Influence of the anion nature on the properties of Sr-containing formamidinium tin halide perovskites
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ARTICLE Influence of the anion nature on the properties of Sr-containing formamidinium tin halide perovskites Lucangelo Dimessoa) Materials and Earth Sciences Department, Technische Universität Darmstadt, Darmstadt D-64287, Germany (Received 8 August 2018; accepted 10 October 2018)
Formamidinium–tin–strontium halides (CH(NH2)2Sn1ySryX3 (FASnX3), X 5 I, Br and 0.0 # y # 0.1) were investigated. X-ray diffraction analysis revealed orthorhombic FASnI3 (space group Amm2) and SnI2 for X 5 I as well as cubic FASnBr3 (space group Pm3m) and SnBr2 for X 5 Br, respectively. For X 5 I, the optical spectra displayed a decrease of the absorption edges with increasing Sr content (1055 nm, y 5 0.0; 950–960 nm, y . 0.0) and a direct semiconducting behavior with narrow band energy gaps (1.31–1.34 eV). For X 5 Br, on increasing the absorption edges (492 nm, y 5 0.0; 975 nm, y 5 0.075), a direct semiconducting behavior with band energy gaps between 2.65 eV (y 5 0.0) and 1.38 eV (y 5 0.075) were observed and the emission photoluminescence (PL) spectra (excitation wavelength kexc 5 380 nm) showed an increase of the luminescence response after the thermal treatment.
I. INTRODUCTION
Organic–inorganic halide perovskites adopt a typical ABX3 structure where the “A” cations reside at the eight corners of a cubic unit while the “B” cations are located at the center of the octahedral [BX6]4 structure. In the most common organic–inorganic halide perovskites, the “A” position represents an organic or metal cation, e.g., CH3NH31 (methylammonium MA), HC(NH2)21 (formamidinium, FA), Cs1 and/or Rb1, “B” represents a divalent metal cation (typically Pb, Sn, and Ge), and “X” represents a halide (I, Br, and/or Cl). Among the advantages of these materials are (i) the abundancy of chemical precursors and (ii) the preparation by easy experimental setups.1,2 The modulation of hybrid perovskite properties, by changing the protonated amine as cation, is a very appealing way of performing such tuning with the aim, for example, of extending and/or changing the absorption edge and improving material stability.3–9 In particular, lead iodide systems based on formamidinium cation have attracted significant interest because of higher symmetry in the crystalline structure compared with the MAPbI3 phase (which is tetragonal at room temperature) and have an improved thermal stability.3–6 Despite these remarkable properties, the toxicity issue of lead (Pb)10–12 has slowed down further developments; indeed PbI2 is the product of the decomposition of perovskites and has a larger solubility than the conventionally toxic Cd21 in CdTe,13,14 which indicates a higher toxic level of Pb21 than Cd21 in their corresponding solar cells materials. Recently tin (Sn) has been reported a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2018.412 J. Mater. Res., 2018
to be a suitable alternative in hybrid perovskites mainly due to the similar ionic radii (6-fold coordination, high spin, Pb21: 1.19 Å15 and Sn21: 1.18 Å16). Besides
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