title: Polaron Stabilization by Cooperative Lattice Distortion and Cation Rotations in Hybrid Perovskite Materials.
abstract: Solution-processed organometallic perovskites have rapidly developed into a top candidate for the active layer of photovoltaic devices. Despite the remarkable progress associated with perovskite materials, many questions about the fundamental photophysical processes taking place in these devices, remain open. High on the list of unexplained phenomena are very modest mobilities despite low charge carrier effective masses. Moreover, experiments elucidate unique degradation of photocurrent affecting stable operation of perovskite solar cells. These puzzles suggest that, while ionic hybrid perovskite devices may have efficiencies on par with conventional Si and GaAs devices, they exhibit more complicated charge transport phenomena. Here we report the results from an in-depth computational study of small polaron formation, electronic structure, charge density, and reorganization energies using both periodic boundary conditions and isolated structures. Using the hybrid density functional theory, we found that volumetric strain in a CsPbI3 cluster creates a polaron with binding energy of around 300 and 900 meV for holes and electrons, respectively. In the MAPbI3 (MA = CH3NH3) cluster, both volumetric strain and MA reorientation effects lead to larger binding energies at around 600 and 1300 meV for holes and electrons, respectively. Such large reorganization energies suggest appearance of small polarons in organometallic perovskite materials. The fact that both volumetric lattice strain and MA molecular rotational degrees of freedom can cooperate to create and stabilize polarons indicates that in order to mitigate this problem, formamidinium (FA = HC(NH2)2) and cesium (Cs) based crystals and alloys, are potentially better materials for solar cell and other optoelectronic applications.
authors.affiliation: Theoretical Physics and Chemistry of Materials, Los Alamos National Laboratory , Los Alamos, New Mexico 87545, United States.
authors.forename: Amanda J
authors.initials: AJ
authors.lastname: Neukirch
authors.affiliation: Materials Physics and Application, Los Alamos National Laboratory , Los Alamos, New Mexico 87545, United States.
authors.forename: Wanyi
authors.initials: W
authors.lastname: Nie
authors.affiliation: Physical Chemistry and Applied Spectroscopy Division, Los Alamos National Laboratory , Los Alamos, New Mexico 87545, United States.
authors.forename: Jean-Christophe
authors.initials: JC
authors.lastname: Blancon
authors.affiliation: Center for Functional Nanomaterials, Brookhaven National Laboratory , Upton, New York 11973, United States.
authors.forename: Kannatassen
authors.initials: K
authors.lastname: Appavoo
authors.affiliation: Materials Physics and Application, Los Alamos National Laboratory , Los Alamos, New Mexico 87545, United States.
authors.forename: Hsinhan
authors.initials: H
authors.lastname: Tsai
authors.affiliation: Center for Functional Nanomaterials, Brookhaven National Laboratory , Upton, New York 11973, United States.
authors.forename: Matthew Y
authors.initials: MY
authors.lastname: Sfeir
authors.affiliation: Institut des Sciences Chimiques de Rennes, ISCR, CNRS , Université de Rennes 1, 35042 Rennes, France.
authors.forename: Claudine
authors.initials: C
authors.lastname: Katan
authors.affiliation: Fonctions Optiques pour les Technologies de l'Information, FOTON UMR 6082, CNRS, INSA de Rennes , 35708 Rennes, France.
authors.forename: Laurent
authors.initials: L
authors.lastname: Pedesseau
authors.affiliation: Fonctions Optiques pour les Technologies de l'Information, FOTON UMR 6082, CNRS, INSA de Rennes , 35708 Rennes, France.
authors.forename: Jacky
authors.initials: J
authors.lastname: Even
authors.affiliation: Physical Chemistry and Applied Spectroscopy Division, Los Alamos National Laboratory , Los Alamos, New Mexico 87545, United States.
authors.forename: Jared J
authors.initials: JJ
authors.lastname: Crochet
authors.affiliation: Materials Physics and Application, Los Alamos National Laboratory , Los Alamos, New Mexico 87545, United States.
authors.forename: Gautam
authors.initials: G
authors.lastname: Gupta
authors.affiliation: Materials Physics and Application, Los Alamos National Laboratory , Los Alamos, New Mexico 87545, United States.
authors.forename: Aditya D
authors.initials: AD
authors.lastname: Mohite
authors.affiliation: Theoretical Physics and Chemistry of Materials, Los Alamos National Laboratory , Los Alamos, New Mexico 87545, United States.
authors.forename: Sergei
authors.initials: S
authors.lastname: Tretiak
keywords: Organic−inorganic perovskite; cation rotations; photovoltaic; polaron