Article pubs.acs.org/JPCC
Neutron Diffraction and X‑ray Absorption Fine Structure Evidence for Local Lattice Distortions and Aperiodic Antisite Substitution in Cu2ZnSnS4 Nanoparticles Francisco J. Espinosa-Faller,§ Dylan R. Conradson,†,⊥ Shannon C. Riha,‡ Mary B. Martucci,†,‡ Sarah J. Fredrick,‡ Sven Vogel,† Amy L. Prieto,*,‡ and Steven D. Conradson*,† †
Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, United States § Universidad Marista de Mérida, Periférico Norte Tablaje 13941, 97300 Mérida, Yucatán, México ‡
ABSTRACT: A thorough structure determination has been performed on Cu2ZnSnS4 nanoparticles, a popular photovoltaic material, using neutron diffractionto characterize the long-range average crystal structureand X-ray absorption fine structure (XAFS) spectroscopy at the Cu, Zn, and Sn K-edges to elucidate the element-specific local structure. This is the first combined multiscale approach on nanoparticles of this material. The results indicate the presence of aperiodic disorder on the cation sites that is diminished by annealing. This disorder involves local lattice distortions around the crystallographic sites rather than the presence of interstitial atoms. It is most consistent with the known antisite substitutions that are integral to CZTS (referring to the ordering of the Cu, Zn, and Sn between planes). However, instead of being confined within single unit cells so as to maintain the crystallographic symmetry, periodicity, and homogeneity, the substitutional disorder appears to extend over larger regions consisting of multiple unit cells but still smaller than the physical dimensions of the nanoparticles. These results therefore imply the presence of nanoscale domains characterized by local fluctuations in composition that cause the individual domains to be enriched in certain metal ions and depleted in others. These will be mirrored by domains with the opposite fluctuations at other locations in the crystal so that the overall composition remains close to the stoichiometric Cu2ZnSnS4. This disorder is likely pronounced in these samples due to the relatively low temperature reaction (300 °C) and annealing (350 °C) conditions and can be expected to have a significant effect on the resulting physical properties of the material and its photovoltaic performance.
1. INTRODUCTION Research concerning nanoparticles is an area of intense scientific interest owning to a wide variety of potential applications in optical, electronic, catalytic, and biological fields.1−7 Nanoparticles often differ from bulk materials both chemically and physically, due to an increased surface to volume ratio. As the number of surface atoms increases with respect to the bulk, a number of size-dependent properties are observed, for example, the size evolution of the optical absorption spectra.8,9 Furthermore, significantly more undercoordinated and spin-/charge-uncompensated sites occur in nanoparticles compared to bulk materials. These sites potentially affect the nanoparticle interatomic interactions, which are often accompanied by modifications to the atomic structure.10 This is relevant as nanoparticle inks of many materials are now being used as precursors for the fabrication of thin films.11 Cu2SnZnS4 (CZTS) is currently under study as an absorber material for thin film solar cells due to its band gap (∼1.5 eV) that is compatible with the solar emission spectrum, high absorption coefficient (>104 cm−1), and p-type conductivity and because its constituent elements are nontoxic and earth abundant.12−15 Extensive effort has been put forth toward the © 2014 American Chemical Society
synthesis of CZTS nanoparticles for a solution-processable route to thin film photovoltaics.16−20 As is most often the case, nanoparticles are cast into a thin film and then annealed to achieve the desired PV performance. The performance enhancement of an annealed film in comparison to an asdeposited thin film is typically attributed to increased grain growth and the removal of the insulating capping ligands used to stabilize the nanoparticles during synthesis and in solutionfacilitating better charge carrier transport through the thin film.21,22 However, dramatic changes in the optical absorption spectra observed in a recent study cannot be explained by grain growth alone, suggesting that the low temperature annealing also is changing the atomic structure of the CZTS nanoparticles.22 Kesterite CZTS forms in the I-4 space group where layers of Cu/Sn located at z = 0, 1/2, and 1, with the atoms on the corners and in the center of the ab planes, alternate with layers of Cu/Zn located at z = 1/4 and 3/4 (see Figure 1). These metallic layers are all held together with intervening S layers on Received: March 2, 2014 Revised: September 26, 2014 Published: September 29, 2014 26292
dx.doi.org/10.1021/jp502150s | J. Phys. Chem. C 2014, 118, 26292−26303
The Journal of Physical Chemistry C
Article
S atoms are displaced to occupy the tetrahedral holes in the S planes. This complicated structure of CZTS is both a problem and a potential asset in its photovoltaic application. Ongoing work has raised the PV efficiency of the CZTS system to the 12.6% level,26 with the possibility of significant enhancements still to come via synthetically controlled modifications to its structure.27 The narrow atomic chemical potential space for the formation of CZTS imposes constraints on the sample growth conditions in order to obtain stoichiometric kesterite without the formation of additional binary or tertiary phases.28−30 Synthetic approaches must find ways to avoid the production of additional phases that would compromise performance both by interfering with the photovoltaic properties (both in terms of light absorption and charge carrier transport) and by altering the stoichiometry of the CZTS.31 The synthesis and postprocessing must also take into consideration the potential for defects32both in the bulk and at grain boundariesand especially for the formation of vacancies and antisites that are coupled to the stoichiometry and the formation of secondary phases. In addition, high temperature annealing promotes the loss of S and SnS.21 The loss of Sn from its native site during annealing, even though in principle energetically unfavorable, could be responsible for the formation of Zn or Cu vacancies with Zn/Cu atoms occupying the Sn sites.33 Even in the stoichiometric compound, facile substitution is responsible for, e.g., the relationship between the kesterite and stannite structures that exhibit, respectively, alternating planes of Cu/Zn and Cu/Sn or Cu and Zn/Sn. Theoretically, it has been calculated that Cu−Zn antisite defects are the dominant intrinsic defects and are actually beneficial at low levels because they contribute to the p-type conductivity of CZTS.28,29 However, Cu−Sn, Zn−Sn, V (vacancy)−Cu, and V−Zn also have relatively low formation energies and are influenced by the preparation conditions. Among defect complexes, the antisite pair [Cu−Zn + Zn+Cu] is the lowest in energy with other complexes less likely to be formed in stoichiometric samples. Regardless, the formation energy is altered by changes in the chemical potential landscape during sample synthesis. This imposes stringent conditions for the formation of high quality CZTS.28−30 However, insofar as perfectly crystalline CZTS may not have the optimum properties as a photovoltaic material, this flexibility offers opportunities for myriad structural variants that may be better, if synthetic strategies can be developed to produce them. Disorder on the largest and smallest scales involves disruption of the CZTS crystal lattice. At the micrometer scale, if a fraction of the precursors react to form other phases, they will deplete CZTS in these elements. If they are very small or amorphous they will not produce Bragg peaks but will contribute to diffuse scattering.34 On the scale of Å, vacancies and other defects in the crystal structure locally affect the charge balance in the unit cell and modify the bonding types and geometries, resulting in local lattice distortions and in an increase of the energy of the material. Because these defects are kinetically trapped due to a large activation energy for them to move to their crystallographic lattice positions, annealing should eliminate them by promoting diffusion of the atoms from unstable configurations into their crystallographic lattice sites. This process should be especially facile for nanoparticles, where the diffusion distances are very short. Nanoscale heterogeneity over several nanometers is also of interest because clustering of one of the elements could
Figure 1. Kesterite structure of CZTS, where copper Cu(2a) is shown in light blue, copper Cu(2c) in dark blue, tin in green, zinc in orange, and sulfur in yellow.
the z = 1/8, 3/8, 5/8, and 7/8 planes with two S atoms in each plane located at the centers of the Cu(2)−Sn−Zn tetrahedron. There are therefore two tetrahedral vacancies or holes in the S planes whose geometry is essentially identical to that of the sites occupied by the S. The S atoms are slightly displaced from the (1/4, 1/4, 1/8) positions to accommodate the Sn ion, which is larger than the Cu and Zn.24 These small displacements are therefore the basis for the longer range ordering of the metal sites. The Cu atoms in the planes with the Sn and Zn form the Cu(2a) and Cu(2c) sites, which occupy the Wyckoff positions 2a and 2c, respectively. The principal structural variants result from antisite substitutions of the metal ions; e.g., the stannite phase has alternating Cu and Sn/Zn planes. The focus of much of the crystallography has been to elucidate these substitutions, under the assumption that they maintain the symmetry by occurring only within single unit cells and that they occur identically within all of the unit cells of the crystal.25 The crystal structure imposes near neighbor environments for the Cu and Zn, consisting of a first neighbor shell of a tetrahedron of S atoms at ∼2.33 Å, a second shell with four Cu, four Sn, and four Zn/Cu atoms at ∼3.84 Å, and a third shell of twelve S at ∼4.52 Å. The Sn environment is simpler because it is bracketed by identical Cu/Zn planes, possessing four S at ∼2.41 Å, eight Cu/Zn atoms at ∼3.84 Å, and twelve S at ∼4.47 Å. The metal−metal sublattice always consists of four other metals forming a square in the same [001]-oriented plane and rectangle in the [010]/ [100]-oriented plane, with two metals in the next metal plane above and two in the one below. The distances within the square are only ∼0.002 Å greater than the ones in the rectangle. Because of their different chemistries and sizes it is unlikely that the Cu and Zn or Sn actually exhibit identical sites in CZTS. Depending on how the “atomic radius” is calculated, Zn ranges from 0.04 Å smaller to 0.03 Å larger than Cu.24 These differences would be a source of local lattice distortions that would not be observed in diffraction because of the crystallographically imposed symmetry and the possibility that the antisite substitutions and the locations of the distortions are aperiodic. A potentially important characteristic of the structure is that it is quite open, raising the possibility of S displacements around its crystallographic sites to accommodate cation substitution, the incorporation of extra S, or defects in which 26293
dx.doi.org/10.1021/jp502150s | J. Phys. Chem. C 2014, 118, 26292−26303
The Journal of Physical Chemistry C
Article
generate regions in the material where the local stoichiometry differs from the bulk average composition. Since domains with an elevated content of one element must be mirrored by others where that particular element is low (to maintain overall charge balance), the material could be described as a patchwork of the various compounds. If the structure within the domains is ordered then the material will exhibit phase separation and be heterogeneous on the nanoscale; i.e., it will contain multiple, coexisting, ordered structures that will generate superlattice peaks in a neutron/X-ray diffraction pattern. However, if the domains are arranged aperiodicially then this type of ordering does not give superlattice peaks and becomes nearly invisible in a diffraction pattern.35 In this article we aim to address whether, and to what extent, local lattice distortions are present in CZTS nanoparticles and attempt to correlate them with their synthesis and subsequent annealing. We have used a combination of the complementary methods of neutron diffraction and X-ray absorption fine structure (XAFS) to explore the structure of the nanoparticles. Neutron scattering is a valuable tool for CZTS because it can distinguish between Cu and Zn atoms due to their difference in neutron scattering cross section, whereas there is only negligible contrast between them with X-rays. For local structure analysis, XAFSvia the extended X-ray absorption fine structure (EXAFS)is extremely valuable because it gives the one-dimensional partial pair distribution function for each element. The precision of EXAFS, 0.01−0.03 Å in distance, 10−30% in the number of atoms, and 0.0−0.06 in the pairwise Debye−Waller factors, allows it to easily identify changes in the chemical speciation of the target elements.
holder was attached to the coldfinger of a liquid nitrogen reservoir cryostat to maintain a sample temperature of 80 K during the measurements. Si [220] crystals were used to obtain the monochromatic X-ray beam. Harmonic rejection was accomplished by changing the relative alignment of the two crystals to reduce the flux to 50% of its maximum level. The data were taken in transmission mode. The reduction of the XAFS data was performed using standard procedures.37 Energy calibration was accomplished by defining the first inflection point in the K edge spectra of Cu, Zn, and Sn metal foils as 8980.3, 9659, and 29 200 eV, respectively. The ionization energies, E0, that define the photoelectron momentum k = ((2m/ℏ2)(E − E0))1/2, were set at 8984, 9661, and 29 203 eV. The data were normalized by shifting the spectra so that the value of a second order polynomial fit through the pre-edge region equals zero and scaling the spectra so that the value of a third order polynomial fit through the region above the edge equals unity at 30. The EXAFS were extracted from the spectra as the difference between the normalized spectra and an adjustable spline function over the region above the edge (after fitting the edge region with a combination of an arctangent and a Gaussian line shape and the higher energy absorbance with a third-order polynomial). The knots of this spline were determined by minimizing the low-frequency residuals (R
