An alternating low band-gap polyfluorene for optoelectronic devices

An alternating polyfluorene (APFO) with low band-gap segments named APFO-Green1 has been designed and synthesized for use in optoelectronic devices. The low band-gap segment consists of an electron acceptor (A), fenced by electron donors (D). This D-A-D configuration leads to a partial charge transfer in the polymer backbone, and thereby a low band-gap (1.3 eV). Results obtained from characterization of APFO-Green1 include light absorption and emission at extended wavelengths as well as high hole mobility. Furthermore, blends of the polymer with different fullerene derivatives exhibit unusually high photovoltaic performance at long wavelengths, making this type of conjugated polymers promising for plastic solar cell applications.     

Enhanced Photocurrent Spectral Response in Low‐Bandgap Polyfluorene and C70‐Derivative‐Based Solar Cells

Plastic solar cells have been fabricated using a low‐bandgap alternating copolymer of fluorene and a donor–acceptor–donor moiety (APFO‐Green1), blended with 3′‐(3,5‐bis‐trifluoromethylphenyl)‐1′‐(4‐nitrophenyl)pyrazolino[70]fullerene (BTPF70) as electron acceptor. The polymer shows optical absorption in two wavelength ranges, λ < 500 nm and 600 < λ < 1000 nm. The BTPF70 absorbs light at λ < 700 nm. A broad photocurrent spectral response in the wavelength range 300 < λ < 1000 nm is obtained in solar cells. A photocurrent density of 3.4 mA cm^–2, open‐circuit voltage of 0.58 V, and power‐conversion efficiency of 0.7 % are achieved under illumination of AM1.5 (1000 W m^–2) from a solar simulator. Synthesis of BTPF70 is presented. Photoluminescence quenching and electrochemical studies are used to discuss photoinduced charge transfer.     

Emissions of organic compounds from the combustion of oats – a comparison with softwood pellets

Oats are a new biofuel possible to use in modified residential wood pellet combustion appliances. The emissions of organic compounds from five sequential combustion stages; initial smouldering, early flaming, late flaming, after-flame smouldering and final glowing, for incomplete burning of oats on a laboratory scale were determined by gas chromatography and compared to those of softwood pellets.High concentrations of 1,6-anhydroglucose and furan-related compounds were released from the initial smouldering of oats, while high concentrations of methoxyphenols were released during the initial smouldering of wood pellets. The results indicate that oats are a biofuel with relatively low emissions during combustion, almost as low as those from wood pellets. After-flame smouldering of oats released lower concentrations of methane, alkenes and aromatic hydrocarbons than the after-flame smouldering of wood pellets. The large differences in emissions from the various combustion stages should be considered when evaluating the environmental aspects and health effects of residential burning of oats and wood pellets.     

Electrospinning of cellulose nanofibers from ionic liquids: The effect of different cosolvents

Cellulose was electrospun with various concentrations of ionic liquid and cosolvent. Three different cosolvents were used in this study; dimethylacetamide (DMAc), dimethyl formamide (DMF), and dimethyl sulfoxide (DMSO). The cosolvents were added to modify the viscosity, electrical conductivity, and surface tension of the solutions. The solubility of cellulose in ionic liquids is highly affected by changes in solvent properties on the molecular level in the binary solvent systems. The difference in molecular structure of the cosolvents and the interactions between cosolvent and ionic liquid can explain the difference in dissolution power of the cosolvents. Scanning electron microscope (SEM) was used to characterize electrospun cellulose fibers. For the systems tested the importance of having a rather high viscosity and high surface tension, and some degree of shear thinning to produce fibers is shown. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Influence of molecular weight and rheological behavior on electrospinning cellulose nanofibers from ionic liquids

Dissolving pulp was depolymerized with 2.5M HCl into cellulose fractions with decreasing molecular weight relative to acid treatment time. The cellulose fractions were dissolved at various concentrations in the ionic liquid 1‐ethyl‐3‐methylimidazolium acetate (EmimAc) with co‐solvent DMSO at ratio 1 : 1 (w/w) and electrospun. Size exclusion chromatography was used to evaluate the molecular weight distributions and the rheological properties were characterized with a cone‐and‐plate rheometer. Scanning electron microscope was used to evaluate the fiber morphology, and thereby spinnability. Zero shear viscosity as a function of cellulose concentration show that all the solutions in this study are in the entangled semi‐dilute regime; where the polymer concentration is large enough for significant overlap necessary for chain entanglement. However, within the intervals studied, neither cellulose concentration nor molecular weight seems to be decisive for if a solution can be electrospun into fibers or not. It is rather the viscosity of the solution that is decisive for electrospinnability, even though the solution is in the entangled semi‐dilute regime. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2303–2310, 2013     

Polymer Solar Cells Based on a Low‐Bandgap Fluorene Copolymer and a Fullerene Derivative with Photocurrent Extended to 850 nm

Polymer solar cells have been fabricated from a recently synthesized low band‐gap alternating polyfluorene copolymer, APFO‐Green2, combined with [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) from organic solutions. External quantum efficiencies (EQEs) of the solar cells show an onset at 850 nm and a peak of > 10 % located at 650 nm, which corresponds to the extended absorption spectrum of the polymer. Photocurrent of 3.0 mA cm^–2, photovoltage of 0.78 V, and power conversion efficiency of 0.9 % have been achieved in solar cells based on this new low‐bandgap polymer under the illumination of air mass 1.5 (AM 1.5) (1000 W m^–2) from a solar simulator.