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International audience; The stereo-electronic control over bisadditions of conjugated polymers to fullerene (C60) is explored in the formation of alternating copolymers. The chemistry, resulting configuration, and properties of poly(3-hexylthiophene)- alt-C60 copolymers prepared by either classic pyrrolidine ring formation or an atom transfer radical addition are compared. Both routes result in controlled additions of polymers to C60. Extensive macromolecular modeling through PM6 methods indicate that there is no conjugation between P3HT and C60 in the systems studied. This along with 2D-NMR, AFM, and photovoltaic characterizations of the materials indicate the importance of the structure of the modified C60 and the nature of the linking group between C60 and P3HT segments in determining the morphology of the copolymers in the solid state. © 2009 Wiley Periodicals, Inc.

We report investigations of a new purposefully designed and synthesized functionalized soluble poly( p -phenylene vinylene) (PPV) derivative bearing polar diode-like molecules (NLO-PPV). The active polar molecule is the 4-( N -butyl- N -2-hydroxyethyl)-1-nitro-benzene group on the copolymer. The grafting of the push–pull molecule with a donor/transmitter/acceptor structure, possessing a large ground state dipole moment, enables the efficient molecular orientation by a DC-electric field. An internal electric field could be stored in such system, facilitating exciton dissociation and improving charge transport in polymer photovoltaic single-layer devices, resulting in increased efficiency. In the test systems an increase of the external quantum efficiency by a factor of about 1.5 to 2 took place upon orientation of the polar molecules. The effect of orientation on the carrier injection and transport properties depending on the chain orientation direction was evidenced.

New functionalized soluble poly(p-phenylene vinylene) derivative bearing polar molecules was designed and synthesized in order to investigate effects of molecular orientation in polymer photovoltaic devices. The active polar molecule is the 4-(N -butyl-N -2-hydroxyethyl)-1nitro-benzene group. The grafting of the push-pull molecule with a donor/transmitter/acceptor structure, possessing a large ground state dipole moment, enables the molecular orientation by a dc electric field. An internal electric field stored in such system facilitates exciton dissociation and improves charge transport in single-layer devices. In our systems an increase in the external quantum efficiency by a factor of about 1.5 to 2 is estimated. The associated effects of orientation on the carrier injection and transport properties were evidenced.

The functionalized soluble poly(p-phenylene vinylene) derivative bearing polar molecules (nlo-ppv) was designed and synthesized to investigate effects of molecular orientation in single-layer polymer photovoltaic devices. The active molecule is the 4-(n-butyl-n-2-hydroxyethyl)-1-nitro-benzene group on the copolymer. The grafting of the molecule with a donor/transmitter/acceptor structure, possessing a large ground state dipole moment, enables their orientation by a dc-electric field. An internal electric field in such layer facilitates exciton dissociation and improves charge transport. We report increase of the external quantum efficiency by a factor of 1.5 to 2 upon orientation. The associated effects on the carrier injection and transport depending on the orientation direction were evidenced.

International audience; We report investigation of optical and electrical properties of poly(9-vinylcarbazole) (PVK) doped with 30 wt% 4-dibutylamino-4′-nitrostilbene (DBANS), depending on the molecular chain orientation of polar DBANS molecules. Orientation of dopant diode-like DBANS dye molecules was achieved by application of a static electric field through the polymer film, while heating the material near its glass transition temperature (Tg). Modification of optical properties due to the orientation was evidenced by the spectral dependencies of absorption coefficient. Appearance of the orientation-induced built-in electrical field was proven optically by the solid electric field induced second harmonic generation (SEFISHG) and electrically by current–voltage (IV) characterization. Measurement of the thermally stimulated currents (TSC) spectra demonstrated that carrier transport and trapping are affected, too. Changes of the TSCs induced by orientation were expressed best in the temperature range of 280–290 K. They could be attributed to the thermally activated process with activation energy of about 0.38 eV. Previous article in issue

The weak absorption of the photoactive layer appears as a one of the main factors limiting organic photovoltaic solar cells performances. In order to increase the interaction of the incident light with the photoactive materials, we investigate the effect of a periodic patterning of the solar cells surface with microstructures in the optical wavelength scale. In this aim, we present an original all optical patterning technique of polymer films. The method is based on a laser controlled mass transport in azopolymer films leading to efficient deformation of the film surface in conjunction with the incoming light interference pattern. The technique is used to pattern one-dimensional gratings on the surface of solar cells. In the work presented here, the cell photoactive material is based on the interpenetrated network of a conjugated donor polymer and a fullerene derivative. The cells investigated are illuminated in a reverse configuration through a semi-transparent top cathode. The effect of the periodic structures onto the incident light propagation has been investigated through optical characterizations. We demonstrate that a part of the incident light can be trapped inside the solar cell layers due to diffraction onto the periodic structures.

We describe in this article the quick optimization of bulk heterojunction organic solar cells based on poly-3-hexylthiophene and [6,6]-phenyl C61 butyric acid methylester blend. Best cells are found to be based on 1:1 in weight ratio and yield 3.6% power conversion efficiency under air-mass 1.5, 100 mW/cm2 illumination. On the optimized cells, ageing measurements have been realized to collect the absolute values of photovoltaic parameters along the time. On this study, accelerated lifetime has been calculated using the Arrhenius model for a first-order kinetic of degradation. According to the model, we can predict a long lifetime keeping more than 80% of the initial short-circuit current density after 1000 h of working.

on a 220-nm-thick film. The conversion efficiency is equal to 0.01% under 100 mW cm 22 white light illumination. Effects of dye loading, film thickness, and light intensity are described. A model based on simple assumptions is used to fit the photoaction spectra. By evaporation under vacuum of a layer of N,N8-dimethyl-3,4,9,10-perylenetetracarboxylic diimide between the ITO and the polymer film, we have fabricated hybrid polymer-molecularp ‐n type photovoltaic cells. The role of this molecule in the photogeneration has been studied. A constant power conversion efficiency of 0.15% has been measured under white light illumination between 0.2 and 200 mW cm 22 .© 2001

We built a new type of light-emitting device in which the emission layer is confined inside an aluminum channel. A 1200 A aluminum layer was first deposited on glass by vacuum evaporation and then carved using microlithographic techniques. Channels were typically 1.5 μm width. A 2000 A thick electroluminescent polymer film was spin coated on top of the Al-channel. The diode emits light under alternative voltages. It can be seen under room illumination. We studied the current–voltage characteristics and LED quantum efficiency. We also studied the glow-discharge effect of the nitrogen gas on the emission spectrum. Efficient electroluminescence may result from a field-induced ionization and recombination process.

Light-emitting diodes made of a single spin-coated layer of poly(9-vinylcarbazole) have been doped with coumarin-515 dye and the effect of the dye concentration on emission and electrical characteristics has been studied. Two different regimes have been identified. For low concentrations, the apparent hole injection barrier rises, holes are trapped, and mobility decreases. External quantum efficiency increases with concentration. At concentrations larger than 10 wt % coumarin aggregates, photoluminescence yield drops and electroluminescence yield increases up to 0.1% photons per electron. As a conclusion, using coumarin dye in a single-layer diode improves electron–hole injection and recombination balance more than using an additional hole-blocking layer.

International audience; In order to build efficient single-layer polymer photovoltaic devices, the realization of an equivalent distributed p-n junction is proposed. Orientation in the initially centrosymmetric material is obtained by ordering polar molecules contained in a polymer matrix with a DC field. The molecular rectification effect induced in an oriented polymer film improves the efficiency of polymeric semiconducting devices like solar cells. The first experiments were conducted with a poly(methylmethacrylate) (PMMA) matrix containing azo-dye compounds (such as the Disperse Red 1, DR1) as the polar molecules, grafted onto the chains of the polymer backbones. Although this material allows for high orientation efficiencies, it is not adapted for photovoltaic applications, since DR1-MMA has very poor semiconducting properties due to the dielectric nature of PMMA. Organic semiconductors are uncommon and it is difficult to find an appropriate polymer system. We present preliminary results obtained for two intrinsically semiconducting and dye-functionalised polymer systems which may be found applicable in the fabrication of organic photovoltaic solar cells using the concept of polar molecular ordering. We use polymer systems with a covalent bond between the dye molecule and conjugated backbone to obtain a high dye content and to avoid phase separation problems. To achieve the largest stable molecular polar orientation possible and to optimise the electric field stored in an oriented structure, we studied the orientation parameters of samples,i.e., the orientation field, temperature, poling duration, and cooling conditions.

Light-emitting diodes made of a single spin-coated layer of poly(9-vinylcarbazole) doped with Coumarin-515 dye have been prepared. The influence of layer thickness and dye doping ratio on blue emission and curent-injection characteristics has been studied.

We report on a joint morphological/photoelectrical study of polymer-based photovoltaic (PV) cells in working conditions. The bulk heterojunction devices investigated are based on an active layer of poly(3-hexyl thiophene) blended with methano-fullerene, combining good PV performances with promising stability. The set-up adopted allowed the electrical properties of the device to be directly correlated to the modification of the electrode morphological parameters (thickness and roughness), which were obtained by in situ energy dispersive X-ray reflectometry (EDXR). The results of this joint time-dependent characterization demonstrated how the observed photo-induced oxidation process, limited to the buried electrode interface, is responsible for a fast decrease in the photo-current. The time-resolved measurements allowed to rule out the dynamics of the morphological changes and showed that the interface morphology may be stabilized by annealing treatments, with a significant improvement of the cell efficiency.

The crucial requirement of device stability in the development of organic electronics was addressed. In particular, the nanoscale morphology of bulk heterojunction organic films for photovoltaic applications was studied by time-resolved energy dispersive X-ray reflectometry in synergy with atomic force microscopy analysis. A reorganization of the organic molecules in the film upon illumination was detected. The occurrence of two distinct processes (characterized by a reorganization of the blend bulk and an increase of its surface roughness, respectively) was revealed. Furthermore, the effect of the morphological instability on the device efficiency over time was quantified. Finally, the effect of thermal annealing treatments and of the choice of different cathodes was verified.

The stability and degradation of calcium/aluminium cathode organic solar cells are investigated in situ by time-resolved energy dispersive X-ray reflectometry. They combine the good charge carrier separation and transport properties of the poly(3-hexylthiophene-2,5-diyl):C 61 -butyric acid methyl ester (P3HT:PCBM) bulk heterojunction blend and the capability of the calcium/aluminium cathode to improve the fill factor and the open circuit voltage, with respect to aluminium cathodes cells. The study focuses on the crucial problem of the device structural/morphological stability in working condition. It aims to detect and control possible morphological variations at the various interfaces and to correlate these changes to the device aging.

International audience; We have investigated properties of poly(9-vinylcarbazole) (PVK) doped with 30 % wt 4-dibutylamino-4'-nitrostilbene (DBANS), depending on the orientation of the polar DBANS molecules. Appearance of the orientation-induced built-in electrical field was proven optically by the Solid Electric Field Induced Second Harmonic Generation and electrically by Current-Voltage characterization. Modification of optical properties was evidenced by the spectral dependencies of absorption coefficient. The Thermally Stimulated Currents spectra demonstrated that carrier transport and trapping are affected, too.

International audience

We have investigated properties of poly(9-vinylcarbazole) (PVK) doped with 30% wt 4-dibutylamino-4'-nitrostilbene (DBANS), depending on orientation of the polar DBANS molecules. Appearance of the orientation-induced built-in electric field was proven optically by the solid electric field induced second harmonic generation and electrically by the current-voltage characterization. Modification of optical properties was evidenced by the spectral dependencies of the absorption coefficient. The thermally stimulated current spectra had demonstrated that carrier transport and trapping are affected, too.

cited By 12; International audience; Chlorobenzene cast films of poly(3-hexylthiophene)-Wend-[6,6]-phenyl C 61 butyric acid methyl ester (P3HT-blend-PCBM) in 1:1 ratios were used in photovoltaic cells with the structure glass/ITO/PEDOT-Wend-PSS/P3HT- Wend-PCBM/LiF/Al. By varying the molecular weights of the PSHTs from 4500 to 280000 g mol-1, it was found that P3HT molecular weight distributions had an important impact on cell characteristics and the most effective annealing temperature required.

cited By 158; International audience; A series of poly(3-hexylthiophene)s (P3HTs) and poly(3-butylthiophene)s (P3BTs) with predetermined molecular weights and varying polydispersities are prepared using a simplified Grignard metathesis chain-growth polymerization. Techniques were elaborated to prepare extremely high molecular weight P3HT (number-average molecular weight of around 280000 g mol-1) with a low polydispersity (< 1.1) without resorting to fractionation. Optimization of the annealing of a series of solar cells based on blends of poly(3- alkylthiophene)s (PSATs) and [6,6]-phenyl C61 butyric acid methyl ester indicates that the polydispersities, molecular weights, and degrees of conjugation of the P3ATs all have an important impact not only on cell characteristics but also on the most effective annealing temperature required. The results indicate that each cell requires annealing treatments specific to the type of polymer and its molecular weight distribution. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA.

Among the class of conjugated polymers, polythiophenes and in particular 3-alkyl-substituted thiophenes seem to focus all the attention in the domain of photovoltaic conversion. At CEA, we are working on the optimization of bulk heterojunction solar cells made of poly-3-hexylthiophene (P3HT)and [6,6]-phenyl C 61 butyric acid methylester (PCBM) blend. First we will describe the influence of the ratio of P3HT and PCBM blend on the efficiency of the resulting bulk heterojunction solar cells. Best cells based on 1:1 in weight ratio yield 3.6 % power conversion efficiency under air-mass 1.5, 100 mW/cm 2 illumination. Then we will compare the efficiency and lifetime of different cells by changing the nature and thickness of cathode (Aluminum or Calcium/Silver). On the optimized cells, we have proceeded to ageing and accelerated lifetime measurements on devices with Ca/Ag cathode. It shows that the current densities decrease less than 3 % and that efficiency is still higher than 1.7 %, after 400 hours under AM 1.5, 100 mW/cm 2 illuminations and at high temperature (60°C).

International audience

International audience

International audience

Though being much less efficient than silicon cells, organic solar cells exhibit a unique combination of interesting properties: low cost, flexibility, and the possibility of large surface coverage. Large progresses have been made over the last years using MDMO-PPV (Poly[2-methoxy-5-(3’,7’-dimethyloctyloxy)-1,4-phenylenevinylene) reaching efficiencies of 2.9% and recently efficiencies over 3%, using poly(3-hexyl thiophene). A great deal of research however has still to be invested to improve the current state of the art. Among the main key-points to be addressed are namely the stability and lifetime of such devices. We are currently working on bulk heterojunction solar cells made from MDMO-PPV and PCBM (methano-fullerene[6,6]-phenyl C61-butyric acid methyl ester). Different batches of MDMO-PPV, originating from different synthesis modes (classical "Gilch" synthesis and "Sulphinyl" synthesis led by IMEC-IMOMEC) have been tested. Evolution of the power efficiency following continuous illumination (AM1.5, 80 mW.cm-2) was characterized under controlled atmosphere of nitrogen. In parallel, photodegradation studies are also investigated and electrical modeling is under way in order to get a better understanding of the relations between photochemical and electrical parameters of the diode that can be deduced from I/V curves.

International audience

In order to improve the photogeneration efficiency and charge transport in polymer photovoltaic cells, we orient diode like molecules inside a polymeric monolayer. Previous results gave experimental evidence of the induction of a rectification behavior as well as an increase in the charge mobility through polar orientation. We present here the first experimental realization where the photovoltaic conversion efficiency is increased by two orders of magnitude in a semiconducting polymer blend.

In the field of organic semiconductor devices, such as electroluminescent diodes or photovoltaic cells, a rectifying junction is mandatory to improve their efficiency. In order to build single polymer film photovoltaic cells, we realize an equivalent distributed p-n junction in a polymeric monolayer. For this purpose, we propose an inter-disciplinary principle derived from the application of nonlinear optical techniques to the technology of polymer semiconductors. We give experimental evidence that molecular rectification induced in an oriented amorphous polymer bearing polar chromophores improves efficiency of polymeric semiconductor device such as photovoltaic cells. Orientation is performed through DC- field ordering of the polar molecules contained in the polymer. Second harmonic generation is used as a probe of the molecular order and permits optimization of the sample orientation parameters, i.e., orientation-field, sample temperature, poling duration and cooling conditions to achieve the largest stable orientation. Additionally, to establish a model which describes charge injection and transport through such device, we propose to use Solid Electric Field Induced Second Harmonic Generation (SEFISHG) as an in situ internal field measurement technique. In this way, before orientation of the molecules, SEFISHG provides the electron work function difference between the two electrode materials, and after orientation, SEFISHG permits the measure of the internal field stored in the structure.

Light emitting diodes made of a single spin-coated layer of poly(9-vinylcarbazole) doped with coumarin-515 dye have been studied. The influence of dye concentration on emission and electrical characteristics is evidenced. Two different regimes are identified. At low concentrations, apparent hole injection barrier raises, holes are trapped and mobility decreases. External quantum efficiency increases with concentration. At concentrations larger than 10%wt, coumarin aggregates, photoluminescence yield drops and electroluminescence yield increases up to 0.1%-photons per electron. We show that using coumarin dye in a single-layer diode improves electron-hole injection and recombination balance more than using an additional hole-blocking layer.

In an attempt to build single polymer film photovoltaic cells, we underline the intrinsic rectifying nature of an oriented polymeric material. Orientation of the initially symmetric structure is performed through DC-field ordering of the polar molecules contained in the polymer. The internal field induced in the polymeric material is evidenced by the induction of a rectifying current-voltage characteristic. Second harmonic generation is used as a probe of the molecular order. Orientation induced rectification is proportional to the molecular order. This preparation technique opens a new route for the improvement of organic-semiconductor devices efficiency.

We developed a new type of light-emitting-diodes in which the emission layer is confined inside an aluminium channel. A 1200 angstrom aluminium layer was first deposited on glass by vacuum evaporation and then carved using microlithography techniques. Channels were typically 1.5 micrometers width. A 2000 angstrom thick electroluminescent polymer film was spin coated on top of the Al channel. The diode emits light under alternative voltages. It can be seen under room illumination. We studied current-voltage characteristics and LED quantum efficiency. Electroluminescence results from a field ionization and recombination process. No charge injection at electrodes takes place in such device.

International audience; Polymeric semiconductor devices are receiving increasing attention in view of potential applications requiring low-cost processing over large areas.'"*] In this respect, unlike with evaporated molecules, the wet-processing capability of polymers offers total compatibility with other complemen-tary technologies. The concepts from which organic-semi-conductor devices are designed are mostly derived from in-organic-semiconductor physics and technology.''] In order to build efficient organic-semiconductor devices such as electroluminescent or photovoltaic solar cells, a rectifying junction is required. Such junction can be of two main typed4] Schottky junctions between an organic semi-conductor and a metal, and p-n junctions between two p-and n-type organic-semiconductors. The Schottky junctions often suffer from degradation problems originating from electrochemical potential differences at the rectifying con-tacts, which induce diffusion of electrode material into the polymer.'" Such a drawback is corrected with p-n junctions in which metal to polymer contacts are ohmic. However, p-n junctions require the use of two polymer layers with wet-processing compatibility. Moreover n-type organic-semiconductors are less common than p-type ones owing to their lower stability under oxygen which is a consequence of their reductive behavior. We report here a different approach in which oriented diode-like molecules, the so-called push-pull molecules used in nonlinear optics,[6] are contained inside a polymer binder. Oriented molecules induce a rectifying effect, be-having as a distributed homojunction within a single poly-mer thin film. In order to demonstrate the principle, we start with an initially symmetric structure: the polymer film is sandwiched between two identical electrodes. The cur-rent-voltage characteristics of the device are also symme-trical. By application of a static electric field through the polymer film, while heating near the glass transition tem-perature (T,,) dopant molecules are oriented in the field with an order parameter (cosO).['] The current-voltage characteristics then become strongly asymmetric. Experimentally, molecular order is controlled using sec-ond harmonic generation (SHG): it appears that

Welcome from the organizers The rapidly developing field of nanotechnology opens up many exciting opportunities and benefits for new materials with significantly improved properties as well as some revolutionary applications in the fields of energy, environment, medicine, etc. These new materials potentially pave the way to considerable innovations in many industries of the 21st century although associated risks must be perfectly under control for workers, consumers and the environment. So, one can easily understand why Nanosafety is now considered as a specific new scientific area, gaining in importance and maturity every day. Following the successful outcome of the two past international conferences on the safe production and use of nanomaterials: Nanosafe 2008 and 2010, the organizing committee has the pleasure of welcoming again to Grenoble some of the most famous specialists of the field in the world. In addition to the standard issues addressed in previous nanosafe conferences such as Toxicology, Eco-toxicology, Expology, Detection, Life Cycle Analysis, Regulation and Standardization, new topics of great interest will be dealt with this year, concerning Governance and practical Risk Management for OSH experts and Societal issues. In order to enhance the exchanges and conviviality three debates werw organized around Nano Governance, Toxicology and Ethics. We hope that you will appreciate this new Nanosafe edition like the previous ones. This is your Nanosafe conference, please enjoy. The Nanosafe conference organisers Francois Tardif and Frederic Shuster Management Vanessa Gaultier Francois TardifFrederic Shuster Vanessa Gaultier Local Organizing Committee Vanessa GAULTIER (CEA) Francois TARDIF (CEA) Dominique BAGUET (CEA) Frededric SHUSTER (CEA) Yves SICARD (CEA) Catherine DURAND (CEA) Didier MOLKO (MINATEC) International Scientific Committee Chair: Frederic SCHUSTER (CEA, FR) Francois TARDIF (CEA, FR) Co-chair: Georgios KATALAGARIANAKIS (EC, BE) Melanie AUFFAN (CEREGE, FR) Jorge BOCZKOWSKI (INSERM, FR) Jean-Yves BOTTERO (CEREGE, FR) Jacques BOUILLARD (INERIS, FR) Derk BROUWER (TNO, NL) Marie CARRIERE (CEA, FR) Claude EMOND (U. MONTREAL, CA) Cassandra ENGEMAN (UCSB, USA) Eric GAFFET (CNRS, FR) Francois GENSDARMES (IRSN, FR) Alexei GRINBAUM (CEA, FR) Antje GROBE (U.STUTTGART, DE) Peter HOET (KUL, BE) Eric QUEMENEUR (CEA, FR) Olivier LE BIHAN (INERIS, FR) Robert MUIR (NANEUM, UK) Tinh NGUYEN (NIST, USA) Bernd NOWACK (EMPA, CH) Gunter OBERDORSTER (U. ROCHESTER, USA) David PUI (U. MINNESOTA, USA) Michael RIEDIKER (IST, CH) Yves SAMSON (CEA, FR) Ken TAKEDA (U. TOKYO, JP) Olivier WITSCHGER (INRS, FR) The PDF contains a list of sponsor logos, the conference programme and planning documents.

In order to build single polymer film photovoltaic cells, we realize an equivalent distributed PN junction in a polymeric monolayer. Orientation is performed through DC-field ordering of the polar molecules contained in the polymer. We describe experimentally an increase by one order of magnitude for the efficiency of our photovoltaic cell by polar ordering of the molecules.

The photovoltaic characteristics of spin-coated thin films of polythiophene sensitized with two different dyes have been measured. We reported on the influence of the affinity between the polymer, and the dye. By changing the dye, the power conversion efficiency is multiplied by 20 to reach 0.01% under a white light of 50 mW.cm −2 .

Cubic susceptibility of poly(butyl)thiophene has been studied by optical third harmonic generation (THG) in solution and in solution cast thin film as a function of fundamental wavelength. The thin film data have been obtained in the polymer absorption range for harmonic wave and show existence of a large three photon resonance. For polymer solutions the THG measurements have been done at 1.064 and 1.907 μm fundamental wavelength. At 1.907 μm the cubic susceptibility is complex with negative real part whereas at 1.064 μm it is real and positive. The observed large three photon resonance is interpreted as a consequence of inhomogenous one photon absorption broadening.

The electrode-active layer interface of organic photovoltaic cells, a critical point in the development of organic devices, was studied by the energy dispersive x-ray reflectivity (EDXR) technique applied in situ. An EDXR-based protocol allowing discrimination between the possible mechanisms that produce the aging process at the interface was established. The study detects photoinduced oxidation of the electrode at the buried interface, to which fading of the device performances could be attributed. This conclusion was further confirmed by results obtained on a new cell, of selectively modified architecture, whose performances turned out to be stable in time.

Carrier transport and trapping effects, as they are influenced by molecular chain orientation, were investigated by Thermally Stimulated Currents (TSC) and Current-Voltage (IV) characteristics in the samples of poly(9-vinylcarbazole) (PVK) doped with 30% wt 4-dibutylamino-4′-nitrostilbene (DBANS). The orientation of DBANS, diode-like molecules, was performed by electric field above the glass transition temperature. We demonstrate that the orientation of polar molecules causes significant changes both in IV dependencies and the TSC spectra. Changes of the TSCs induced by orientation were expressed best in the temperature range of 280–290 K. They could be attributed to the thermally activated process with activation energy of about 0.38 eV.

Submicrometric periodic patterning of an organic solar cell surface is investigated in order to optimize the photovoltaic conversion efficiency of the device. Patterning is achieved using a single-step all-optical technique based on photoinduced mass transport in azopolymer films. The polymer film with a structured surface is used as a substrate for an organic solar cell based on a copper phthalocyanine/C60 heterojunction. The effect of periodic patterning is investigated through the solar-cell optical-absorption properties and external quantum efficiency measurements. The possibility to increase the short circuit current density and the corresponding photovoltaic conversion efficiency is evidenced with one-dimensional periodic structures.

Second- and third-order hyperpolarizabilities that are obtained with various experimental and theoretical techniques are compared. This comparison requires care in the conventions used in defining the various parameters that enter the data analysis. Experimental values of γ are obtained for p-dioxane, n-hexane, acetone, CHCl3, CCl4, and benzene with both third-harmonic generation and electric-field-induced second-harmonic generation techniques. In addition, values of γ and β for 2-methyl, 4-nitroaniline in p-dioxane and CHCl3 are reported. These experimental results are compared with values of γ and β obtained from a finite-field semiempirical calculation.

International audience

We underline the intrinsic rectifying nature of an oriented polymeric material. Orientation of the initially symmetric structure is performed through dc-field ordering of the polar molecules contained in the polymer. The internal field induced in the polymeric material is evidenced by the induction of a rectifying current-voltage characteristic. Second harmonic generation is used as a probe of the molecular order. Orientation induced rectification is proportional to the molecular order. This preparation technique opens a new route for the improvement of organic-semiconductor devices efficiency. Additionally, electric-field induced second-harmonic generation (EFISHG) is used as an internal field measurement technique. It offers an original characterization tool for charge injection and transport studies in organic semiconductors.

We study the influence of the doping ratio on current-voltage characteristics and electroluminescence quantum efficiency of single-layer poly(9-vinylcarbazole) (PVK)-polymer blue light-emitting diodes. Driving voltage increases monotonically with coumarin doping ratio, up to 10 −1% photons/electron for a 30 wt % doping ratio. Time-of-flight experiments give evidence of a hole-trapping effect of the coumarin. It improves the balance for injection of holes and electrons. Holes which drift through the film are expected to have a higher probability of recombination with electrons on the coumarin luminescent centers.

International audience

A submicrometric periodical patterning of the surface of an organic solar cell is investigated towards optimisation of the photovoltaic conversion efficiency. Patterning is achieved using a technique based on photoinduced mass transport in azopolymer films.

Polymeric semiconductor devices are receiving increasing attention in view of potential applications requiring low-cost processing over large areas.'"*] In this respect, unlike with evaporated molecules, the wet-processing capability of polymers offers total compatibility with other complemen-tary technologies. The concepts from which organic-semi-conductor devices are designed are mostly derived from in-organic-semiconductor physics and technology.''] In order to build efficient organic-semiconductor devices such as electroluminescent or photovoltaic solar cells, a rectifying junction is required. Such junction can be of two main typed4] Schottky junctions between an organic semi-conductor and a metal, and p-n junctions between two p-and n-type organic-semiconductors. The Schottky junctions often suffer from degradation problems originating from electrochemical potential differences at the rectifying con-tacts, which induce diffusion of electrode material into the polymer.'" Such a drawback is corrected with p-n junctions in which metal to polymer contacts are ohmic. However, p-n junctions require the use of two polymer layers with wet-processing compatibility. Moreover n-type organic-semiconductors are less common than p-type ones owing to their lower stability under oxygen which is a consequence of their reductive behavior. We report here a different approach in which oriented diode-like molecules, the so-called push-pull molecules used in nonlinear optics,[6] are contained inside a polymer binder. Oriented molecules induce a rectifying effect, be-having as a distributed homojunction within a single poly-mer thin film. In order to demonstrate the principle, we start with an initially symmetric structure: the polymer film is sandwiched between two identical electrodes. The cur-rent-voltage characteristics of the device are also symme-trical. By application of a static electric field through the polymer film, while heating near the glass transition tem-perature (T,,) dopant molecules are oriented in the field with an order parameter (cosO).['] The current-voltage characteristics then become strongly asymmetric. Experimentally, molecular order is controlled using sec-ond harmonic generation (SHG): it appears that