Article Cite This: Chem. Mater. 2018, 30, 549−558
pubs.acs.org/cm
Substituting Copper with Silver in the BiMOCh Layered Compounds (M = Cu or Ag; Ch = S, Se, or Te): Crystal, Electronic Structure, and Optoelectronic Properties J. Gamon,*,†,‡ D. Giaume,† G. Wallez,†,§ J.-B. Labégorre,‡,∥ O. I. Lebedev,∥ R. Al Rahal Al Orabi,⊥,# S. Haller,†,‡ T. Le Mercier,‡ E. Guilmeau,∥ A. Maignan,∥ and P. Barboux† †
Chimie ParisTech, PSL Research University, CNRS, Institut de Recherche de Chimie Paris (IRCP), F-75005 Paris, France Solvay, Research and Innovation Center Paris, 52 rue de La Haie Coq, 93308 Aubervilliers Cedex, France § Sorbonne University, UPMC Université, Paris 06, 75005 Paris, France ∥ Laboratoire CRISMAT, UMR-CNRS 6508, ENSICAEN, UNICAEN, Normandie Université, 6 bd du Maréchal Juin, F-14050 Caen Cedex 4, France ⊥ Solvay, Design and Development of Functional Materials Department, Axel’One, 87 avenue des Frères Perret, 69192 Saint Fons Cedex, France # Department of Physics, Central Michigan University, Mt. Pleasant, Michigan 48859, United States ‡
S Supporting Information *
ABSTRACT: The synthesis of BiAgOCh (Ch = S or Se) compounds has been successfully achieved via the ion exchange of copper with silver in aqueous solutions, starting from the copper parent phase. Optical and electrical measurements of BiAgOCh powders confirm an increase in both the bandgap and the electrical resistivity, as compared to those of the copper compounds. The structure of the BiAgOS phase has been clearly examined. X-ray diffraction synchrotron measurements coupled with advanced high-resolution transmission electron microscopy analysis evidenced a Ag-deficient structure, as well as Bi-rich defects, both types of defects being oppositively charged. Silver atoms are also found in interstial sites, which explains the two-dimensional ionic conductivity. This structural study combined with theoretical calculations explains the intrinsic conductivity behavior of these semiconductors linked to the mutual compensation of both defect types in the structure and to the increase in the hole effective mass. This study shows the feasibility of modifying the optoelectronic properties of the BiMOCh compounds, with the goal of integrating them in heterojunction solar cells. Moreover, it provides very precise insight into the complexity of the relationship between structural defects and optoelectronic properties.
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INTRODUCTION The oxysulfide family BiCuOCh, where Ch = S, Se, or Te, was identified for the first time by Kusainova et al.1 in 1994. The related compounds crystallize in the same tetragonal P4/nmm space group. The structure consists of alternating layers of (Bi2O2)2+ and (Cu2Ch2)2−. BiCuOCh phases have been widely studied for their thermoelectric properties,2−8 owing to their intrinsically low thermal conductivity as well as their high electronic conductivity. Indeed, they are naturally degenerated p-type semiconductors with low bandgap values (from 0.4 eV for BiCuOTe to 1.1 eV for BiCuOS).2,9 Among these compounds, Ba-doped BiCuOSe exhibits the best reported figure of merit (ZT = 1.4 at 923 K).10 The p-type conductivity originates from the creation of copper vacancies2,5,6,11,12 following Kroeger Vink’s equation: X CuCu
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→ V′Cu + h +
As the valence band is composed of the 3d orbitals of copper and p orbitals of the chalcogenide atom, copper vacancies will induce the creation of holes delocalized in the [Cu2Ch2]2− layer. In the QCuOCh system (Q = Bi or La), the copper vacancies appear to be thermodynamically favored because of their low formation energy.13 In particular, the copper vacancies in BiCuOS seem to be intrinsically induced by the structure, as evidenced by high-resolution transmission electronic microscopy (HR-TEM) analysis.6 The actual measured composition is BiCu1−δOS with δ ≈ 0.06, as shown by neutron diffraction studies12 and confirmed by Rietveld measurements on our powders. Received: November 27, 2017 Revised: December 20, 2017 Published: December 20, 2017
Cu 0surface © 2017 American Chemical Society
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DOI: 10.1021/acs.chemmater.7b04962 Chem. Mater. 2018, 30, 549−558
Article
Chemistry of Materials Recently, Le Bahers et al.14,15 also identified BiCuOS as a promising material for heterojunction photovoltaic cells. Band structure calculations showed that this compound possesses in theory all the requirements for an absorber (p-type) layer: a bandgap between 1.1 and 1.4 eV, a high mobility of charge carriers (hole effective mass mh < 0.5m0), and a high dielectric constant (ε > 10).16 However, the carrier concentration, ∼1021 cm−3 (considering that each copper vacancy induces one delocalized hole in the structure), is found to be too high (optimum value between 1015 and 1017 cm−3), so the space charge layer length at the junction with the n-type semiconductor will be decreased, as well as the diffusion length of photogenerated electron holes. To decrease the concentration of p-type carriers in the structure, we have investigated the feasibility of substituting copper with silver, this element being less likely to oxidize and to yield a p-type doping.17 While LaAgOS can be synthesized by a high-temperature solid-state annealing process,18,19 this synthetic route does not allow crystallization of BiAgOCh materials. Mechanochemical synthesis17 and a wet chemistry route20 were reported for the preparation of BiAgOS and could therefore allow us to overcome this issue. We report herein on an alternative method of ion exchange, at room temperature, for the synthesis of BiAgOCh compounds, together with their crystal structures and optical and electrical properties.
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180.676−181.978 nm; Bi, 190.178 nm; Ag, 328.068−338.289 nm; Cu, 324.754−327.396 nm; Se, 196.026 nm. X-ray and Synchrotron Diffraction. Preliminary X-ray diffraction (XRD) experiments were performed in Bragg−Brentano geometry on a Panalytical X’Pert Pro apparatus with a monochromatized Cu Kα1 beam in the 8° ≤ 2θ ≤ 140° scan range, step δ(2θ) = 0.013°, with a total counting time of 13 h. Synchrotron diffraction (SD), implemented to extract high-precision structural data from the thiosulfate-washed sample, was performed at the European Synchrotron Radiation Facility (ERSF) in Grenoble, France, on highresolution beamline ID22, at λ = 0.40013 Å. The sample was introduced into a 500 μm diameter glass capillary to record the pattern in transmission mode [2° ≤ 2θ ≤ 40°, and δ(2θ) = 0.002°] with a 12 rps spinning, giving 425 reflections. All experiments were performed at room temperature. Optical Properties. The optical properties of the powders were investigated by diffuse reflectance spectroscopy with a Cary 5000 ultraviolet−visible−near-infrared spectrophotometer (Agilent Technologies) equipped with an integrating sphere. Reflection data have been treated following a combined approach of the Tauc21 and Kumar method.22 Indeed, absorbance α of the material is proportional to ln[(Rmax − Rmin)/(R − Rmin)], where R is the diffuse reflection intensity and Rmax and Rmin are the maximum and minimum reflectance values, respectively. As for an indirect bandgap material, one can write following Tauc formula21 αhν = A(hν − Eg)2. Therefore, if we plot the square root of hν{ln[(Rmax − Rmin)/(R − Rmin)]} versus hν, the bandgap of the material can be obtained as the extrapolation of a straight line to the zero value of the ordinate. Transport Properties. Resistivity measurements were performed with the four-probe method on a pelletized sample (300 MPa, relative density of 70%). Contacts were made with a silver paint (purchased from SPI Supplies). Transmission Electron Microscopy. Electron diffraction (ED) studies were performed on a FEI Tecnai G2 30 UT (LaB6) microscope operated at 300 kV with 0.17 nm point resolution and equipped with an EDAX EDX detector. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) together with EDX elemental mapping was performed on a JEOL ARM-200F cold FEG double aberration corrected electron microscope operated at 200 kV and equipped with a large solid-angle CENTURIO EDX detector and a Quantum GIF system. TEM samples were prepared by crushing the material with butanol in an agate mortar, and the resulting dispersion was transferred to holey carbon films fixed on 3 mm copper grids. Computational Procedures. Our calculations are based on density functional theory (DFT). We used the full-potential linearized augmented plane wave (FLAPW) approach, as implemented in the WIEN2K code.23 A plane wave cutoff corresponding to RMTKmax = 7 was used in all calculations. The radial wave functions inside the nonoverlapping muffin-tin spheres were increased to lm = 12. The charge density was Fourier expanded to Gmax = 16 Å−1. Total energy convergence was achieved with respect to the Brillouin zone (BZ) integration mesh with 10 × 10 × 4 k-points. The atomic positions were relaxed (with the CASTEP code)24 by using a set of ultrasoft pseudopotentials25,26 with the PBEsol exchange-correlation functional27 until all atomic forces were
