna, Faculty of Physics and Center for Computational Materials Science, Sensengasse 8/12, 1090 Vienna, Austria.
authors.forename: Cesare
authors.initials: C
authors.lastname: Franchini
authors.affiliation: University of Vienna, Faculty of Physics and Center for Computational Materials Science, Sensengasse 8/12, 1090 Vienna, Austria.
authors.forename: Georg
authors.initials: G
authors.lastname: Kresse