The use of size and shape tunable quantum confinement nanocrystals has many potential applications for use in semiconductors, optics and sensors. The synthesis of lead sulfide (PbS) and copper indium diselenide (CuInSe���) nanoparticles are of particular interest for use in semiconductor, optoelectronics and bio-medical applications. The continuous synthesis of lead sulfide (PbS) and copper indium diselenide (CuInSe���) nanocrystals was undertaken in this work. Quality colloidal nanocrystal synthesis requires three components: precursors, organic surfactants and solvents. The synthesis of the nanocrystals can be thought of as a nucleation event, followed by a subsequent growth period. Both the nucleation and growth rates were found to be dependent upon factors such as temperature, growth time, and precursor concentration. For a continuous flow system the residence time (at nucleation and growth conditions) was also found to be important. In order to separate the nucleation and growth events, injection techniques were employed to achieve rapid nucleation of nanocrystals with final size dictated by the growth temperature and/or residence time through the growth zone of the reaction system. Experimental parameters to investigate the size, shape, and composition of synthesized nanocrystals included injection temperature, growth temperature, residence time, and concentration of organic surfactants. Size tunability was accomplished for both PbS and CuInSe��� nanocrystals where particle sizes less than 10 nm were achieved and the resulting nanocrystal compositions were found to be at the approximate stoichiometric ratios for both PbS and CuInSe���. The materials used for the process tubing and pumps were found to be important as chlorinated reaction byproducts were found to react with the stainless steel tubing and pump heads. Post processing was also found to be important in order to remove any possible reaction by-products and residual precursors from the surface of synthesized nanocrystals. When at least one dimension of the nanocrystal approaches the exciton Bohr radius, the bandgap for the nanocrystal increases. UV-VIS spectroscopy was used to optically characterize synthesized PbS nanocrystals from our continuous flow synthesis. The absorption spectra for the particles demonstrated an absorption onset showing a large blueshift compared to that of bulk PbS. The blueshift matches closely with literature reports of the quantum confinement effect that would be desired when synthesizing PbS nanoparticles at diameters that are less than the PbS exciton Bohr radius of 18 nm. / Graduation date: 2013

The maximum sensitivity of a thermal IR sensor can be available either by means of the sensor material, having its own absorbing properties, or by the deposition of an additional absorber structure on the detector surface. In this thesis, the theory of two absorption structures is discussed. The first is called the interferometric absorber structure. The second structure under investigation uses a lead selenide layer for the IR absorption. In the interferometric structure, a new epoxy material SU8-2002 was used as a dielectric medium. This material has a very low thermal conductivity of 0.3 W/mK, which makes it suitable for thermal detectors. The interferometric structure is based on three layers, a 40–60 Å thick Ti layer, a SU8–2002 layer with a thickness of 2000 Å thick and a 2000Å Al layer. Using standard cleanroom processing an interferometric structure was fabricated. Transfer matrix theory was used in order to simulate the interferometric structure and the lead selenide was fabricated by means of an argon-plasma sputtering process. Both fabricated samples were characterized through Fourier transfer infrared (FTIR) spectroscopy together with a specular reflectance accessory. The thicknesses of the added layers were measured using Atomic force microscopy (AFM) for both the interferometric and lead selenide structure.  It was determined  that by changing the reflective index value of the SU8-2002 from the reported value of 1.575 to about 2.40 that this provided a better agreement with the experimental results. The absorption results for the interferometric structure were determined to be approximately 82–98% for the wavelength region of 2-20µm at 30 degree. The PbSe absorption spectra showed 30%–50% absorption for the wavelength region 2.5 – 6.67μm.

No description available.

In this study, photoemission spectroscopy (PES) was used to investigate the electronic properties of nanocrystalline titanium dioxide (TiO2), zinc oxide (ZnO), and cadmium selenide (CdSe). Electrospray deposition technique enabled the preparation of thin films in vacuum from a dispersion prepared outside the vacuum chamber. This method also allowed the step-wise formation of interfaces and the monitoring of the evolution of the electronic structure with intermittent PES characterization. The work function of nanocrystalline TiO2 and ZnO was measured with ultraviolet photoemission spectroscopy (UPS) and low-intensity x-ray photoemission spectroscopy (LIXPS). Measurements on environmentally contaminated surfaces revealed an instantaneous and permanent work function decrease of 0.3-0.5 eV upon exposure to ultraviolet radiation during a UPS measurement. The work function reduction is likely to be related to the formation of a surface dipole caused by the photo-chemical hydroxylation of surface defects. This phenomenon was further investigated with regard to its influence on the electronic structure of the indium tin oxide (ITO)/TiO2 interface found in dye-sensitized solar cells. The experiments suggest that UV radiation can cause a small but significant change of the charge injection barriers at the interface. The determined band line-ups revealed electron injection barriers of ~0.3-0.5 eV, while UV radiation caused an increase of about 0.15 eV. This might have the potential to further impede electron transfer to the ITO electrode and affect the performance of solar cell device. Another type of photovoltaic cell using nanocrystalline material is a heterojunction bulk solar cell. Conversion efficiencies of such devices are currently only about 3% due to the inefficient charge separation at interfaces formed by blending organic and inorganic material. An approach to improve efficiencies in such devices is the use of covalently bonded conductive polymer/inorganic hybrid nanocrystals. In this study a prototypical model system was investigated with PES with the aim to develop a measurement protocol that allows the determination of electronic properties for such hybrid materials. The comparison of the relative core-level binding energies of the organics-functionalized CdSe nanocrystal compared to the ligand-free CdSe nanocrystal and the arylselenophosphate ligand material enabled the determination of the electronic structure at the interface. Core-level measurements support the hypothesis that the Se functionality of the organic ligand coordinates to the Cd sites on the nanopthesis surface.

The synthesis of nanocrystals is unique compared to the formation of larger micron-sizesspecies as the final crystal sizes are not much larger than the primary nuclei. As a consequencethe final outcome of a nanocrystal synthesis i.e mean crystal size, concentrationand standard deviation is almost solely determined by the end of the nucleation phase. Directingthe growth of crystals beginning from aggregates of only tens of atoms into maturemonodisperse nanocrystals requires that the governing kinetics are strictly controlled at everymoment of the reaction. To effect this task various different ligands need to be employed,each performing a particular function during both nucleation and growth. (For complete abstract open document)

Σκοπός της παρούσας διπλωματικής εργασίας είναι η μελέτη της επίδρασης της γωνίας πρόσπτωσης της ηλιακής ακτινοβολίας στη λειτουργία φωτοβολταϊκών κυττάρων, σε περιοχές μεσαίου γεωγραφικού πλάτους. Συγκεκριμένα μελετήθηκε η επίδραση της γωνίας αυτής στη λειτουργία δύο ΦΒ πλαισίων ενός μονοκρυσταλλικού πυριτίου (sc-Si) και ενός δισεληνοϊδιούχου χαλκού (CIS). Τα πλαίσια τοποθετήθηκαν σε ταράτσα κτηρίου του Τμήματος Ηλεκτρολόγων Μηχανικών και Τεχνολογίας υπολογιστών του Πανεπιστημίου Πατρών. Οι μετρήσεις ξεκίνησαν τον Οκτώβριο του 2009 και ολοκληρώθηκαν το Σεπτέμβριο του 2010. Τα ΦΒ πλαίσια είχαν νότιο προσανατολισμό δεδομένου ότι τοποθετήθηκαν σε τόπο του βόρειου ημισφαιρίου με γεωγραφικό πλάτος 38ο 32’ . Κατά την πειραματική διαδικασία οι μετρήσεις γίνονταν σε πολλαπλές κλίσεις κάθε φορά και για τα δύο πλαίσια . Μετρήθηκαν το ρεύμα βραχυκύκλωσης, η τάση ανοιχτοκυκλώσεως, η θερμοκρασία πλαισίου και η ηλιακή ακτινοβολία. Υπολογίστηκε ο βαθμός απόδοσης , ο συντελεστή ποιότητας και η γωνία πρόσπτωσης, ανά κλίση ΦΒ πλαισίου. Υλοποιήθηκε αλγόριθμος για τον θεωρητικό υπολογισμό της βέλτιστης γωνίας κλίσης έτσι ώστε η γωνία πρόσπτωσης να γίνεται ελάχιστη ανά χρονική στιγμή (μέγιστης παραγόμενης ενέργειας ) και έγινε σύγκριση της με τα πειραματικά αποτελέσματα. Διαπιστώσαμε πειραματικά την εξάρτηση της παραγόμενης ισχύος από την γωνία πρόσπτωσης της ηλιακής ακτινοβολίας . Επιπλέον μέσω της πειραματικής διαδικασίας υπολογίστηκε η βέλτιστη κλίση πλαισίων ανά εποχή για τόπους γεωγραφικού πλάτους ίδιου με τον τόπο των πειραματικών μετρήσεων . / -

La crise de l’énergie ainsi que les problèmes écologiques sont considérés comme les défis les plus importants de demain. Cependant, les sources d’énergies renouvelables et respectueuses de l’environnement ne sont pas suffisamment développées, ce qui entraîne une contribution faible à la production d’énergie. Les cellules solaires font partie des sources d’énergies renouvelables les plus attractives et prometteuses. Cependant, les panneaux solaires existants ont toujours un facteur de qualité négatif, c’est-à-dire que leur production et leur entretien demandent plus d’énergie qu’ils ne sont capables de produire pendant leur cycle de fonctionnement. Malgré la réduction exponentielle du prix des panneaux solaires, leur efficacité de conversion n’est pas suffisante. Les succès récents dans la science des matériaux ont beaucoup contribué à son amélioration, néanmoins des études sur les nouveaux matériaux photovoltaïques sont nécessaires. Les travaux de recherche présentés dans ce manuscrit concernent les vitrocéramiques de chalcogénures dans le système GeSe2-Sb2Se3-CuI. De nombreuses compositions chimiques différentes ont été préparées et analysées du point de vue structural et électrique dans le cadre de ce système. Il a été démontré que la phase Sb2Se3, responsable des propriétés photoélectriques dans les vitrocéramiques, possède un grand potentiel pour les applications photovoltaïques grâce à ses propriétés optiques bien adaptées. L’influence des modifications de la composition des cristaux de Sb2Se3 sur la structure et les propriétés électriques a été étudiée. La flexibilité du type de conductivité et la résistivité des cristaux massifs de Sb2Se3 ont été montrées. Enfin, les couches minces à base de vitrocéramiques et cristaux massifs étudiés ont été préparées par la pulvérisation cathodique RF magnétron. Leur structure et les propriétés électriques sont également décrites dans cette thèse. Une cellule solaire complètement fonctionnelle, basée sur Sb2Se3 dopé iode et sur la vitrocéramique de 40GeSe2-40Sb2Se3-20CuI, a été préparée et caractérisée. / Energy crisis and global ecological problems are considered as important challenges of nearest future. Eco-friendly and renewable sources of energy are still severely undeveloped and contribute weakly to the energy production. One of the most attractive and promising domains of renewable energy is a solar light harvesting. However, existing solar panels still possess negative quality factor, i.e. their fabrication and maintenance require more energy that they are capable to produce during their life cycle. Despite exponential reduction of the price, solar cells are not efficient enough in terms of light to energy conversion. Recent breakthroughs in material science contributed a lot to the increase of efficiency, however further investigation of novel materials are needed. Here, chalcogenide glassceramics of GeSe2-Sb2Se3-CuI system were studied in details. Within this system various chemical compositions were prepared and analyzed for their structure and photoelectric properties. We found that Sb2Se3 phase, responsible for the appearance of photoelectric effect in glass-ceramics, have a great potential for light harvesting due to its suitable optical properties. In the present work we demonstrated the influence of various dopants on photoelectric properties of Sb2Se3 crystals. We showed a possibility of conductivity type and resistivity tuning of bulk Sb2Se3 crystals in a wide range of values. Thin film devices based on studied bulk compositions were prepared by RF sputtering and characterized structurally and electrically as well. We demonstrated a fully functional thin film solar device based on iodine doped Sb2Se3 and 40GeSe2-40Sb2Se3-20CuI glass-ceramic.

Investigations performed in order to understand the electronic and optoelectronic properties of field effect transistors based on few layers of 2D Copper Indium Selenide (CuIn7Se11) are reported. In general, field effect transistors (FETs), electric double layer field effect transistors (EDL-FETs), and photodetectors are crucial part of several electronics based applications such as tele-communication, bio-sensing, and opto-electronic industry. After the discovery of graphene, several 2D semiconductor materials like TMDs (MoS2, WS2, and MoSe2 etc.), group III-VI materials (InSe, GaSe, and SnS2 etc.) are being studied rigorously in order to develop them as components in next generation FETs. Traditionally, thin films of ternary system of Copper Indium Selenide have been extensively studied and used in optoelectronics industry as photoactive component in solar cells. Thus, it is expected that atomically thin 2D layered structure of Copper Indium Selenide can have optical properties that could potentially be more advantageous than its thin film counterpart and could find use for developing next generation nano devices with utility in opto/nano electronics. Field effect transistors were fabricated using few-layers of CuIn7Se11 flakes, which were mechanically exfoliated from bulk crystals grown using chemical vapor transport technique. Our FET transport characterization measurements indicate n-type behavior with electron field effect mobility µFE ≈ 36 cm^2 V^-1 s^-1 at room temperature when Silicon dioxide (SiO2) is used as a back gate. We found that in such back gated field effect transistor an on/off ratio of ~ 10^4 and a subthreshold swing ≈ 1 V/dec can be obtained. Our investigations further indicate that Electronic performance of these materials can be increased significantly when gated from top using an ionic liquid electrolyte [1-Butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6)]. We found that electron field effect mobility µFE can be increased from ~ 3 cm^2 V^-1 s^-1 in SiO2 back gated device to ~ 18 cm^2 V^-1 s^-1 in top gated electrolyte devices. Similarly, subthreshold swing can be improved from ~ 30 V/dec to 0.2 V/dec and on/off ratio can be increased from 10^2 to 10^3 by using an electrolyte as a top gate. These FETs were also tested as phototransistors. Our photo-response characterization indicate photo-responsivity ~ 32 A/W with external quantum efficiency exceeding 10^3 % when excited with a 658 nm wavelength laser at room temperature. Our phototransistor also exhibit response times ~ tens of µs with specific detectivity (D*) values reaching ~ 10^12 Jones. The CuIn7Se11 phototransistor properties can be further tuned & enhanced by applying a back gate voltage along with increased source drain bias. For example, photo-responsivity can gain substantial improvement up to ~ 320 A/W upon application of a gate voltage (Vg = 30 V) and/or increased source-drain bias. The photo-responsivity exhibited by these photo detectors are at least an order of magnitude better than commercially available conventional Si based photo detectors coupled with response times that are orders of magnitude better than several other family of layered materials investigated so far. Further photocurrent generation mechanisms, effect of traps is discussed in detail.

Ce travail porte sur les composés semi-conducteurs à base de séléniures pour la conversion d'énergie par effet photovoltaïque ou thermoélectrique. Une nouvelle famille de céramiques Cu₂GeSe₃-Sb₂Se₃ avec une microstructure unique a été synthétise par un procédé de fusion-trempe. L'influence de la composition du matériau et de l'ajout de l’iode sur la microstructure et les propriétés photoélectriques a été étudiée. Le réseau d’hétérojonctions formé par deux semi-conducteurs à bande interdite relativement étroite a un effet évident sur les propriétés photoélectriques. Le système Cu₃SbSe₄-Sb₂Se₃ a également été étudié avec l'objectif d'éliminer le germanium qui est un élément relativement rare. Et les résultats indiquent que le Cu3SbSe4 peut remplacer le Cu2GeSe3 pour former des hétérojonctions avec Sb₂Se₃, en maintenant une séparation et un transport de charges efficaces. Une approche basée sur une injection à chaud a été utilisée pour la synthèse de matériaux semi-conducteurs à base de séléniures pour des applications photovoltaïque ou thermoélectrique. Des nanoparticules et nano-feuillets monocristallins bidimensionnels de CuSe de haute qualité et ont été obtenus. La structure des nanocristaux de Cu₃Sb₁₋ₓSnxSe₄ a été étudiée et le mécanisme de formation des nano-feuillets a été proposé. Des nanoparticules de Cu₃Sb₁₋ₓSnxSe₄ avec une distribution de taille étroite ont été également synthétisées avec le procédé d’injection à chaud. Ces nanoparticules ont été utilisées comme précurseurs pour la préparation de matériaux massifs par pressage à chaud. Leur performance thermoélectrique a été étudiée. / This work was focused on selenide semiconducting compounds for energy conversion by photovoltaic or thermoelectric effect. A totally new family of Cu₂GeSe₃-Sb₂Se₃ ceramics with a unique microstructure was fabricated directly by melt-quenching method. The influence of the material composition and the iodine addition on the microstructure and photoelectrical properties was investigated. The interpenetrating heterojunction network formed by two relatively narrow bandgap semiconductors has an obvious enhancement effect on the photoelectrical properties. The Cu₃SbSe₄-Sb₂Se₃ system has also been studied with the objective to eliminate the germanium which is a relatively rare element. And the results indicated that Cu₃SbSe₄ can substitute the Cu₂GeSe₃ to form heterojunctions with Sb₂Se₃, maintaining the efficient charge separation and transport. A hot-injection based-approach has been used for the synthesis of selenide semiconducting materials for photovoltaic or thermoelectric applications. CuSe nanoparticles and CuSe nanoplates with high quality single-crystals and two dimensional nanostructure were prepared. The structure of the nanocrystals has been studied and the mechanism of the nanoplates formation has been proposed. Cu₃Sb₁₋ₓSnxSe₄ nanoparticles with a narrow size distribution had also been synthesized through the hot-injection route. They have been used as precursors for the preparation of bulk materials by hot-pressing and their thermoelectric performances have been studied.

As existing energy sources have been depleting at a fast pace, thermoelectric (TE) materials have received much attention in recent years because of their role in clean energy generation and conversion. Thermoelectric materials hold promise in terrestrial applications such as waste heat recovery. Bismuth selenide (Bi2Se3), lead telluride (PbTe), skutterudites CoSb3, and Bi-Sb alloys are among the widely investigated thermoelectric materials. Synthesis of above mentioned thermoelectric materials in nanostructured form and their characterization were investigated. Highly crystalline Bi2Se3, undoped and indium (In) doped PbTe, unfilled and ytterbium (Yb) filled CoSb3 nanomaterials were synthesized using hydrothermal/solvothermal technique and Ca-doped Bi-Sb alloy was synthesized using ball milling method. The mechanism of indium doping to the PbTe matrix was investigated using X-ray diffraction, laser-induced breakdown spectroscopy (LIBS) and a first principle calculation. It was found that indium doping, at a level below 2%, is substitution on Pb site. The effects of the amount of sodium borohydride (NaBH4) as the reducing agent and the annealing treatment on the phase transition of CoSb3 were investigated. It was found that a sufficient amount of NaBH4 along with the specific annealing condition was needed for the formation of pure phase CoSb3. Thermoelectric properties of Bi2Se3 and Ca-doped Bi85Sb15 were also investigated. A lower thermal conductivity and a higher Seebeck coefficient were achieved for a Bi2Se3 sample prepared in dimethyl formamide (DMF) at 200ºC for 24 h as compared to bulk Bi2Se3. The decrease in thermal conductivity can be attributed to the increased phonon scattering at the interfaces of the nanostructures and at the grain boundaries in the bulk nanocomposite. The increase in the Seebeck coefficient of Bi2Se3 nanostructures is likely the result of the quantum confinement of the carriers in nanostructures. The effect of calcium doping on Bi85Sb15 nanostructures were investigated. It was found that 2% calcium doped Bi-Sb alloy showed the best TE efficiency due to the enhanced power factor and reduced thermal conductivity.