Olivier Douhéret

Nanoscale investigation of the electrical properties in semiconductor polymer–carbon nanotube hybrid materials

The morphology and electrical properties of hybrids of a semiconducting polymer (namely poly(3-hexylthiophene) P3HT) and carbon nanotubes are investigated at the nanoscale with a combination of Scanning Probe Microscopy techniques, i.e., Conductive Atomic Force Microscopy (C-AFM) and time-resolved Current Sensing Force Spectroscopy Atomic Force Microscopy (CSFS-AFM, or PeakForce TUNA™). This allows us to probe the electrical properties of the 15 nm wide P3HT nanofibers as well as the interface between the polymer and single carbon nanotubes. This is achieved by applying controlled, low forces on the tip during imaging, which allows a direct comparison between the morphology and the electrical properties at the nanometre scale.

Synthesis and Characterization of Nanocomposites Based on Functional Regioregular Poly(3-hexylthiophene) and Multiwall Carbon Nanotubes.

New functionalized poly(3-hexylthiophene)s (P3HT) have been designed and synthesized with the aim of increasing the dispersion of carbon nanotubes (CNT) in solutions and in thin films of semiconducting polymers. Dispersion in solution has been assessed by sedimentation tests while the thin film morphology has been analyzed by TEM and AFM. Both the physisorption of P3HT chains (via pyrene end-groups) or their chemical grafting (onto amine functions generated on the CNT surface) lead to a much better dispersion in solution and in the solid. In thin films, P3HT fibrils are observed to arrange perpendicular to the CNT surface, which can be understood on the basis of molecular modeling simulations. Finally, the effect of dispersing those P3HT/CNT nanocomposites in bulk-heterojunction P3HT-based photovoltaic devices has been evaluated.

Efficient bulk heterojunction photovoltaic cells with a pre-organized poly(3- hexylthiophene) phase

This paper presents the controlled generation of nanostructured poly(3-hexylthiophene) (P3HT) from solution to fully fibrillate solid-state films. The fibrillate morphology of P3HT is further maintained when the deposition is carried out from a mixed solution with the acceptor material ([6,6]-phenyl-C61-butyric acid methyl ester, PCBM). This enables the fabrication of active layers for bulk heterojunction organic photovoltaic devices with a controlled morphology. The contribution of these nanostructures to the photovoltaic performances is evidenced. High-resolution electrical characterization with conductive atomic force microscopy (C-AFM) confirms an improved charge transport throughout the fibrillate P3HT matrix. A local hole mobility value of 0.9 × 10−4 cm2/V s is derived from the C-AFM measurements.

Solution processed liquid metal-conducting polymer hybrid thin films as electrochemical pH-threshold indicators

A global and accurate mapping of the environment could be achieved if sensors and indicators are mass-produced at low cost. Printed electronics using polymeric (semi)conductors offer a platform for such sensor/indicator based circuits. Herein, we present the material concept for an electrochemical pH-threshold indicator based on a printable hybrid electrode which comprises a liquid metal alloy (GaInSn) embedded in a conducting polymer matrix (PEDOT). This hybrid electrode displays a large variation in open circuit potential versus pH in an electrochemical cell, which when connected to the gate of an electrochemical transistor leads to a dramatic change in the drain current in a narrow range of pH.

Bilayer solvent and vapor-triggered actuators made of cross-linked polymer architectures via Diels-Alder pathways

A simple and straightforward approach to produce solvent and vapor-based actuating materials is developed in this work. These actuators are based on rigidity gradients created in bilayer architectures made of reversibly cross-linked poly(e-caprolactone) (PCL) networks into which functional nanofillers, i.e. multi-walled carbon nanotubes (MWCNTs), are incorporated using simple processing techniques. A key element of the bilayer functionality lies in, by taking advantage of thermo-reversible Diels–Alder reactions (between furfuryl and maleimide moieties), ensuring good adhesion between the layers. Thereby, the produced material instantaneously swells in an anisotropic way due to the rigidity gradient, resulting in reproducible bending actuations. Besides, it is shown that the percolating network of electrically conductive MWCNTs offers the possibility of implementing these bilayers as solvent detector sensors. This opens the door to the development of multi-responsive devices with tunable actuating behaviors and potential application in the robotics field as self-deployable structures in different environments (water, organic solvents, etc.).

A scanning probe microscopy study of nanostructured TiO2/poly(3-hexylthiophene) hybrid heterojunctions for photovoltaic applications

The nanoscale morphology of photoactive hybrid heterojunctions plays a key role in the performances of hybrid solar cells. In this work, the heterojunctions consist of a nanocolumnar TiO2 surface covalently grafted with a monolayer of poly(3-hexylthiophene) (P3HT) functionalized with carboxylic groups (–COOH). Through a joint analysis of the photovoltaic properties at the nanoscale by photoconductive-AFM (PC-AFM) and surface photovoltage imaging, we investigated the physical mechanisms taking place locally during the photovoltaic process and the correlation to the nanoscale morphology. A down-shift of the vacuum level of the TiO2 surface upon grafting was measured by Kelvin probe force microscopy (KPFM), evidencing the formation of a dipole at the TiO2/P3HT-COOH interface. Upon in situ illumination, a positive photovoltage was observed as a result of the accumulation of photogenerated holes in the P3HT layer. A positive photocurrent was recorded in PC-AFM measurements, whose spatial mapping was interpreted consistently with the corresponding KPFM analysis, offering a correlated analysis of interest from both a theoretical and material design perspective.