CRTI Library

Étude des propriétés structurales électroniques et thermodynamiques de l’alliage quaternaireZn1-xMgxSeyTe1-y

First-principles calculations are performed to study the structural, electronic and thermodynamic properties of Zn1-xMgxSeyTe1-yalloys using the full potential-linearized augmented plane wave method (FP-LAPW) within the density functional theory (DFT). In this approach the Perdew-Burke-Ernzerhorf generalized gradient approximation (PBE-GGA) was used for the exchange-correlation potential. Moreover, the modified Becke Johnson approximation (mBJ) was also used for band structure calculations. First for the MgX binary compoundsvarious phases were considered in order to confirm the most stable one and to predict the transition pressure between different phases. The lattice constant for theternary alloys exhibits a small deviation from the Vegard's law. The microscopic origins of the gap bowing were explained by using the approach of Zunger and co-workers. The bowing of the fundamental gap versus composition predicted by our calculations is in good agreement with available theoretical data. In addition, we have studied the thermal properties of these alloys using the Debye model implemented in Gibbs program. Finally, the energy band gap of Zn1-xMgxSeyTe1-y quaternary alloys lattice matched to InAsandZnTesubstrats was investigated. To our knowledge this is the first quantitative theoretical investigation on Zn1-xMgxSeyTe1-y quaternary alloys and still awaits experimental confirmations. Voir les détails

Mots clés : Zn1-xMgxSeyTe1-y Quaternary alloys, Functional density (DFT), Approximation of the generalized gradient (PBE-GGA), mBJ approximation, InAs and ZnTe substrates

Sulfide precursor concentration and lead source effect on PbS thin flms properties

Lead sul?de (PbS) thin ?lms were synthesized using chemical bath deposition (CBD). Bath solutions are formed of various concentrations of thiourea, sul?de source, ranged from 0.6 to 1.2 M and two different salts as Pb source (lead acetate and lead nitrate). From the growth mechanism, we inferred that PbS is formed through the ion by ion process when using acetate lead source, while, using nitrate source yields to ?lms growth through the complex decomposition process. Due to the difference in the involved growth process, lead acetate produces ?lms with larger crystallite size (from 4 to 16 nm), smooth and dense ?lms. However, lead nitrate produces rough ?lms with smaller crystallite size (from 1 to 4 nm).Increasing the thiourea concentration results in crystallinity improvement when using lead acetate and, oppositely, in crystallinity degradation when using lead nitrate. Due to the quantum effect caused by the small crystallite sizes, the ?lms optical gap is varied from 0.5 to 0.9 eV. Voir les détails

Mots clés : Thin ?lms Lead sul?de Chemical bath deposition

Gap states density measurement in copper oxide thin films

The density of gap states near the Fermi level have been measured in copper oxide (CuO) thin ?lms deposited by spray pyrolysis technique. The measurement method is based on the exploitation of the current–voltage characteristics of the space charge limited current (SCLC) measured in a sandwich Au/CuO/Au structure. The measured gap states density is equal to 1.5x1014cm-3 and 2.0 x1014 ev-1 respectively in films prepared at 300 and 400 °C substrate temperature, while the defect position are located at 16 and 20meV above Fermi level. The carriers mobility and concentration are also determined from SCLC, the obtained results are in good agreement with Hall effect measurement ones. Voir les détails

Mots clés : Density of states Thin ?lms Spray pyrolysis Solar cells

Contribution à la caractérisation et synthèse de pigment de peinture à base de calamine. Evaluation de la qualité et analyse des incertitudes sur les propriétés.

The iron oxide pigment consists of 53.18% iron with a siliceous matrix. The calamine itself contains 73.83% iron. The density of the pigment and calamine is respectively ρ=3.0665 g/cm3 and ρ=5.4203 g/cm3. The pigment is of oolitic structure. Calamine has a homogeneous structure of the three iron oxides. The grain size of the compounds is between 0.6 and 40 μm with a D50 of less than 8 μm. Their specific surfaces are 1.6 and 1.5 m2/g. The calamine has an ability to keep its magnetization unlike the pigment which has a low magnetization.Simultaneous Thermal analysis shows a mass increase of 3.602% between 400 and 1000 °C for calamine and exotherm equal to 1.128 W/g. For the iron pigment, this analysis shows an overall mass loss equal to 11.05% accompanied by heat absorption equal to 1.926 W/g. For the iron pigment, this analysis shows an overall mass loss equal to 11.05% accompanied by a heat absorption equal to 1.926 W/g. Concerning the mixtures with 5, 10, 15, 20, 25 and 35% of calamine, we find a decrease of the loss of mass. The flow of heat takes a polynomial pace increasing.The SEM observations show a homogeneous structure of the calamine composed of grains of iron oxides of different sizes ranging from 1 μm to 10 μm and aggregates of small particles. As for the iron oxide pigment, it is composed of aggregates of grains more or less rounded formed of oxides of iron and gangue. The EDS analysis shows a dominance of the iron element for both materials. The optical measurement spectra show that the three compounds do not absorb any visible radiation and absorb significant fluxes in the near UV. They reflect all the incident radiation in the visible but reflect very little near UV radiation. The X-ray diffraction of calamine shows that the crystalline phases of its constitution are mixtures of wustite, magnetite and hematite. As for the iron oxide pigment, it is composed of the following crystalline phases: goethite, hematite, fayalite, silica, phosphorus pentoxide, and hausmannite. The prediction model that we developed deals with the modeling of mass loss and thermal behavior as a function of the mixing rate by a combined model of "neuronal network NN" and the MTCS simulation, which allowed us to calculate this mass loss and the corresponding energy variation. The model based on the neural network approach, by its high precision, offers us a reduced range of uncertainties compared to the linear model. Voir les détails

Mots clés : Pigment iron, scale (calamine), iron oxides, grinding, Simultaneous thermal analysis, X-ray diffraction, spectrophotometry.

Deposition of tin(II) sulfide thin films by ultrasonic spraypyrolysis: Evidence of sulfur exo-diffusion

Tin Sulfide (SnS) thin films were deposited by ultrasonic spray pyrolysis technique, on glass substrate heated at 280 °C, with different deposition times. The used precursor SnCl2 and thiourea are dissolved in methanol. X-ray diffraction (XRD) analysis indicates that films are mainly composed with orthorhombic SnS phase at low deposition time. With increasing deposition time, the hexagonal SnS2 phases become dominant. SnO2 and metallic Sn phases have been detected with increasing deposition time. Scanning electron microscopy (SEM) observations reveal that films surfaces are rough with the presence of bubbles due to S2 gas exo-diffusion from the bulk during film growth. A model of S gas formation is presented. Voir les détails

Mots clés : Tin Sulfide Thin films Spray pyrolysis

Elaboration et Caractérisation de Couches Minces SnS

The present work deals with the deposition and characterization of tin sulfide thin film for the photovoltaic application. Films were synthesized by spray pyrolysis technique due to its simplicity and low cost. The starting solutions, were composed of tin chloride (SnCl2) and thiourea (CS (NH2)2) as a source of Sn and S respectively. Sets of films were prepared by varying four deposition parameters namely deposition time, solvent, spray flow rate and substrate temperature. After preparation films were subjected of various characterizations: structural, morphological, optical and electrical. The structural characterization indicates that SnS films have an orthorhombic structure with a preferential orientation (111) when prepared at low flow rate, high substrate temperature and distilled water as solvent. However, secondary phases such as Sn2S3, SnO2 and Sn metal are detected in films prepared with methanol as solvent. SEM results indicate the presence of bubbles and craters on the surface of films prepared with methanol, this is due to the sulfur exo-diffusion in form gas of S2. The optical studies show that SnS film has a large absorption coefficient in the visible range; the optical gap value is in the order of 1.3eV suggesting that SnS thin films can be a good candidate for thin films solar cells. The electrical measurements indicate that films prepared at high substrate temperature have a large conductivity. The I-V characterization of the two realized SnS/CdS and SnS/ZnS heterostructures reveal a typical behavior of a heterojunction with a good ideality factor; their characteristics are improved after thermal annealing. The Au/SnS/CdS/FTO heterojunction exhibits a photovoltaic effect but with a poor efficiency. Voir les détails

Mots clés : Tin sulfide, Spray pyrolysis, Thin films, solar cells.

Simulation des propriétés électriques d'une jonction à base de nitrure de gallium. Application au détecteur de rayonnement

Le nitrure de gallium (GaN) est l'un des composés semi-conducteurs les plus intéressants, bien adaptés pour fonctionner dans n'importe quelle application dans laquelle la température ou le niveau de radiation pourrait endommager l'électronique conventionnelle. Récemment, la détection du rayonnement ultraviolet (UV) au moyen de dispositifs à base de GaN a suscité un grand intérêt dans de nombreux domaines importants, nécessitant des détecteurs à haute performance, à faible courant d'obscurité et à haute sensibilité. Dans ce travail, nous avons évalué, au moyen d'une étude de simulation en utilisant le simulateur de conception assistée par ordinateur (TCAD) Silvaco-Atlas, les caractéristiques électriques de deux types de photodétecteurs UV: le premier est un photodétecteur à barrière de Schottky (Pt / n-GaN) tandis que le deuxième est un photodétecteur "p+-n-n+" à base du composé ternaire Al0.17Ga0.83N. Les caractéristiques électriques densité de courant - tension et réponse spectrale monochromatique, sous différentes intensités de lumière UV en polarisation inverse et directe, ainsi que les effets de la température et les pièges sont présentées. Les résultats de simulations, ont montré que les densités de courants sous des tensions de polarisation inverse et dans l’obscurité, sont extrêmement faibles. Par contre, en illuminant les détecteurs par des rayons UV avec différentes intensités, nous avons montré que les densités de photo-courants augmentent de plusieurs ordres de grandeur, ce qui confirme l'utilité des structures étudiées pour la détection des rayons UV. D'autre part, on a constaté une réduction significative de la réponse spectrale des détecteurs étudiés, sous l'effet de la présence de certains niveaux de piégeage. Voir les détails

Mots clés : simulation, GaN, Photodétecteur, Ultraviolet, température, Réponse spectrale

Characterization of the Polylactic acid stretched uniaxial and annealed by Raman spectrometry and Differential scanning calorimetry

in this work, we have been interested in the characterization of the effect of heat treatment and mechanical treatment on the crystallinity of a polylactic acid (PLA) film by two techniques, DSC and Raman spectroscopy. the results obtained by the DSC for the stretched film shows the appearance of a broad peak of crystallization around 120 ° C, a rise in melting peak in a significant way, which shows that the uniaxial stretching has increased the crystallinity of the PLA, whereas for the annealed film appearance of a double melting peak. The results obtained by Raman spectroscopy show new peak appears at 922 cm−1 after stretching process, indicating crystallization process occurs. Voir les détails

Mots clés : Raman, DSC, PLA, crystallinity

Effet de la température sur les propriétés électriques d’une photopile

Le but principale de ce travail est d’étudier l’effet de la température sur la performance d'une cellule solaire à base de Silicium, à travers deux méthodes de caractérisation qui sont la caractéristique courant tension et la réponse spectrale. Dans cette optique, nous avons élaboré un logiciel « RS-Cellule.V2 » sous Visual Basic pour l’acquisition expérimentale et la modélisation numérique de la réponse spectrale, ainsi qu’un code de calcul « FIVT.1 » pour la modélisation de l’effet de la température sur la caractéristique courant tension selon les quatre modèles de la cellule.Les résultats obtenus montrent une amélioration de la réponse spectrale, notamment dans la région des longues longueurs d’ondes, avec l'accroissement de la température pour les deux types d’éclairements AM0 et AM1.5, ce qui traduit une augmentation du courant de court circuit.Ce résultat est confirmé lors de l’étude de l’effet de la température sur la caractéristique courant tension en utilisant les différents modèles. D'autre part, les résultats de simulation montrent la bonne approximation du fonctionnement réel de la cellule solaire par le modèle à deux diodes. L'emploi de la méthode de Marquardt-Levenberg pour extraire les paramètres de la cellule nous a donné de bons résultats en bon accord avec la littérature. Voir les détails

Mots clés : Cellule solaire, Silicium, Réponse spèctrale, Caractéristique I-V, température

Theoretical design and performance of InxGa1-xN single junction solar cell

The insertion of optimized Window and a back surface field (BSF) layers on an InxGa1-xN p-n basic single junction (BSJ) solar cell is the chief reason behind the reduction of front and back recombination. In this context, this work is focused on the selection of the suitable parameters including the indium (In) content, thickness and doping concentration for the InxGa1-xN inserted layers, that gives the best photovoltaic performances. At this aim, numerical simulations were performed using the computational numerical modeling TCAD Silvaco-Atlas to design, optimize the InxGa1-xN BSJ and extract the above Window and BSF parameters that enhance the BSJ performances. A short circuit current density ?(J?_sc) of 26.15 mA⁄?cm?^2 , an open circuit voltage ?(V?_oc) value of 0.904 V and a fill factor (FF) value of 79.67 % are obtained under AM1.5G illumination, exhibiting a maximum conversion efficiency (η) of 19.62 %. Other parameters like the external quantum efficiency (EQE), electric field developed, the current density-voltage (J-V) and the power density-voltage (P-V) characteristics are also calculated and plotted for the designed solar cell. Voir les détails

Mots clés : InGaN, solar cell, BSF layer, Window layer, simulation, Silvaco