Olivier Margeat

Ultrasmall iron nanoparticles: Effect of size reduction on anisotropy and magnetization

Stable iron nanoparticles have been synthesised by the decomposition of {Fe(N[Si(CH3)3]2)2}2 under dihydrogen pressure. Those conditions lead to a system of monodisperse and metallic nanoparticles which diameter is less than 2nm and stabilized by HN[Si(CH3)3]2. The magnetization is found to be MS=1.92μB∕at., i.e., 10% lower than the bulk value. The Mossbauer spectrum is fitted by two contributions of metallic iron. The magnetic anisotropy energy constant increases up to 5.2×105J∕m3, i.e., ten times the bulk one.

Ligand effects on the air stability of copper nanoparticles obtained from organometallic synthesis

Low-polydispersity copper nanoparticles (NPs) are prepared through hydrogenolysis of various organometallic copper precursors in an organic medium at moderate temperature. The effects of the precursor composition and the nature of the additional surfactants on the structure and stability of NPs are characterized by TEM and UV-Vis analysis. The improved air stability of copper NPs originating from amidinate copper and stabilized by an alkylamine compound (hexadecylamine) is evidenced and compared with the effect of a long chain carboxylic acid (oleic acid).

Enhanced Ultraviolet Stability of Air-Processed Polymer Solar Cells by Al Doping of the ZnO Interlayer.

Photostability of organic photovoltaic devices represents a key requirement for the commercialization of this technology. In this field, ZnO is one of the most attractive materials employed as an electron transport layer, and the investigation of its photostability is of particular interest. Indeed, oxygen is known to chemisorb on ZnO and can be released upon UV illumination. Therefore, a deep analysis of the UV/oxygen effects on working devices is relevant for the industrial production where the coating processes take place in air and oxygen/ZnO contact cannot be avoided. Here we investigate the light-soaking stability of inverted organic solar cells in which four different solution-processed ZnO-based nanoparticles were used as electron transport layers: (i) pristine ZnO, (ii) 0.03 at %, (iii) 0.37 at %, and (iv) 0.8 at % aluminum-doped AZO nanoparticles. The degradation of solar cells under prolonged illumination (40 h under 1 sun), in which the ZnO/AZO layers were processed in air or inert atmosphere, is studied. We demonstrate that the presence of oxygen during the ZnO/AZO processing is crucial for the photostability of the resulting solar cell. While devices based on undoped ZnO were particularly affected by degradation, we found that using AZO nanoparticles the losses in performance, due to the presence of oxygen, were partially or totally prevented depending on the Al doping level.

Copper nanoparticles and organometallic chemical liquid deposition (OMCLD) for substrate metallization

We present a facile, room temperature and “fully liquid” method to specifically produce either copper nanoparticles or thin conductive copper films on silicon substrates by using a dedicated reduction process of mesitylcopper by H2 or an aminoborane.

Effects of ultraviolet soaking on surface electronic structures of solution processed ZnO nanoparticle films in polymer solar cells

We systematically show the effect of UV-light soaking on surface electronic structures and chemical states of solution processed ZnO nanoparticle (ZnONP) films in UHV, dry air and UV–ozone. UV exposure in UHV induces a slight decrease in work function and surface-desorption of chemisorbed oxygen, whereas UV exposure in the presence of oxygen causes an increase in work function due to oxygen atom vacancy filling in the ZnO matrix. We demonstrate that UV-light soaking in combination with vacuum or oxygen can tune the work function of the ZnONP films over a range exceeding 1 eV. Based on photovoltaic performance and diode measurements, we conclude that the oxygen atom vacancy filling occurs mainly at the surface of the ZnONP films and that the films consequently retain their n-type behavior despite a significant increase in the measured work function.

Growth of Pt-Pd Nanoparticles Studied In Situ by HRTEM in a Liquid Cell.

The growth of Pt-Pd nanoparticles from organometallic precursors is studied in situ in real time by HRTEM in a graphene oxide liquid cell. The reduction of the metal precursors is induced by the electron beam. During the growth, the particles rearrange their internal structure to form faceted single crystals. The growth is compatible with the Lifshitz-Slyozov-Wagner (LSW) mechanism in the limiting case of a reaction-limited process. The same particles are also synthesized ex situ by using a chemical reducing agent and observed in HRTEM.

Competing Forces in the Self-Assembly of Coupled ZnO Nanopyramids

Self-assembly (SA) of nanostructures has recently gained increasing interest. A clear understanding of the process is not straightforward since SA of nanoparticles is a complex multiscale phenomenon including different driving forces. Here, we study the SA between aluminum doped ZnO nanopyramids into couples by combining inorganic chemistry and advanced electron microscopy techniques with atomistic simulations. Our results show that the SA of the coupled nanopyramids is controlled first by morphology, as coupling only occurs in the case of pyramids with well-developed facets of the basal planes. The combination of electron microscopy and atomistic modeling reveals that the coupling is further driven by strong ligand-ligand interaction between the bases of the pyramids as dominant force, while screening effects due to Al doping or solvent as well as core-core interaction are only minor contributions. Our combined approach provides a deeper understanding of the complex interplay between the interactions at work in the coupled SA of ZnO nanopyramids.

Direct Hydrogen Evolution from Saline Water Reduction at Neutral pH using Organic Photocathodes

Here, we have developed an organic photocathode for water reduction to H2 , delivering more than 1 mA cm-2 at 0 V versus RHE and above 3 mA cm-2 at -0.5 V versus RHE with moderate stability under neutral pH conditions. The initial competitive reduction of water to H2 and ZnO to metallic Zn is responsible for the dynamic behaviour of both photocurrent and Faradaic efficiency of the device, which reaches 100 % Faradaic efficiency after 90 min operation. In any case, outstanding stable H2 flow of approximately 2 μmol h-1 is measured over 1 h at 0 V versus RHE and at neutral pH, after equilibrium between the Zn2+ /Zn0 concentration in the AZO film is reached. This achievement opens new avenues for the development of allsolution-processed organic photoelectrochemical cells for the solar generation of H2 from sea water.

Time evolution studies of dithieno[3,2-b:2′,3′-d]pyrrole-based A–D–A oligothiophene bulk heterojunctions during solvent vapor annealing towards optimization of photocurrent generation

Solvent vapor annealing (SVA) is one of the main techniques to improve the morphology of bulk heterojunction solar cells using oligomeric donors. In this report, we study time evolution of nanoscale morphological changes in bulk heterojunctions based on a well-studied dithienopyrrole-based A–D–A oligothiophene (dithieno[3,2-b:2 0 ,3 0-d]pyrrole named here 1) blended with [6,6]-phenyl-C 71-butyric acid methyl ester (PC 71 BM) to increase photocurrent density by combining scanning transmission electron microscopy and low-energy-loss spectroscopy. Our results show that SVA transforms the morphology of 1 : PC 71 BM blends by a three-stage mechanism: highly intermixed phases evolve into nanostructured bilayers that correspond to an optimal blend morphology. Additional SVA leads to completely phase-separated micrometer-sized domains. Optical spacers were used to increase light absorption inside optimized 1 : PC 71 BM blends leading to solar cells of 7.74% efficiency but a moderate photocurrent density of 12.3 mA cm A2. Quantum efficiency analyses reveal that photocurrent density is mainly limited by losses inside the donor phase. Indeed, optimized 1 : PC 71 BM blends consist of large donor-enriched domains not optimal for exciton to photocurrent conversion. Shorter SVA times lead to smaller domains; however they are embedded in large mixed phases suggesting that introduction of stronger molecular packing may help us to better balance phase separation and domain size enabling more efficient bulk heterojunction solar cells.

The influence of branched alkyl side chains in A–D–A oligothiophenes on the photovoltaic performance and morphology of solution-processed bulk-heterojunction solar cells

Besides providing sufficient solubility, branched alkyl chains also affect the film-forming and packing properties of organic semiconductors. In order to avoid steric hindrance as it is present in wide-spread alkyl chains comprising a branching point position at the C2-position, i.e., 2-ethylhexyl, the branching point can be moved away from the π-conjugated backbone. In this report, we study the influence of the modification of the branching point position from the C2-position in 2-hexyldecylamine (1) to the C4-position in 4-hexyldecylamine (2) connected to the central dithieno[3,2-b:2′,3′-d]pyrrole (DTP) moiety in a well-studied A–D–A oligothiophene on the optoelectronic properties and photovoltaic performance in solution-processed bulk heterojunction solar cells (BHJSCs) with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the acceptor material. Post-treatment of the photoactive layers is performed via solvent vapor annealing (SVA) in order to improve the film microstructure of the bulk heterojunction. The time evolution of nanoscale morphological changes is followed by combining scanning transmission electron microscopy with low-energy-loss spectroscopic imaging (STEM-SI), solid-state absorption spectroscopy, and two-dimensional grazing incidence X-ray diffraction (2D-GIXRD). Our results show an improvement of the photovoltaic performance that is dependent on the branching point position in the donor oligomer. Optical spacers are utilized to increase light absorption inside the co-oligomer 2-based BHJSCs leading to increased power conversion efficiencies (PCEs) of 8.2% when compared to the corresponding co-oligomer 1-based devices. A STEM-SI analysis of the respective device cross-sections of active layers containing 1 and 2 as donor materials indeed reveals significant differences in their respective active layer morphologies.