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Etude et optimisation de la croissance du silicium amorphe hydrogéné sur substrats de silicium cristallin texturé par voie chimique pour l'augmentation du rendement de cellules solaires à hétérojonction a-Si:H/c-Si
No full textThis thesis is the result of the work carried out on the passivation of anisotropically wet-etched crystalline silicon by hydrogenated amorphous silicon deposited by low-temperature RF PECVD. Firstly, the (100) and (111) crystalline orientations were studied to understand the impact thereof on a-Si:H growth. Wet-chemical etching of crystalline silicon was then addressed in order to choose the best recipe to texture silicon wafers and thereby increase their light-trapping properties. Surprisingly, the deposition of a-Si:H on textured wafers was tested and passivation under our standard conditions was not evident at all. Indeed, we experienced huge losses in effective lifetime and implied Voc. We could at first explain this difference by the larger surface and thus larger number of dangling bonds exhibited by textured wafers. However, we imagined that randomly distributed pyramid landscapes could be imperfectly defined in their natural {111} crystallographic orientation and that they rather contained a particular set of specific surface orientations. Such being the case, this would in turn foster particular growth modes locally. Consequently, we decided to use a method that is counter-intuitive for textured c-Si wafers in order to obtain a perfectly abrupt a- Si:H/c-Si interface. The improvements were striking and showed great reproducibility while HR-TEM/STEM analyses clearly indicated that the interface was made more abrupt indeed. This allowed us to procede towards our main goal: improving a-Si:H/c- Si heterojunction (SHJ) solar cell efficiencies on textured c-Si n-type wafers. A proper cleaning routine was defined, the cleanliness of the whole fabrication chain processes (no clean-room conditions) was optimized and adequate layer stacks were developed to enhance SHJ solar cells. As a consequence, a 2x2 cm2 (n)c-Si HJ solar cell was fabricated with a conversion efficiency as high as 20.1% with a Voc of 701 mV, a Jsc of 37.5 mA/cm2 and a FF of 76.3%. This cell contains the two major improvements we have introduced in the conventional HJ solar cell architecture. Implementation of these improvements required a study of their impact on the SHJ solar cell resilience to TCO deposition and on their final J(V) characteristics. A new semi-industrial PECVD cluster tool was installed and new PECVD processes were defined to assess its potential for passivation studies and to check its reproducibility features. As a result, we obtained a 11.3 ms effective lifetime with a 734 mV implied Voc on a symmetrical i/i stack.Cette thèse est le résultat de travaux menés sur la passivation du silicium cristallin texturé par voie chimique, par le silicium amorphe hydrogéné déposé par RF PECVD à basse température. Tout d’abord, les orientations cristallines (100) et (111) ont été étudiées pour comprendre leur impact sur la croissance du a-Si:H. Ensuite, la gravure du silicium par voie humide a été explorée pour définir la recette de texturation du silicium cristallin la plus adaptée en vue d’améliorer ses propriétés de piégeage optique. La croissance du a-Si:H sur des wafers texturés a été étudiée et, à notre grand étonnement, leur passivation dans nos conditions standard n’a pas été évidente au premier abord. En effet, cela est passé par des pertes importantes en termes de temps de vie et de Voc implicite. Cela pouvait être tout d’abord expliqué par la surface développée plus importante du substrat de silicium après texturation. Cependant, nous avons imaginé que la surface du silicium cristallin pouvait être imparfaitement définie dans son orientation naturelle faite de plans {111} et qu’elle pouvait au contraire contenir une distribution d’orientations cristallines. Cela aurait pour conséquence de favoriser localement des modes de croissances particuliers. Par conséquent, nous avons décidé d’utiliser une méthode contre-intuitive pour obtenir des interfaces a-Si :H/c-Si abruptes lors de la passivation de wafers c-Si texturés par voie chimique. Les améliorations ont tout de suite été très nettes avec une grande reproductibilité tandis que des analyses HR-TEM/STEM ont démontré l’obtention d’une interface plus abrupte. Cela nous a permis de poursuivre notre objectif principal fixé : augmenter le rendement de cellules solaire à hétérojonction a-Si :H/c-Si (SHJ) sur wafers c-Si texturés de type n. Une procédure de nettoyage chimique des wafers adaptée à notre laboratoire a été définie, la propreté de la chaîne complète de fabrication a été optimisée et des couches de a-Si :H adéquates ont été développées pour améliorer les performances de cellules solaires SHJ. Par la suite, une cellule solaire de 2x2 cm2 a pu être fabriquée avec un rendement de 20.1%, un Voc de 701 mV, un Jsc of 37.5 mA/cm2 et un FF de 76.3%. Cette cellule contient les deux améliorations majeures que nous avons introduites dans l’architecture SHJ. Leur implémentation a requis l’étude de leur résistance aux procédés de dépôts du TCO et de leur impact sur les caractéristiques J(V) finales. Dans cette quête vers les hauts rendements, un nouveau réacteur semi-industriel de type cluster PECVD a été installé. De nouveaux procédés PECVD ont été définis pour à la fois estimer ses capacités pour les études de passivation et également vérifier ses caractéristiques de reproductibilité. Par la suite, un temps de vie de 11.3 ms a pu être obtenu avec un Voc implicite de 734 mV sur un stack symétrique i/i sur substrat lisse (111)
Effect of input pulse chirp on nonlinear energy deposition and plasma excitation in water
No full textInternational audienceWe analyze numerically and experimentally the effect of the input pulse chirp on the nonlinear energy transfer from 5 µJ fs-pulses at 800 nm to water. Numerical results are also shown for pulses at 400 nm, where linear losses are minimized, and for different focusing geometries. Input chirp is found to have a big impact on the transmitted energy and on the plasma distribution around focus, thus providing a simple and effective mechanism to tune the electron density and energy deposition. We identify three relevant ways in which plasma features may be tuned
Lasing of ambient air with microjoule pulse energy pumped by a multi terawatt IR femtosecond laser
No full textInternational audienceWe report on the lasing action of atmospheric air pumped by an 800 nm femtosecond laser pulse with peak power up to 4 TW. Lasing emission at 428 nm increases rapidly over a small range of pump laser power, followed by saturation above ∼1.5 TW. The maximum lasing pulse energy is measured at 2.6 μJ corresponding to an emission power in the MW range, while a maximum conversion efficiency of 3.5×10−5 is measured at moderate pump pulse energy. The optical gain inside the filament plasma is estimated to be in excess of 0.7/cm. Lasing emission shows a doughnut profile, reflecting the spatial distribution of the pump-generated white-light continuum that acts as a seed for the lasing. We attribute the pronounced saturation to the defocusing of the seed in the plasma amplifying region and to the saturation of the seed intensity
Instantaneous Band Gap Collapse in Photoexcited Monoclinic VO2 due to Photocarrier Doping
No full textInternational audienceUsing femtosecond time-resolved photoelectron spectroscopy we demonstrate that photoexcitation transforms monoclinic VO2 quasi-instantaneously into a metal. Thereby, we exclude an 80 fs structural bottleneck for the photoinduced electronic phase transition of VO2. First-principles many-body perturbation theory calculations reveal a high sensitivity of the VO2 band gap to variations of the dynamically screened Coulomb interaction, supporting a fully electronically driven isostructural insulatorto-metal transition. We thus conclude that the ultrafast band structure renormalization is caused by photoexcitation of carriers from localized V 3d valence states, strongly changing the screening before significant hot-carrier relaxation or ionic motion has occurred
Inertial-sensor bias estimation from brightness/depth images and based on SO(3)-invariant integro/partial-differential equations on the unit sphere
No full textInternational audienceConstant biases associated to measured linear and angular velocities of a moving ob- ject can be estimated from measurements of a static scene by embedded brightness and depth sensors. We propose here a Lyapunov-based observer taking advantage of the SO(3)-invariance of the partial differential equations satisfied by the measured brightness and depth fields. The resulting asymptotic observer is governed by a non-linear integro/partial differential system where the two independent scalar variables indexing the pixels live on S2. The observer design and analysis are strongly simplified by coordinate-free differential calculus on S2 equipped with its natural Riemannian structure. The observer convergence is investigated under C1 regularity assumptions on the object motion and its scene. It relies on Ascoli-Arzela theorem and pre-compactness of the observer trajectories. It is proved that the estimated biases converge towards the true ones, if and only if, the scene admits no cylindrical symmetry. The observer design can be adapted to realistic sensors where brightness and depth data are only available on a subset of S2. Preliminary simulations with synthetic brightness and depth images (corrupted by noise around 10%) indicate that such Lyapunov-based observers should be robust and convergent for much weaker regularity assumptions
Structure and Thermodynamics of Mg:Phosphate Interactions in Water: A Simulation Study
No full textInternational audienceThe association of Mg2+ and H2PO4− in water can give insights into Mg:phosphate interactions in general, which are very widespread, but for which experimental data is surprisingly sparse. It is studied through molecular dynamics simulations (>100 ns) by using the polarizable AMOEBA force field, and the association free energy is computed for the first time. Explicit consideration of outer-sphere and two types of inner-sphere association provides considerable insight into the dynamics and thermodynamics of ion pairing. After careful assessment of the computational approximations, the agreement with experimental values indicates that the methodology can be extended to other inorganic and biological Mg:phosphate interactions in solution
Linear-time computation of minimal absent words using suffix array
No full textInternational audienceBackground: An absent word of a word y of length n is a word that does not occur in y. It is a minimal absent word if all its proper factors occur in y. Minimal absent words have been computed in genomes of organisms from all domains of life; their computation also provides a fast alternative for measuring approximation in sequence comparison. There exists an O(n)-time and O(n)-space algorithm for computing all minimal absent words on a fixed-sized alphabet based on the construction of suffix automata (Crochemore et al., 1998). No implementation of this algorithm is publicly available. There also exists an O(n^2)-time and O(n)-space algorithm for the same problem based on the construction of suffix arrays (Pinho et al., 2009). An implementation of this algorithm was also provided by the authors and is currently the fastest available. Results: Our contribution in this article is twofold: first, we bridge this unpleasant gap by presenting an O(n)-time and O(n)-space algorithm for computing all minimal absent words based on the construction of suffix arrays; and second, we provide the respective implementation of this algorithm. Experimental results, using real and synthetic data, show that this implementation outperforms the one by Pinho et al. The open-source code of our implementation is freely available at http://github.com/solonas13/maw. Conclusions: Classical notions for sequence comparison are increasingly being replaced by other similarity measures that refer to the composition of sequences in terms of their constituent patterns. One such measure is the minimal absent words. In this article, we present a new linear-time and linear-space algorithm for the computation of minimal absent words based on the suffix array
A novel description of FDG excretion in the renal system: application to metformin-treated models
No full textInternational audienc
Electromagnetic inverse shape problem for coated obstacles
No full textInternational audienceWe address the inverse problem of retrieving the shape of an obstacle with impedance in the form of a surface wave operator using the knowledge of electromagnetic scattering amplitude at a fixed frequency. We prove unique reconstructions from infinitely many measures. We then provide a characterization of the scattering amplitude derivative with respect to the obstacle shape. This derivative includes the case of shape dependent impedance parameters. We then employ a gradient-descent algorithm with H 1 boundary regularisation of the descent direction to numerically solve the inverse problem. The procedure is validated for three dimensional geometries using synthetic data
On the controllability of quantum transport in an electronic nanostructure
No full textInternational audienceWe investigate the controllability of quantum electrons trapped in a two-dimensional device, typically a MOS field-effect transistor. The problem is modeled by the Schrödinger equation in a bounded domain coupled to the Poisson equation for the electrical potential. The controller acts on the system through the boundary condition on the potential, on a part of the boundary modeling the gate. We prove that, generically with respect to the shape of the domain and boundary conditions on the gate, the device is controllable. We also consider control properties of a more realistic nonlinear version of the device, taking into account the self-consistent electrostatic Poisson potential