From PbI2 to MAPbI3 through Layered Intermediates - The Journal of

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From PbI2 to MAPbI3 through Layered Intermediates Giovanna Pellegrino, Stefania D\'Angelo, Ioannis Deretzis, Guglielmo G. Condorelli, Emanuele Smecca, Graziella Malandrino, Antonino La Magna, and Alessandra Alberti J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03798 • Publication Date (Web): 15 Aug 2016 Downloaded from http://pubs.acs.org on August 16, 2016

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

From PbI2 to MAPbI3 through Layered Intermediates

Giovanna Pellegrino1, Stefania D’Angelo2, Ioannis Deretzis1, Guglielmo Guido Condorelli2, Emanuele Smecca1, Graziella Malandrino2, Antonino La Magna1and Alessandra Alberti1*

1

CNR-IMM Zona Industriale, VIII Strada, No. 5, 95121 Catania, Italy

2

Dipartimento Scienze Chimiche, Università degli studi di Catania and INSTM UdR Catania, V.le

A. Doria 6, 95125 Catania, Italy

*Corresponding author: email address: [email protected] telephone number: +39 095 5968214

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ABSTRACT

In the present work, we describe the process of formation of methyl ammonium lead iodide perovskite (MAPbI3) by reaction between [001] oriented PbI2 thin films and MAI. The reaction leads to a rapid change in the optical properties with the formation of a brown phase having a large band in the visible range and a contribution in the near- UV range (375 nm). X-ray Diffraction analyses indicate that the MAI+PbI2 as-obtained material mainly consists of a highly oriented matrix of MAI that intermixes with the Pb-I precursor. A chemical exfoliation of the initial PbI2 structure with the massive intercalation of the methyl-ammonium iodide ions suitably describes the optical and structural findings. Short thermally treating (100°C) of this as-obtained sample results in the formation of a second oriented intermediate phase, attributed to the presence of a lowdimensional perovskite structure. Such low-dimensional metastable phase is no longer visible by increasing the annealing time or the annealing temperature (150°C), with the single 3D MAPbI3 tetragonal phase present in the XRD pattern.


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INTRODUCTION

In the last years, organic/inorganic hybrid perovskites have encountered remarkable attention within the scientific community for their attractive properties which find applications in the opto-electronic devices and solar cells.1,2 The first results in the field of photovoltaics (PV) were published in 2009 by Miyasaka’s group which reports on the sensitization of mesoporous TiO2 films by two different types of organo-lead halide perovskite nanocrystals, namely methyl-ammonium lead bromide (CH3NH3PbBr3) and iodide(CH3NH3PbI3).3 From that date, in a short lapse of time, the use of organo/lead/halide- based materials as solid state heterojunction has registered improving performances and efficiencies nowadays comparable to those of silicon.4 More recently, near-infrared photoemission and electro-luminescence properties of the hybrid halide perovskite have found application as light-emitting compounds and as sensitive components in photo-detectors.5-8 The conspicuous interest towards the family of organic/ halide perovskites is mainly due to the unusual combination of properties typically belonging to both the molecular compounds (optical tunability, highlight-absorbing coefficient, easy and cheap manufacturing) and to the inorganic semiconductors (robustness, high charge transport character and diffusion lengths).9,10 The versatility of the hybrid perovskite engineering is demonstrated by the huge number of works in literature. Such variability is mainly due to the easy synthetic interchange of the metal, the organic cation and the halide anion, to obtain a final material with controlled absorption and charge transport characteristics. For example, the band gap of CH3NH3PbI3 can be tailored by replacing the methylammonium cation (MA+) with the slightly larger formamidinium cation (H2N-CH=NH2)11 or by chemically managing the Br-content.12



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Besides that, also the stoichiometric proportion between the elements and the consequent structural atomic arrangement becomes a crucial aspect for the characteristics of the final material. In the large family of organo lead-iodide-based perovskites, in fact, the [PbI6]4- units can form threedimensional (3-D), two-dimensional (2-D),one-dimensional (1-D) or zero-dimensional (0-D) networks, depending on the sharing of the iodine at the corners of the octahedral units. The n-D architectures have different stoichiometry, crystallographic habits and optical properties.13 In particular, the three-dimensional (3D) lattices consist of networks of all corner-connected octahedra extended along the three spatial coordinates with the small organic ammonium moiety (e.g. CH3NH3+, HC(NH2)2+) filling the holes between the octahedra. In the case of MAPbI3, for instance, the structure shows the I4-mcm tetragonal symmetry with a=b=8.86 Å and c=12.64 Å.18 Two main methods are reported in literature for the synthesis of the MAPbI3 3D perovskite, based on spin-coating and thermal evaporation of the MAI and PbI2 precursors.19-22 In particular, both the evaporation and the wet procedure can be performed by following a single step or sequential deposition. Very recently, Patel and al. reported on the dynamics of MAPbI3 formation from the PbI2 through the interaction with MAI by following a two- step thermal evaporation procedure. 23 The two-dimensional (2D) lattices are self-organized layers formed from a simple intercalation of RNH3+ into the PbI2 sheets to yield well-ordered films (quantum wells). 17,24,25 Such hybrid structures take the generic structural formula of A2MX4, where A is a primary (alkyl or aromatic) ammonium cation (RNH3+), M is a metal of the IV group and X a halogen. Such materials are known as monolayer perovskites, consisting of a single network of connected octahedral units sandwiched by frames of organic monoamine. The organic/inorganic alternate stacking is organized by attractive ionic interactions between the RNH3+ and PbI64- whilst van der Waals interactions occurring within the organic chains of the molecular layer (spacer).This confines the perovskite in bi-dimensionality because of steric effects. Nevertheless, the ammonium moieties in those structures present long alkyl chains which stabilize the organic framework by van der Waals interaction. In such lead/iodide-based 2D structures, the 4 Environment ACS Paragon Plus

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lattice parameters a= b are very similar, being in the range of 8.7÷9.0 Å 26,27.The value of the c-axis is, instead, related to the nature of the intercalated organic molecule. Interestingly, a and b parameters are very close to those belonging to the tetragonal MAPbI3 phase. The 2.5 dimensionality (2.5D) is also reported for perovskite.28,29,30 Such hybrids structures contain a bilayer of inorganic network and take the generic formula (A)2(MA)Pb2I7, with the presence of the MA moiety. 15 Thanks to the reduced size of the MA+ cation (small hindrance), this ion can be accommodated inside the lead-iodide inorganic network, occupying the same crystallographic position of that occurring in MAPbI3. Structural similarities are, then, recognizable between the (A)2(MA)Pb2I7 and the 3D arrangement 31 for the presence of MA+ ion inside the lattice. The variability of the organo/lead/iodide structures encourages a deep comprehension of the phenomena regarding the interaction between the two initial precursors PbI2 and MAI. It is expected, for instance, that the small gas phase methylamine molecules and hydrogen iodide (HI) can intercalate in the sheet-like PbI2 structure with relative easiness.32 In this paper, the interaction between PbI2 and MAI through a chemical two step method is presented. To this purpose, simplified systems consisting of uniform PbI2 thin layers (20 ±5 nm), grown [00l]-oriented on TiO2 flat substrates were used. We analyzed the layers as-obtained by the PbI2/MAI reaction at room temperature and after mild annealing processes. All the reactions and the thermal treatments, here described, have been carried out in inert atmosphere (H2O, O2< 1 ppm) to prevent contaminations and to avoid the effects attributable to the presence of water. We observed the formation of highly oriented MA/lead/iodide intermediate phases from PbI2 as the result of the intercalation of the organic parts in the inorganic Pb-I framework. The approach described in the paper has allowed us to follow the steps of the initial interaction between the PbI2 and MAI from the early stage. Phase modifications of the inorganic/organic self-assembled structures as induced by thermal treatments have been also discussed in the paper providing a detailed structural,



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morphological and optical characterization of the organo/lead/iodide structures all along the phasetransformations.

EXPERIMENTAL SECTION

Preparation of TiO2layers TiO2 layers with a thickness of ~37 nm have been deposited on 1737 Corning glass (0.9 mm-thick, having Root Mean Square roughness (RMS) 99%) and anhydrous isopropyl alcohol (>99.7%) were purchased by Aldrich. All the products were opened and kept inside a Glove-box (O2

From PbI2 to MAPbI3 through Layered Intermediates - The Journal of

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