Alain Lafond

A low-temperature order-disorder transition in Cu2ZnSnS4 thin films

Cu2ZnSnS4 (CZTS) is an interesting material for sustainable photovoltaics, but efficiencies are limited by the low open-circuit voltage. A possible cause of this is disorder among the Cu and Zn cations, a phenomenon which is difficult to detect by standard techniques. We show that this issue can be overcome using near-resonant Raman scattering, which lets us estimate a critical temperature of 533 ± 10 K for the transition between ordered and disordered CZTS. These findings have deep significance for the synthesis of high-quality material, and pave the way for quantitative investigation of the impact of disorder on the performance of CZTS-based solar cells.

Structure flexibility of the Cu2ZnSnS4 absorber in low-cost photovoltaic cells: from the stoichiometric to the copper-poor compounds.

Here we present for the very first time a single-crystal investigation of the Cu-poor Zn-rich derivative of Cu(2)ZnSnS(4). Nowadays, this composition is considered as the one that delivers the best photovoltaic performances in the specific domain of Cu(2)ZnSnS(4)-based thin-film solar cells. The existence of this nonstoichiometric phase is definitely demonstrated here in an explicit and unequivocal manner on the basis of powder and single-crystal X-ray diffraction analyses coupled with electron microprobe analyses. Crystals are tetragonal, space group I ̅4, Z = 2, with a = 5.43440(15) Å and c = 10.8382(6) Å for Cu(2)ZnSnS(4) and a = 5.43006(5) Å and c = 10.8222(2) Å for Cu(1.71)Zn(1.18)Sn(0.99)S(4).

X-ray resonant single-crystal diffraction technique, a powerful tool to investigate the kesterite structure of the photovoltaic Cu2ZnSnS4 compound

Cu/Zn disorder in the kesterite Cu2ZnSnS4 derivatives used for thin film based solar cells is an important issue for photovoltaic performances. Unfortunately, Cu and Zn cannot be distinguished by conventional laboratory X-ray diffraction. This paper reports on a resonant diffraction investigation of a Cu2ZnSnS4 single crystal from a quenched powdered sample. The full disorder of Cu and Zn in the z = 1/4 atomic plane is shown. The structure, namely disordered kesterite, is then described in the I42m space group.

Surfactant-free CZTS nanoparticles as building blocks for low-cost solar cell absorbers

A process route for the fabrication of solvent-redispersible, surfactant-free Cu₂ZnSnS₄ (CZTS) nanoparticles has been designed with the objective to have the benefit of a simple sulfide source which advantageously acts as (i) a complexing agent inhibiting crystallite growth, (ii) a surface additive providing redispersion in low ionic strength polar solvents and (iii) a transient ligand easily replaced by an carbon-free surface additive. This multifunctional use of the sulfide source has been achieved through a fine tuning of ((Cu²⁺)(a)(Zn²⁺)(b)(Sn⁴⁺)(c)(Tu)(d)(OH⁻)(e))(t⁺), Tu = thiourea) oligomers, leading after temperature polycondensation and S²⁻ exchange to highly concentrated (c > 100 g l⁻¹), stable, ethanolic CZTS dispersions. The good electronic properties and low-defect concentration of the sintered, crack-free CZTSe films resulting from these building blocks was shown by photoluminescence investigation, making these building blocks interesting for low-cost, high-performance CZTSe solar cells.

New three-dimensional thiostannates composed of linked Cu8S12 clusters and the first example of a mixed-metal Cu7SnS12 cluster.

Three new compounds (enH)(6+n)Cu(40)Sn(15)S(60) (1), (enH)(3)Cu(7)Sn(4)S(12) (2), and (trenH(3))Cu(7)Sn(4)S(12) (tren = tris(2-aminoethyl)amine) (3) containing Cu(8)S(12) and Cu(7)SnS(12) clusters have been prepared from direct solvothermal reaction of the elements in amine solvents. In 1, the cubic close-packed arrangement of Cu(8)S(12) clusters, interconnected by capping SnS(4) tetrahedra and CuS(3) triangles, form two interpenetrating channel networks that are presumably filled with disordered solvent molecules. Structures 2 and 3 contain well-ordered, protonated amine molecules and Cu(7)SnS(12) clusters. The clusters are connected by SnS(4) tetrahedra to form a three-dimensional structure with ReO(3) topology. (119)Sn Mössbauer measurement is consistent with Sn(IV) atoms linking, and Sn(II) atoms within, the mixed-metal Cu(7)SnS(12) clusters.

In Situ XRD, XAS, and Magnetic Susceptibility Study of the Reduction of Ammonium Nickel Phosphate NiNH4PO4·H2O into Nickel Phosphide

The reduction of the ammonium nickel phosphate NiNH(4)PO(4) x H(2)O precursor into nickel phosphide (Ni(2)P), a highly active phase in hydrotreating catalysis, was studied using a combination of magnetic susceptibility and in situ X-ray diffraction and X-ray absorption spectroscopy (XAS) techniques. The transformation of NiNH(4)PO(4) x H(2)O into Ni(2)P could be divided into three distinguishable zones: (1) from room temperature to 250 degrees C, the NiNH(4)PO(4) x H(2)O structure was essentially retained; (2) from 300 to 500 degrees C, only an amorphous phase was observed; (3) above 500 degrees C, a crystallization process occurred with the formation of Ni(2)P. An in situ XAS study and magnetic susceptibility measurements clearly revealed for the first time that the amorphous region corresponds to the nickel pyrophosphate phase alpha-Ni(2)P(2)O(7). The phosphate reduction into phosphide did not start before 550 degrees C and led to the selective formation of Ni(2)P at 650 degrees C.

Charge Transfer in Misfit Layer Chalcogenides, [(MX)n]1+x(TX2)m: a Key for Understanding their Stability and Properties

Abstract Stability of misfit layer chalcogenides (M.L.C.) is always governed by a charge transfer (C.T.) mechanism. A comparison between M.L.C. and parent intercalated layered binary systems shows comparable characteristics, which illustrate the role of electron donor for the MX part (or its corresponding A-intercalant species) towards the TX2 acceptor part. In that way, intercalation chemistry within the vdW gap of multilayered TX2 misfit derivatives is also considered. The donor character of the MX part, easy to understand when trivalent M cations are present, can be also assumed if a divalent M cation is partly substituted for a trivalent one, as revealed from precise electron microprobe analysis. Examples of M.L.C. with enlarged MX part (with mixed M+II, M+III) emphasize the reality of the C.T. This M-to-T donor effect necessarily controls the transport and intercalation properties.

Substitution of Li for Cu in Cu2ZnSnS4: Toward Wide Band Gap Absorbers with Low Cation Disorder for Thin Film Solar Cells

The substitution of lithium for copper in Cu2ZnSnS4 (CZTS) has been experimentally and theoretically investigated. Formally, the (Cu1-xLix)ZnSnS4 system exhibits two well-defined solid solutions. Indeed, single crystal structural analyses demonstrate that the low (x < 0.4) and high (x > 0.6) lithium-content compounds adopt the kesterite structure and the wurtz-kesterite structure, respectively. For x between 0.4 and 0.6, the two aforementioned structure types coexist. Moreover, 119Sn NMR analyses carried out on a (Cu0.7Li0.3)2ZnSnS4 sample clearly indicate that lithium replaces copper preferentially on two of the three available 2-fold crystallographic sites commonly occupied by Cu and Zn in disordered kesterite. Furthermore, the observed individual lines in the NMR spectrum suggest that the propensity of Cu and Zn atoms to be randomly distributed over the 2c and 2d crystallographic sites is lowered when lithium is partially substituted for copper. Additionally, the first-principles calculations provide insights into the arrangement of Li atoms as a function of the Cu/Zn disorder and its effect on the structural (lattice parameters) and optical properties of CZTS (band gap evolution). Those calculations agree with the experimental observations and account for the evolutions of the unit cell parameters as well as for the increase of band gap when the Li-content increases. The calculation of the formation enthalpy of point defect unambiguously indicates that Li modifies the Cu/Zn disorder in a manner similar to the change of Cu/Zn disorder induced by Ag alloying. Overall, it was found that Li alloying is a versatile way of tuning the optoelectronic properties of CZTS making it a good candidate as wide band gap materials for the top cells of tandem solar cells.

X-ray structure determination of a new misfit layer compound: Bismuth chromium sulfide

(BiS)1.24CrS2 is a new misfit layer phase in which two types of slabs, |BiS| and |CrS2|, alternate along the stacking direction (c = 1098.5 pm). The misfit between the two slabs occurs along the a in-plane direction with the periodicities a1 = 558.24 pm and a2 = 347.05 pm, respectively. The a1a2 ratio is irrational but close to 85. The second in-plane cell parameter (b = 590.0 pm) is common to both slabs. The |BiS| slab arrangement is similar to a {001} slice (half an edge thick) of a NaCl-type fcc cell. The |CrS2| slab is a sandwich with the Cr in trigonally distorted sulfur octahedra like in 1T-TiS2. Such a structure resembles that of (LaS)1.20CrS2.

Evidence for a modified-stannite crystal structure in wide band gap Cu-poor CuIn1−xGaxSe2: Impact on the optical properties

The crystal structure of high Ga-content CuIn1−xGaxSe2 (CIGSe) compounds has been further investigated with the help of single crystal x-ray diffraction technique. It is known that CIGSe compounds adopt the chalcopyrite crystal structure. In the case of Cu-poor, Ga-rich CIGSe, the present study shows that an alternative structure should be considered. This structure is derived from that of stannite in which there is a Ga∕In segregation on two different atomic planes. The diffuse reflectance measurements of the Cu-poor compound reveal a slightly different band gap and a smoother transition compared with those of the stoichiometric compound.