Inducing molecular orientation in solution-processed thin films of fluorene-bithiophene-based copolymer: thermal annealing vs. solvent additive.

A deep understanding of the factors influencing the morphology of thin films based on conjugated polymers is essential to boost their performance in optoelectronic devices. Herein, we investigated the electronic structure and morphology of thin films of the copolymer poly(9,9-dioctyl-fluorenyl-co-bithiophene) (F8T2) in its pristine form as well as samples processed with the solvent additive 1,8-diiodooctane (DIO) or post-processed through thermal annealing treatment. Measurements were carried out using angle-resolved S K-edge NEXAFS (near-edge X-ray absorption fine structure) in total electron yield (TEY) and fluorescence yield (FY) detection modes. Two main transitions were observed at the S 1s NEXAFS spectra: S 1s → π* and S 1s → σ* (S-C). The observed dichroism pointed to a face-on orientation of the conjugated backbone, which was significantly increased for F8T2 films processed with DIO. Resonant Auger decay spectra were obtained and analyzed using the core-hole clock (CHC) method. An enhancement in the charge transfer process was observed for thermally annealed films, especially for samples processed with DIO, corresponding to an increase in film ordering. Furthermore, the investigated films were characterized using X-ray photoelectron spectroscopy, attesting to the presence of the thiophene unit in the samples and demonstrating that some of its sulfur atoms were positively polarized in the F8T2 films. All these experimental findings were compared with molecular dynamics (MD) simulations of film evaporation with and without DIO. The use of MD, together with mathematical modeling, was able to explain the major effects found in the experiments, including the polarization of sulfur atoms. The simultaneous use of powerful spectroscopic techniques and theoretical methods shed light on key aspects linking film morphology with fabrication procedures.

Molecular orientation and femtosecond charge transfer dynamics in transparent and conductive electrodes based on graphene oxide and PEDOT:PSS composites.

The conducting polymer, poly(3,4-ethylenedioxythiophene)-polystyrenesulfonic acid (PEDOT:PSS), is certainly one of the most important substitute materials for indium tin oxide in organic devices. Its metallic conductivity and transmittance bring favorable perspectives for organic photovoltaic applications. Although graphene oxide (GO) is not a good conductor, it can form high-quality thin films and can be transparent, and additionally, GO is an inexpensive material and can be easily synthesized. This study investigated how the conductivity of a composite film of graphene oxide (GO) and different amounts of PEDOT:PSS can be modified. The effects of GO:PEDOT:PSS composites with several PEDOT:PSS proportions were analyzed in regards to the composite molecular structure and ordering, charge transfer dynamics (in the femtosecond range), electrical properties and morphology. For the best conductivity ratio GO found with 5% PEDOT:PSS, a solvent treatment was also performed, comparing the resistivity of the film when treated with dimethyl sulfoxide (DMSO) and with ethylene glycol.

Photoanode for Aqueous Dye-Sensitized Solar Cells based on a Novel Multicomponent Thin Film.

Most of the dye-sensitized solar cells (DSSCs) developed so far use organic electrolytes and water-sensible sensitizers. The search for aqueous DSSCs, a promising technology for solar-energy conversion, implies finding materials that are stable in aqueous solution. In this study, Prussian blue (PB) was utilized as an innovative sensitizer in a photoanode for DSSCs and a novel synthetic approach to a carbon nanotubes/TiO2 /PB nanocomposite thin film was developed. The photoresponse was evaluated in a total aqueous electrolyte, and photocurrents of 600 µA cm-2 were achieved.

Ultrafast interface charge transfer dynamics on P3HT/MWCNT nanocomposites probed by resonant Auger spectroscopy.

The interfacial electronic structure and charge transfer dynamics of poly-3-hexylthiophene (P3HT) and multi-walled carbon nanotube (Fe-MWCNT) nanocomposites were investigated by near-edge X-ray absorption fine structure (NEXAFS) and resonant Auger (RAS) spectroscopies around the sulfur K-edge. Nanocomposites with 5 wt% (P3HT/Fe-MWCNT-5%) and 10 wt% (P3HT/Fe-MWCNT-10%) of Fe-MWCNT species were prepared and compared with pristine P3HT film. The quantitative NEXAFS analysis shows a strong π-π interchain interaction of the pristine P3HT polymer film, which is reduced by the presence of the Fe-MWCNT. S-KL2,3L2,3 RAS spectra were measured at photon energies corresponding to the main electronic transitions appearing in the S-K edge NEXAFS spectrum. Ultrafast charge transfer times were estimated from the RAS spectra using the core-hole clock approach with the S 1s core-hole lifetime as an internal clock. The π-π interchain charge transfer time increases from 4.7 fs on pristine P3HT polymer to 6.5 fs on the P3HT/Fe-MWCNT-5% nanocomposite. The electronic coupling between P3HT and Fe-MWCNT species occurs mainly through the P3HT π* molecular orbital. The increase of Fe-MWCNT concentration from 5 to 10 wt% reduces the charge transfer rate at the resonance maximum due probably to Fe-MWCNT aggregation, reducing the P3HT and Fe-MWCNT electronic coupling.

Thermally induced anchoring of fullerene in copolymers with Si-bridging atom: Spectroscopic evidences.

We use X-ray photoelectron spectroscopy (XPS), Near-edge X-ray absorption fine structure (NEXAFS), resonant Auger spectroscopy (RAS), Attenuation Total Reflection Infrared (ATR-IR) and Atomic Force Microscopy (AFM) to study the blend between the copolymer poly[2,7-(9,9-bis(2-ethylhexyl)-dibenzosilole)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole] (PSiF-DBT) and the fullerene derivative PC71BM submitted to different annealing temperatures. Those measurements indicate that there is an incidental anchoring of a fullerene derivative to the Si-bridging atoms of a copolymer induced by thermal annealing of the film. Insights about the physical properties of one possible PSiF-DBT/PC71BM anchored structure are obtained using Density Functional Theory calculations. Since the performance of organic photovoltaic based on polymer-fullerene blends depends on the chemical structure of the blend components, the anchoring effect might affect the photovoltaic properties of those devices.

The total chemical synthesis of polymer/graphene nanocomposite films.

A versatile and room temperature synthesis of thin films of polymer/graphene is reported. Drastically differing from other methods, not only the polymer but also the graphene are completely built from their simplest monomers (thiophene and benzene) in a one-pot polymerization reaction at a liquid-liquid interface. The materials were characterized and electronic properties are presented.

The effect of thermal annealing on the charge transfer dynamics of a donor-acceptor copolymer and fullerene: F8T2 and F8T2:PCBM.

Ultrafast charge delocalization dynamics in an internal donor-acceptor copolymer poly(9,9-dioctylfluorenyl-co-bithiophene) (F8T2) and its blend with the fullerene derivative [6,6]-phenyl C61 butyric acid methyl ester (PCBM) was studied by resonant Auger spectroscopy measured around sulfur K-edge using the core-hole clock approach. The effect of thermal annealing on the charge transfer delocalization times (τCT) was also investigated. Two main transitions with S 1s → π* and S 1s → σ*(S-C) character were measured at the S 1s NEXAFS spectra. Poor charge delocalization was observed for as cast polymeric films at photon energies corresponding to the S 1s → π* transition, which may suggest a weak π-electronic coupling due to weak polymer crystallinity and chain stacking. Enhancement in the charge transfer process for photon energies close to the resonance maximum was observed for thermally annealed F8T2 and its blends. Atomic Force Microscopy (AFM) topography for as cast F8T2:PCBM shows a top position of PCBM units relative to the polymer, homogeneously distributed on the film surface. This configuration improves the charge delocalization through S 1s → π* molecular orbitals for the as cast blended film, suggesting a strong π-electronic coupling. A new rearrangement of F8T2:PCBM film was found after thermal annealing, leading to a more efficient electron transfer channel through σ* molecular orbitals.

Resistive switching in iron-oxide-filled carbon nanotubes.

Iron-oxide-filled carbon nanotubes exhibit an intriguing charge bipolarization behavior which allows the material to be applied in resistive memory devices. Raman analysis conducted with an electric field applied in situ shows the Kohn anomalies and a strong modification of the electronic properties related to the applied voltage intensity. In addition, the I(D)/I(G) ratio indicated the reversibility of this process. The electrical characterization indicated an electronic transport governed by two main kinds of charge hopping, one between the filling and the nanotube and the other between the nanotube shells.

Interactions of iron-oxide filled carbon nanotubes with gas molecules.

This work presents a study on iron-oxide filled carbon nanotubes (CNTs) and their interaction with the surrounding atmosphere. Theoretical and experimental methods were employed to determine the interaction mechanism between the CNTs and some gases, such as O2 and N2. The electrical behavior of these CNTs under different atmospheric conditions was studied through resistance measurements, and for comparison, similar studies were conducted on non-filled carbon nanotubes. The iron-oxide filled CNTs were found to be more sensitive to the presence of O2 than the non-filled carbon nanotubes. This behavior was confirmed by the first-principles simulations based on density functional theory with local spin density approximations for CNTs filled with hematite and magnetite iron-oxides. The theoretical study on the interactions of iron-oxide filled CNTs with gas molecules demonstrated a physisorption regime between the nanotube and the surrounding atmosphere, resulting in modifications of the electronic properties of this material.

Development of a chemiresistor sensor based on polymers-dye blend for detection of ethanol vapor.

The conductive blend of the poly (3,4-ethylene dioxythiophene) and polystyrene sulfonated acid (PEDOT-PSS) polymers were doped with Methyl Red (MR) dye in the acid form and were used as the basis for a chemiresistor sensor for detection of ethanol vapor. This Au | Polymers-dye blend | Au device was manufactured by chemical vapor deposition and spin-coating, the first for deposition of the metal electrodes onto a glass substrate, and the second for preparation of the organic thin film forming ∼1.0 mm2 of active area. The results obtained are the following: (i) electrical resistance dependence with atmospheres containing ethanol vapor carried by nitrogen gas and humidity; (ii) sensitivity at 1.15 for limit detection of 26.25 ppm analyte and an operating temperature of 25 °C; and (iii) the sensing process is quickly reversible and shows very a low power consumption of 20 µW. The thin film morphology of ∼200 nm thickness was analyzed by Atomic Force Microscopy (AFM), where it was observed to have a peculiarly granulometric surface favorable to adsorption. This work indicates that PEDOT-PSS doped with MR dye to compose blend film shows good performance like resistive sensor.

A simple two-phase route to silver nanoparticles/polyaniline structures.

Novel silver nanoparticles/polyaniline composites were obtained through a two-phase water/toluene interfacial reaction. We show that by rigorously controlling the reaction time, different structures of the nanocomposites can be obtained, such as a thin sheet of polyaniline around the silver nanoparticles or a polymer mass with nanoparticles homogeneously embedded within it. Samples were characterized by FT-IR, UV-vis-NIR and Raman spectroscopy, X-ray diffraction, cyclic voltammetry, TEM, and HRTEM. Conductivity and current-voltage characteristics of the nanocomposites were measured, and the results indicate that different properties result from the different structures in which the nanocomposites were formed.