Christine Videlot-Ackermann

Pulsed-laser printing of organic thin film transistors

Organic thin-film transistors have been fabricated using laser-induced forward transfer as spatially resolved laser-printing method. Using this technique, source and drain electrodes were deposited from silver nanoparticle ink and the copper phthalocyanine (CuPc) was used to form the active layer. Both kinds of materials were transferred from a donor substrate onto a receiver substrate upon irradiation with laser pulses in the picosecond regime. The latter substrate formed the gate and the dielectric of the transistor. Electrical characterizations showed that the transistors are fully operative, showing well-defined linear and saturation regimes in the I-V curves.

A “Kite” Shaped Styryl End-Capped Benzo[2,1-b:3,4-b′]dithiophene with High Electrical Performances in Organic Thin Film Transistors

The bridging of distyryl-bithiophene leads to a kite shape of the conjugated system. This twist is not like twistacenes or twist deformation in alpha-oligothiophenes but a curvature such as in bowl shaped systems. Initially perceived to be undesirable, this new semiconductor in OTFT devices provides excellent performances in air, mu = 0.1 cm(2)/V.s, I(on)/I(off) > 10(6), S < 4 V/decade, higher by a factor of 5 to the parent unbridged coplanar analogue.

Synthesis and thin film electronic properties of two pyrene-substituted oligothiophene derivatives

The synthesis and the electric properties of two new pyrene end-substituted oligothiophene derivatives (Py-nT, n = 2,4) are described. The highest hole mobility (ca. 10−3 cm2 V−1 s−1) was obtained for vacuum evaporated thin films of Py-4T as the active layer in field effect transistors based on hexamethyldisilazane-treated SiO2/Si substrates. Moreover, Py-nT thin films were doped with molecular iodine, which led to a 106 fold increase of electric conductivity. In the case of Py-4T, a value of 1 S cm−1 was obtained after 21 minutes exposure to iodine vapor.

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.

Structure properties relationships of liquid crystal bent core organic semiconductors based on benzo[2,1-b:3,4-b′]dithiophene-4,5-dione

We report the synthesis of a new diketone bridged dithiophene end-capped with the mesogenic functionalities: 2,7′-bis(alkoxy-biphenyl) and 2,7′-bis(alkoxy-phenyl)-benzo[2,1-b:3,4-b′]dithiophene-4,5-dione. Optical and electrochemical properties in solution were investigated by UV-visible absorption and cyclic voltammetry. Liquid crystal properties of these new materials were investigated by differential scanning calorimetry, optical polarizing microscopy, and X-ray diffraction studies. Both compounds exhibit layered phases though there are differences in the organisation of the phase structures. Thin films were implemented as active layers into organic thin-film transistors to evaluate the charge transport properties.

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.

Crystal structure of oligothiophene thin films characterized by two-dimensional grazing incidence X-ray diffraction

Crystal structure in an highly oriented organic thin film was determined using two dimensional grazing incidence X-ray diffraction (2D-GIXD). α,ω-Hexyl-distyryl-bithiophene (DH-DS2T) was chosen as the material for the structural analysis, because it is a typical organic semiconductor showing a high crystallinity and an ordered layer-growth behavior. A 2D-GIXD pattern over large range of scattering angles was obtained by using high-brightness synchrotron radiation in SPring-8 and high-sensitive 2D X-ray detector (PILATUS 300 K). The analysis of the observed 2D-GIXD pattern was made to clarify that the crystal structure of highly oriented DH-DS2T thin films belongs to a monoclinic unit-cell with a = 0.58 nm, b = 0.78 nm, c = 3.43 nm, and β = 94.3°, and the space group is determined to be P21/a by considering the extinction rule. Furthermore, by fitting the simulated Bragg peak intensities to the experimental data, the molecular structure is determined. In this structure, two molecules are included in the unit cell and they tilt about 25° against substrate normal.

Liquid Crystal α,ω-Hexyl-Distyryl-Bithiophene: Morphology and Charge Tranport Properties in Organic Thin Film Transistors

The solution and solid-state properties as well as the organic thin film transistor (OTFT) behavior of α,ω-hexyl-distyryl-bithiophene (DH-DS2T) are presented. Liquid Crystal properties were studied by differential scanning calorimetry (DSC) and optical polarising microscopy (OPM). High vacuum evaporated thin films were studied by optical absorption, scanning electron microscopy (SEM) and implemented as p-type semiconducting layers into organic thin film transistors (OTFTs).