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Recently developed organic photovoltaics (OPVs) show distinct advantages over their inorganic counterparts due to their lighter weight, flexible shape, versatile materials synthesis and device fabr ...

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.

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.

A conjugated polymer was prepd. based on 2-thia-1,3,5,8-tetraaza-cyclopenta[b]naphthalene central electron accepting unit and electron donating thiophene units on each side, LBPP-1, with unusually low band-gap (ca. 1.0 eV). Light absorption and photovoltaic response up to 1200 nm in composites with fullerene were demonstrated. The performance of solar cells contg. LBPP-1 is described and the suitability of LBPP-1 for photodetection in the IR region is discussed.

Charge transport in a near infrared absorbing polyfluorene copolymer (APFO-Green1) and its blends with methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM), and 3′-(3,5-bis-trifluorom ...

Low-bandgap alternating polyfluorenes with different donor-acceptor-donor moieties were synthesized. Electrochem. and optical absorption measurement show that onset bandgaps of these polymers range from 1.2 to 1.5 eV. These polymers, blended with a C70-deriv. as acceptor, are used for solar cell fabrication. Devices show promising photovoltaic properties, and the spectral response of photocurrent covers all visible and near-IR wavelength regions with its onset extended to 1 mm. The best data gives a photocurrent d. of 3.4 mA/cm2, open circuit voltage of 0.58 V and power conversion efficiency of 0.7% under illumination of AM1.5 (1000 W/m2) from a solar simulator. [on SciFinder (R)]

An alternating polyfluorene (APFO) with low band-gap segments. APFO-Green1, was 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 partial charge transfer in the polymer backbone and a low band-gap of 1.3 eV. Characterization of APFO-Green1 include measurement of light absorption and emission at extended wavelengths and high hole mobility was found. Blends of the polymer with different fullerene derivs. exhibit unusually high photovoltaic performance at long wavelengths, making this type of conjugated polymers promising for application in plastic solar cells. [on SciFinder (R)]

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 fullerene (BTPF70) as electron acceptor. The polymer shows optical absorption in two wavelength ranges, l < 500 nm and 600 < l < 1000 nm. The BTPF70 absorbs light at l < 700 nm. A broad photocurrent spectral response in the wavelength range 300 < l < 1000 nm is obtained in solar cells. A photocurrent d. 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 electrochem. studies are used to discuss photoinduced charge transfer. [on SciFinder (R)]

Two new alternating polyfluorene copolymers containing different low band gap segments have been designed and synthesized for use in optoelectronic devices. The low band gap segments consist of electron donating and accepting moieties which induce a partial charge transfer in the polymer backbone and thereby, a lower band gap. Results obtained from characterization of the new materials, called APFO-Green1 and APFO-Green2, include absorption and emission at extended wavelengths as well as high stability. Furthermore, blends of the new copolymers with different fullerene derivatives exhibit unprecedented photovoltaic behavior at long wavelengths.

Polymer solar cells were fabricated from a low band-gap alternating polyfluorene copolymer, APFO-Green2, combined with [6,6]-phenyl-C61-butyric acid Me ester (PCBM), from org. solns. External quantum efficiencies 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. A photocurrent of 3.0 mA/cm2, photovoltage of 0.78 V, and power conversion efficiency of 0.9% were obtained with solar cells based on this new low-bandgap polymer under an illumination of AM 1.5 (1000 W/m2) from a solar simulator. [on SciFinder (R)]

We report on transistors and light-emitting diodes using a conjugated polymer consisting of alternated segments of fluorene units and low-band gap donor–acceptor–donor (D–A–D) units. The D–A–D segment includes two electron-donating thiophene rings combined with a thiadiazolo-quinoxaline unit, which is electron withdrawing to its nature. The resulting polymer is conjugated and has a band gap of around 1.27 eV. Here we present the corresponding electro- and photoluminescence spectra, which both peak at approximately 1 μm. Single layer light-emitting diodes demonstrated external quantum efficiencies from 0.03% to 0.05%. The polymer was employed as active material in thin film transistors, a field-effect mobility of 3 × 10−3 cm2/V s and current on/off ratio of 104 were achieved at ambient atmosphere.

Two new crescent-shaped unsymmetrical cyanine dyes have been synthesised and their interactions with DNA have been investigated by different spectroscopic methods. These dyes are analogues to a minor groove binding unsymmetrical cyanine dye, BEBO, recently reported by us. In this dye, the structure of the known intercalating cyanine dye BO was extended with a benzothiazole substituent. To investigate how the identity of the extending heterocycle affects the binding to DNA, the new dyes BETO and BOXTO have a benzothiazole group and a benzoxazole moiety, respectively. Whereas BEBO showed a heterogeneous binding to calf thymus DNA, linear and circular dichroism studies of BOXTO indicate a high preference for minor groove binding. BETO also binds in the minor groove to mixed sequence DNA but has a contribution of non-ordered and non-emissive species present. A non-intercalative binding mode of the new dyes, as well as for BEBO, is further supported by electrophoresis unwinding assays. These dyes, having comparable spectral properties as the intercalating cyanine dyes, but bind in the minor groove instead, might be useful complements for staining of DNA. In particular, the benzoxazole substituted dye BOXTO has attractive fluorescence properties with a quantum yield of 0.52 when bound to mixed sequence DNA and a 300-fold increase in fluorescence intensity upon binding.

The amplified spontaneous emission (ASE) of optically pumped films of poly(2-(2′,5′-bis(octyloxy)benzene)-1,4-phenylenevinylene (BOP-PPV) was studied in structures comprising a gate electrode, a thin film of gate insulator material (SiO2) and the polymer film as luminescent semiconducting layer (i.e. a field effect transistor without the source and drain electrodes). The influences of different gate electrodes on the threshold and the wavelength of the amplified emission were measured for variable thickness of the gate insulator. An exponential increase in ASE threshold (It) with decreasing separation between electrode and polymer layer was observed. In structures with 200 nm SiO2 gate insulator, It=300 kW/cm2 with an n-Si gate electrode and 200 kW/cm2 with Au electrode (100 nm thick). Compared to the same polymer film on pure SiO2 (It=2 kW/cm2), this increase results from waveguide losses in the nearby gate electrode. With an indium–tin–oxide (ITO) gate electrode (140 nm thick) on glass, again with a 200...

We report photo- and electroluminescence from an alternating conjugated polymer consisting of fluorene units and low-band gap donor-acceptor-donor (D–A–D) units. The D–A–D segment includes two elec ...

Time-resolved terahertz spectroscopy and Monte Carlo simulations of charge-carrier motion are used to investigate photoinduced transient conductivity in a blend of a low-band-gap polyphenylene copolymer and fullerene derivative. The optical excitation pulse generates free holes delocalized on polymer chains. We show that these holes exhibit a very high initial mobility as their initial excess energy facilitates their transport over defects (potential barriers) on polymer chains. The conductivity then drops down rapidly within 1 ps, and we demonstrate that this decrease occurs essentially by two mechanisms. First, the carriers loose their excess energy and they thus become progressively localized between the on-chain potential barriers-this results in a mobility decay with a rate of (180 fs)(-1). Second, carriers are trapped at defects (potential wells) with a capture rate of (860 fs)(-1). At longer time scales, populations of mobile and trapped holes reach a quasiequilibrium state and further conductivity decrease becomes very slow. .

In this study we report the synthesis, characterization, and photovoltaic properties of a series of six conjugated polymers based on donor-acceptor-donor (DAD) structure. The polymers are obtained via Suzuki polymerization of different alkoxy-substitutedDADmonomers together with a substituted fluorene or phenylene monomer. Application of polymers as light-harvesting and electron-donating materials in solar cells, in conjunction with both [60]PCBMand [70]PCBMas acceptors, show powerconversion efficiencies (PCEs) up to 2.9%, values obtained without extensive optimization work. Furthermore, atomic force microscopy and field-effect transistor (FET) mobility measurements of acceptor-polymer mixtures show that differences in substitution on the polymers affect morphology, mobility, and device performance. Within the series of polymers, all showing similar optical absorption and redox behavior, substituents play an important role in phase separation on a micrometer scale, which in turn has a large impact on device performance. The phase-separation behavior is clearly seen in [70]PCBM devices where the best-performing devices are obtained using the polymers with short alkoxy groups or no substituents together with a high speed of spin coating during device preparation. Refereed/Peer-reviewed

Solar cells based on organic molecules or conjugated polymers attract great attention due to their unique advantages, such as low cost, and their use in flexible devices, but are still limited by their low power conversion efficiency (PCE). To improve the PCEs of polymer solar cells, more efforts have been made to increase short-circuit current (Jsc) or open-circuit voltage (Voc). However, the trade-off between Jsc and Voc in bulk heterojunctions solar cells makes it tricky to find a polymer with a low band gap to efficiently absorb photons in the visible and near infrared region of the solar spectrum, while maintaining a high Voc in solar cells. Therefore, it is crucial to design and synthesize polymers with energy levels aligning with the LUMO (lowest unoccupied molecular orbital) of an electron acceptor to minimize the LUMO level difference between donor and acceptor to keep enough driving force for charge generation, thereby maximizing photovoltage in solar cells. Here a novel copolymer APFO-Green 9 was synthesized. Polymer solar cells based on APFO-Green 9 blended with a derivative of fullerene demonstrate high photovoltage by fine tuning the HOMO and LUMO level of APFO-Green 9. Solar cells based on APFO-Green 9 and [6,6]-phenyl-C71-butyric acid methyl ester ([70]PCBM) present a photoresponse extended to 900 nm with Jsc of 6.5 mA cm 2, Voc of 0.81 V and PCE of 2.3% under illumination of AM1.5 with light intensity of 100 mW cm 2. As a low band gap polymer with a Voc bigger than 0.8 V, APFO-Green 9 is a promising candidate for efficient tandem solar cells. Refereed/Peer-reviewed

We report on transistors and light-emitting diodes using a conjugated polymer consisting of alternated segments of fluorene units and low-band gap donor-acceptor-donor (D-A-D) units. The D-A-D segment includes two electron-donating thiophene rings combined with a thiadiazolo-quinoxaline unit, which is electron withdrawing to its nature. The resulting polymer is conjugated and has a band gap of around 1.27 eV. Here we present the corresponding electro- and photoluminescence spectra, which both peak at approximately 1 micrometer. Single layer light-emitting diodes demonstrated external quantum efficiencies from 0.03% to 0.05%. The polymer was employed as active material in thin film transistors, a field-effect mobility of 0.003 cm 2 /Vs and current on/off ratio of 104 were achieved at ambient atmosphere.

The authors report polarized red, electroluminescence peak at 705 nm and near IR, electroluminescence peak at 950 nm, light emission from light emitting diodes based on two polyfluorene copolymers. The copolymers are synthesized from a fluorene monomer combined with donor-acceptor-donor comonomers and designed to have a low band gap and form birefringent liq. cryst. phases. Emission occurs from aligned thin films of polymer layers. The emissive layers are aligned by spin coating on a layer of rubbed conducting polymer poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate) and thermally converted into glassy nematic liq. cryst. state. [on SciFinder (R)]

A conjugated polymer with a low band gap of 1.21 eV, i.e., absorbing infrared light, is demonstrated as active material in field-effect transistors (FETs). The material consists of alternating fluo ...