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International audience; Three conjugated compounds based on carbazole, cyclopentadithiophene and dithienopyrrole substituted by branched alkyl chains have been synthesized by the Knoevenagel condensation of malonodinitrile with the appropriate dicarboxaldehyde. Electronic properties of the target compounds have been analyzed in solution by UV–vis absorption spectroscopy and cyclic voltammetry and their potentialities as a donor material in donor–acceptor heterojunction solar cells have been evaluated in bilayer devices involving both solution processed or vacuum deposited donor layers and thermally evaporated fullerene C60 as an acceptor.

International audience; Conjugated polymers of hybrid structure containing cyanovinylene linkages have been synthesized by Knoevenagel condensation of diformyl terthienyls with para-dicyanomethylbenzene. UV–vis and cyclic voltammetric data show that these polymers combine reduced band gap, improved light-harvesting properties and low lying HOMO level. Whereas the very low solubility of the polymers did not allow the fabrication of bulk heterojunction solar cells, bilayer heterojunction solar cells have been realized using thermally evaporated films of fullerene C60 as acceptor material. The best devices show a maximum external quantum efficiency of ∼20% and a power conversion efficiency of 0.40% under simulated AM 1.5 solar illumination.

Easily adjustable parameters such as area and design can affect the determination of the efficiency of donor-acceptor organic solar cells. Devices with crossing electrodes and unpatterned (semi)conducting organic layers can collect a non-negligible current from regions usually not considered as part of the photovoltaic element, a fact that might lead to an overestimation of the power conversion efficiency.

Organic semiconductors based on π-conjugated systems are the focus of considerable interest in the emerging area of soft or flexible photonics and electronics. Whereas in recent years the performances of devices such as organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), or solar cells have undergone considerable progress, a number of technical and fundamental problems related to the low dimensionality of organic semiconductors based on linear π-conjugated systems remain unsatisfactorily resolved. This low dimensionality results in an anisotropy of the optical and charge-transport properties, which in turn implies a control of the material organization/molecular orientation during or after device fabrication. Such a constraint evidently represents a problem when device fabrication by solution-based processes, such as printing techniques, is envisioned. The aim of this short Review is to illustrate possible alternative strategies based on the development of organic semiconductors with higher dimensionality, capable to exhibit isotropic electronic properties.

3D conjugated architectures consisting of short oligothiophene chains attached to a connecting node formed by a sterically twisted bithiophenic system have been synthesized by various combinations of bromination and organometallic couplings. The analysis of the electronic properties of these systems by UV-vis absorption spectroscopy and cyclic voltammetry shows that the effective conjugation length depends on the twist angle in the central bithiophene node and on the number, length, and composition of the conjugated branches. A first evaluation of the potential of this new class of compounds for the realization of organic solar cells has been carried out. To this end two prototype bulk heterojunctions based on two representative compounds as donors and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as acceptor have been realized. A short-circuit current density of 1.33 mA cm -2 and a power conversion efficiency of 0.20 % have been measured under AM 1.5 (air-mass 1.5) simulated solar irradiation.

Conjugated polymers are of increasing interest as semiconductors for soft (opto)electronic devices, including photovoltaic elements. A promising conversion of solar energy into electrical energy is possible with blends of soluble electron donor-type conjugated polymers and fullerenes as electron-acceptor, transporting component. This approach, called bulk-heterojunction, suggested the preparation of intrinsic ambipolar materials to control simultaneously the electronic and morphological properties. On these bases, the covalent grafting of acceptor moieties onto conjugated backbones seemed attractive for the preparation of intrinsically ambipolar polymeric materials (‘double-cable’ polymers) as an alternative to donor–acceptor composites. The design, characterisation and application of this novel class of polymers are reviewed taking into account the current understanding of organic photovoltaics. Copyright © 2007 Society of Chemical Industry

Two novel “hybrid” systems consisting of a triphenylamine core carrying π-conjugated terthienyl branches have been synthesized and characterized by differential scanning calorimetry, X-ray diffraction, cyclic voltammetry, UV−vis absorption, and fluorescence spectroscopy. The semiconductor potentialities of these compounds, which lead to glassy or amourphous films, have been evaluated by their implementation in very simple prototype devices that display electroluminescence at low voltage as well as a photovoltaic effect. Moreover, field-effect transistors based on one of these novel molecules display a high hole mobility (1.1 × 10-2 cm V-1 s-1). These results suggest that molecules leading to amorphous materials could represent a valid alternative as compared to systems that require control of the molecular orientation/organization.

Star-shaped molecules based on a triphenylamine core derivatized with various combinations of thienylenevinylene conjugated branches and electron-withdrawing indanedione or dicyanovinyl groups have been synthesized. UV-vis absorption and fluorescence emission data show that the introduction of the electron-acceptor groups induces an intramolecular charge transfer that results in a shift of the absorption onset toward longer wavelengths and a quenching of photoluminescence. Cyclic voltammetry shows that all compounds present a reversible first oxidation process whose potential increases with the number of electron-withdrawing groups in the structure. Prototype bulk and bilayer heterojunction solar cells have been realized using fullerene C60 derivatives as acceptor material. The results obtained with both kinds of devices show that the introduction of electron-acceptor groups in the donor structure induces an extension of the photoresponse in the visible spectral region, an increase of the maximum external quantum efficiency, and an increase of the open-circuit voltage under white light illumination. These synergistic effects allow reaching power conversion efficiencies of approximately 1.20% under simulated AM 1.5 solar irradiation at 100 mW cm(-2).

Poly(3,4-ethylenedithiathiophene) (PEDTT) is a polythiophene-like conjugated polymer in which each thiophene ring is functionalized with an ethylenedithia bridge. As such, PEDTT is the sulfur analogue of the well-known poly(3,4-ethylenedioxythiophene) (PEDOT). Substituent effects, namely the presence of sulfur atoms in PEDTT replacing the oxygen atoms of PEDOT, do not provide a simple explanation for the different electronic properties of the two polymers in the neutral state. This paper reports the spectroscopic properties of PEDTT, studied by in situ techniques such as IR-, Vis-, and electron spin resonance (ESR) spectroelectrochemistry. The differences observed upon electrochemical oxidation of PEDTT and PEDOT (e.g., the different infrared active vibrational band patterns in IR spectroelectrochemistry as well as the different nature of the charged states) are even more marked than those observed in the neutral state. These results, with AM1 calculations, indicate conformational effects as a possible explanation for the different electronic and spectroscopic properties of PEDTT and PEDOT.

Using the photon energy induced blue shift of the resonant Raman scattering frequencies in inhomogeneous films ('RRS dispersion'), we extract the energy of the lowest even parity state of pi-conjugated polymers. We show that in the small band gap (approximate to1.1 eV) poly(dithienothiophenes), the experimentally observed unusually large RRS dispersion yields for the lowest even parity state (2A(g)) an energy, which is in the vicinity of the optically allowed state (1B(u)). (C) 2004 Elsevier B.V. All rights reserved.

Spectroelectrochemical studies of polydithienothiophenes (pDTTs), low band-gap conjugated polymers with polythiophene-like chain, using in situ FTIR-ATR and ESR spectroscopy, are shown. Spectral results obtained during p- and n-doping, as well as photoinduced spectral effects, are compared. The formation of paramagnetic positive and negative charge carriers with unusually high g-values is described.

To enhance the efficiency of polymer photovoltaics, much effort is put into synthesis of novel compounds which show a better harvesting of solar light. In this respect, a new low band gap polymer, namely, poly(5,6-dithiooctylisothianaphtene), was synthesised. This work focusses on the spectroscopic characterisation of the material. The dynamics of the photoexcitations were studied by monitoring the dependence of the photoinduced absorption band on the laser modulation frequency and indicated dispersive recombination kinetics in this material. An appropriate model was used to describe the observed behaviour. To investigate the nature of the photogenerated species more profound, photoinduced absorption spectroscopy in the infrared was performed, showing an infrared active vibrational pattern (IRAV). Solar cells were constructed with an active layer consisting of the pristine material or a mixture with an electron accepting moiety. For this purpose, (6,6)-phenyl-C61-butyric-acid (PCBM) in 1:1 a (w/w) ratio with respect to the polymer was used. A clear improvement of the diode behaviour was observed going from the pristine material to the mixture. Photocurrent action spectra of the solar cells with the polymer:PCBM-mixture indicated an active contribution of the polymer to the photocurrent.

Conjugated polymers are used increasingly for the fabrication of electronic and optoelectronic devices like light emitting diodes (LEDs) and photovoltaic (PV) elements. A breakthrough in realising a promisingly efficient conversion of solar energy into electrical energy has been achieved by using blends of soluble electron-donor type conjugated polymers with fullerenes as electron-acceptor, transporting component. This “bulk heterojunction” approach suggests the preparation of intrinsically bipolar materials as a way to control both electronic and morphological properties at once. On these bases, the covalent grafting of fullerene moieties to conjugated backbones seems promising for the preparation of intrinsically bipolar polymeric materials (double-cable polymers) alternative to conjugated polymer/fullerene composites. The recent developments on the design, the characterisation and the application of this novel class of fullerene functional materials are reviewed.

Poly(dithienothiophene)s (PDTTs), low band-gap conjugated polymers with polythiophene-like chain where an aromatic thienothiophene moiety is fused to each thiophene ring, were studied using Raman spectroscopy, photoinduced infrared absorption, as well as attenuated total reflection Fourier transform infrared (ATR-FTIR) and electron spin resonance (ESR) spectroelectrochemistry. The spectroelectrochemical studies were performed in situ during p- and n-doping (electrochemical oxidation and reduction, respectively). Raman lines of the pristine polymers are compared to infrared active vibration (IRAV) bands due to the charge carriers injected by the electrochemical doping processes or by illumination. The different π-electron distribution along the polythiophene-like chain, which determines the different band-gap sizes, also account for the different lattice relaxations and vibrational behaviors shown by these polymers. By means of in situ ESR spectroscopy, the formation of paramagnetic positive and negative c...

A new series of regioregular poly(terthiophenes), bearing bis(thioether) side groups, has been prepared by electrochemical oxidation. Cyclic voltammetry of the parent trimers reveals an increase in electron donating ability in cyclic thioether derivatives. For example, one compound containing a central ethylenedithiothiophene (EDTT) unit gives two irreversible oxidation peaks which are significantly lower than the corresponding values for the structurally analogous bis(methylthio) derivative (ΔE1ox = 170 mV, ΔE2ox = 30 mV). The difference in oxidation potentials of each terthiophene in acetonitrile solution can be explained by examining the highest occupied molecular orbital (HOMO) energies. The electrochemical behaviour of the polymers illustrates the increase in p-doping ability of the EDTT-containing polymer. The cyclic voltammograms and electronic absorption spectra of the polymers show that the polymers containing cyclic thioether units have the lowest bandgap in the series (ca. 1.4 V and 1.5 eV, respectively). Photoinduced IR spectroscopy of poly(dithienyl-3,4-ethylenedithiothiophene) provides evidence of a long-living photoexcited charged state in the polymer.

To efficiently harvest the terrestrial solar emission in photovoltaic devices, the absorption spectrum of the photoactive material has to match the solar spectrum as well as possible. Materials which absorb at photon energies where the maximum of the solar emission occurs (around 1.8 eV are very important for photon harvesting. The synthesis of Poly-( N -dodecyl-2,5-bis(2′-thienyl)pyrrole-(2,1,3-benzothiadiazole)) (PTPTB), a low bandgap polymer, and its use in 'bulk heterojunction' photovoltaic devices was recently reported. For better understanding of the properties of this material, spectroelectrochemical, optical spectroscopic and cyclovoltammetric measurements as well as electrochemical voltage spectroscopy (EVS) were carried out to determine both the electrochemical and the optical bandgap. The comparison of the results allows the prediction of charge carrier formation via light exposure.

To circumvent the problems of morphology and donor−acceptor phase segregation in organic bulk heterojunction solar cells, double-cable materials, consisting of a hole conducting conjugated polymer chain carrying pendant electron accepting moieties, are proposed. We report the electrochemical synthesis of a double-cable material consisting of a polythiophene backbone with covalently bound tetracyanoanthraquinodimethane (TCAQ) moieties. Cyclic voltammetry and UV−vis absorption measurements reveal that in this functional material both the polythiophene chain and the TCAQ moieties maintain their individual electrochemical and electronic ground state properties. Photoinduced absorption (PIA) and light-induced electron spin resonance (LESR) studies clearly indicate photoinduced electron transfer from the polythiophene backbone to the TCAQ moieties upon photoexcitation. The results of the PIA are comparatively studied with UV/vis and near-infrared absorption spectra upon electrochemical doping.

A novel polythiophene bearing a strong electron-accepting fluorene unit has been synthesized. Poly(2d) can be prepared by chemical (iron(III) chloride) or electrochemical oxidation. Intramolecular charge-transfer (ICT) within the polymer was studied by electronic absorption spectroscopy and compared to a series of model monomer derivatives. The redox properties of the materials were studied by cyclic voltammetry; polythiophene main chain in poly(2d) is oxidized at ca. +0.7 V vs Ag/AgCl, while the nitrofluorene repeat unit is reduced at ca. −0.5 and −0.7 V. The model compounds 3a−e show a Hammett-type correlation for ICT energies, E1red1/2 and E2red1/2 vs the substituents on the fluorene ring, with the sensitivity parameters ρICT- ≈ 0.17−0.20 eV and ρCV- ≈ 0.17−0.25 V. In contrast, the oxidation process (Eox) displays very low sensitivity to the structure of the fluorene ring (ρCV- ≈ 0.04 V), indicating that ICT can be substantially tuned without a noticeable effect on the redox properties of the thiophene...

Homogeneous blends of a processable methanofullerene, [6,6]-phenyl C61-butyric acid methyl ester (PCBM), with poly(para-phenylenevinylene) (PPV) synthesized using a novel nonionic precursor route were produced. These photoactive blends have been investigated by excited-state spectroscopy and by photocurrent measurements. UV-vis and IR absorption measurements, as well as luminescence spectroscopy, were used to monitor the conversion process of the precursor polymer to the PPV in blends with PCBM. The presence of PCBM did not influence the successful conversion process. The occurrence of photoinduced charge transfer, well-known for blends of substituted PPV with fullerenes, was evidenced in composites of PPV/PCBM by the strong quenching of the PPV luminescence. LESR (light induced electron spin resonance) and PIA (photoinduced absorption) studies confirmed the occurrence of photoinduced electron transfer from the PPV to PCBM within the bulk of solid state composite films. Photovoltaic devices made from PPV/PCBM blends showed power efficiencies of up to 0.25% under intense white light illumination. The spectral-photocurrent excitation profile was observed to follow closely the absorption spectrum of the PPV.

A comparison of doping and photoinduced IRAV spectra of conjugated polymers with more complex structures is presented. Differences in behavior regarding the comparison of p- and n-doped states of different materials were observed. In contrast, a comparison between doped and photoinduced IRAV spectra often show a similar band pattern. Our results do not agree with the predictions given by theoretical descriptions, which were developed for simpler structures. From these theories, the same IRAV pattern for p- and n-doping, but differences to photoinduced IRAV should be expected.

Attenuated total reflection Fourier transform infrared studies of the electrochemical redox processes of poly(dithieno[3,4-b:2‘,3‘-d]thiophene) (pDTT3), a low band gap conjugated polymer with a “polythiophene-like” backbone, where an aromatic moiety is fused to each thiophene ring, are reported. Vibrational spectroscopic studies are performed in situ during electrochemical oxidation (p-doping) and reduction (n-doping). Infrared active vibration bands due to charge carriers injected to the polymer chain are compared to photoinduced IR bands and to Raman spectra of the neutral polymer. The similar spectral features, observed during p- and n-doping processes, indicate a high charge carrier delocalization for both signs of carriers. Furthermore, the spectral evidence of the strong involvement of the fused aromatic rings to the π-electronic structure of this low band gap polymer has been obtained.

The synthesis of the monomer 1,6-bis(3,6-dihexadecyl-N-carbazolyl)-2,4-hexadiyne, its polymerization and the purification of the polymer, which is soluble in common organic solvents, are reported. Results from powder X-ray diffraction studies carried out on the red form of the polymer are discussed. The red polymer chains self-assemble into cylindrical shapes which produce hexagonal columnar mesophases with a transition from a more (Colho) to a less ordered (Colhd) structure around 85 C. The role played by the long alkyl chains in the formation of the supramolecular hexagonal mesophase is emphasized by comparing these results with those obtained from an analogous polymer with dodecyl substituents which exhibits only the Colhd structure over the whole range of temperature explored.

The frequency and intensity of the infrared and Raman CC modes have been investigated for a large number of molecules. A series of symmetrical and asymmetrical substituted diacetylenic monomers having the carbazole group attached directly or indirectly to the diacetylenic backbone have been studied in their microcrystalline or liquid state. A qualitative assignment has been proposed and particular attention has been given to those systems for which a peculiar behaviour of the infrared and Raman intensities has been observed. This behaviour is found when the diacetylenic skeleton is directly attached to either the 9H-carbazolyl or triethylsilyl group.

Fixation of a hexyl-2,2′-bithienyl unit on a conjugated BODIPY donor increases the conversion efficiency of the resulting molecular bulk heterojunction solar cell significantly.

The frequency and intensity of the infrared and Raman CC modes have been investigated for a large number of molecules. A series of symmetrical and asymmetrical substituted diacetylenic monomers having the carbazole group attached directly or indirectly to the diacetylenic backbone have been studied in their microcrystalline or liquid state. A qualitative assignment has been proposed and particular attention has been given to those systems for which a peculiar behaviour of the infrared and Raman intensities has been observed. This behaviour is found when the diacetylenic skeleton is directly attached to either the 9H-carbazolyl or triethylsilyl group.

International audience; Photoinduced energy conversion from multilayers of organic dye on dense TiO2 films was investigated in bilayer hybrid solar cells. Dye layers of varying thicknesses were prepared by spin-casting the star-shaped dye [tris(dicyano-vinyl-2-thienyl)phenyl]amine (1) from solutions onto dense TiO2 on conducting glass substrates. A spin-cast layer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and graphite powder was used for contacting the devices. Excitons generated in the dye multilayer contribute to the power conversion efficiency, reaching a maximum of ca. 0.3% at a dye layer thickness of ca. 8 nm for the devices described herein. For dye layers exceeding 5 nm, the cell performance becomes limited by the exciton diffusion length LED and the hole mobility in the organic layer. Using dye multilayers is a viable way to increase light harvesting in solid-state dye-sensitized solar cells.

International audience; BODIPY derivatives have been used as donor in solution-processed bulk heterojunction solar cells using PCBM as acceptor. A power conversion efficiency of 1.34% has been obtained under simulated solar irradiation.

OFETs built from extended TTF derivatives containing N-methylpyrrole, furan, thiophene and benzene with meta or para linkages as spacers are presented. The aromatic spacer plays a crucial role in the characteristics and stability of the OFETs. The less conjugated derivative with a meta-benzene moiety as spacer has the best mobility, with a higher on/off ratio and strongly enhanced stability in air.

International audience; Regioregular poly[3-(3,6-dioxaheptyl)thiophene] (PDHT) has been synthesized by Grignard metathesis (GRIM). The UV–vis absorption spectrum of spin-cast films of PDHT presents a well-resolved vibrational fine structure typical of well-ordered polymers. The characterization of bulk heterojunction solar cells fabricated using PCBM as acceptor shows that PDHT leads to power conversion efficiencies in ca 4–5 times lower than that obtained with regioregular poly(3-hexylthiophene) (P3HT). In contrast, the characterization of bilayer solar cells realized by thermal evaporation of C60 on spin-cast polymer films shows that PDHT leads to performances similar or even better than P3HT both before and after thermal treatment.

International audience; 3D-conjugated systems based on oligothiophene segments grafted on a phosphorus or on a phosphine oxide node have been synthesized. Under Stille coupling conditions, bromide terminated thienyl phosphine derivatives undergo a breaking of the phosphorus–carbon bond attributed to a ligand exchange with the Pd catalyst. The electronic properties of the new compounds have been analyzed by UV-vis and fluorescence spectroscopy and cyclic voltammetry.

Introduction of electroaccepting groups at the periphery of triphenylamine-based derivatives leads to an internal charge-transfer band. Syntheses and spectroscopic, electrochemical, and theoretical studies of various derivatives which differ by the strength and the number of electroacceptor groups are presented. These various results show that the ICT band and the acceptor/donor abilities of derivatives can be finely tuned.

The cover image illustrates the dual photovoltaic and electroluminescence function of a single-layer device based on a thienylenevinylene–triphenylamine with internal charge transfer (ICT), as reported by Cravino, Roncali, and co-workers on p. 3033. The material forms an organic glass with isotropic electronic properties while ICT leads simultaneously to an extension of the photoresponse to the red and to an increase of the open circuit voltage. The use of an additional layer of C60 further improves the photovoltaic. Images of the sun and moon courtesy NASA/JPL–Caltech.

International audience; Simple single-layered diodes based on a glass-forming triphenylamine-thienylenevinylene with internal charge transfer show a photovoltaic effect in solar light and red electroluminescence when forward biased (see figure and cover). The use of an additional layer of C60 as electron acceptor improves the photovoltaic performance without affecting the electroluminescence.

Introduction of dicyanovinyl groups on a triphenylamine-based conjugated system leads to an intramolecular charge transfer which extends the spectral response and raises the open-circuit voltage of the resulting hetero-junction solar cells.

The usage of fullerenes as electron mediator in photoelectrochemical cells with a conjugated polymer as electron donor and a dissolved redox couple as acceptor is described. To prevent solubility problems, the fullerenes are covalently linked to the conjugated polymer polybithiophene (“double cable”). Copolymers with different amount of fullerenes are studied in respect to light absorption and photovoltaic device properties. Compared with polybithiophene, the introduction of fullerenes in the devices enhances the photovoltaic response significantly.

We report the synthesis and the spectroscopic properties of a novel soluble thiophene-based copolymer carrying pendant fullerene moieties (donor–acceptor double-cable polymer). Photoinduced absorption (PIA) experiments on spin cast films reveal a photoinduced electron transfer from the polythiophene chain onto the fullerene moieties. The performance of photovoltaic devices produced with this copolymer is also discussed.

Information on doping- and photoinduced phenomena and processes in materials used in organic polymeric photovoltaic systems (plastic solar cells) can be obtained using infrared spectroscopy. The methods and results for the identification of doping- and photoinduced charge carriers, investigation of charge carriers in electrochemical systems, and studies on degradation processes are shown.

We have prepared a novel bithiophene carrying a fulleropyrrolidine substituent. Its electrochemical polymerization affords an electron donor−acceptor material that has been studied by means of electrochemical and spectroscopic techniques. The results from photoinduced absorption (PIA) and light-induced electron spin resonance (LESR) experiments demonstrate a photoinduced electron transfer from the polythiophene backbone to the tethered acceptor moieties.

A novel double-cable polythiophene carrying fullerene side groups has been prepared electrochemically. Electrochemical and spectroscopic data show that this bipolar material, where phase separation cannot occur, retains the favorable optoelectronic properties of polythiophene/fullerene blends. Excited state spectroscopy reveals clear evidence of a photo-induced electron transfer from the donor cable (polythiophene) to the pendant acceptor cable (fullerene substituents).

The solution properties of a novel soluble polydiacetylene, poly[1,6-bis(3,6-dihexadecyl-9H-carbazol-9-yl)hexa-2,4-diyne] (polyDCHD-HS), have been investigated by using UV–Vis absorption, fluorescence, and FT-Raman spectroscopies. The polymer gives rise to stable, brilliant red solutions in aromatic solvents and in chloroform at room temperature with no evidence of the colour transition to yellow that usually takes place by increasing the temperature with other soluble polydiacetylenes. A particular role appears to be played by benzene–polyDCHD-HS interactions, because in this solvent a very narrow and intense excitonic peak is observed at room temperature in the electronic spectrum. Interestingly, these solutions show also a well-resolved fluorescence spectrum, with a very small Stokes shift and a rather high quantum yield (≅10−2). Furthermore, a very efficient transfer of the excitation energy from the carbazolyl substituents to the conjugated backbone is found to take place. On account of the observation that by increasing the temperature of the benzene solutions up to 75 °C the excitonic feature in both the absorption and the emission spectra undergoes fully reversible broadening and intensity reduction accompanied by the increase of the Stokes shift, without any dramatic colour change, it is likely that the rates of torsional motions in the polymer chains are thermally activated without modifying the average conjugation length of the polymer chains. FT-Raman measurements in benzene solutions in the same temperature range have confirmed this conclusion. The photophysical properties of polyDCHD-HS in benzene around 60 °C are quite similar to those observed in other aromatic solvents at room temperature. By heating up the red solutions in toluene or in chlorobenzene, no spectral shift is observed but only a limited increase of the absorption band broadening. All these findings strongly suggest that by the appropriate choice of the substituents it is possible to obtain stable solutions of the red form of polydiacetylenes.

Homogenous blends of a processable methanofullerene, [6,6]-Phenyl C61 - butyric acid methyl ester (PCBM), with poly (p-phenylene vinylene) (PPV) made from a novel nonionic precursor route were investigated by optical spectroscopy and by photocurrent measurements. The conversion process of the precursor polymer to the PPV in the blends with PCBM was monitored by IR absorption. In composites of PPV/PCBM strong quenching of the PPV luminescence was observed. LESR (light induced electron spin resonance) and PIA (photoinduced absorption) studies confirmed the occurrence of photoinduced electron transfer from the PPV to PCBM. Photovoltaic devices made from PPV/PCBM blends showed monochromatic power conversion efficiencies of app. 0.3%. The spectral photocurrent was observed to follow the absorption profile of the PPV.

In this article, photoinduced infrared absorption (PIA-FTIR) and light induced electron spin resonance (LESR) measurements on an asymmetric carbazolyl substituted polydiacetylene (PDA), the poly{1-(N-carbazolyl) penta-1,3-diyn-5-ol} (polyCPDO) are presented. Results from both experimental techniques imply that contrary to other well-known polydiacetylene derivatives, in polyCPDO there exist only long-living, highly localized charged photoexcitations with spin 1/2 (g value∼2.0026), indicative for polaronic excitations. PolyCPDO also shows a strong dark ESR signal with the same g value as the LESR signal and with a Curie susceptibility of approximately 10−3 spins per CPDO monomer unit. This unusually high number of defects may come from intermediates of the polymerization. Furthermore, the photoexcited states of composites from polyCPDO mixed with a special solubilized methano-fullerene [6,6]-Phenyl C61–butyric acid methyl ester (PCBM) with excellent acceptor properties are investigated and compared with th...

Composite films of conducting polymers and fullerenes are attractive for making optoelectronic devices, such as solar cells. A new approach mixes the two components at the molecular level.

Polymers with narrow band gap are expected to posses appreciably high RT conductivities, luminescence in the NIR and improved solar energy harvesting properties. Here we report the spectroscopic properties of a soluble and environmentally stable copolymer (PEDOTEHIITN) with a band-gap of ca. 1.1 eV. The combination of this outstandingly narrow band gap, processability and stability are promising for future electronic and optoelectronic applications.

The use of low bandgap polymers in organic photovoltaics for better light harvesting is getting more and more important. For determination of the electrochemical bandgap cyclovoltammetry (CV) and electrochemical voltage spectroscopy (EVS) was applied. The optical bandgap was determined by optical absorption measurements. The results are compared in terms of energy vs. vacuum level to predict the suitability of these materials for organic photovoltaics.

International audience; Fixation of a 5-hexyl-2,2′-bithienyl unit on a conjugated BODIPY donor increases the conversion efficiency of the resulting molecular bulk heterojunction solar cells from 1.30 to 2.20%.

The synthesis of a novel bithiophene with a pendant fullerene substituent and its electrochemical polymerisation are reported; light induced ESR measurements on the electrodeposited films reveal a photoinduced electron transfer from the donor cable (polythiophene) to the pendant acceptor cable (fullerene moieties).

This letter analyzes the influence of the energy of the highest-energy occupied molecular orbital (HOMO) and the single occupied molecular orbital (SOMO) (i.e., the low-energy polaronic level) of different electron donors (D) on the open circuit voltage (VOC) value delivered by bilayer and bulk-heterojunction solar cells containing fullerenes as acceptor (A). The analysis further confirms that the VOC of these devices scales with the energy difference between the HOMO of the D and the lowest-energy unoccupied molecular orbital of the A. As observed, a better relationship is obtained if the SOMO energy of the D—extracted from photoinduced absorption spectra—is considered.