Publications

2011

• White Organic Light‐Emitting Diodes Based on Quench‐Resistant Fluorescent Organophosphorus Dopants
  
  • Joly Damien
  • Tondelier Denis
  • Deborde Valérie
  • Delaunay Wylliam
  • Thomas Anup
  • Bhanuprakash Kotamarthi
  • Geffroy Bernard
  • Hissler Muriel
  • Réau Régis
  Advanced Functional Materials, Wiley, 2011, 22 (3), pp.567-576. The control of the doping ratio of a blue-emitting matrix by an orange emitter with high accuracy still remains very challenging in the development of reproducible white organic light-emitting diodes (WOLEDs). In this work, the development of an organophosphorus dopant that presents a high doping rate in order to reach white emission is reported. The increase of the doping rate has a small impact on the CIE co-ordinates and on the EQE. These results are very appealing towards the development of "easy-to-make" WOLEDS. (10.1002/adfm.201102005)
  DOI : 10.1002/adfm.201102005
• Design and Fabrication of a 1 THz Backward Wave Amplifier
  
  • Paoloni Claudio
  • Di Carlo Aldo
  • Brunetti Francesca
  • Mineo Mauro
  • Ulisse Giacomo
  • Durand Alain
  • Krozer Victor
  • Kotiranta M.
  • Fiorello Anna Maria
  • Dispenza M.
  • Secchi Alberto
  • Zhurbenko Vitaly
  • Bouamrane Faycal
  • Bouvet Thomas
  • Megtert Stephan
  • Tamburri E.
  • Cojocaru Costel Sorin
  • Gohier Aurelien
  Terahertz Science and Technology, 2011, 4, pp.149-163. The THz frequency range represents a true challenge for designers, fabrication technologies and characterization systems. So far, huge technological obstacles have prohibited any system realization different from laboratory one. Furthermore, most of the applications in the THz frequency range require a level of power not achievable by optoelectronic devices at room temperature or by solid-state technology. The recent availability of three-dimensional simulators and high aspect ratio micro-fabrication techniques has stimulated a class of vacuum electron devices operating in the THz regime, to get a level of output power to enable applications at these frequencies. The OPTHER (Optically driven THz amplifier) project, funded by the European Community, is on the road to realize the first 1 THz vacuum tube amplifier. Technology at the state of the art has been used for the realization of the parts with dimensions supporting THz frequencies. A backward wave amplifier configuration is chosen to make the parts realizable. A carbon nanotube cold cathode has been considered for electron generation. A thermionic micro electron gun is designed to test the tube. A novel slow-wave structure (SWS), the double corrugated rectangular waveguide, is devised to support a cylindrical electron beam and to guarantee high interaction impedance with limited losses. Both LIGA and UV SU-8 photolithography have been tested to realize the SWS.
• Imagerie polarimétrique par fibre optique
  
  • Desroches Jérôme
  • Lefort Claire
  • Ducourthial Guillaume
  • Mansuryan Tigran
  • Brevier Julien
  • de Martino Antonello
  • Louradour Frédéric
  • Pagnoux Dominique
  • Barthélémy Alain
  , 2011.
• Etude et réalisation de transistors à nanotubes de carbone pour la détection sélective de gaz.
  
  • Gorintin Louis
  , 2011. Ce travail de thèse porte sur la réalisation d'un capteur d'espèces chimiques gazeuses à partir de transistors à nanotubes de carbone à effet de champs (i.e. CNTFET). Cette nouvelle génération de capteurs présente de nombreux avantages : compacts et bas coût, ils peuvent être intégrés dans des systèmes de détection ultrasensibles et autonomes. Ils sont destinés à des applications de sécurité civile ponctuelle ou en réseau comme la détection de gaz d'attaque ou d'explosifs. Nous proposons la réalisation de transistors à tapis aléatoires de nanotubes de carbone. L'utilisation de ces tapis permet de résoudre les problèmes de fabrication en quantité et de reproductibilité rencontrée avec les transistors à un seul nanotube. La première partie de nos travaux repose sur la mise au point d'une méthode de dépôt de tapis de nanotubes à l'aide d'un aérographe assisté d'un robot automatisé: des poudres commerciales de nanotubes de carbone sont mises en solution puis déposées de manière aléatoire par atomisation de micro gouttes de solvant. Les tapis ainsi obtenus permettent de réaliser en grand nombre de transistors avec des performances électriques reproductibles. La seconde partie présente le développement d'une matrice de transistors composée par des électrodes de différente nature (platine, palladium, or, nickel, titane) afin de répondre au problème de la sélectivité pour la détection gazeuse rencontré par ce type de dispositif. Ainsi nous discriminons les gaz en réalisant une empreinte électronique exploitant la modification spécifique des caractéristiques des différentes jonctions métal/nanotubes, et donc des caractéristiques de transfert des CNTFETs. Notre capteur présente une sensibilité et une sélectivité à l'ammoniac, au dioxyde d'azote, au diméthyle methylphosphonate (simulant du gaz sarin) et à l'eau oxygénée. La production en grand nombre de ces éléments permet de valider le potentiel industriel de ces capteurs.
• Ethanol-mediated metal transfer printing on organic films
  
  • Aldakov Dmitry
  • Tondelier Denis
  • Palacin Serge
  • Bonnassieux Yvan
  ACS Applied Materials & Interfaces, Washington, D.C. : American Chemical Society, 2011, 3 (3), pp.740-745. Ethanol-mediated metal transfer printing (mTP) is a soft method, which allows to efficiently deposit metals onto various organic surfaces for applications in organic electronics. This simple approach in based on the stronger adhesion of the metals to the organic materials in the presence of thin ethanol layer between the metallized PDMS and the substrate due to the capillary action. Patterns with a resolution of at least 20 μm have been obtained on organic polymeric materials and photoresists without heating or applied pressure. Compared to other methods ethanol mediated mTP is considerably faster and has smaller limitations on the stamp depth. Residual silicone layer detected on the metal surface after the transfer by XPS studies has been mostly removed by UV/ozone treatment. Organic field-effect transistors (OTFTs) based on the metal electrodes deposited by mTP have been successfully fabricated and tested. (10.1021/am101085k)
  DOI : 10.1021/am101085k
• Operating mechanism of the organic metal-semiconductor field-effect transistor (OMESFET)
  
  • Kim Chang Hyun
  • Tondelier Denis
  • Geffroy Bernard
  • Bonnassieux Yvan
  • Horowitz Gilles
  European Physical Journal: Applied Physics, EDP Sciences, 2011, 56, pp.34105. Organic metal-semiconductor field-effect transistors (OMESFETs) were fabricated with a polycrystalline organic semiconductor (pentacene) and characterized in order to systematically analyze their operation mechanism. Impedance measurements confirmed full depletion of the thick pentacene film (1 μm) due to the low doping concentration of unintentional doping (typically less than 10^14 cm^−3). The necessity of developing a specific device model for OMESFET is emphasized as the classical (inorganic) MESFET theory based on the depletion modulation is not applicable to a fully-depleted organic semiconductor. By means of joint electrical measurements and numerical simulation, it is pointed out that the gate voltage controls the bulk distribution of injected carriers, so that the competition between the gate and drain currents is critical for determining the operation mode. Finally, the geometrical effect is investigated with comparing a number of transistors with various channel widths and lengths.
• Comparaison des outils optique et radar en polarimétrie bistatique
  
  • Trouvé Nicolas
  , 2011. L'information polarimétrique des ondes électromagnétiques est fondamentalement semblable pour les micros ondes (radar), l'infrarouge et le visible (optique). Cependant, les échelles et les moyens de mesures impliqués varient très fortement ce qui explique que les divers outils développés (étalonnage, configuration géométrique, filtrage du bruit) divergent entre les différentes communautés. Diffèrent notamment : la géométrie d'acquisition, classiquement bistatique en optique et majoritairement monostatique en radar, la nature de la mesure, cohérente pour le radar (intensité et phase) et en énergie pour l'optique ; les traitements. Cette thèse exploite l'utilisation transverse des outils appartenant à ces deux mondes afin d'améliorer notre compréhension globale de la polarimétrie et son exploitation. Nous avons d'abord adapté une méthode de calibration inspirée de l'optique au cas du radar bistatique. Ensuite nous avons analysé l'impact des géométries bistatiques et des choix de référence, pour proposer le choix du plan de diffusion dans l'analyse des images radar. Puis nous nous sommes penchés sur l'estimation de la matrice de covariance polarimétrique dans le cadre du radar. Ceci a permis de proposer une méthode originale de segmentation des images SAR ainsi que de nouveaux types de détecteurs. Enfin, nous présentons les différents "traitements polarimétriques" à proprement parler, et diverses applications auxquelles ces traitements peuvent servir : caractérisation de milieux dépolarisants en optique (tissus) ou radar (forêt), aide à la restitution 3D.
• Growth-in-place deployment of in-plane silicon nanowires
  
  • Yu Linwei
  • Chen Wanghua
  • O'Donnell Benedict
  • Patriarche Gilles
  • Bouchoule Sophie
  • Pareige Philippe
  • Rogel Regis
  • Salaün Anne-Claire
  • Pichon Laurent
  • Roca I Cabarrocas Pere
  Applied Physics Letters, American Institute of Physics, 2011, 99, pp.203104. Up-scaling silicon nanowire (SiNW)-based functionalities requires a reliable strategy to precisely position and integrate individual nanowires. We here propose an all-in-situ approach to fabricate self-positioned/aligned SiNW, via an in-plane solid-liquid-solid growth mode. Prototype field effect transistors, fabricated out of in-plane SiNWs using a simple bottom-gate configuration, demonstrate a hole mobility of 228 cm2/V s and on/off ratio >103. Further insight into the intrinsic doping and structural properties of these structures was obtained by laser-assisted 3 dimensional atom probe tomography and high resolution transmission electron microscopy characterizations. The results could provide a solid basis to deploy the SiNW functionalities in a cost-effective way. (10.1063/1.3659895)
  DOI : 10.1063/1.3659895
• Modeling the low-voltage regime of organic diodes: Origin of the ideality factor
  
  • Kim Chang Hyun
  • Yaghmazadeh Omid
  • Bonnassieux Yvan
  • Horowitz Gilles
  Journal of Applied Physics, American Institute of Physics, 2011, 110, pp.093722. This paper investigates the physics of single-layer organic diodes in the low-voltage regime. A simple analytical model is developed to describe the current-voltage characteristics of the device. At variance with what is often reported in the literature, the operating mechanism of the organic diode is closer to that of the p-n junction than that of the conventional Schottky diode. The influence of an exponential distribution of traps is also analyzed. Alongside a drastic reduction of the current at above-diffusion-potential regime, traps introduce a substantial ideality factor in the low-voltage current. Two-dimensional physically based simulations are carried out in order to ascertain the validity of our model. By including trap effects, device simulation could fairly fit the experimental data of the organic diodes made of vacuum-evaporated pentacene. (10.1063/1.3660221)
  DOI : 10.1063/1.3660221
• Iron catalysts for the growth of carbon nanofibers: Fe, Fe3C or both?
  
  • He Zanbing
  • Maurice Jean Luc
  • Gohier Aurelien
  • Lee Chang Seok
  • Pribat Didier
  • Cojocaru Costel Sorin
  Chemistry of Materials, American Chemical Society, 2011, 23 (24), pp.5379-5387. Iron is a widely used catalyst for the growth of carbon nanotubes (CNTs) or carbon nanofibers (CNFs) by catalytic chemical vapor deposition. However, both Fe and Fe−C compounds (generally, Fe3C) have been found to catalyze the growth of CNTs/CNFs, and a comparison study of their respective catalytic activities is still missing. Furthermore, the control of the crystal structure of iron-based catalysts, that is α-Fe or Fe3C, is still a challenge, which not only obscures our understanding of the growth mechanisms of CNTs/CNFs, but also complicates subsequent procedures, such as the removal of catalysts for better industrial applications. Here, we show a partial control of the phase of iron catalysts (α-Fe or Fe3C), obtained by varying the growth temperatures during the synthesis of carbon-based nanofibers/nanotubes in a plasma-enhanced chemical vapor deposition reactor. We also show that the structure of CNFs originating from Fe3C is bamboo-type, while that of CNFs originating from Fe is not. Moreover, we directly compare the growth rates of carbon-based nanofibers/nanotubes during the same experiments and find that CNFs/ CNTs grown by α-Fe nanoparticles are longer than CNFs grown from Fe3C nanoparticles. The influence of the type of catalyst on the growth of CNFs is analyzed and the corresponding possible growth mechanisms, based on the different phases of the catalysts, are discussed. (10.1021/cm202315j)
  DOI : 10.1021/cm202315j
• Synthesis and characterization of all-conjugated copolymers of 3-hexyl-thiophene and EDOT by grignard metathesis polymerization
  
  • Miozzo L.
  • Battaglini N.
  • Braga D.
  • Kergoat Laurent
  • Suspène C.
  • Yassar A.
  Journal of Polymer Science Part A: Polymer Chemistry, Wiley, 2011, 50 (3), pp.534-541. Random regioregular copolymers of 3-hexylthiophene and 3,4-ethylenedioxythiophene were synthesized by grignard metathesis polymerization. Soluble copolymers were obtained with a high degree of regioregularity and with a monomer ratio close to the feed ratio. A comparison between the optical properties and the thin film morphologies of these copolymers and poly(3-hexylthiophene) is also presented. (10.1002/pola.25062)
  DOI : 10.1002/pola.25062
• Angle resolved Mueller Polarimetry and applications to periodic structures
  
  • Fallet Clément
  , 2011. With the constant decrease of the size of the transistors in microelectronics, the characterization tools have to be more and more accurate and have to provide higher and higher throughput. Semiconductor manufacturing being a layer-by-layer process, the fine positioning of the stack is crucial. The misalignment of the stack is called overlay and we here propose a new tool and method to accurately characterize overlay by measuring a single target built in the scribe lines. The method uses the fundamental symmetry properties of the Mueller matrix acquired in the back focal plane of a high-aperture microscope objective and enables a characterization of the overlay with a total measurement uncertainty of 2nm. After a brief introduction to polarization and the Mueller matrix, we describe the new design of the instrument and its complete calibration. The main body of this manuscript is dedicated to the overlay characterization but the applications of this instrument are very diverse so we also detail how our instrument can shed some light on the characterization and the understanding of the auto-organization of some scarab beetles' exoskeleton. These beetles exhibit a very strong circular dichroism and many research groups around the world try to mimic their exoskeleton. We conclude this manuscript with a brief overview of the main perspectives from our instrument.
• Biomedical applications of polarimetric imaging contrast. Initial studies for scattering media and human tissues
  
  • Antonelli Maria Rosaria
  , 2011. Improving the visualization of dysplastic regions of uterine cervix in vivo is essential for a better identification of the locations to biopsy and a better definition of the boundaries of the anomalous regions to be surgically removed. For this purpose we propose an innovative optical technique based on multispectral full Mueller polarimetric imaging in backscattering configuration. Measurements on ex vivo samples were performed to define the best acquisition procedures and data treatments for in vivo diagnosis. As this optimization requires thorough understanding of polarimetric contrasts between healthy and anomalous tissues, we simulated the propagation of polarized light in multilayer structures representative of real tissues. These structures typically involve an uppermost layer describing the epithelium and/or superficial connective tissue, on top of a totally depolarizing lambertian surface lumping the contribution of deeper layers. The simulations were performed by using a Monte Carlo code which has been modified and adapted to our topic. We thus showed that the contribution of the cell nuclei is in fact quite small in the backscattering geometry. For connective tissue, collagen fibers were modelled as 200 nm radius scatterers. Once more this contribution alone could not reproduce the main experimental features. Very small scatterers (50 nm typical radius) have to be included to account for the Rayleigh-like polarimetric response observed in all tissues, both healthy and diseased. These scatterers may be representative of proteins, whose concentration seems to be a crucial parameter to account for the observed contrasts. In this sense, polarimetric imaging may reflect not only the tissue morphology as seen by optical microscopy, but also its physiological state, which may be a very important point for cancer detection and staging.
• Overlay measurements by Mueller polarimetry in back focal plane
  
  • Fallet Clément
  • Novikova Tatiana
  • Foldyna Martin
  • Manhas Sandeep
  • Haj Ibrahim Bicher
  • de Martino Antonello
  • Vannuffel Cyril
  • Constancias Christophe
  Journal of Micro/Nanolithography, MEMS, and MOEMS, Society of Photo-optical Instrumentation Engineers, 2011, 10 (3), pp.033017. Angle resolved Mueller polarimetry implemented as polarimetric imaging of a back focal plane of a high NA microscope objective has already demonstrated a good potential for CD metrology. Here we present the experimental and numerical results indicating that this technique may also be competitive for the measurements of overlay error delta. A series of samples of superimposed gratings with well controlled overlay errors have been manufactured and measured with the angle resolved Mueller polarimeter. The overlay targets were 20-mu m wide. When the overlay error is delta is equal to 0, absolute values of elements of real 4x4 Mueller matrix M are invariant by matrix transposition. Otherwise this symmetry breaks down. Consequently, we define the following overlay estimator matrix as E = |M| - |M|(t). The simulations show that matrix element E-14 is the most sensitive to the overlay error. The scalar estimator of E-14 was calculated by averaging the pixel values over a specifically chosen mask. This estimator is found to vary linearly with d for overlay values up to 50 nm. Our technique allows entering small overlay marks (down to 5-mu m wide). Only one target measurement is needed for each overlay direction. The actual overlay value can be determined without detailed simulation of the structure provided two calibrated overlay structures are available for each direction. (10.1117/1.3626852)
  DOI : 10.1117/1.3626852
• Growth of graphene using a nickel thin film
  
  • Lee C.S.
  • Maurice J.L.
  • He Z.B.
  • Pribat D.
  • Châtelet Madeleine
  • Le Normand F.
  • Speisser C.
  • Cojocaru C.S.
  , 2011.
• Optical absorption in vertical silicon nanowires for solar cell applications
  
  • Foldyna Martin
  • Yu Linwei
  • O'Donnell Benedict
  • Roca I Cabarrocas Pere
  Proceedings of SPIE, the International Society for Optical Engineering, SPIE, The International Society for Optical Engineering, 2011, 8111, pp.10. Photovoltaic research has moved from popular solar cells, based on crystalline silicon substrates with thicknesses of around 250 μm, to the thin film structures saving large amount of the active material. The next generation of solar cells requires substantial increase of the energy conversion efficiency, which can be achieved by enhancing of the optical trapping inside the cell. In this work we study the efficiency of light trapping inside vertical silicon nanowire solar cells. The main focus is on the optical trapping inside single vertical nanowires, which can enhance optical absorption far beyond capabilities of a thin film. Spectral optical absorption modeling based on RCWA together with the electromagnetic field distribution analysis gave insight into the light trapping inside the nanowires. Results provide a guide for the optimization of nanowires diameters, density and length for maximal short circuit currents with minimal material demands. (10.1117/12.892690)
  DOI : 10.1117/12.892690
• Fundamental Benefits of the Staggered Geometry for Organic Field-Effect Transistors
  
  • Kim Chang Hyun
  • Bonnassieux Yvan
  • Horowitz Gilles
  IEEE Electron Device Letters, Institute of Electrical and Electronics Engineers, 2011, 32 (9), pp.1302. In this letter, decisive advantages of the staggered type organic field-effect transistors (OFETs) over the coplanar type are elucidated by 2-D device simulation. It is found that the charge transport in the channel is not limited by the contact electrode in staggered OFETs, whereas coplanar OFETs show strongly contact-limited behavior. This dissimilarity originates from the continuity (staggered) or discontinuity (coplanar) of the carrier concentration at the channel ends, which is directly connected to the channel potential profile. The calculated current-voltage curves also support these arguments as the current in coplanar OFETs follows the contact-limited transistor model. (10.1109/LED.2011.2160249)
  DOI : 10.1109/LED.2011.2160249
• Nanosensors for nanoscale structural health monitoring in civil engineering: new insight on carbon nanotubes devices
  
  • Lebental Bérengère
  • Bourquin Frédéric
  • Ghis Anne
  • Cojocaru Costel-Sorin
  , 2011, pp.1p.. Nanoporosities play a most significant role in the durability of cementitious materials, so that nanoscale features are a promising target for structural health monitoring (SHM). However, to this day, no sensor features the resolution required to investigate non-destructively these features. To fill in this loophole, IFSTTAR is devising innovative nanoscale SHM solutions based on nanosensors. Two types of carbon nanotubes (CNT) devices are being studied: ultrasonic nanotransducers for microporosity assessment and field-effect transistors for relative humidity (RH) monitoring. A detailed numerical model of the CNT-based ultrasonic nanotransducers we are devising shows that upon embedding in a cementitious material it could determine the volume and content of the porosity in its vicinity. Such information would be invaluable in the evaluation of structural durability (Lebental et al., 2011). Our characterizations of the field-effect transistors devices we fabricated with low density CNT networks indicate that they could be exploited as a high sensitivity means to probe high RH environment such as concrete (Bondavalli et al., 2009) With such promising outputs, these two studies open up the path toward in-situ morphology and composition monitoring of the microporosity of cementitious materials by nanosensors. This nanosensors-based nanoscale knowledge is the key to detect and prevent degradations of cement-based structures. As such, it will significantly contribute to an improved sustainability in civil engineering.
• A Simple Route to Rod‐Coil Block Copolymers of Oligo‐and Polythiophenes with PMMA and Polystyrene
  
  • Antoun T.
  • Iraqi A.
  • Kergoat Laurent
  • Miozzo L.
  • Yassar A.
  Macromolecular Chemistry and Physics, Wiley-VCH Verlag, 2011, 212 (11), pp.1129-1136. A simple and efficient approach for the preparation of rod-coil block copolymers comprising oligo- and polythiophenes blocks together with PMMA or PS blocks is described. The block copolymers were prepared using a two-step procedure. α,ω-dicarboxy-terminated oligothiophenes and carboxy terminated poly(3-hexylthiophene) were first prepared. These were then reacted with P4S10 in a second step to generate the α,ω-thioester terminated oligothiophenes and poly(3-hexylthiophene)s which were subsequently used in a one-pot reaction as RAFT polymerization agents with methylmethacrylate and styrene. The di- and tri-block copolymers hence obtained were fully characterized, both in solution and as thin films. (10.1002/macp.201100078)
  DOI : 10.1002/macp.201100078
• Direct epitaxial growth of SrTiO3 on Si (001): Interface, crystallization and IR evidence of phase transition
  
  • Niu G.
  • Peng W. W.
  • Saint-Girons G.
  • Penuelas J.
  • Roy Pascal
  • Brubach J. B.
  • Maurice J.-L.
  • Hollinger G.
  • Vilquin B.
  Thin Solid Films, Elsevier, 2011, 519 (17), pp.5722-5725. The work reports the direct epitaxial growth of SrTiO3 on Si (001) substrate by molecular beam epitaxy. The impact of the growth temperature and the initial oxygen partial pressure on the heteroepitaxy is studied in detail using different in-situ and ex-situ characterization methods. The optimal growth condition has been identified as 360 °C with the initial oxygen partial pressure of 5×10−8 Torr to achieve a high-quality single crystalline SrTiO3 film and a coherent interface between SrTiO3 and Si. The THz Infrared (IR) measurements show that the biaxial strained SrTiO3 commensurately grown on silicon undergoes a cubic-tetragonal phase transition. (10.1016/J.TSF.2010.12.208)
  DOI : 10.1016/J.TSF.2010.12.208
• Membrane de nanotubes de carbone pour les Micro-commutateurs MEMS
  
  • Verger Aurélie
  • Pothier Arnaud
  • Blondy Pierre
  • Cojocaru C.-S.
  • Gothier A.
  , 2011, pp.Présentation Orale - 2A-54-Plumee2011.
• Effects of Roughness on Scatterometry Signatures
  
  • Foldyna M.
  • Germer T. A.
  • Bergner B. C.
  , 2011, 1395, pp.49-53. We used azimuthally-resolved spectroscopic Mueller matrix ellipsometry to study a periodic silicon line structure with and without artificially-generated line edge roughness (LER). Grating profiles were determined from multiple azimuthal configurations, focusing the incident beam into a 60 μm spot. We used rigorous numerical modeling, taking into account the finite numerical aperture and determining the profile shape using a four trapezoid model for the line profile. Data obtained from the perturbed and unperturbed gratings were fit using the same model, and the resulting root-mean-square error (RMSE) values were compared. The comparison shows an increase in RMSE values for the perturbed grating that can be attributed to the effects of LER. (10.1063/1.3657865)
  DOI : 10.1063/1.3657865
• Silicon surface passivation and epitaxial growth on c-Si by low temperature plasma processes for high efficiency solar cells
  
  • Labrune Martin
  , 2011. This thesis presents a work which has been devoted to the growth of silicon thin films on crystalline silicon for photovoltaic applications by means of RF PECVD. The primary goal of this work was to obtain an amorphous growth on any c-Si surface in order to provide an efficient passivation, as required in heterojunction solar cells. Indeed, we demonstrated that epitaxial or mixed phase growths, easy to obtain on (100) Si, would lead to poor surface passivation. We proved that growing a few nm thin a-Si1-xCx:H alloy film was an efficient, stable and reproducible way to hinder epitaxy while keeping an excellent surface passivation by the subsequent deposition of a-Si:H films. Process optimization mainly based on Spectroscopic Ellipsometry, Effective lifetime measurements (Sinton lifetime tester) and current-voltage characterization led us to demonstrate that it was possible to obtain a-Si:H/c-Si heterojunction solar cells with stable VOC of 710 mV and FF of 76 % on flat (n) c-Si wafers, with solar cells of 25 cm2 whose metallization was realized by screen-printing technology. This work has also demonstrated the viability of a completely dry process where the native oxide is removed by SiF4 plasma etching instead of the wet HF removal. Last but not least, the epitaxial growth of silicon thin films, undoped and n or p-type doped, on (100)-oriented surfaces has been studied by Spectroscopic Ellipsometry and Hall effect measurements. We have been able to fabricate homojunction solar cells with a p-type emitter as well as p-i-n structures with an undoped epitaxial absorber on a heavily-doped (p) c-Si wafers.
• The Rubrenic Synthesis: The Delicate Equilibrium between Tetracene and Cyclobutene
  
  • Braga D.
  • Jaafari A.
  • Miozzo L.
  • Moret M.
  • Rizzato S.
  • Papagni A.
  • Yassar A.
  European Journal of Organic Chemistry, Wiley-VCH Verlag, 2011, 2011 (22), pp.4160-4169. Herein we describe the synthesis of new substituted tetraaryltetracenes, obtained by the dimerization of triarylchloroallenes, prepared from propargyl alcohols. The propargyl alcohols were prepared by two different synthetic strategies and then the alcohols were treated to obtain the corresponding acenes. In addition to the expected tetracene derivatives, we observed the formation of bis(alkylidene)cyclobutenes. When strong electron-donating substituents were present, the main product was the cyclobutene. We discuss a reaction mechanism that accounts for the formation of the cyclobutenes. (10.1002/ejoc.201100033)
  DOI : 10.1002/ejoc.201100033
• Solution, Solid State, and Film Properties of a Structurally Characterized Highly Luminescent Molecular Europium Plastic Material Excitable with Visible Light
  
  • Zucchi Gael
  • Murugesan Vajjiravel
  • Tondelier Denis
  • Aldakov Dmitry
  • Jeon Taewoo
  • Yang Feng
  • Thuéry Pierre
  • Ephritikhine M.
  • Geffroy Bernard
  Inorganic Chemistry, American Chemical Society, 2011, 50 (11), pp.4851-4856. The synthesis and X-ray crystal structure of the ligand L (4,7-dicarbazol-9-yl-[1,10]-phenanthroline) are reported, as well as those of the molecular complex, [Eu(tta)3(L)] (1), (tta = 2-thenoyl trifluoroacetylacetonate). Their photophysical properties have been investigated both in solution and in the solid state. It was shown that the ligands used for designing 1 are well-suited for sensitizing the EuIII ion emission, thanks to a favorable position of the triplet state as investigated in the GdIII complex [Gd(tta)3(L)], (2). The low local symmetry of the EuIII ion shown by the X-ray crystal structure of 1 is also revealed by luminescence spectroscopy. Because of interesting volatility and solubility properties, 1 is shown to behave as a real molecular material that can be processed both by thermal evaporation and from solution. When doped in poly(methylmethacrylate) (PMMA), 1 forms air-stable and highly red-emitting plastic materials that can be excited in a wide range of wavelengths from the UV to the visible part of the electromagnetic spectrum (250-560 nm). Absolute quantum yields of 80% have been obtained for films comprising 1-3% of 1. Ellipsometry measurements have been introduced to gain information on physical data of 1. They have been performed on thin films of 1 deposited by thermal evaporation and gave access to the refractive index, n, and the absorption coefficient, k, as a function of the wavelength. A value of 1.70 has been found for n at 633 nm. These thin films also show interesting air-stability. (10.1021/ic2000415)
  DOI : 10.1021/ic2000415