Mohammad Javad Jafari

Imaging the Phase Separation Between PEDOT and Polyelectrolytes During Processing of Highly Conductive PEDOT:PSS Films.

UNLABELLED Treating PEDOT PSS (Clevios) with certain additives, such as ethylene glycol (EG), dimethyl sulfoxide (DMSO) and sorbitol, has been shown to increase the conductivity of this material from roughly 1 to nearly 1000 S/cm. Using a slow drying method, we show that the additive induced a separation between free PSS and reorganized PEDOT PSS complexes in the highly conductive PEDOT PSS films. Additives (DMSO, DEG, and PEG 400) were included in PEDOT PSS aqueous dispersions at large volume fractions. The mixtures were slowly dried under room conditions. During drying, the evaporation of water resulted in an additive-rich solvent mixture from which the reorganized PEDOT PSS complexes aggregated into a dense film while free PSS remained in the solution. Upon complete drying, PSS formed a transparent rim film around the conducting PEDOT film. The chemical compositions of the two phases were studied using an infrared microscope. This removal of PSS resulted in more compact packing of PEDOT molecules, as confirmed by X-ray diffraction measurements. X-ray photoelectron spectroscopy and atomic force microscope measurements suggested the enrichment of PEDOT on the film surface after PSS separation. Through a simple drying process in an additive-containing dispersion, the conductivity of PEDOT films increased from 0.1 to 200-400 S/cm. Through this method, we confirmed the existence of two phases in additive-treated and highly conductive PEDOT PSS films. The proper separation between PSS and PEDOT will be of relevance in designing strategies to process high-performance plastic electrodes.

Acido-basic control of the thermoelectric properties of poly(3,4-ethylenedioxythiophene)tosylate (PEDOT-Tos) thin films

PEDOT-Tos is one of the conducting polymers that displays the most promising thermoelectric properties.

Spectroelectrochemical investigation of redox states in a polypyrrole/lignin composite electrode material

We report spectroelectrochemical studies to investigate the charge storage mechanism of composite polypyrrole/lignin electrodes. Renewable bioorganic electrode materials were produced by electropolymerization of pyrrole in the presence of a water-soluble lignin derivative acting as a dopant. The resulting composite exhibited enhanced charge storage abilities due to a lignin-based faradaic process, which was expressed after repeated electrochemical redox of the material. The in situ FTIR spectroelectrochemistry results show the formation of quinone groups, and reversible oxidation–reduction of these groups during charge–discharge experiments in the electrode materials. The most significant IR bands include carbonyl absorption near 1705 cm−1, which is attributed to the creation of quinone moieties during oxidation, and absorption at 1045 cm−1 which is due to hydroquinone moieties.

Time-Resolved Chemical Mapping in Light-Emitting Electrochemical Cells

An understanding of the doping and ion distributions in light-emitting electrochemical cells (LECs) is required to approach a realistic conduction model which can precisely explain the electrochemical reactions, p-n junction formation, and ion dynamics in the active layer and to provide relevant information about LECs for systematic improvement of function and manufacture. Here, Fourier-transform infrared (FTIR) microscopy is used to monitor anion density profile and polymer structure in situ and for time-resolved mapping of electrochemical doping in an LEC under bias. The results are in very good agreement with the electrochemical doping model with respect to ion redistribution and formation of a dynamic p-n junction in the active layer. We also physically slow ions by decreasing the working temperature and study frozen-junction formation and immobilization of ions in a fixed-junction LEC device by FTIR imaging. The obtained results show irreversibility of the ion redistribution and polymer doping in a fixed-junction device. In addition, we demonstrate that infrared microscopy is a useful tool for in situ characterization of electroactive organic materials.

Ground-state charge transfer for NIR absorption with donor/acceptor molecules: interactions mediated via energetics and orbital symmetries

The interactions between electron donors (D) and acceptors (A) of organic semiconducting molecules are of great interest to organic electronics, e.g. electrical doping of organic semiconductors (OSCs), photo-generation of charges in organic solar cells, and light-emitting/detecting devices based on OSCs. A blend of D/A OSC is typically characterized by weak van der Waals interactions or integer charge transfer (ICT) between neighboring D/A molecules. In between these two scenarios of physical blends and ICT complexes, orbital hybridization between adjacent D/A molecules serves as a third alternative, characterized by an in situ formation of a ground state complex featuring partial charge transfer between participating donor and acceptor molecules. In this work is presented a comprehensive experimental study on partial charge-transfer complex (CPX) formed via orbital hybridization. Thiophenes and phthalocyanines are used as electron donors, while acceptor molecules of different geometries and electron affinities are employed with the aim to clarify how orbital symmetry, energy level alignment and steric hindrance affect orbital hybridization and subsequent tuning of the optical band-gap into the near infrared (NIR) region.

Doping Polypyrrole Films with 4-N-Pentylphenylboronic Acid to Enhance Affinity towards Bacteria and Dopamine.

Here we demonstrate the use of a functional dopant as a fast and simple way to tune the chemical affinity and selectivity of polypyrrole films. More specifically, a boronic-functionalised dopant, 4-N-Pentylphenylboronic Acid (PBA), was used to provide to polypyrrole films with enhanced affinity towards diols. In order to prove the proposed concept, two model systems were explored: (i) the capture and the electrochemical detection of dopamine and (ii) the adhesion of bacteria onto surfaces. The chemisensor, based on overoxidised polypyrrole boronic doped film, was shown to have the ability to capture and retain dopamine, thus improving its detection; furthermore the chemisensor showed better sensitivity in comparison with overoxidised perchlorate doped films. The adhesion of bacteria, Deinococcus proteolyticus, Escherichia coli, Streptococcus pneumoniae and Klebsiella pneumoniae, onto the boric doped polypyrrole film was also tested. The presence of the boronic group in the polypyrrole film was shown to favour the adhesion of sugar-rich bacterial cells when compared with a control film (Dodecyl benzenesulfonate (DBS) doped film) with similar morphological and physical properties. The presented single step synthesis approach is simple and fast, does not require the development and synthesis of functional monomers, and can be easily expanded to the electrochemical, and possibly chemical, fabrication of novel functional surfaces and interfaces with inherent pre-defined sensing and chemical properties.

The contraction of PEDOT films formed on a macromolecular liquid-like surface

Vapour phase polymerized (VPP) PEDOT obtained using triblock copolymer PEG–PPG–PEG:Fe(III) tosylate polymeric oxidative layers has shown record-high conductivity and unique thermoelectric properties. These properties are related to the molecular weight, morphology and doping of PEDOT. Here we show that in its unwashed condition, the PEDOT chain adopts a neutral benzenoid conformation. The polymer chain converts into the charged quinoid structure after the removal of oxidizers with solvent washing. X-ray diffraction results suggest that the dopant is also incorporated into the packed polymer after the washing process. The changes in the chain structure and doping lead to the characteristic polaron and bipolaron absorption in the 800 and 1200 nm range. We observed a large contraction of the film after washing that is likely due to these changes, along with the removal of excessive polymer:oxidizer trapped in the PEDOT matrix. The contraction of films can be completely suppressed by mechanical clamping. PEDOT films without contraction show both a higher conductivity and higher optical transparency.

Light-induced degradation of fullerenes in organic solar cells: a case study on TQ1:PC71BM

The stability of organic solar cells (OSCs) is critical for practical applications of this emerging technology. Unfortunately, in spite of intensive investigations, the degradation mechanisms in OSCs have not been clearly understood yet. In this report, we employ a range of spectroscopic and transport measurements, coupled with drift-diffusion modelling, to investigate the light-induced degradation mechanisms of fullerene-based OSCs. We find that trap states formed in the fullerene phase under illumination play a critical role in the degradation of the open-circuit voltage (VOC) in OSCs. Our results indicate that the degradation is intrinsic to the fullerenes in OSCs and that alternative acceptor materials are desired for the development of stable OSCs.