Chemical Structure and Physical Properties of Organic-Inorganic Metal Halide Materials for Solid State Solar Cells
Majid Safdari a, James Gardner a
a KTH The Royal Institute of Technology, Roslagstullsbacken 21, Stockholm, Sweden
Proceedings of Perovskite Thin Film Photovoltaics (ABXPV17)
València, Spain, 2017 March 1st - 2nd
Organizers: Henk Bolink and David Cahen
Oral, Majid Safdari, presentation 029
Publication date: 18th December 2016

The application of methylammonium lead (II) iodide as a light-absorbing layer in solid-state solar cells leads to impressive efficiency of over 22% in laboratory devices. However, for industrial applications, fundamental issues regarding their thermal and moisture stability need to be addressed. MAPbI3 belongs to the perovskite family of materials with the general formula ABX3 ,where is the organic cation (methylammonium) which is reported to be a major source of instability. In this work, a variety of alkyammonium lead (II) iodide materials have been synthesized by changing the organic cation, to study the relationship between the structural and physical properties of these materials. Methylammonium, ethylammonium and propylammonium were used for the [(A)PbI3] series. In another set of experiments, butyldiammonium, hexyldiammonium, and octyldiammonium cations were studied as (A)PbI4 perovskite materials. Various dimensionalities for the structures of these materials were found; three dimensional (3D) networks (MAPbI3, MAPbBr3), two dimensional (2D) layered systems (BdAPbI4, HdAPbI4, OdAPbI4), and one dimensional (1D) columns (EAPbI3, PAPbI3, EAPb2I6). Several new lower dimensional materials (2D and 1D) were investigated and reported for the first time. X-ray single crystallography was used to obtain the detailed structures. Bulk structures were confirmed by comparison of the X-ray diffraction patterns with single crystal data. [PbI6] octahedral structural units were repeated through the material network depending on the dimensionality and connectivity of the materials. Where a bulkier cation was introduced, the crystallographic unit cell increased in size which resulted lower symmetry crystals. The connectivity of the unit cells along the material networks was found to be based on corner-sharing and face-sharing. Lower dimensionality resulted in larger bandgaps and lower photoconductivity, and hence a lower light conversion efficiency for the related solar cells. The thermal and moisture stability was greater in the 1D and 2D materials with bulkier organic cations than with methylammonium. The electronic structure of the new 2D layered perovskites was investigated by X-ray photoelectron spectroscopy, X-ray absorption spectroscopy and X-ray emission spectroscopy. These findings were in agreement with the experimental part, indicating that the valence band is composed mainly of iodine orbitals, while lead orbitals predominate in the conduction band. The iodide/lead ratio obtained from surface analysis of the material deposited on the TiO2 films matched the proposed general formula from single crystal data. In total, an overview is provided of the relationship between the chemical dimensionality and physical properties of the organic-inorganic lead halide materials with focus on the solar cell application.

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