Stéphane Jobic

CuGaO2: a promising alternative for NiO in p-type dye solar cells

CuGaO2 is used here as photocathode in place of NiO for a p-type dye-sensitized solar cell with iodide/triiodide (I−/I3−) or tris(4,4′-bis-tert-butyl-2,2′-bipyridine)cobalt (Co2+/Co3+) as redox mediator, and PMI or PMI-NDI as sensitizer. Both photovoltaic characteristics and flat band potentials suggest that CuGaO2 can be viewed as a promising substitute for NiO.

P-Type Nitrogen-Doped ZnO Nanoparticles Stable under Ambient Conditions

Zinc oxide is considered as a very promising material for optoelectronics. However, to date, the difficulty in producing stable p-type ZnO is a bottleneck, which hinders the advent of ZnO-based devices. In that context, nitrogen-doped zinc oxide receives much attention. However, numerous reviews report the controversial character of p-type conductivity in N-doped ZnO, and recent theoretical contributions explain that N-doping alone cannot lead to p-typeness in Zn-rich ZnO. We report here that the ammonolysis at low temperature of ZnO(2) yields pure wurtzite-type N-doped ZnO nanoparticles with an extraordinarily large amount of Zn vacancies (up to 20%). Electrochemical and transient spectroscopy studies demonstrate that these Zn-poor nanoparticles exhibit a p-type conductivity that is stable over more than 2 years under ambient conditions.

Kinetics of coloration in photochromic organoammonium polyoxomolybdates.

The excellent photochromic properties of (H(2)DABCO)(2)(HDMA)(0.5)Na(0.75)(H(3)O)(0.75)[Mo(8)O(27)] x 3 H(2)O (4), a new member of the (H(2)DABCO)(2)(A)(x)[Mo(8)O(27)] x n H(2)O series, are compared with those of (H(2)DABCO)(2)(NH(4))(2)[Mo(8)O(27)] x 4 H(2)O (1), (H(2)DABCO)(2)(H(2)pipz)[Mo(8)O(27)] (2), and (H(2)pipz)(3)[Mo(8)O(27)] (3). All these powdered materials turn from white to purple under illumination at 365 nm, which is associated with photoreduction of Mo(6+) cations into Mo(5+) cations. We show that the rates of coloration, which increase in the order 1 < 3, 2 < 4, are related to the decrease in the concentration of reducible Mo(6+) centers with irradiation time and follow a second-order reaction law because the event of light absorption at a reducible Mo(6+) site does not necessarily coincide with that of the N(+)-H bond breaking in the N(+)-H...O hydrogen bond associated with the Mo(6+) site. First-principles density functional electronic structure calculations were carried out to find that this trend correlates with the homolytic dissociation energies of the N(+)-H bonds in the organic cations HDMA(+), H(2)pipz(2+), H(2)DABCO(2+), and NH(4)(+). This observation is consistent with a photochromic mechanism based on the homolytic cleavage of N(+)-H bonds rather than on the heterolytic cleavage of N(+)-H bonds.

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).

Tuning the size and color of the p-type wide band gap delafossite semiconductor CuGaO2 with ethylene glycol assisted hydrothermal synthesis

Delafossite CuGaO2 is a prototype p-type transparent semiconducting oxide. We report here a new ethylene glycol (EG) mediated hydrothermal synthesis route to this material from soluble hydrated metal nitrates. We found that EG acts as a reducing agent which eases the formation of CuGaO2 by stabilizing Cu(I) at low temperature in hydrothermal conditions. Moreover, we observed that the initial pH of the precursors solution is a key parameter to tune the particle sizes (from 2 µm to 300 nm) and subsequently the color (from dark brown to light gray) of the CuGaO2 powder. The optimal synthesis conditions to obtain the brightest product are described. In these conditions, the CuGaO2 powder exhibits a pale grayish color and contains 300 nm large nano-layered particles approximately 20 nm thick.

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.

Occurrence and characterization of anionic bondings in transition metal dichalcogenides

Abstract Transition metal dichalcogenides MX2 crystallize in either two-dimensional or three-dimensional (3D) structures. This originates from the competition between cationic d levels and anionic sp levels. The occurrence of a chalcogen pairing may be obtained through oxidation of a ternary phase: Li2FeS2 leads to a metastable new binary compound Fe3+S2−(S2)2−. Such an electronic situation may also be found within the 3D family, the IrX2 (XS, Se) and RhSe2 compounds with quite elongated XX bonds attributed to strong strains. IrTe2 should confirm the structural type presented by sulphur and selenium derivatives. Its previously reported CdI2-like structure is in fact based on a polymeric network with multiple TeTe bonds (Irn3+(Te−1.5)2n) as confirmed by integrated overlap population calculations. This polymeric modification is presented by several other MTe2 phases and explains the very low c a value (1.38) of the hexagonal cell observed in this family. The polymerization phenomena must be generalized to most pyrite-like MTe2 with the noticeable exception of MnTe2. The layered binary Cr 2 3 □ 1 3 Te 2 is another example of tellurium polymeric bondings. Finally a classification of structures taking into account not only the dimensionality but also the polymerization degree of such materials is suggested. From many examples, it is shown that the polymerizing behaviour of the heavy chalcogen anion seems to be much more general than expected and should lead to many charge transfer studies.

Photo- and electrochromic properties of covalently connected symmetrical and unsymmetrical spiropyran-polyoxometalate dyads.

Polyoxometalates covalently linked to one or two spiropyran entities have been isolated. These organic-inorganic hybrids exhibit multi-electrochromic and photochromic properties.

Anionic polymeric bonds in transition metal ditellurides

Whereas sulfur and selenium tend, in some dichalcogenides, to bond to give ( X 2 ) 2− or ( X 2 ) − pairs, tellurium is more prone to setting multiple bonds. Reported CdI 2 -like and pyrite-like ditellurides are actually polymeric modifications leading to two new structural subgroups within these two families. A structures classification taking into account the dimensionality and the polymerization degree of such materials is suggested. A consequence of such anionic behavior is the occurrence of a partial oxidation state of tellurium, a frequent response of that element to a very stable cationic oxidation state. From many examples, it is shown that the polymerizing behavior of that heavy chalcogen anion seems to be much more general than expected and should lead to many charge transfer studies.

Smart Heterostructures for Tailoring the Optical Properties of Photochromic Hybrid Organic–Inorganic Polyoxometalates

A new concept of photoresponsive composites has been elaborated by intimately connecting a Photochromic Phase (PP), (H(2)DABCO)(2)(HDMA)(0.5)Na(0.75)(H(3)O)(0.75)[Mo(8)O(27)]·3H(2)O (1), with a second hybrid organic-inorganic molybdate material, (H(2)DABCO)(HDABCO)[Fe(OH)(6)Mo(6)O(18)]·4H(2)O (2) acting as an Oxidation Catalytic Phase (OCP) toward the former once photoexcited. The association of both the PP and the OCP in the composite drastically improves the bleaching process of the PP alone because of efficient electronic transfers through the OCP-PP interface without affecting significantly its photoinduced color change characteristic. Two OCP-PP composites with different PP weight percents have been obtained by associating 1 with 2. The optical properties of these composites before and after UV irradiation have been investigated by Diffuse Reflectance Spectroscopy, and the strong impact of the OCP on the fading kinetics of the PP has been clearly highlighted.