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Directional crystallization from the melt has been used as a tool to grow parallel crystalline stripes of p- and n-type molecular semiconductors. To start, the phase behavior of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT-C8): tetracyanoquinodimethane (TCNQ) blends (molar ratio from 20:1 to 1:1) has been investigated by variable-temperature X-ray diffraction, differential scanning calorimetry, and polarized optical microscopy. The partial charge transfer between the C8-BTBT-C8 donor and the TCNQ acceptor as a function of temperature has been studied. Blends of 10:1 and 20:1 have been selected for directional crystallization because they show similar thermotropic and phase behavior comparable to that of pure C8-BTBT-C8. Directional crystallization results suggest that moderate cooling rates (6 and 12 °C min-1) leads to a digitated growth mode that gives rise to parallel crystalline stripes of C8-BTBT-C8 and C8-BTBT-C8-TCNQ charge-transfer complexes (C8-BTBT-C8-TCNQ CT), as confirmed by confocal Raman imaging. X-ray diffraction reveals high preferential orientation and good in-plane alignment for both C8-BTBT-C8 and C8-BTBT-C8-TCNQ CT crystallites., SCOPUS: ar.j, info:eu-repo/semantics/published

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.

Accurate structural models for rubrene, the benchmark organic semiconductor, derived from synchrotron X-ray data in the temperature range of 100–300 K, show that its cofacially stacked tetracene backbone units remain blocked with respect to each other upon cooling to 200 K and start to slip below that temperature. The release of the blocked slippage occurs at approximately the same temperature as the hole mobility crossover. The blocking between 200 and 300 K is caused by a negative correlation between the relatively small thermal expansion along the crystallographic b-axis and the relatively large widening of the angle between herringbone-stacked tetracene units. DFT calculations reveal that this blocked slippage is accompanied by a discontinuity in the variation with temperature of the electronic couplings associated with hole transport between cofacially stacked tetracene backbones.

A directional crystallization of the compound 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT-C8) based on a temperature gradient approach has been used as a post-deposition process to control the crystalline morphology of thin films. C8-BTBT-C8 films of arbitrary thicknesses have been fabricated by spin-coating and subjected to the directional crystallization process. Dewetting is prevented by the liquid crystal phase of C8-BTBT-C8, allowing the formation of flat and rather uniform thin films with large crystalline domains. Spin-coating concentration, gradient magnitude, and cooling rate have been varied to explore a large set of crystallization conditions and to correlate them with the thin film morphology. The latter has been characterized by a combined use of optical profilometry, polarized optical microscopy, X-ray reflectometry, and X-ray diffraction measurements. The characterization results confirm that the roughness and crystalline order of the thin films are improved after the temperature gradient treatments: (1) high cooling rate treatments (≥9 °C min−1) significantly reduce the roughness of high thickness films, leading to good continuity and uniformity of the films; (2) dendritic growth dominates not only the films with low thickness but also films with high thickness treated at low cooling rates (, info:eu-repo/semantics/published

International audience; Bulk heterojunction polymer solar cells based on a novel combination of materials are fabricated using industry-compliant conditions for large area manufacturing. The relatively low-cost polymer PTQ10 is paired with the non-fullerene acceptor 4TIC-4F. Devices are processed using a non-halogenated solvent to comply with industrial usage in absence of any thermal treatment to minimize the energy footprint of the fabrication. No solvent additive is used. Adding the well-known and low-cost fullerene derivative PC61BM acceptor to this binary blend to form a ternary blend, the power conversion efficiency (PCE) was improved from 8.4% to 9.9% due to increased fill factor (FF) and open circuit voltage (VOC), while simultaneously improving the stability. The introduction of PC61BM is able to balance the hole-electron mobility in the ternary blends, which is favourable for high FF. This charge transport behavior is correlated with the bulk heterojunction (BHJ) morphology deduced from Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS), Atomic Force Microscopy (AFM) and surface energy analysis. In addition, the industrial figure of merit (i-FOM) of this ternary blend was found to increase drastically upon addition of PC61BM due to an increased performancestability-cost balance.

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; Organic semiconductors (OSCs) are promising transducer materials when applied in organic field-effect transistors (OFETs) taking advantage of their electrical properties which highly depend on the morphology of the semiconducting film. In this work, the effects of OSC thickness (ranging from 5 to 15 nm) on the piezoresistive sensitivity of a high-performance p-type organic semiconductor, namely dinaphtho [2,3-b:2,3-f] thieno [3,2-b] thiophene (DNTT), were investigated. Critical thickness of 6 nm thin film DNTT, thickness corresponding to the appearance of charge carrier percolation paths in the material, was demonstrated to be highly sensitive to mechanical strain. Gauge factors (GFs) of 42 ± 5 and −31 ± 6 were measured from the variation of output currents of 6 nm thick DNTT-based OFETs engineered on top of polymer cantilevers in response to compressive and tensile strain, respectively. The relationship between the morphologies of the different thin films and their corresponding piezoresistive sensitivities was discussed.

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.

Investigation of mechanical strain on the electrical characteristics of ferroelectric OFET base memory is crucial for novel flexible printed circuit. In this regards, the effects of compressive and tensile strain applied in parallel, 45° angle and perpendicular to the semiconductor channel were studied showing critical consideration to be taken into account before designing flexible memories.

Hybrid perovskites form an extremely attractive class of materials for large scale, low-cost photovoltaic applications. Fullerene-based charge extraction layers have emerged as a viable n-type charge collection layer, and in “inverted” p–i–n device architectures the solar cells are approaching efficiencies of 20%. However, the regular n–i–p devices employing fullerenes still lag behind in performance. Here, we show that partial solubility of fullerene derivatives in the aprotic solvents used for the perovskites makes it challenging to retain integral films in multilayer solution processing. To overcome this issue we introduce cross-linkable fullerene derivatives as charge collection layers in n–i–p planar junction perovskite solar cells. The cross-linked fullerene layers are insolubilized and deliver improved performance in solar cells enabled by a controllable film thickness.

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; Two new triarylamine-based wide bandgap small molecules differing by the position of their substituents were investigated as hosts for solution-processed organic light-emitting diodes (OLEDs). Blue-green, green and red OLEDs were realized with FIrpic, Ir(ppy)2(acac) and Ir(ppy)2(dbm) as triplet emitters respectively and the three layers constituting the device stacking were successively deposited with orthogonal solvents. Interestingly, one of the two bis(diphenylamino)naphthalene-based compounds, (NAP-1,5-DPA), furnished significantly enhanced EL performances compared to its isomeric counterpart. A maximum luminance of 3905 cd m−2 at 21 V was notably achieved with this material for devices comprising FIrpic as dopant. To get a deeper insight into these major differences in devices, the two host materials were characterized by UV-visible absorption and luminescence spectroscopy, and cyclic voltammetry. Thin films of the two materials were also examined by optical and atomic force microscopy. Thermal properties of the two hosts were also investigated as well as their electronic characteristics by theoretical calculations.

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.

We report a series of molecules that spontaneously self-organize into small electroluminescent domains of sub-micrometer dimensions when dissolved in tetrahydrofuran. The self-assembled spherical aggregates have an average diameter of 300 nm and exhibit efficient energy transfer from the blue to the green or red component. The aggregates can be chromatically addressed or patterned by selective bleaching of the energy-acceptor component using a laser source. This allows the fabrication of electroluminescence devices by directly photopatterning the active layer without the need of additional steps. Submicron features (700 nm) can be achieved using a collimated light source.

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.

The use of novel organosilica materials embedding π-conjugated moieties as semiconductor into field-effect transistors is demonstrated. The chosen π-conjugated core is a [1]benzothieno[3,2-b][1]benzothiophene that is modified with hydrolyzable and crosslinkable triethoxysilyl moieties. After polycondensation, this compound forms a hybrid material composed of charge transport pathways as well as insulating sublayers made of silicon oxide (SiOx). The hybrid material behaves as a semiconductor and is subsequently integrated as active layer into field-effect transistors. These precursors show J-type aggregation that evolves toward H-type aggregates during the sol–gel process, which improve charge transport. Taking advantage of the sol–gel chemistry involved here, hybrid field-effect transistors that are fully crosslinked with covalent bonds are built. Molecules are crosslinked to each other, covalently bonded to the silicon oxide dielectric, and also covalently bonded to the gold electrodes, thanks to the use of an appropriate additional interfacial monolayer. This is the first report of fully covalent transistors. Those devices with modest mobilities show impressive resilience against polar, aliphatic, and aromatics solvents even under sonication. This study opens the route toward a new class of hybrid materials to create highly robust electronic applications., SCOPUS: ar.j, info:eu-repo/semantics/published

A trimer of diketopyrrolopyrole (DPP), Tri-BTDPP, was synthesized and characterized. Tri-BTDPP has a HOMO level of −5.34 eV, a low band gap of 1.33 eV, and a hole mobility of ∼10 −3 cm 2 V −1 s −1 in organic field effect transistors (OFETs). Organic photovoltaic (OPV) devices using the donor/acceptor blends of Tri-BTDPP and PC 71 BM exhibited low power conversion efficiencies (PCE) of up to 0.72%, even though the desirable optical and electronic characteristics of this compound as a donor semiconductor for achieving high performance for OPV. Through an intensive study of the active layer using AFM, XRD, and DSC, it was found that Tri-BTDPP and PC 71 BM were unable to intermix and formed oversized Tri-BTDPP phases, resulting in poor charge separation. Some guidance on how to improve the OPV performance of Tri-DPP compounds is provided.

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.

cited By 8; International audience; Two donor-acceptor (D-A) copolymers containing dithienosilole (DTS) donor unit and unsubstituted benzothiadiazole (BT) or fluorinated benzothiadiazole (ffBT) acceptor unit, PDTSBT and PDTSffBT, were synthesized by Stille cross coupling polymerization and tested for application in polymer solar cells (PSCs). The new alternating copolymer (PDTSffBT) possesses both a low optical bandgap (Eg = 1.54 eV) and a deep highest occupied molecular orbital energy level (HOMO) (-5.46 eV). It was found that the introduction of the two electron-withdrawing fluorine atoms on the benzothiadiazole unit results in a decrease of the HOMO energy level with slight effect on the lowest unoccupied molecular orbital (LUMO) as observed through cyclic voltammetry (CV) analysis. Inclusion of fluorine atoms also leads to an increase in the interchain interaction in the PDTSffBT copolymer with respect to its analogue PDTSBT copolymer according to the X-ray diffraction measurements (XRD). When PDTSffBT:PC71BM blends are tested in bulk heterojunction (BHJ)-polymer solar cells, the cells display a short-circuit current (J SC) of 8.82 mA/cm2, a high open circuit voltage (V OC) of 0.74 V, and a Fill Factor (FF) of 45%, giving an overall power conversion efficiency (PCE) of 2.93%, compared to 1.64% for the BT-containing cells prepared in parallel under identical conditions. The significant performance enhancement results from the higher VOC and J SC in the ffBT-containing cells. These results unambiguously indicate that the fluorination is an efficient method to modulate the energetic levels and improve the photovoltaic performances of the widely used benzothiadiazole-based low bandgap copolymers. © 2014 Elsevier Ltd. All rights reserved.

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 linear array of aluminum discs is deposited between the driving electrodes of an extremely large planar polymer light-emitting electrochemical cell (PLEC). The planar PLEC is then operated at a constant bias voltage of 100 V. This promotes in situ electrochemical doping of the luminescent polymer from both the driving electrodes and the aluminum discs. These aluminum discs function as discrete bipolar electrodes (BPEs) that can drive redox reactions at their extremities. Time-lapse fluorescence imaging reveals that p- and n-doping that originated from neighboring BPEs can interact to form multiple light-emitting p–n junctions in series. This provides direct evidence of the working principle of bulk homojunction PLECs. The propagation of p-doping is faster from the BPEs than from the positive driving electrode due to electric field enhancement at the extremities of BPEs. The effect of field enhancement and the fact that the doping fronts only need to travel the distance between the neighboring BPEs to form a light-emitting junction greatly reduce the response time for electroluminescence in the region containing the BPE array. The near simultaneous formation of multiple light-emitting p–n junctions in series causes a measurable increase in cell current. This indicates that the region containing a BPE is much more conductive than the rest of the planar cell despite the latter’s greater width. The p- and n-doping originating from the BPEs is initially highly confined. Significant expansion and divergence of doping occurred when the region containing the BPE array became more conductive. The shape and direction of expanded doping strongly suggest that the multiple light-emitting p–n junctions, formed between and connected by the array of metal BPEs, have functioned as a single rod-shaped BPE. This represents a new type of BPE that is formed in situ and as a combination of metal, doped polymers, and forward-biased p–n junctions connected in series.

Organic photovoltaic (OPV) solar cells are a promising option for cheap, renewable energy, but must improve in their stability. This study examines changes in the IV parameters of inverted OPV devices with thermal ageing and correlates them to changes in the mechanical stability of the devices observed by fracture analysis. In particular, the role that the use of different materials in the hole transport layer (HTL) and metal electrode has in determining the stability (both mechanical and electronic) of the device is studied. Data from a range of characterization techniques (including Kelvin probe analysis and X-ray photoelectron spectroscopy elemental depth profiling) are used to correlate changes in device structure and performance, demonstrating the presence of inter-layer diffusion when silver is used as an electrode material. This inter-diffusion has the beneficial effect of improving the adhesion of the electrode to the device, but is correlated to declines in the performance of the device when used in conjunction with MoO3 as an HTL. An improvement in adhesion is also seen with aluminium electrodes, but without any signs of diffusion, showing that an improvement in the mechanical stability of a device when thermally aged need not come at the expense of performance stability.

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.

Great improvements in the development of organic photovoltaic (OPV) devices have been reported over the years; however, the overall efficiency and operational lifetimes of the devices must be improved. Maintaining a stable power conversion efficiency (PCE) in bulk heterojunction OPV devices can be achieved by utilizing cross-linkable ternary additives that freeze the optimal morphology. However, these additives currently do not contribute to improving the PCE of the device therefore limiting their overall effectiveness. In this study we present a dual functional bis(6-azidohexanoate)silicon phthalocyanine ((HxN3)2-SiPc) with cross-linking groups and near IR absorption as a ternary additive in poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PC61BM) devices. As an additive at 10 wt% with respect to PC61BM, (HxN3)2-SiPc increased the short circuit current density (Jsc) by ≈9% due to increased photocurrent generation in the near IR region. In addition, devices utilizing (HxN3)2-SiPc exhibited a 97% PCE retention after thermal ageing at 150 °C for 23 h (compared to 47% retention for baseline devices) showing the compound is an effective cross-linker. These findings represent the first example of a multifunctional dye additive in an OPV device that simultaneously broadens the spectral coverage, resulting in added photogeneration, and stabilizes the active layer morphology, resulting in increased PCE retention.

International audience; Electron injection by tunneling from a gold electrode and hole transport properties in polycrystalline MAPbI3 has been investigated using variable temperature experiments and numerical simulations. The presence of a large and unexpected band bending at the Au/MAPbI3 interface is revealed and attributed to the trapping of holes, which enhances the injection of electrons via tunneling. These results elucidate the role of volume and interface defects in state-of-the-art hybrid perovskite semiconductors.

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.

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

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.

We report here an original approach to dope the semiconducting polymer-metal interface in an inverted bulk-heterojunction (BHJ) organic solar cell. Solution-processed 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ), is deposited on top of a P3HT:PC61BM layer before deposition of the top electrode. Doping of P3HT by F4-TCNQ occurs after thermally induced diffusion at 100 °C of the latter into the BHJ. Diffusion and doping are evidenced by XPS and UV-vis-NIR absorption. XPS highlights the decrease in Fluorine concentration on top of the BHJ after annealing. In the same time, a charge transfer band attributed to doping is observed in the UV-vis-NIR absorption spectrum. Inverted polymer solar cells using solution-processed F4-TCNQ exhibit power conversion efficiency of nearly 3.5% after annealing. This simple and efficient approach, together with the low annealing temperature required to allow diffusion and doping, leads to standard efficiency P3HT:PC61BM polymer solar cells, which are suitable for printing on plastic flexible substrate.

International audience

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