Search

Controlled synthesis of the electroluminescent polymer PFO-MEH-PPV (poly[(9,9-dioctyl-2,7-divinylenefluorenylene)-alt-co-(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene)]) provided samples of varying molecular weight (Mw) in the range 20–360 kDa, as determined by gel-permeation chromatography and light scattering. The samples were used as the active layers in organic light-emitting diodes (OLEDs), and the performance of the devices was examined as a function of Mw. Turn-on voltages fell in the range 1.92–2.78 V, luminances varied from 231 to 5826 cd/m2, and luminous efficacies ranged from 0.06 to 0.90 lm/W. The emitted colour was found to vary from green to yellow as Mw increases. Optimal performance was attained by using PFO-MEH-PPV with Mw = 100 kDa. To help reveal how Mw determines the performance of OLEDs, relative quantum yields of photoluminescence in solutions and films were measured, and films were characterized by atomic force microscopy and transmission electron microscopy.

Organic mixed conductors find use in batteries, bioelectronics technologies, neuromorphic computing, and sensing. While great progress has been achieved, polymer-based mixed conductors frequently experience significant volumetric changes during ion uptake/rejection, i.e., during doping/de-doping and charging/discharging. Although ion dynamics may be enhanced in expanded networks, these volumetric changes can have undesirable consequences, e.g., negatively affecting hole/electron conduction and severely shortening device lifetime. Here, the authors present a new material poly[3-(6-hydroxy)hexylthiophene] (P3HHT) that is able to transport ions and electrons/holes, as tested in electrochemical absorption spectroscopy and organic electrochemical transistors, and that exhibits low swelling, attributed to the hydroxylated alkyl side-chain functionalization. P3HHT displays a thickness change upon passive swelling of only +2.5%, compared to +90% observed for the ubiquitous poly(3,4-ethylenedioxythiophene):polystyrene sulfonate, and +10 to +15% for polymers such as poly(2-(3,3'-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)-[2,2'-bithiophen]-5-yl)thieno[3,2-b]thiophene) (p[g2T-TT]). Applying a bias pulse during swelling, this discrepancy becomes even more pronounced, with the thickness of P3HHT films changing by10% while that of p(g2T-TT) structures increases by +75 to +80%. Importantly, the initial P3HHT film thickness is essentially restored after de-doping while p(g2T-TT) remains substantially swollen. The authors, thus, expand the materials-design toolbox for the creation of low-swelling soft mixed conductors with tailored properties and applications in bioelectronics and beyond.

International audience; Fullerene derivatives have been ubiquitous as an electron-accepting material in organic photovoltaic solar cells (OSCs). We consider whether and why traces of PCBM oxidation products should be seen as electronic defects impairing the performance of OSCs. Thin PCBM deposits were first illuminated under ambient air for a few minutes, thus revealing the extraordinary easiness of oxidizing PCBM. The charge transfer in polymer:PCBMox bulk heterojunctions was then studied. As a result of a few minutes of PCBM photooxidation, the electron transfer from the polymer to two types of PCBMox species was shown to occur at the expense of the transfer to pristine PCBM. Such modifications to the molecular structure of PCBM and to the charge transfer at the nanoscale were finally correlated with a dramatic loss in the device's photovoltaic performance at the macroscale. This study clearly indicates the need to integrate photooxidation-resistant electron-accepting materials into OSCs to extend their lifetime

The relationship between charge transport and surface morphology is investigated by utilizing rubrene single crystals of varying thicknesses. In the case of pristine crystals, the surface conductivities decrease exponentially as the crystal thickness increases until ∼4 μm, beyond which the surface conductivity saturates. Investigation of the surface morphology using optical and atomic force microscopy reveals that thicker crystals have a higher number of molecular steps, increasing the overall surface roughness compared with thin crystals. The density of molecular steps as a surface trap is further quantified with the subthreshold slope of rubrene air-gap transistors. This thickness-dependent surface conductivity is rationalized by a shift from in-plane to out-of-plane transport governed by surface roughness. The surface transport is disrupted by roughening of the crystal surface and becomes limited by the slower vertical crystallographic axis on molecular step edges. Separately, we investigate surface-doping of rubrene crystals by using fluoroalkyltrichrolosilane and observe a different mechanism for charge transport which is independent of surface roughness. This work demonstrates that the correlation between crystal thickness, surface morphology, and charge transport must be taken into account when measuring organic single crystals. Considering the fact that these molecular steps are universally observed on organic/inorganic and single/polycrystals, we believe that our findings can be widely applied to improve charge transport understanding.

Thermal expansion coefficients of most materials are usually small, typically up to 50 parts per million per kelvin, and positive, i.e. materials expand when heated. Some materials show an atypical shrinking behavior in one or more crystallographic directions when heated. Here we show that a high mobility thiophene-based organic semiconductor, BHH-BTBT, has an exceptionally large negative expansion between 95 and 295 K (-216 < α2 = αb < -333 MK-1), being compensated by an even larger positive expansion in the perpendicular direction (287 < α1 < 634 MK-1). It is shown that these anomalous expansivities are completely absent in C8-BTBT, a much studied organic semiconductor with a closely related molecular formula and 3D crystallographic structure. Complete theoretical characterization of BHH-BTBT using ab initio molecular dynamics shows that below ∼200 K two different α and β domains exist of which one is dominant but which dynamically exchange around and above 210 K. A supercritical-like transition from an α dominated phase to a β dominated phase is observed using DSC measurements, UV-VIS spectroscopy, and X-ray diffraction. The origin of the extreme negative and positive thermal expansion is related to steric hindrance between adjacent tilted thiophene units and strongly enhanced by attractive S···S and S···C interactions within the highly anharmonic mixed-domain phase. This material could trigger the tailoring of optoelectronic devices highly sensitive to strain and temperature.

none 10 si Organic semiconductors are intensively studied as promising materials for the realisation of low-cost flexible electronic devices. The flexibility requirement implies either performance stability towards deformation, or conversely, detectable response to the deformation itself. The knowledge of the electromechanical response of organic semiconductors to external stresses is therefore not only interesting from a fundamental point of view, but also necessary for the development of real world applications. To this end, in this work we predict and measure the variation of charge carrier mobility in rubrene single crystals as a function of mechanical strain, applied selectively along the crystal axes. We find that strain induces simultaneous mobility changes along all three axes, and that in some cases the response is higher along directions orthogonal to the mechanical deformation. These variations cannot be explained by the modulation of intermolecular distances, but only by a more complex molecular reorganisation, which is particularly enhanced, in terms of response, by π-stacking and herringbone stacking. This microscopic knowledge of the relation between structural and mobility variations is essential for the interpretation of electromechanical measurements for crystalline organic semiconductors, and for the rational design of electronic devices. mixed Matta, Micaela; Pereira, Marco José; Gali, Sai Manoj; Thuau, Damien; Olivier, Yoann; Briseno, Alejandro; Dufour, Isabelle; Ayela, Cedric; Wantz, Guillaume; Muccioli, Luca Matta, Micaela; Pereira, Marco José; Gali, Sai Manoj; Thuau, Damien; Olivier, Yoann; Briseno, Alejandro; Dufour, Isabelle; Ayela, Cedric; Wantz, Guillaume; Muccioli, Luca

Highly efficient inverted organic light-emitting diodes (iOLEDs) are reported by including in the structure a surface modifier, polyethylenimine-ethoxylated (PEIE), to decrease the cathode work function and a hole blocking layer, 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi) to increase the efficiency of the device. The two compounds have been processed in a single step, by using a mixture PEIE:TPBi spun from the same solution. It is demonstrated by time-of-flight secondary-ion mass spectrometry (TOF-SIMS) that a bilayer is formed and same performances as the separately processed materials are obtained. This technic enables to reach high luminances (40 000 cd m−2) and high current efficiencies (13 cd/A) using the conjugated Super Yellow (SY) polymer as the emissive layer while reducing the number of processing steps.

International audience; Borondifluoride complexes of curcuminoid derivatives end-capped with triphenylamine groups were designed for solution-processed bulk-heterojunction organic solar cells. They were obtained very simply in a one-pot synthesis from cheap building blocks. Compared to push-pull systems based on borondifluoride complexes of hydroxychalcones, curcuminoids present the donor-accept-or-donor electronic structure and exhibit significantly improved chemical and thermal stability and photovoltaic performance. Indeed, power conversion efficiency up to 4.14% and high open-circuit voltage over 1.0 V have been achieved using PC61BM as acceptor.

International audience; Benzochalcogenopheno[3,2-b][1]benzochalcogenophenes (BXBXs) have been the key π-conjugated core structures in the development of superior organic semiconductors for organic field-effect transistors (OFETs). The semiconducting properties of parent BXBXs, however, have not been well examined. In this work, we focus on the parent system and investigate the effect of different chalcogen atoms, i.e., sulphur, selenium or tellurium atoms, in the BXBX core on molecular electronic properties, crystal structures, intermolecular interactions, solid-state electronic structures, and carrier transport properties. Replacing the sulphur atoms in [1]benzothieno[3,2-b][1]benzothiophene (BTBT) with selenium atoms marginally changes the molecular properties and the intermolecular interactions, thus resulting in similar herringbone packing structures in the solid state. The carrier mobilities of single-crystal (SC)-OFETs are higher for [1]benzoselenopheno[3,2-b][1]benzoselenophene (BSBS) than those for BTBT, which can be understood by the increase in the intermolecular electronic coupling in BSBS, originating from the larger atomic radius and more diffused electron cloud of selenium atoms than sulphur atoms. On the other hand, the packing structure of [1]benzotelluropheno[3,2-b][1]benzotellurophene (BTeBTe) is determined to be a dimeric herringbone structure. The crystal structure of BTeBTe being strikingly different from those of BTBT and BSBS can be explained by a drastic change in the intermolecular interaction in the solid state. Furthermore, the BTeBTe-based SC-OFETs do not show transistor response. To elucidate these unexpected results, various experimental and theoretical approaches, e.g., evaluation of ionization potentials and band calculations, are examined. Through these approaches, a comprehensive view of the parent BXBX system is given, and also both the pros and cons of incorporation of heavy chalcogen atoms, positive and negative “heavy-atom effects”, in developing organic semiconductors are discussed

International audience; A key challenge in the field of organic photovoltaics (OPVs) is making them efficient and stable devices despite their being composed of organic materials, which are susceptible to becoming photodegraded in the presence of atmospheric oxygen. It is therefore essential to determine to what extent the donor material used in the active layer can be oxidized before the oxidation results in a loss of solar cell performance. Here we mainly focused on thieno[3,4-b]thiophene-alt-benzodithiophene polymer (PTB7), and compared it to the well-known poly(3-hexylthiophene) (P3HT). The complexity of the PTB7 chemical structure, based on an alternation of benzodithiophene (BDT) and thienothiophene (TT) and flanked with alkoxy and alkyl side chains, necessitated a re-investigation of the first step of the photooxidative process. Neither the intrinsic photochemical process nor the presence of an alkoxy side chain was found to be critical for the photostability. The high initial sensitivity of PTB7 in photooxidative conditions was instead related to attack of singlet oxygen on the conjugated backbone, with this attack shown to give rise to the formation of carbonylated species. In addition, traces of PTB7 oxidation, resulting from processing or very short durations of irradiation under ambient air, were found to result in a significant drop in solar cell performance. Also in this work, PTB7 was found to be more susceptible to photooxidation than was P3HT, in line with the higher instability of PTB7-based solar cells. The novel bottom-up approach implemented in this work revealed the importance of the formation of traces of polymer oxidation products in altering solar cell efficiency. The use of unstable materials is suspected to play a key role in the poor initial performances and/or reduced lifetimes of organic solar cells.

International audience; The synthesis and characterization of a series of [1]benzothieno[3,2-b][1]benzothiophene (BTBT) molecules disubstituted by hydroxy aliphatic chains in positions 2 and 7 (BTBT-CnOH), where the intralayer molecular stacking alternates between a classical and an inverted herringbone mode as a function of whether the alkyl sides chains have an even or an odd number of carbon atoms are reported. This odd–even effect does not only affect the interlayer distance of the lamellar structures and the melting points, but also the electronic properties. The BTBT-CnOH odd series develops a classical herringbone pattern with edge-to-edge S⋯S interaction chains linked together by face-to-edge S⋯S interaction chains with 2D mobility. However, the even series has only edge-to-edge interactions in an inverted herringbone organization and thus only a 1D conducting character. These two types of herringbone patterns have different field effect transistor characteristics and mobilities, those of the odd members being systematically higher than their even neighbors. This is the first example of an odd–even effect impacting the electronic properties of an organic semiconductor.

Micro-electromechanical systems (MEMS) made of organic materials have attracted efforts for the development a new generation of physical, chemical, and biological sensors, for which the electromechanical sensitivity is the current major concern. Here, we present an organic MEMS made of a rubrene single-crystal air-gap transistor. Applying mechanical pressure on the semiconductor results in high variations in drain current: an unparalleled gauge factor above 4000 has been measured experimentally. Such a high sensitivity is induced by the modulation of charge injection at the interface between the gold electrode and the rubrene semiconductor as an unusual transducing effect. Applying these devices to the detection of acoustic pressure shows that force down to 230 nN can be measured with a resolution of 40 nN. This study demonstrates that MEMS based on rubrene air-gap transistors constitute a step forward in the development of high-performance flexible sensors.

International audience; In the present paper, four fluorescent materials currently used in organic light emitting diodes (OLEDs) are presented in an original way as high performance photocatalysts usable in polymerization photoinitiating systems. Their performance is excellent in free radical polymerization, cationic polymerization but also in the synthesis of interpenetrating polymer networks (IPNs). A coherent picture of the chemical mechanisms involved in these new photocatalytic systems is provided. Remarkably, an oligomeric and copolymerizable photocatalyst (PVD2) is proposed here for the first time, i.e., both the high molecular weight of PVD2 and the presence of reactive double bonds as end groups (which could be involved in a copolymerization reaction) ensure a very low migration of the catalyst from the synthesized polymer.

International audience; Upon exposure to photooxidative conditions, organic materials are susceptible to undergo degradation via processes involving radical oxygen species and/or reaction with singlet oxygen (1O2). In this frame, the work herein presents a new and straightforward methodology to clarify the role of highly-reactive 1O2 in the photodegradation mechanism of conjugated materials applied in organic electronics. The general methodology consists in the comparison of the infrared signatures of the conjugated materials after the materials are exposed to photooxidative and thermooxidative conditions and in situ generated 1O2. The methodology was validated by analysing the behaviour of four donor materials commonly used in organic solar cells. Analysis of the degradation mechanism of these materials allowed exemplifying the three possible case scenarios, namely (1) both 1O2 and radical oxygen species are involved in the general photooxidation mechanism of the studied material, (2) the material is unreactive towards 1O2 and thus this species plays no role in the photooxidation process, and (3) the conjugated material is reactive towards chemically produced 1O2 but this species is not the main responsible for the photooxidative degradation of the material. In the latter two cases, a free-radical oxidation process accounts for the photooxidation of the investigated materials. The results derived from this simple, yet enlightening, methodology provide fundamental understanding about the degradation pathways of conjugated materials, which is a key point to develop not only efficient but also stable organic electronic devices.

International audience; A novel bisazide molecule to be used in polymer–fullerene bulk heterojunction (BHJ) solar cells with two distinct functionalities is reported here. Firstly, it acts as a solvent-additive to reach optimized BHJ morphology and power conversion efficiencies without further requirements of annealing post-treatment of the active layer. Secondly, this molecule is a powerful thermally-triggered cross-linker for fullerenes enabling to freeze the BHJ morphology in its optimized form making active layers thermally stable.

International audience; Understanding the degradation mechanisms in organic photovoltaics is crucial in order to develop stable organic semiconductors and robust device architectures. The rapid loss of efficiency, referred to as burn-in, is a major issue to be addressed. This study reports on the influence of the electron transport layer (ETLs) and UV light on the drop of open-circuit voltage (Voc) for P3HT:PC60BM-based devices. The results show that Voc loss is induced by the UV and, more importantly, that the ETL can amplify it, with TiOx yielding a stronger drop than ZnO. Using impedance spectroscopy (IS) and X-ray photoelectron spectroscopy (XPS), different degradation mechanisms were identified according to whether the ETL is TiOx or ZnO. For TiOx-based devices, the formation of an interface dipole was identified, resulting in a loss of the flat-band potential (Vfb) and, thus, of the Voc. For ZnO-based devices, chemical modifications of the metal oxide and active layer at the interface were detected, resulting in a doping of the active layer which impacts the Voc. This study highlights the role of the architecture and, more specifically, of the ETL in the severity of burn-in and degradation pathways.

Bipolar electrochemistry has recently emerged as a very unique method to address conducting particles in a wireless manner. The technique is often applied to the fabrication of Janus particles; however the chemical nature of the bipolar electrode has been essentially limited to carbon- or metal-based materials. Here, we report for the first time the use of conducting organic single crystals as bipolar electrodes for the preparation of a new generation of Janus objects. Fabre and Bechgaard salts involving respectively tetrathia- and tetraselenafulvalene were selected for proof-of-concept experiments. Such an approach allows to preserve the integrity of these fragile substrates because it necessitates neither electronic wiring nor mechanical contact. The site-selective electrodeposition of copper is successfully achieved, leading thus to a new metal–organic Janus structure. Subsequently, asymmetric generation of photovoltage under illumination is achieved due to the anisotropic presence of copper, making th...

We synthesized a novel bis-azide low-band gap cross-linkable molecule N3-[CPDT(FBTTh2)2] with wide absorption. This compound is of interest as an additive in polymer/fullerene bulk heterojunction solar cells. In addition to providing efficient thermal stabilization of the morphology, the additive can harvest additional solar light compared with pristine poly(3-hexyl thiophene) to improve the power-conversion efficiency (PCE). The additional donor material was visualized from the appearance of additional external quantum efficiency contributions between 650 and 800 nm. An open-circuit voltage increase of ∼2% compensates the decrease in the short-circuit current of ∼2% to achieve a fully thermally stabilized PCE of 3.5% after 24 h of annealing at 150 °C.

Novel light emitting electrochemical cells (LECs) are fabricated using CdSe-CdS (core-shell) quantum dots (QDs) of tuned size and emission blended with polyvinylcarbazole (PVK) and the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6). The performances of cells constructed using sequential device layers of indium tin oxide (ITO), poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS), the QD/PVK/IL active layer, and Al are evaluated. Only color saturated electroluminescence from the QDs is observed, without any other emissions from the polymer host or the electrolyte. Blue, green, and red QD-LECs are prepared. The maximum brightness (≈1000 cd m-2) and current efficiency (1.9 cd A-1) are comparable to polymer LECs and multilayer QD-LEDs. White-light QD-LECs with Commission Internationale d'Eclairage (CIE) coordinates (0.33, 0.33) are prepared by tuning the mass ratio of R:G:B QDs in the active layer and voltage applied. Transparent QD-LECs fabricated using transparent silver nanowire (AgNW) composites as the cathode yield an average transmittance greater than 88% over the visible range. Flexible devices are demonstrated by replacing the glass substrates with polyethylene terephthalate (PET).

Solution processing is mandatory to continue the development of large scale and low cost organic solar cells. Tungsten oxide (WO3) has shown good performance when used as a hole transporting layer in organic solar cells. Here this oxide has been deposited by a sol–gel technique on top of the active layer in an inverted structure device. This layer has been characterised by XPS and UPS to understand the effect of the stoichiometry and of the energy levels on the device performance. In the optimised structure, an average power conversion efficiency (PCE) of 3.5% has been achieved, which is comparable to devices with vacuum deposited transition metal oxide using poly(3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PCBM) as an active layer. Moreover in our study no annealing is needed after the deposition of the sol–gel processed oxide, which is really promising for the development of industrial roll-to-roll production on flexible substrates.

Reviewed are crosslinking strategies to stabilize the active-layer morphology of organic solar cells, namely donor-to-donor, donor-to-acceptor and acceptor-to-acceptor. The active layer of a polymer photovoltaic cell is mainly based on a blend of two components: a semiconducting polymer (electron donor) and a fullerene derivative (electron acceptor) to form the bulk heterojunction (BHJ). To offer optimum photovoltaic performances, the morphology of this layer has to be very carefully controlled at the nanoscale. The materials of the BHJ require specific phase segregation enabling the optimum photogenerated exciton diffusion and dissociation, and also to ensure pathways for charge carriers to electrodes. However, such a specific morphology is thermodynamically unstable over time and phase segregation occurs with thermal cycling under solar operating conditions inducing a decrease of solar cell efficiency. This review reports on the recent progress towards obtaining a stable optimized BHJ morphology and improved efficiency stability, using different chemical routes for crosslinking the organic semiconductors. © 2014 Society of Chemical Industry.

The objective of this mini-review is to outline current major ternary blends used in the active layer of polymer bulk heterojunction photovoltaic solar cells and to give an insight into the direction of the field. The use of a third-component material in polymer − fullerene blends is described in two sections. On the one hand, the first family of solid state additives enables us to enlarge photon collection by expanding the action spectra of the solar cells. The second section deals with materials used to engineer bulk heterojunction morphology at the nanoscale. The different approaches explored for many of the ternary blend systems suggest the great potential of such mixtures to significantly improve the optoelectronic properties of solar cells on a long-term basis. © 2013 Society of Chemical Industry

International audience; Two neutral heteroleptic iridium(III) complexes bearing substituted pyridine 1,2,4-triazoles as the ancillary ligands have been designed and investigated as emitters for PhOLEDs. Interestingly, blue and blue–green devices were obtained with the two complexes. Best devices were obtained with C1 and reached a maximum brightness of 10,200 cd/m2, a current efficiency of 4.3 cd/A at an operation voltage of 7.9 V while exhibiting the unusual low turn-on voltage of only 2.8 V. UV–visible absorption and photoluminescence spectroscopy as well as the cyclic voltammetry of the two iridium(III) complexes were also investigated.

A series of donor-acceptor-donor (D-A-D) structured small-molecule compounds, with 3,3'-(ethane-1,2-diylidene)bis(indolin-2-one) (EBI) as a novel electron acceptor building block coupled with various electron donor end-capping moieties (thiophene, bithiophene and benzofuran), were synthesized and characterized. When the fused-ring benzofuran is combined to EBI (EBI-BF), the molecules displayed a perfectly planar conformation and afforded the best charge tranport properties among these EBI compounds with a hole mobility of up to 0.021 cm2 V-1 s-1. All EBI-based small molecules were used as donor material along with a PC61BM acceptor for the fabrication of solution-processed bulk-heterojunction (BHJ) solar cells. The best performing photovoltaic devices are based on the EBI derivative using the bithiophene end-capping moiety (EBI-2T) with a maximum power conversion efficiency (PCE) of 1.92%, owing to the broad absorption spectra of EBI-2T and the appropriate morphology of the BHJ. With the aim of establishing a correlation between the molecular structure and the thin film morphology, differential scanning calorimetry, atomic force microscopy and X-ray diffraction analysis were performed on neat and blend films of each material.

Chemical design criteria for materials for bioelectronics applications using a series of copolymer derivatives based on poly(3-hexylthiophene) are established. Directed chemical design via side-chain functionalization with polar groups allows manipulation of ion transport and ion-to-electron transduction. Insights gained will permit increased use of the plethora of materials employed in the organic electronics area for application in the bioelectronics field.

Two cationic iridium (III) complexes [Ir(dfppy)(tpy)(bpy)](PF6) and [Ir(dfppy)(tpy)(phen)](PF6) bearing three different ligands were tested as triplet emitters for Light-Emitting Electrochemical Cells (LECs). These two phosphorescent materials only constitute the third and fourth examples of triple heteroleptic cationic iridium complexes to be tested in electroluminescent devices. LECs fabricated with this almost unknown class of iridium complex furnished green-emitting devices. Parallel to investigations devoted to electroluminescent properties, photophysical and electrochemical properties of the two new complexes were examined. Density functional theory calculations were also performed to provide insight into the electronic structure of the two emitters.

In this study, direct and inverted OLEDs (iOLEDs) were fabricated by using the polymer Super Yellow (SY) as the emissive material. By adding a 5 nm-thick layer of polyethylenimine ethoxylated (PEIE) on top of ITO/ZnO (indium tin oxide/zinc oxide) in the iOLEDs, its cathode work function was reduced of 1 eV. A thin layer of 1,3,5-tris(Nphenylbenzimidazol- 2-yl)benzene (TPBi) was further added by wet-process in the iOLEDs, blocking the holes at the PEIE/SuperYellow interface, so that the iOLEDs could finally reach much higher luminance compared to the direct OLEDs, maximum 40 000 cd/m2, with a constant maximum efficiency of 15 cd/A (13 cd/A maximum for the direct OLEDs). Temperature dependent transient electroluminescence measurements were conducted in order to compare charge carriers mobilities and disorder in direct OLEDs and iOLEDs. It was shown that the holes mobilities are higher for direct OLEDs (4 10-6 cm²/(V.s)) than iOLEDs (1.5 10-6 cm²/(V.s)) while the width of the distribution of energy states (DOS), σ, and the positional disorder parameter, Σ, are comparable for both structures.

Preparations of benzannulated cycloheptanones starting from unique binaphthyl molecular platforms are described. Binaphthyl acetic acids proved suitable percursors for fused cycloheptanone architectures. Seven-membered rings embedded in binaphthyl units were selectively generated by use of Eaton's reagent. Isomeric helical architectures arising from electrophilic cyclisation processes at second reaction sites in the precursors could also be obtained under different acidic conditions. Unambiguous discrimination between isomeric geometries was provided by multiple quantum NMR sequences. DFT calculations were performed and gave evidence of different behaviour of the substrates towards intramolecular electrophilic substitution. The theoretical approach confirmed the experimental results, agreeing completely with X-ray data.

International audience; Polymeric photovoltaic (PV) solar cells have been fabricated using six solvents: chloroform (CHCl3), toluene (T), chlorobenzene (CB), orthodichlorobenzene (ODCB), 1,2,3,4-tetrahydronaphthalene (THN) and 1,2,4-trichlorobenzene (TCB). The active layers were composed of poly(3-hexyl)thiophene (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). Special care has been taken to keep all experimental parameters constant (thickness of the active layers, donor/acceptor weight ratio, area of active surface and electrodes) in order to avoid artefacts and truly study the effect of solvents. Studies using atomic force microscopy (AFM) and optical absorption (UV-vis) showed the relationship between the photovoltaic performance and the evaporation rate of solvents. The use of solvents with high boiling point results in a higher degree of organization in the structure of P3HT. A direct comparison with devices processed with thermal treatment has also been performed. As often reported thermal annealing increases photo-conversion efficiency of devices created from common solvents, due to better separation of phase between the two materials of the blend. In the case of solvents with high boiling point such as THN and TCB, neither phase separation nor modification of P3HT crystallization induced by thermal annealing has been observed. However thermal treatment appears to enhance performance, ensuing the evaporation of remaining solvent in the active layers. An overview of the effect of solvent on the electrical properties of films containing pure P3HT and P3HT:PCBM blend reported in the literature has been completed for the discussion.

International audience; A phosphorescent cyclometalated heteroleptic Ir(III) complex was investigated as emitter for light-emitting electrochemical cells (LECs) and polymer organic light-emitting diodes (POLEDs). By exploring the deposition conditions of the emissive layer in multilayered electroluminescent devices, we directly impacted the emission color by mixing or not the complex with poly(vinylcarbazole) (PVK). Strategy of emission color tuning enabled to produce blue or yellow emitting devices. Heteroleptic complex was characterized by UV-visible absorption and luminescence spectroscopy. Cyclic voltammetry and DFT calculations were also performed.

In this article, we have studied the temperature and illumination dependence of open-circuit voltage ( V OC ) in polymer–fullerene based solar cells. It has been observed that V OC at higher illumination intensities gets converged at 0 K which gives information about maximum achievable V OC in a particular donor–acceptor blend. Besides this, recombination processes have been studied by transient open-circuit voltage decay (TOCVD) and the transition between recombination regimes has been observed for the first time. At low V OC carrier lifetime exhibits a constant value around 500 μs, which is interpreted in terms of a monomolecular recombination regime. At higher V OC carrier lifetime decreases as derived from a bimolecular relaxation law. The method allows estimating the recombination coefficient, which results in 2×10 −13 cm 3 s −1 . The results have been explained by considering Gaussian density-of-states (DOS) for highest-occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO).

International audience; In the field of polymer-based photovoltaic cells, poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)propyl-1-phenyl[6,6]C61 (PCBM) are, to date, the most-studied active materials around the world for the bulk-heterojunction structure. Various power-conversion efficiencies are reported up to approximately 5%. This Research News article is focused on a survey of the tremendous literature published between 2002 and 2010 that exhibits solar cells based on blends of P3HT and PCBM.

International audience; The anisotropy of the electronic interactions between fullerenes in crystalline solids was examined using a confocal fluorescence microscope by probing the polarization of the fluorescence emission arising from fullerene excimer-like emitting states. Crystals of C60 obtained by vacuum-sublimation or from chloroform solution exhibited no or little polarization (p = 0 or 0.11, respectively), as expected from the high symmetry of the C60 fcc lattice or the low degree of anisotropy induced by included solvent molecules. The use of hydrogen-bonding to supramolecularly control interfullerene electronic interactions was explored using a fullerene derivative (1) combining a solubilizing 3,4-di-tert-butylbenzene group and a barbituric acid hydrogen-bonding (H-B) moiety. The crystal structure of 1 establishes the existence of fullerene H-B tapes along which interfullerene electronic interactions are expected to be large. In agreement with this, we observe very strong polarization of the fullerene excimer-like emission (p = 0.78), indicative of a high degree of anisotropy in the fullerene interactions. The charge-carrier mobility of 1 as determined from OFET devices was found to be lower than that of C60 (1.2 × 10−4 vs 1.2 × 10−2 cm2/s V), which is rationalized on the basis of the reduced dimensionality of 1 as a wire-like semiconductor and variations in the morphology of the device active layer revealed by AFM measurments

International audience; We have studied the use of water and solvent-based polyaniline (PANI) dispersions as a transparent hole-injecting layer in polymer light emitting diodes (PLEDs). We have investigated the effects of the pH, the conductivity, the work function and the nature of the solvent and the dopant on the hole-injection properties. The water-dispersed PANI at a low pH (pH 1.8) showed an efficiency similar to that of PEDOT-based devices due to its high HOMO level and high conductivity. For the solvent-based PANI dispersions, the influence of the nature of the solvent and the dopant on the crystallinity, the conductivity of the PANIs and on the performances of the light emitting devices is discussed.

This paper reports the photoluminescence properties and the performance enhancement of organic light-emitting diodes (OLEDs) of a series of well-defined p-conjugated rigid rod oligomers analogous to poly(p-phenylenevinylene) (PPV). They are based on benzene-1,4- bis(phenylene vinylene) (OPPV) with electron-withdrawing (nitro) or electron-donating (methyl and dimethylamino) substituents in para-position of the terminal rings of the molecule. Substitution with methoxy and a long alkoxy group (hexadecyloxy) in 2,5-positions is also examined. Photoluminescence spectra obtained from films are discussed and compared to those previously reported in tetrahydrofuran solutions. OPPV oligomers were then used as emissive material in OLEDs and the I-V-L characteristics of the devices are presented. The results show that donating groups placed on the OPPV backbone increases the performances of the devices.

International audience

International audience

With the development of new technologies, researchers have recently focused their interest on flexible electronics to improve the interaction between users and devices. In this context, integrated ultra-low-pressure sensors remain a technological challenge. Here, we show that organic single crystal-based air-gap transistors fabricated by soft lithography are extremely sensitive to mechanical strain and pressure. When low forces are applied to the suspended semiconducting crystal (rubrene), the gated transistors show that drain currents as output signals are extremely sensitive to the deformations. Incredibly high gauge factors, over 4000, are measured using this technique. Further investigation shows that the contact resistance at the rubrene-gold interface is responsible for the high sensing performances. Such a platform is also able to detect sound waves with high sensitivity, making these simple devices suitable for applications in pressure sensing.

This paper reports an organic MEMS sensor with a cutting-edge electro-mechanical transducer based on an active organic field effect transistor (OFET). The strategy lies in the integration of a piezoelectric Poly (VinyliDene Fluoride/TriFluoroEthylene) (P(VDF/TrFE)) copolymer as active gate dielectric layer in an OFET device mounted on a polymer micro-cantilever. The charges generated by the piezoelectric layer due to surface strain are converted and amplified into drain current. Such an advanced scheme enables highly efficient integrated electromechanical transduction with record relative sensitivity ((ΔID/IDS)/e) over 600 in the low strain regime, constituting a key-step for the development of highly sensitive MEMS sensors.

The study of a [1]benzothieno[3,2-b][1]benzothiophene (BTBT) derivative decorated with hexyl chains functionalized with hydroxyl end groups is reported. A rapid and inexpensive functionalization of the BTBT in positions 2 and 7 has been developed. This compound is able to self-organize into a lamellar structure through σ-π stacking and van der Waals interactions but also through hydrogen bonding interactions. The hydrogen-bonded network controls the interlamellar region in terms of organization and stability. The liquid-crystal phase and structural changes observed by DSC have been characterized using an original approach combining FTIR and powder XRD measurements as a function of temperature. Thermally evaporated diol based OFETs exhibited good mobilities of up to 0.17 cm2 V-1 s-1 measured under an inert atmosphere but also in ambient air. The diol derivative is considered to be a very promising platform for the design of new functionalized BTBT., SCOPUS: ar.j, info:eu-repo/semantics/published

International audience; The efficiency of organic photovoltaic (OPV) solar cells is constantly improving; however, the lifetime of the devices still requires significant improvement if the potential of OPV is to be realised. In this study, several series of inverted OPV were fabricated and thermally aged in the dark in an inert atmosphere. It was demonstrated that all of the devices undergo short circuit current-driven degradation, which is assigned to morphology changes in the active layer. In addition, a previously unreported, open circuit voltage-driven degradation mechanism was observed that is highly material specific and interfacial in origin. This mechanism was specifically observed in devices containing MoO3 and silver as hole transporting layers and electrode materials, respectively. Devices with this combination were among the worst performing devices with respect to thermal ageing. The physical origins of this mechanism were explored by Rutherford backscattering spectrometry and atomic force microscopy and an increase in roughness with thermal ageing was observed that may be partially responsible for the ageing mechanism.

International audience; For efficient organic photovoltaic (OPV) solar cells, a low work function electrode is necessary to enhance the built-in voltage of the active layer, thereby improving the overall efficiency. Calcium is often used for this purpose in the laboratory; however, its development on a larger scale is impaired by its high reactivity with oxygen and water and the resulting low stability of solar cells under operation. The influence of a novel interlayer, lanthanum hexaboride (LaB6), on the electronic properties of OPV is studied in this work. Similarly to calcium, when LaB6 is used as an interlayer, it enhances the built-in voltage in the device, leading to a higher fill factor (FF) and optimal open circuit voltage (Voc). As a result, optimized LaB6-based devices present significantly improved power conversion efficiencies. More importantly, while calcium/aluminum (Ca/Al) and aluminum (Al) cathodes lose their capacity to enhance the internal electrical field during thermal aging, the LaB6/aluminum (LaB6/Al) electrode remains stable. This remarkable effect results in a highly stable Voc and flat-band potential during aging.

International audience; In organic photovoltaic (PV) devices based on solution-processed small molecules, we report here that the physicochemical properties of the substrate are critical for achieving high-performances organic solar cells. Three different substrates were tested: ITO coated with PEDOT:PSS, ZnO sol-gel, and ZnO nanoparticles. PV performances are found to be low when the ZnO nanoparticles layer is used. This performance loss is attributed to the formation of many dewetting points in the active layer, because of a relatively high roughness of the ZnO nanoparticles layer, compared to the other layers. We successfully circumvented this phenomenon by adding a small quantity of polystyrene (PS) in the active layer. The introduction of PS improves the quality of film forming and reduces the dark currents of solar cells. Using this method, high-efficiency devices were achieved, even in the case of substrates with higher roughness.

International audience; A new carbazole-based polymer PEPEK varying from the previously reported PEPK by the length of the spacer between the polymer backbone and the pendent carbazole moiety was investigated as polymeric host for solution-processed devices. Interestingly, if the two polymers are structurally close since the length of the alkyl chain only differs from one carbon atom, the previously reported PEPK gave higher performances than the newly synthesized PEPEK when tested as host for the wide bandgap triplet emitter FIrpic. To optimize electroluminescence performances, two device configurations were examined. On doping the emissive layer of phosphorescent organic light-emitting devices (OLEDs) at 16 wt% with FIrpic, best PEPK-based OLEDs gave an efficacy of 15.14 cd/A whereas PEPEK-based devices furnished an efficiency of 12.17 cd/A in the same conditions. To determine the origin of this unexpected behavior, the new polymer PEPEK was characterized by UV–visible absorption and luminescence spectroscopy as well as cyclic voltammetry. Thermal properties of PEPEK were also examined and compared to those of PEPK.

International audience; Luminescent materials based on 3,4-ethylenedioxythiophene (EDOT) and dihexylfluorene were designed. EDOT–fluorene– EDOT molecule, C3, was synthesized in high yields by a Stille cross-coupling reaction. The formylation of EDOT units yielded C4 molecule and a subsequent coupling reaction of C4 with a Wittig reagent produced C5. Electrochemical and optical experiments allowed us to accurately determine the energy level diagram corresponding to these three new molecules. Finally, in order to compare the electroluminescent behaviour of the bricks C3, C4 and C5, a simple monolayer OLED structure [ITO/PEDOT-PSS(50 nm)/Cx(50 nm)/Ca(100 nm)/Al(100 nm)] was designed. In the case of C5, energetic barriers at both interfaces were significantly reduced. For these reasons, devices based on C5 exhibit very high luminance values of approximately 1000 Cd/m2 at low applied voltage (below 10 V) with a turn-on voltage of 4.5 V. Maximum quantum and luminous efficiencies of 0.3 Cd/A and 0.1 lm/W are reported. According to the basic single layer structure, such values are very promising if C5 is used as emissive material in a multilayer-device.

Nous decrivons dans cette communication la realisation et l'evaluation de diodes electroluminescentes organiques monocouches a hautes performances basees sur le 1,1-dimethyl-2,5-bis(p-2,2'-dipyridylaminophenyl)-3,4-diphenylsilole agissant a la fois en tant que transporteur de trous et d'electrons. Ce compose presente de tres bonnes proprietes en electroluminescence dans le domaine de couleur jaune-vert avec une efficacite de 16 Cd/A et 4 Im/W a 100 Cd/m 2 . En comparaison avec les autres siloles decrits par Tamao et al. une telle amelioration des performances peut etre expliquee par un mecanisme de transfert d'energie entre les groupes aryles en position-2,5 et le noyau silole central.

International audience; Poly(2-(N-carbazolyl)ethyl acrylate), a poly(acrylate) comprised of carbazole-side groups attached via a flexible chain to the polymer backbone (PVAK) has been tested as host for solution-processed polymer light-emitting devices (PLEDs). This non-conjugated polymer proved to be an excellent candidate to host wide-bandgap phosphors. Notably, this polymer exhibited a high thermal stability (Td = 322 °C), a glass transition temperature (Tg) of 91 °C and a wide bandgap corresponding to the pendent carbazole units and the disrupted π-conjugation of the polymer main chain, making this polymeric host a suitable candidate for wide bandgap triplet emitters. When tested as a host for FIrpic and Ir(ppy)3, the resulting blue and green light-emitting devices showed a maximum luminous efficiency of 18.25 and 17.74 cd/A, respectively, which are comparable to recent reports of devices made using other carbazole-based oxygen-rich polymeric hosts. The polymer was also characterized by UV–visible absorption, photoluminescence spectroscopy as well as cyclic voltammetry

International audience; A series of organic–metal chelate complexes of symmetric tetradentate Schiff base ligands derived from salicylaldehyde and 2-hydroxy-1-naphthaldehyde were designed and investigated. The combination of five nonconjugated flexible spacers and five conjugated rigid spacers with four salicylaldehyde derivatives provided a series of forty complexes. The series of blue to red photoluminescent complexes was investigated by UV/Vis absorption and luminescence spectroscopy in solution and in the solid state as well as cyclic voltammetry. The nature of the bridge in the ligand scaffold was determined to be the main parameter that influences the absorption and the emission color of the investigated zinc(II) complexes