title: Room Temperature Phase Transition in Methylammonium Lead Iodide Perovskite Thin Films Induced by Hydrohalic Acid Additives.
abstract: Although reactive additives have been employed in perovskite solar cells to enhance film morphology and significantly increase device performance, little is known about the effect of these additives on perovskite structural and optical properties. Here we report a systematic study of how the properties of methylammonium lead iodide perovskite (CH3NH3PbI3) are influenced by hydrohalic acid additives (HX; X=I, Br, Cl) in the precursor solution. Detailed structural and optical spectroscopic analysis reveals that all three acids affect the optical properties and alter the unit cell lattice parameters. Depending on the identity and concentration of HX, optical bandgaps widen or compress: addition of HBr yields a wider bandgap, whereas HI compresses the gap at high concentrations; HCl, on the other hand, has no significant effect on the bandgap. These changes can be understood by correlating them with the types of defects present in polycrystalline perovskite thin films in combination with the structural strain induced in very small crystallites. The presence of extra halides from HX in the precursor solution enables filling of the lattice vacancies in the perovskite, thereby altering metal-halogen-metal bond connectivity and consequently cell volumes and optical bandgaps. Remarkably, a room temperature tetragonal→cubic phase transition is observed for CH3NH3PbI3films treated with high HX concentrations. Further insights into this anomalous phase transformation are obtained from in situ variable-temperature X-ray diffraction in the 25-55 °C (298-328 K) range, revealing a monotonic fall in transition temperature with increasing precursor solution HX concentration.
authors.affiliation: Department of Chemistry, Northwestern University, Evanston, IL, 60628, USA.|Argonne-Northwestern Solar Energy Research Center, Northwestern University, Evanston, Illinois, 60208, USA.
authors.forename: Chan Myae Myae
authors.initials: CM
authors.lastname: Soe
authors.affiliation: Department of Chemistry, Northwestern University, Evanston, IL, 60628, USA.
authors.forename: Constantinos C
authors.initials: CC
authors.lastname: Stoumpos
authors.affiliation: Argonne-Northwestern Solar Energy Research Center, Northwestern University, Evanston, Illinois, 60208, USA.|Departments of Physics and Astronomy and Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, USA.
authors.forename: Boris
authors.initials: B
authors.lastname: Harutyunyan
authors.affiliation: Department of Chemistry, Northwestern University, Evanston, IL, 60628, USA.|Argonne-Northwestern Solar Energy Research Center, Northwestern University, Evanston, Illinois, 60208, USA.|Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 S. Cass Ave., Lemont, IL, 60439, USA.
authors.forename: Eric F
authors.initials: EF
authors.lastname: Manley
authors.affiliation: Department of Chemistry, Northwestern University, Evanston, IL, 60628, USA.|Argonne-Northwestern Solar Energy Research Center, Northwestern University, Evanston, Illinois, 60208, USA.|Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 S. Cass Ave., Lemont, IL, 60439, USA.
authors.forename: Lin X
authors.initials: LX
authors.lastname: Chen
authors.affiliation: Argonne-Northwestern Solar Energy Research Center, Northwestern University, Evanston, Illinois, 60208, USA.|Departments of Physics and Astronomy and Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, USA.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Bedzyk
authors.affiliation: Department of Chemistry, Northwestern University, Evanston, IL, 60628, USA. t-marks@northwestern.edu.|Argonne-Northwestern Solar Energy Research Center, Northwestern University, Evanston, Illinois, 60208, USA. t-marks@northwestern.edu.
authors.forename: Tobin J
authors.initials: TJ
authors.lastname: Marks
authors.affiliation: Department of Chemistry, Northwestern University, Evanston, IL, 60628, USA. m-kanatzidis@northwestern.edu.|Argonne-Northwestern Solar Energy Research Center, Northwestern University, Evanston, Illinois, 60208, USA. m-kanatzidis@northwestern.edu.
authors.forename: Mercouri G
authors.initials: MG
authors.lastname: Kanatzidis
mesh_terms: D000148:Acids, Noncarboxylic / Q000737:chemistry*; D007454:Iodides / Q000737:chemistry*; D007854:Lead / Q000737:chemistry*; D008744:Methylamines / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D055620:Optical Phenomena; D044367:Phase Transition; D013696:Temperature
chemical_list: D000148:Acids, Noncarboxylic; D007454:Iodides; D008744:Methylamines; D007854:Lead; C027451:methylamine
keywords: additives; perovskite; phase transition; thin films; x-ray diffraction