title: Spatially resolved studies of the phases and morphology of methylammonium and formamidinium lead tri-halide perovskites.
abstract: The family of organic-inorganic tri-halide perovskites including MA (MethylAmmonium)PbI3, MAPbI3-xClx, FA (FormAmidinium)PbI3and FAPbBr3are having a tremendous impact on the field of photovoltaic cells due to the combination of their ease of deposition and high energy conversion efficiencies. Device performance, however, is known to be still significantly affected by the presence of inhomogeneities. Here we report on a study of temperature dependent micro-photoluminescence which shows a strong spatial inhomogeneity related to the presence of microcrystalline grains, which can be both bright and dark. In all of the tri-iodide based materials there is evidence that the tetragonal to orthorhombic phase transition observed around 160 K does not occur uniformly across the sample with domain formation related to the underlying microcrystallite grains, some of which remain in the high temperature, tetragonal, phase even at very low temperatures. At low temperature the tetragonal domains can be significantly influenced by local defects in the layers or the introduction of residual levels of chlorine in mixed halide layers or dopant atoms such as aluminium. We see that improvements in room temperature energy conversion efficiency appear to be directly related to reductions in the proportions of the layer which remain in the tetragonal phase at low temperature. In FAPbBr3a more macroscopic domain structure is observed with large numbers of grains forming phase correlated regions.
authors.affiliation: Laboratoire National des Champs Magnétiques Intenses, CNRS-UGA-UPS-INSA, 143, avenue de Rangueil, 31400 Toulouse, France. paulina.plochocka@lncmi.cnrs.fr and Institute of Experimental Physics, Faculty of Physics, University of Warsaw, Pasteura 5, 02-093 Warsaw, Poland.
authors.forename: K
authors.initials: K
authors.lastname: Galkowski
authors.affiliation: Laboratoire National des Champs Magnétiques Intenses, CNRS-UGA-UPS-INSA, 143, avenue de Rangueil, 31400 Toulouse, France. paulina.plochocka@lncmi.cnrs.fr and Institute of Applied Physics, Academiei Str. 5, Chisinau, MD-2028, Republic of Moldova.
authors.forename: A A
authors.initials: AA
authors.lastname: Mitioglu
authors.affiliation: Laboratoire National des Champs Magnétiques Intenses, CNRS-UGA-UPS-INSA, 143, avenue de Rangueil, 31400 Toulouse, France. paulina.plochocka@lncmi.cnrs.fr.
authors.forename: A
authors.initials: A
authors.lastname: Surrente
authors.affiliation: Laboratoire National des Champs Magnétiques Intenses, CNRS-UGA-UPS-INSA, 143, avenue de Rangueil, 31400 Toulouse, France. paulina.plochocka@lncmi.cnrs.fr.
authors.forename: Z
authors.initials: Z
authors.lastname: Yang
authors.affiliation: Laboratoire National des Champs Magnétiques Intenses, CNRS-UGA-UPS-INSA, 143, avenue de Rangueil, 31400 Toulouse, France. paulina.plochocka@lncmi.cnrs.fr.
authors.forename: D K
authors.initials: DK
authors.lastname: Maude
authors.affiliation: Institute of Experimental Physics, Faculty of Physics, University of Warsaw, Pasteura 5, 02-093 Warsaw, Poland.
authors.forename: P
authors.initials: P
authors.lastname: Kossacki
authors.affiliation: University of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, UK. robin.nicholas@physics.ox.ac.uk.
authors.forename: G E
authors.initials: GE
authors.lastname: Eperon
authors.affiliation: University of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, UK. robin.nicholas@physics.ox.ac.uk.
authors.forename: J T-W
authors.initials: JT
authors.lastname: Wang
authors.affiliation: University of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, UK. robin.nicholas@physics.ox.ac.uk.
authors.forename: H J
authors.initials: HJ
authors.lastname: Snaith
authors.affiliation: Laboratoire National des Champs Magnétiques Intenses, CNRS-UGA-UPS-INSA, 143, avenue de Rangueil, 31400 Toulouse, France. paulina.plochocka@lncmi.cnrs.fr.
authors.forename: P
authors.initials: P
authors.lastname: Plochocka
authors.affiliation: University of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, UK. robin.nicholas@physics.ox.ac.uk.
authors.forename: R J
authors.initials: RJ
authors.lastname: Nicholas