Publications

2016

• Physically-Based Compact Modelling of Organic Electronic Devices
  
  • Jung Sungyeop
  , 2016. In spite of a remarkable improvement in the performance of organic electronic devices, there is still a lack of rigorous theoretical understanding on the device operation. This thesis is dedicated to establishing practical models of organic electronic devices with a full physical basis, namely a physically-based compact model. A physically-based compact model of a circuit element is a mathematical equation that describes the device operation, and is generally assessed by three criteria: whether it is sufficiently simple to be incorporated in circuit simulators, accurate to make the outcome of the simulators useful to circuit designers, and rigorous to capture physical phenomena occuring in the device. In this context, distinctive features of charge carrier injection and transport in organic semiconductors are incorporated in the models with a particular effort to maintain mathematical simplicity. The concomitant effect on the current-voltage characteristics of prototypical organic diodes and transistors are studied. Parameter extraction methods consistent to the models are presented which enable unambiguity determination of device parameters used for modeling device operation and assessing device performance and properties of organic thin-films and interfaces. The approaches encompass analytical developement of physical equations, two-dimensional numerical simulation based on finite-element method and experimental validation. The original and fully analytical compact models and parameter extraction methods provide fundamental understanding on how energetic disorder in an organic semiconductor thin-film, described by the Gaussian density of states, affects the observable current-voltage characteristics of the devices.Keywords : Organic electronics, device physics, analytical modeling, diodes, field-effect transistors, Gaussian density-of-states
• Stabilizing a graphene platform toward discrete components
  
  • Mzali Sana
  • Montanaro Alberto
  • Xavier Stéphane
  • Servet Bernard
  • Mazellier Jean-Paul
  • Bezencenet Odile
  • Legagneux Pierre
  • Piquemal-Banci Maëlis
  • Galceran Regina
  • Dlubak Bruno
  • Seneor Pierre
  • Martin Marie-Blandine
  • Hofmann Stephan
  • Robertson John
  • Cojocaru Costel-Sorin
  • Centeno Alba
  • Zurutuza Amaia
  Applied Physics Letters, American Institute of Physics, 2016, 109 (25), pp.253110. (10.1063/1.4972847)
  DOI : 10.1063/1.4972847
• Transmission electron microscopy study of low-temperature silicon epitaxy by plasma enhanced chemical vapor deposition
  
  • Haddad Farah
  , 2016. This thesis focuses on low temperature (LT, ~200°C) epitaxial growth of silicon thin films by plasma enhanced chemical vapor deposition (PECVD) for solar cell applications. Our starting goal was to acquire a better understanding of epitaxial growth, by using transmission electron microscopy (TEM) as the main experimental tool. First, we investigated the initial stages of epitaxial growth using SiF4/H2/Ar chemistry by performing a series of short depositions – from few tens to few hundred of seconds – on different types of substrates. We made a correlation between cross-sectional and plan-view TEM images and in-situ ellipsometry measurements. We discussed the growth mechanisms under the hypotheses of the traditional growth mediated by atoms, radicals and ions and the relatively new approach based on the melting of plasma generated nanoparticles upon impact with the substrate. Additionally, in order to understand how epitaxy by LT-PECVD is sustained, we studied how it is lost or how it breaks down. For that, experiments of intentional breakdown of epitaxy were performed by either increasing the RF power or the hydrogen flow rate using the same SiF4/H2/Ar chemistry. In both cases, the breakdown mechanism was based on the development of twins and stacking faults thus disrupting epitaxial configuration; this was accommodated with surface roughening. Thanks to this new understanding of epitaxy breakdown, we can propose some ways to sustain epitaxy for higher thicknesses. Moreover, we fascinatingly observed a quasi-fivefold symmetry in the diffraction patterns for these layers and for layers deposited using SiH4/H2/HMDSO/B2H6/Ar plasma chemistry as well. We attributed such symmetry to the breakdown of epitaxy through multiple twinning. We developed a quantitative analysis method to discriminate twin positions from random microcrystalline ones in the diffraction patterns and to estimate the number of twin operations. We also discussed some probable reasons for the occurrence of twinning and multiple twinning in a fivefold symmetry fashion. Finally, one important achievement to the TEM world is the optimization, during this doctoral work, of the traditional TEM sample preparation (tripod polishing), transforming it from a long and boring method to a fast method that is competitive with the relatively expensive focus ion beam (FIB) technique.
• Investigation of crystalline silicon solar cells at the nano-scale using scanning probe microscopy techniques
  
  • Narchi Paul
  , 2016. This thesis focuses on the investigation of crystalline silicon solar cells at the nano-scale using scanning probe microscopy (SPM) techniques. In particular, we chose to investigate electrical properties at the nano-scale using two SPM techniques: Kelvin Probe Force Microscopy (KPFM) and Conducting Probe Atomic Force Microscopy (CP-AFM).First, we highlight the strengths and weaknesses of both these techniques compared to electron microscopy techniques, which can also help investigate electrical properties at the nano-scale. This comprehensive comparison enables to identify measurements where KPFM and CP-AFM are particularly adequate. These measurements are divided in two categories: material investigation and devices investigation.Then, we focus on materials investigation at the nano-scale using SPM techniques. We first present doping measurements at the nano-scale using an advanced CP-AFM technique called Resiscope. We prove that this technique could detect doping changes in the range 1015 and 1020 atoms.cm-3 with a nano-scale resolution and a high signal/noise ratio. Then, we highlight decay time measurements on passivated crystalline silicon wafers using KPFM. Measurements are performed on the unpassivated cross-section. We show that, even though the cross-section is not passivated, decay times measurements obtained with KPFM are in good agreement with lifetimes measured by microwave photoconductivity decay.Subsequently, we focus on device measurements. Using KPFM, we investigate two different crystalline silicon solar cell architectures: epitaxial silicon (epi-Si) solar cells and interdigitated back contact (IBC) heterojunction solar cells. In particular, we focus on measurements on devices under operating conditions. We first study the influence of the applied electrical bias. We study the sensitivity of surface potential to electrical bias and we show that diode and resistance effects can be detected at the nano-scale. KPFM measurements are compared to scanning electron microscopy (SEM) measurements in the same conditions since SEM is also sensitive to surface potential. We show that KPFM measurements on the cross-section of epi-Si solar cells can help detect electric field changes with electrical bias. Besides, if the electrical bias is frequency modulated, we show that lifetime measurements can be performed on the cross-section of epi-Si solar cells and can help detect limiting interfaces and layers. Then, we study the influence of illumination on KPFM and CP-AFM measurements. We perform photovoltage and photocurrent measurements on the cross-section of epi-Si solar under different values of illumination intensity and illumination wavelength. We show a good sensitivity of KPFM measurements to illumination. However, we show that measurements for different wavelengths at a given open circuit voltage, are not correlated with the internal quantum efficiency, as we could have expected.Finally, we summarize our work in a table showing the impact of strengths and weaknesses of the techniques for the different measurements highlighted. From this table, we imagine an “ideal” microscopy setup to investigate crystalline silicon solar cells in a reliable, versatile and accurate way. We propose investigations of interest that could be carried out using this “ideal” setup.
• Méthodologie de fabrication de transistors à base de Graphène : application aux composants optoélectroniques hyperfréquences
  
  • Mzali Sana
  , 2016. Depuis sa découverte en 2004, le graphène n’a cessé de capter l’intérêt de la communauté scientifique grâce à ses innombrables propriétés et à la diversité de ses applications potentielles. Néanmoins, son implémentation à l’échelle industrielle exige encore beaucoup de contraintes et notamment concernant la stabilité de ses performances.L’objectif de cette thèse est de développer un procédé de fabrication de dispositifs intégrant une couche de graphène faiblement dopée et présentant des caractéristiques électriques stables. Le graphène, étant un matériau extrêmement sensible à l’environnement, il s’est avéré primordial de le protéger afin d’avoir un bon contrôle sur ses propriétés. Pour ce faire, plusieurs approches technologiques ont été abordées et analysées à l’aide d’une étude statistique des caractéristiques de plus de 500 transistors. Le procédé optimal intègre une couche de « protection » du graphène réalisée après son transfert et la passivation des dispositifs fabriqués avec une couche d’oxyde. Grâce à cette méthode, 75% des transistors fabriqués sont fonctionnels, présentent une faible hystérèse et sont stables dans le temps, ce qui constitue des critères indispensables pour l’intégration du graphène dans des composants discrets en particulier pour l’optoélectronique.Par la suite, le procédé technologique développé a été adapté à la fabrication de lignes coplanaires à base de graphène pour la photodétection hyperfréquence. Des valeurs de photo-courant, proches de celles de la littérature (0.15 mA/W), ont été mesurées avec un laser 1.55 µm modulé à des fréquences allant jusqu’à 40 GHz. Cette technologie est maintenant évaluée pour la fabrication de mixeurs optoélectroniques haute fréquence.
• Amélioration de la passivation de cellules solaires de silicium à hétérojonction grâce à l’implantation ionique et aux recuits thermiques
  
  • Defresne Alice
  , 2016. Les cellules solaires à hétérojonction a-Si:H/c-Si atteignent un rendement record de 24.7% en laboratoire. La passivation de la surface du c-Si est la clé pour obtenir de hauts rendements. En effet, la brusque discontinuité de la structure cristalline à l'interface amorphe/cristal induit une forte densité de liaisons pendantes créant une grande densité de défauts dans la bande interdite. Ces défauts sont des centres de recombinaison pour les paires électron-trou photogénérées dans le c-Si. Différentes couches diélectriques peuvent être utilisées pour passiver les wafers dopés n et dopés p : (i) le SiO₂ réalisé par croissance thermique, (ii) l’Al₂O₃ déposé par ALD, (iii) le a-SiNₓ:H et l’a-Si:H déposés par PECVD. La couche de passivation la plus polyvalente est a Si:H puisqu’elle peut passiver aussi bien les wafers dopés n que ceux dopés p. De plus sa production est peu coûteuse en énergie car sa croissance est réalisée à une température d’environ 200°C. L’inconvénient de cette couche de passivation est que lorsqu’elle est dopée p elle ne supporte pas des températures supérieures à 200°C, en raison de l’exodiffusion des atomes d’hydrogène qu’elle contient. Cependant, afin d'avoir un bon contact électrique, TCO et électrodes métalliques, il est souhaitable de recuire à plus haute température (entre 300°C et 500°C). Nous avons implanté des ions Argon de façon contrôlée dans des précurseurs de cellules solaires à des énergies comprises entre 1 et 30 keV, pour contrôler la profondeur à laquelle nous créons les défauts. En variant la fluence entre 10¹² Ar.cm⁻² et 10¹⁵ Ar.cm⁻² nous contrôlons la concentration de défauts créés. Nous montrons qu’une implantation à une énergie de 5 keV avec une fluence de 10¹⁵ Ar.cm⁻² n’est pas suffisante pour endommager l’interface a-Si:H/c-Si. La durée de vie effective des porteurs minoritaires mesurée par photoconductance (temps de décroissance de la photoconductivité) passe de 3 ms à 2,9 ms après implantation. En revanche les implantations à 10 keV, 10¹⁴ Ar.cm⁻² ou à 17 keV, 10¹² Ar.cm⁻² sont suffisantes pour dégrader la durée de vie effective de plus de 85%. Suite aux implantations les cellules solaires ont subi des recuits sous atmosphère contrôlée à différentes températures et ce jusqu’à 420°C. Nous avons découvert que le recuit permet de guérir les défauts introduits par l’implantation. Mais surtout, dans certains cas, d’obtenir des durées de vie après implantation et recuit supérieures aux durées de vies initiales. En combinant l’implantation ionique et les recuits, nous conservons de bonnes durées de vies effectives des porteurs de charges (supérieures à 2 ms) même avec des recuits jusqu’à 380°C. Nous avons utilisé une grande variété de techniques telles que la photoconductance, la photoluminescence, l’ellipsométrie spectroscopique, la microscopie électronique en transmission, la Spectroscopie de Masse d’Ions Secondaires, la spectroscopie Raman et l’exodiffusion de l’hydrogène pour caractériser et analyser l’ensemble des résultats et phénomènes physico-chimique intervenant dans la modification des précurseur de cellules solaires. Nous discutons ici de plusieurs effets tels que l’augmentation de la durée de vie et la tenue en température par la conservation de l’hydrogène dans la couche de silicium amorphe et ceci même après les recuits. Cette conservation peut s’expliquer par l’augmentation du nombre de liaisons Si-H au sein du silicium amorphe et par la formation de cavités lors de l’implantation. Durant les recuits l’hydrogène qui diffuse est piégé puis libéré par les cavités et/ou les liaisons pendantes, ce qui limite son exo-diffusion et le rend de nouveau disponible pour la passivation des liaisons pendantes.
• Modelling of multijunction cells
  
  • Lachaume Raphaël
  • Foldyna Martin
  • Hamon Gwenaëlle
  • Vaissière Nicolas Vaissiere
  • Cariou Romain
  • Decobert Jean
  • Roca I Cabarrocas Pere
  • Alvarez J
  • Kleider Jean-Paul
  , 2016.
• 2D‐TEM investigations of CNTs synthetized within vertical‐PAA templates for devices applications
  
  • Florea Ileana
  • Leandro Nicolas
  • Châtelet Marc
  • Cojocaru Costel-Sorin
  , 2016, 5. Since their discovery in 1991 carbon nanotubes (CNTs) become one of the most emblematic type of nanostructures due to their unique electrical, mechanical, thermal and optical properties. Despite these promising and astonishing properties, the critical subject of their manipulation remains a primordial challenge. Very expensive and time consuming techniques, like e-beam lithography, have been developed in order to precisely control their growth, or to obtain a nano-object from the etching of a bulk material. Such techniques are suitable as proofs of concept; however the object-by-object manipulation approach is unrealistic because they are not compatible with a massive integration. On the other hand the miniaturization of the fabricated devices is reaching a bottleneck since the scaling-down for enhancing their performance is approaching many physical limitations. The efforts concentrated to achieve device fabrication by continuous miniaturization are nowadays struggling with performance decline in the electrical conductivity transports values and a significantly loss of reliability. Self-organized templates such as porous anodic alumina (PAA) templates provide several advantages for controlling the nanostructures growth.[1,2] Its well-ordered structure and the confinement imposed by the nanopores the PAA template offer a promising approach for cost-effective, stable and efficient fabrication of carbon nanotubes based devices.[3] Here we present a complete 2D analysis based on advanced electron microscopy techniques devoted to the full characterization of both the PAA structure and the as-grown CNTs using a dHF-CVD (double-Hot Filament assisted CVD) synthesis method. More exactly, we combine the FIB (Focused Ion Beam) preparation technique with advanced TEM characterization techniques such as STEM-EDX and EELS spectroscopy for the assessment of an accurate correlation between the synthesis parameters and the morphological, structural and chemical characteristics of both the PAA structure and the as-grown CNTs. In a first step, the TEM analysis of different PAA cross-sections prepared using the FIB technique, allowed us accessing precise characteristics such as the pore length (800nm) and their diameter (30nm) as well as the inter-pore distance (30nm) (see figure1). A more detailed analysis on the bottom part of the PAA structure helped us evidencing the presence of a branched nanopores structure product of an exponential voltage decrease process, applied in order to thin the oxide barrier layer at the bottom of the pores. For the CNTs, we first examined the impact of the catalyst pretreatment step performed prior to the CNTs growth step. Secondly, by varying the hot-filaments power applied during the growth, we investigated the impact of the additional gas phase activation conditions over the synthesized carbon nanostructures. The results revealed that the pretreatment conditions determine the catalyst distribution at the bottom pores of the PAA membranes, with a strong impact on the CNTs growth within the PAA templates. Another important finding concerns the amount of defects incorporated into the grown CNTs walls which could be related to the hot-filament power applied during the synthesis. (10.1002/9783527808465.EMC2016.6882)
  DOI : 10.1002/9783527808465.EMC2016.6882
• Detailed analysis of III-V/epi-SiGe tandem solar cell performance including light trapping schemes
  
  • Lachaume R.
  • Foldyna M.
  • Hamon G.
  • Decobert J.
  • Cariou R.
  • Roca I Cabarrocas P.
  • Alvarez J.
  • Kleider J.P.
  Solar Energy Materials and Solar Cells, Elsevier, 2016, 166, pp.276-285. Recent developments have unlocked the main issues arising from the combination of III-V and silicon and have opened a new way to fabricate tandem solar cells. In this study we provide a detailed analysis of III-V/epi-SiGe tandem devices performance using opto-electrical models and parameters acquired from previous experimental realizations of single junction devices. At first, we present the validation of our top and bottom cells models by comparison with previously published solar cells. The analysis of the current matching and the impact of the Al content in AlGaAs absorber on the open circuit voltage is performed on a very wide range of thickness and Al content. The optimal configurations for tandems with thin film absorbers are found with an empirical expression. This expression relates the required bottom absorber thickness to the Al content for current matching in a flat tandem device. Low-temperature epitaxial SiGe growth on III-V materials is an inverted growth technique, meaning that the last material grown is the Si(Ge) bottom cell. We can thus easily texture the back of the bottom cell for higher photon absorption. The proposed nanostructurization of the back reflector shows that, to reach the same efficiency, only half of the thickness is required if a 2D grating is combined with a silver reflector. The detailed influence of the bulk and interface electrical quality in the epi-SiGe bottom cell is also assessed. Finally, the prediction of the tandem device performance according to different realistic scenarios is presented. (10.1016/j.solmat.2016.11.023)
  DOI : 10.1016/j.solmat.2016.11.023
• Direct Growth of Crystalline Silicon on GaAs by Low Temperature PECVD: Towards Hybrid Tunnel Junctions for III-V/Si Tandem Cells
  
  • Hamon Gwenaëlle
  • Decobert Jean
  • Vaissière Nicolas Vaissiere
  • Lachaume Raphaël
  • Cariou Romain
  • Chen Wanghua
  • Alvarez J
  • Kleider Jean-Paul
  • Roca I Cabarrocas Pere
  , 2016.
• Tandem radial-junction silicon nanowire solar cells fabricated by PECVD
  
  • Dai Letian
  • Foldyna Martin
  • Al-Ghzaiwat Mutaz
  • Chen Wanghua
  • Maurin Isabelle
  • Levtchenko Alexandra
  • Le Gall Sylvain
  • Lachaume Raphaël
  • Alvarez J
  • Kleider Jean-Paul
  • Roca I Cabarrocas Pere
  • Gacoin Thierry
  , 2016.
• Si1-xGex alloys on III-V (100): first steps in film growth by low temperature PECVD epitaxy
  
  • Vaissière Nicolas Vaissiere
  • Hamon Gwenaëlle
  • Lachaume Raphaël
  • Chen Wanghua
  • Decobert Jean
  • Alvarez J
  • Kleider Jean-Paul
  • Roca I Cabarrocas Pere
  , 2016.
• Etude sur la tranche de cellules solaires en silicium cristallin à l'échelle nanométrique à l'aide de techniques de microscopie à sonde locale
  
  • Narchi Paul
  • Alvarez J
  • Roigé A
  • Chrétien Pascal
  • Foldyna Martin
  • Prod'Homme Patricia
  • Kleider Jean-Paul
  • Cabarrocas Pere Roca I.
  , 2016.
• Inkjet Printing NiO-Based p-Type Dye-Sensitized Solar Cells
  
  • Brisse Romain
  • Faddoul Rita
  • Bourgeteau Tiphaine
  • Tondelier Denis
  • Leroy Jocelyne
  • Campidelli Stéphane
  • Berthelot Thomas
  • Geffroy Bernard
  • Jousselme Bruno
  ACS Applied Materials & Interfaces, Washington, D.C. : American Chemical Society, 2016, 9, pp.2369 - 2377. Fabrication at low cost of transparent p-type semiconductors with suitable electronic properties is essential toward the scalability of many electronic devices, especially for photovoltaic and photocatalytic applications. In this context, the synthesis of mesoporous NiO films through inkjet printing of a sol−gel ink was investigated for the first time. Nickel chloride and Pluronic F-127, used as nickel oxide precursor and pore-forming agent, respectively, were formulated in a water/ ethanol mixture to prepare a jettable ink for Dimatix printer. Multilayer NiO films were formed, and different morphologies could be obtained by playing on the interlayer thermal treatment. At low temperature (30 °C), a porous nano-particulate−nanofiber dual-pore structure was observed. On the other hand, with a high temperature treatment (450 °C), nanoparticulate denser films without any dual structure were obtained. The mechanism for NiO formation during the final sintering step, investigated by means of X-ray photolectron spectroscopy, shows that a Ni(OH) 2 species is an intermediate between NiCl 2 and NiO. The different morphologies and thicknesses of the NiO films were correlated to their performance in a p-DSSC configuration, using a new push−pull dye (so-called " RBG-174 ") and an iodine-based electrolyte. Moreover, the positive impact of a nanometric NiO x layer deposited by spin-coating and introduced between FTO and the NiO mesoporous network is highlighted in the present work. The best results were obtained with NiO x /four layer-NiO mesoporous photocathodes of 860 nm, with a current density at the short circuit of 3.42 mA cm −2 (irradiance of 100 mW cm −2 spectroscopically distributed following AM 1.5). ■ INTRODUCTION Solar energy is the ideal source of energy because of its cleanliness, high power, and ready availability. However, for a massive use, the development of low-cost and efficient conversion devices is still needed. Among the different emerging cost-effective solar-cell technologies, nanocrystalline dye-sensitized solar cells (DSSCs) are promised to an industrial success. Indeed, DSSCs that consist of a photoactive anode and a passive cathode sandwiching an electrolyte (n-DSSCs) have shown high photon to energy conversion efficiencies (PCE), up to 13%. 1 Moreover, the propensity of DSSCs to operate under diffuse light conditions is an important asset regarding market-dominating silica-based technology, which require direct light for optimized performance. The idea of tandem-DSSC (t-DSSC), in which both the anode and cathode are photoactive, has been raised over the last twenty years. 2−4 These devices show theoretical PCEs over 40%, 5,6 which is superior to the one photoelectrode configuration. However, with record PCE at 2.42%, 3 t-DSSCs have not beaten yet classical n-DSSCs. Indeed p-DSSCs (which consist of one photocathode and a passive anode) have presented moderate yields compared to n-DSSCs, with record PCE at 2.51%, with an iron-based electrolyte. 7 These low yields stem from (1) the low short-circuit (J sc) current densities that can be delivered, where J sc values of 7.65 mA cm −2 were reported with the record p-DSSC device, and (2) the poor fill factors (FF) obtained, as FF of 0.51 in the same device were hardly reached. It is commonly assumed that the p-type oxide which is mostly used to build the photocathode, nickel oxide (NiO), is mainly responsible for the poor activity of the devices. However, a replacing material to NiO is yet to be found as p-type oxides are scarce. 8−10 Thus, research on NiO itself remains an important axis of work in the field of DSSCs. 11−16 This wide band gap semiconducting oxide should indeed also be a choice material for low-cost and carbon-free solar fuel production devices, 17 and then research on NiO, especially on its synthesis, is also crucial for this domain. A wide range of methods exists for the preparation of NiO, 16 among which is polymer-templated sol−gel synthesis, which has provided one of the current best NiO-based DSSCs. 4 However, for this preparation method, Scotch tape doctor-blading is used to deposit the ink onto the TCO glass substrate, and several authors reported a reproducibility issue. An alternative to doctor-blading is inkjet printing (IJP), and it (10.1021/acsami.6b12912)
  DOI : 10.1021/acsami.6b12912
• Cathode commutable à nanotubes de carbone pour tube à rayons X
  
  • Sabaut Lucie
  , 2016. Les systèmes d'imagerie à rayons X (RX) sont des appareils volumineux et contraignants en termes de contrôle du faisceau. L'industrie des tubes électroniques est donc à la recherche de solutions pour assurer la stabilité du courant tout en permettant la miniaturisation du système.Ce travail opte pour l'amélioration de la source d'électrons, en remplaçant l'émission thermoïonique historique par l'émission de champ. En particulier, les cathodes froides à base de nanotubes de carbone possèdent l'avantage de pouvoir délivrer de forts courants (>1A/cm^2), tout en ayant un faible temps de réponse.A travers le développement d'une structure innovante de cathodes à nanotubes de carbone à grille intégrée, l'objectif de cette étude est de réaliser des sources commutables et régulées, pour des sources de rayons X miniatures, portables ou polyvalentes.La modélisation électrostatique de la nouvelle structure a conduit à la fabrication de cathodes à grille optimisées, sur lesquelles est cru un réseau vertical de nanotubes de carbone. L'analyse de défaillance permet finalement d'obtenir des dispositifs isolés fiables. Leur caractérisation en émission de champ indique des performances de modulation de courant inégalées, de l'ordre de 10^6 pour +/-40V de polarisation de grille. La régulation du courant a également été démontrée avec l'obtention d'une stabilité à 0,02% sur 100 h.Pour pallier les limitations rencontrées (courant de fuite et croissance parasite), une structure de grille enterrée a été proposée avec succès, ainsi qu'une nouvelle méthode de fabrication d'émetteurs courts et fins. Ces cathodes fonctionnelles ont finalement été intégrées en tube à rayons X et ont montré pour la première fois une modulation de courant de 2000 à une haute tension fixe de 60 kV.
• Polarimetric imaging technique through an optical fiber: towards an endoscopic tool for the diagnosis of cancers of inner tissus
  
  • Vizet Jérémy
  • Pierangelo Angelo
  • Buckley Colman
  • Manhas Sandeep
  • Pagnoux Dominique
  , 2016.
• Nanoscale Investigation of Carrier Lifetime on the Cross Section of Epitaxial Silicon Solar Cells Using Kelvin Probe Force Microscopy
  
  • Narchi Paul
  • Cariou Romain
  • Foldyna Martin
  • Prodhomme Patricia
  • Roca I Cabarrocas Pere
  IEEE Journal of Photovoltaics, IEEE, 2016, 6 (6), pp.1576 - 1580. (10.1109/JPHOTOV.2016.2598258)
  DOI : 10.1109/JPHOTOV.2016.2598258
• Electrostatic Energy Harvesting Circuit with DC-DC Convertor for Vibration Power Generation System
  
  • Wei J
  • Lefeuvre E.
  • Mathias H.
  • Costa François
  Journal of Physics: Conference Series, IOP Science, 2016, 773. This paper presents an interface circuit with power control features for electrostatic vibration energy harvesting. A DC-DC convertor is used to control the output voltage of a diode-based charge pump circuit. Therefore, the maximum and minimum voltage across the variable capacitor of the energy harvester may be adjusted to track the maximum power point of the system. The power conversion function of the DC-DC convertor depends on the switches configuration. An example of Maximum Power Point Tracking (MPPT) for different conversion function is presented in this paper. Simulation results show that at least 10 µW is generated. (10.1088/1742-6596/773/1/012045)
  DOI : 10.1088/1742-6596/773/1/012045
• Using Low Temperature Photoluminescence Spectroscopy to Investigate CH3NH3PbI 3 Hybrid Perovskite Degradation
  
  • Jemli Khaoula
  • Diab Hiba
  • Lédée Ferdinand
  • Trippé-Allard Gaëlle
  • Garrot Damien
  • Geffroy Bernard
  • Lauret Jean‐sébastien
  • Deleporte Emmanuelle
  • Audebert Pierre
  , 2016. Investigating the stability and evaluating the quality of the CH3NH3PbI3 perovskite structures is quite critical both to the design and fabrication of high-performance perovskite devices [1] and to fundamental studies of the photophysics of the excitons. In particular, it is known that, under ambient conditions, CH3NH3PbI3 degrades producing some PbI2 [2]. We show here that low temperature photoluminescence (PL) spectroscopy is a powerful tool to detect PbI2 traces in hybrid perovskite layers and single crystals [3]. Because PL spectroscopy is a signal detection method on a black background, small PbI2 traces can be detected, when other methods currently used at room temperature fail. Our study highlights the extremely high stability of the single crystals compared to the thin layers and defects and grain boundaries are thought to play an important role in the degradation mechanism.
• 9H-Quinolino[3,2,1-k]phenothiazine: A New Electron-Rich Fragment for Organic Electronics
  
  • Poriel Cyril
  • Rault-Berthelot Joëlle
  • Thiery Sébastien
  • Quinton Cassandre
  • Jeannin Olivier
  • Biapo Urelle
  • Tondelier Denis
  • Geffroy Bernard
  Chemistry - A European Journal, Wiley-VCH Verlag, 2016, 22 (50), pp.17930 - 17935. A new electron-rich fragment, namely the qui-nolinophenothiazine (QPTZ) is reported. TheQ PTZ fragment incorporated in spiroconfigured materials leads to higher performance in blue Phosphorescent OLEDs than structurally related phenylacridine and indoloacridine based materials (increasing the HOMO energy level,m od-ulating the spin-orbit coupling, etc.) and leads to highly efficient blue phosphorescent organic light emitting diodes, indicatingt he strongp otential of this new molecular fragment in organic electronics. Over the last 30 years,t he fantastic emergence of organic electronics is assignable to the design,t he synthesis and the study of thousands of organic semiconductors (OSCs) with specific properties fitting with the different electronic devices. [1–4] Since the discoveryo ft he " host/guest concept " in phosphorescent organic light-emitting diodes (PhOLEDs), [5] the designo fO SCs usable as host materials for blue-emitting phosphors has been an intense research field worldwide. [4] In addition to its high thermal/morphological stabilitya nd its high triplet energy (E T), an ideal host should also possess ab ipolar character,w ith HOMO and LUMO energy levels adapted to the Fermi levels of the electrodes. [6–9] However,t here is ac ertaina ntinomy between ah igh E T (essential to insure efficient energy transfers) and as hort HOMO/LUMO gap (essential to insure efficient charge injection) and the best trade-off should be found for the ideal host. If the nature of the linkage [10] between the electron rich and the electron-poor units is of key importance to control the E T as well as the HOMO/LUMO gap, their intrinsic properties remainn everthelesst he most important characteristics. In this context, the widely known electron-rich carbazole fragment has led to impressive breakthroughs and is nowadays an important buildingb lock in organic electronics. [11, 12] Around the molecular structure of carbazole, many other car-bazole-like fragments have been developed in the recent years sometimes surpassing the performance of the carbazole fragment in electronics. Thus, indolocarbazole (IC), [13] phenylacri-dine (PA) [14–16] and more recentlyi ndoloacridine (IA) [17] are nowadays key building blocks, widely used in all organic electronic devices (Scheme 1). As the future of organic electronics strongly relies on the discoveryo fn ovel elementarym olecular fragments, such as the new bithiophenesulfonamide building block recently reported by Marks,F acchettie ta l., [19] we wish to report herein an ew promising electron-rich core, namely the quinolinophe-Scheme1.Phenylacridine (PA, left), [14, 18] indoloacridine(IA, middle) [17] and quinolinophenothiazine (QPTZ,r ight) based semiconductors. (10.1002/chem.201603659)
  DOI : 10.1002/chem.201603659
• Plasma-Assisted Growth of Silicon Nanowires by Sn Catalyst: Step-by-Step Observation
  
  • Tang Jian
  • Maurice Jean-Luc
  • Chen Wanghua
  • Misra Soumyadeep
  • Foldyna Martin
  • Johnson Erik V.
  • Roca I Cabarrocas Pere
  Nanoscale Research Letters, SpringerOpen, 2016, 11 (1). (10.1186/s11671-016-1681-5)
  DOI : 10.1186/s11671-016-1681-5
• Electronic properties of embedded graphene: doped amorphous silicon/CVD graphene heterostructures
  
  • Arezki Hakim
  • Boutchich Mohamed
  • Alamarguy David
  • Madouri Ali
  • Alvarez José
  • Cabarrocas Pere Roca I
  • Kleider Jean-Paul
  • Yao Fei
  • Hee Lee Young
  Journal of Physics: Condensed Matter, IOP Publishing [1989-....], 2016, 28 (40), pp.404001. Large-area graphene film is of great interest for a wide spectrum of electronic applications, such as field effect devices, displays, and solar cells, among many others. Here, we fabricated heterostructures composed of graphene (Gr) grown by chemical vapor deposition (CVD) on copper substrate and transferred to SiO2/Si substrates, capped by n- or p-type doped amorphous silicon (a-Si:H) deposited by plasma-enhanced chemical vapor deposition. Using Raman scattering we show that despite the mechanical strain induced by the a-Si:H deposition, the structural integrity of the graphene is preserved. Moreover, Hall effect measurements directly on the embedded graphene show that the electronic properties of CVD graphene can be modulated according to the doping type of the a-Si:H as well as its phase i.e. amorphous or nanocrystalline. (10.1088/0953-8984/28/40/404001)
  DOI : 10.1088/0953-8984/28/40/404001
• The selective low cost gas sensor based on functionalized graphene
  
  • Woo Heechul
  , 2016. Recent advances in nanomaterials provided a strong potential to create a gas sensor with many advantages such as high sensitivity of single molecule detection, low cost, and low power consumption. Graphene, isolated in 2004, is one of the best promising candidate for the future development of nanosensors applications because of its atom-thick, two-dimensional structures, high conductivity, and large specific surface areas. Every atom of a monolayer graphene can be considered as a surface atom, capable of interacting even with a single molecule of the target gas or vapor species, which eventually results in the ultrasensitive sensor response.In this thesis work, graphene films were synthesized by Chemical Vapor Deposition (CVD) on the glass substrate. Raman spectroscopy was used to analyze the quality and number of layers of graphene. Atomic Force Microscope (AFM) and Scanning Electron Microscopy (SEM) were also performed to analyze the quality of graphene. After the characterization of graphene films, graphene based resistive devices (four identical electrodes are thermally evaporated directly onto the graphene film as metal electrodes) were fabricated. The electrical characterization has been carried out using Keithley-4200.Intrinsic device response was studied with different external condition changes (pressure, humidity, light illumination). The device was non-covalently functionalized with organometallic complex (Ru(II) trisbipyridine) and the its light exposure response was studied. The observed device response was reproducible and similar after many cycles of on and off operations. The theoretical and experimental approaches and the results obtained during the thesis are opening up a way to understand and fabricate future gas sensing devices based on the non-covalentely functionalized graphene.
• Ultrathin Epitaxial Silicon Solar Cells with Inverted Nanopyramid Arrays for Efficient Light Trapping
  
  • Gaucher Alexandre
  • Cattoni Andrea
  • Dupuis Christophe
  • Chen Wanghua
  • Cariou Romain
  • Foldyna Martin
  • Lalouat Loïc
  • Drouard Emmanuel
  • Seassal Christian
  • Roca I Cabarrocas Pere
  • Collin Stéphane
  Nano Letters, American Chemical Society, 2016, 16 (9), pp.5358-5364. Ultrathin c-Si solar cells have the potential to drastically reduce costs by saving raw material while maintaining good efficiencies thanks to the excellent quality of monocrystalline silicon. However, efficient light trapping strategies must be implemented to achieve high short-circuit currents. We report on the fabrication of both planar and patterned ultrathin c-Si solar cells on glass using low temperature (T<275°C), low-cost, and scalable techniques. Epitaxial c-Si layers are grown by PECVD at 160°C and transferred on a glass substrate by anodic bonding and mechanical cleavage. A silver back mirror is combined with a front texturation based on an inverted nanopyramid array fabricated by nanoimprint lithography and wet etching. We demonstrate a short-circuit current density of 25.3mA/cm2 for an equivalent thickness of only 2.75µm. External quantum efficiency (EQE) measurements are in very good agreement with FDTD simulations. We infer an optical path enhancement of 10in the long wavelength range. A simple propagation model reveals that the low photon escape probability of 25% is the key factor in the light trapping mechanism. The main limitations of our current technology and the potential efficiencies achievable with contact optimization are discussed. (10.1021/acs.nanolett.6b01240)
  DOI : 10.1021/acs.nanolett.6b01240
• Flexible Photodiodes Based on Nitride Core/Shell p-n Junction Nanowires
  
  • Zhang H
  • Dai Xing
  • Guan Nan
  • Messanvi Agnes
  • Neplokh Vladimir
  • Piazza Valerio
  • Vallo Martin
  • Bougerol Catherine
  • Julien François H
  • Babichev Andrey
  • Cavassilas Nicolas
  • Bescond Marc
  • Michelini Fabienne
  • Foldyna Martin
  • Gautier Eric
  • Durand Christophe
  • Eymery Joël
  • Tchernycheva Maria
  ACS Applied Materials & Interfaces, Washington, D.C. : American Chemical Society, 2016, 8, pp.26198 - 26206. A flexible nitride p-n photodiode is demonstrated. The device consists of a composite nanowire/polymer membrane trans- ferred onto a flexible substrate. The active element for light sensing is a vertical array of core/shell p−n junction nanowires containing InGaN/ GaN quantum wells grown by MOVPE. Electron/hole generation and transport in core/shell nanowires are modeled within nonequilibrium Green function formalism showing a good agreement with experimental results. Fully flexible transparent contacts based on a silver nanowire network are used for device fabrication, which allows bending the detector to a few millimeter curvature radius without damage. The detector shows a photoresponse at wavelengths shorter than 430 nm with a peak responsivity of 0.096 A/W at 370 nm under zero bias. The operation speed for a 0.3 × 0.3 cm2 detector patch was tested between 4 Hz and 2 kHz. The −3 dB cutoff was found to be ∼35 Hz, which is faster than the operation speed for typical photoconductive detectors and which is compatible with UV monitoring applications. (10.1021/acsami.6b06414)
  DOI : 10.1021/acsami.6b06414