La cellule photovoltaïque à porteurs chauds est un dispositif de conversion de l’énergie solaire en énergie électrique dont les rendements théoriques approchent les 86%. Additionnellement à une cellule photovoltaïque standard, ce dispositif permet de convertir l’excédent d’énergie cinétique des porteurs photogénérés, en énergie électrique. Pour cela, le phénomène de thermalisation doit être réduit et des contacts électriques sélectifs en énergie ajoutés. Afin de déterminer les performances potentielles des absorbeurs, tout en surmontant le défi de fabrication des contacts électriques sélectifs, un montage et une méthode de cartographie d’intensité absolue de photoluminescence résolue spectralement ont été utilisés. Ceci a permis d’obtenir la température d’émission et la séparation des quasi-niveaux de Fermi, les deux grandeurs thermodynamiques caractéristiques de la performance des absorbeurs. Dans cette étude, des absorbeurs à base de puits quantiques d’InGaAsP sur substrat d’InP sont utilisés. Les grandeurs thermodynamiques sont estimées et la technique de caractérisation utilisée permet l’accès à des grandeurs tel que le facteur de thermalisation mais aussi un coefficient thermoélectrique, appelé photo-Seebeck. L’analyse quantitative de porteurs chauds dans des conditions pertinentes pour le photovoltaïque est une première ; le dispositif étudié permettrait de dépasser la limite de Schockley-Queisser. Enfin, le dispositif étant muni de contacts des caractérisations électriques sont faites et comparé aux mesures optiques. Afin de mieux comprendre l’évolution des grandeurs thermodynamiques étudiées, une première simulation est proposée. / The hot carrier solar cell is an energy conversion device where theoretical conversion efficiencies reach almost 86%. Additionally to a standard photovoltaic cell, the device allows the conversion of kinetic energy excess of photogenerated carriers into electrical energy. To achieve this, the thermalisation process must be limited and electrical energy selective contacts added. In order to determine potential absorber performances and overcome the fabrication challenge of energy selective contacts, a set-up and the related method of mapping absolute photoluminescence spectra were used. This technique allows getting quasi-Fermi levels splitting and temperature of emission, both thermodynamic quantities characteristic of the performance of the absorbers. In this study, absorbers based on InGaAsP multiquantum wells on InP substrate were used. The thermodynamic quantities are determined and allow to access at quantities such as thermalisation rate but also a thermoelectric coefficient, so-called Photo-Seebeck. The quantitative analysis of the hot carriers regime, in relevant conditions for photovoltaic is a first: the analysed device indicates a potential photovoltaic conversion over the Schockley-Queisser limit. At last, as the device is supplied with electrical contacts, electrical characterization are made and compared to optical measurements. A first simulation is proposed to better understand the thermodynamic quantities evolution as a function of the electrical bias.

Optical polymers are a subject of research and industry implementation for many decades. Optical polymers are inexpensive, easy to process and flexible enough to meet a broad range of application-specific requirements. These advantages allow a development of cost-efficient polymer photonic integrated circuits for on-chip optical communications. However, low refractive index contrast between core and cladding limits light confinement in a core and, consequently, integrated polymer device miniaturization. Also, polymers lack active functionality like light emission, amplification, modulation, etc. In this work, we improved a performance of integrated polymer waveguides and demonstrated active waveguide devices. Also, we present novel Si QD/polymer optical materials. In the integrated device part, we demonstrate optical waveguides with enhanced performance. Decreased radiation losses in air-suspended curved waveguides allow low-loss bending with radii of only 15 µm, which is far better than &gt;100 µm for typical polymer waveguides. Another study shows a positive effect of thermal treatment on acrylate waveguides. By heating higher than polymer glass transition temperature, surface roughness is reflown, minimizing scattering losses. This treatment method enhances microring resonator Q factor more than 2 times. We also fabricated and evaluated all-optical intensity modulator based on PMMA waveguides doped with Si QDs. We developed novel hybrid optical materials. Si QDs are encapsulated into PMMA and OSTE polymers. Obtained materials show stable photoluminescence with high quantum yield. We achieved the highest up to date ~65% QY for solid-state Si QD composites. Demonstrated materials are a step towards Si light sources and active devices. Integrated devices and materials presented in this work enhance the performance and expand functionality of polymer PICs. The components described here can also serve as building blocks for on-chip sensing applications, microfluidics, etc. / <p>QC 20171207</p>

This thesis focuses on the analysis of the external quantum efficiency of quantum dot-enhanced multi-junction solar cells. Divided in four major parts, it uses the experimental methodology developed in the SUNLAB. At first, a model is introduced to calculate the external quantum efficiency of single and multi-junction solar cells. This model takes into account the semiconductor physics governing the electrical property of the solar cell. It furthermore takes into account the optical transmission and reflection in the semiconductor structure using a transfer matrix method. The calculated curve fits a single junction GaAs solar cell's external quantum efficiency to a high degree of precision. Finally, an InGaP/GaAs/Ge solar cell's external quantum efficiency is calculated and it reproduces accurately the behavior of a measured cell. Second, the reflectivity of a solar cell is studied. An analysis technique involving using the fast Fourier transform of the oscillation in the reflectivity is introduced. This technique extracts the thicknesses of the top and middle subcells. The reflectivity is subsequently calculated using the transfer matrix method and it reproduces the behavior of the measured samples. Third, the effect of the addition of quantum dots in the middle subcell is studied. It is demonstrated that they extend the absorption range of the middle subcell. This is completed by first modeling the quantum mechanical behavior of the electrons and holes in the nanostructure. Their emission and absorption properties are derived. Those derived properties are verified by experimentally measured photoluminescence and electroluminescence of the nanostructures. The resulting model is then compared to experimentally measured external quantum efficiencies of single junction and multi-junction quantum dot-enhanced solar cells. Finally, a study of the bottom subcell artifact is completed. Using the fill-factor bias experiment, each of the contribution of the light coupling and the internal voltage biasing is decoupled. For the measured sample, an optimal voltage of 2.1 V is found to minimize the artifact. At this point, the internal voltage biasing creates an artifact of 1 % and the light coupling artifact is 8 %.

M. Tech (Department of Chemistry, Faculty of Applied and Computer Sciences) Vaal University of Technology. / Quantum dots (QDs) have already proven features that can be considered to improve their properties for biological applications. Metal selenide nanoparticles possess semiconducting behaviors that can vary with structural and optical properties evolving from their synthesis. An aqueous medium through a simple, non-toxic and environmentally friendly colloidal route for the preparation of water-soluble CdSe, Cu2Se, FeSe semiconductor nanoparticles has been developed. Different capping molecules with multi-functional moieties (-COOH, -NH2 and -OH) namely, L-cysteine, L-glutamic acid and L-phenylalanine, have been employed in the preparation of cadmium selenide, copper selenide and iron selenide semiconductor nanoparticles as capping molecules. The synthesized metal selenide nanoparticles were characterized by Fourier Transform Infrared (FTIR), UV-Vis spectroscopy, Photoluminescence spectroscopy (PL), X-ray Diffraction (XRD), Vibrating Sample Magnetometer (VSM) and Transmission Electron Microscopy (TEM). The FTIR spectroscopy confirmed the binding moiety through the surface of the nanoparticles which is pH dependent. The XRD patterns confirmed a cubic phase of CdSe and Cu2Se while FeSe revealed a hexagonal phase for the synthesized nanoparticles. The optical absorption as a function of wavelength for the prepared nanoparticles at different temperature is investigated. The morphology of the nanoparticles dominated through this method was spherical in shape. Amino acids capped metal selenide nanoparticles were successfully synthesized by aqueous medium through a simple colloidal route. The absorption spectra of all samples prepared were blue shifted as compared to their bulk counterparts which signify quantum confinement effect. The optical absorption measurements show some dependency of the temperature values used in the synthesis of nanoparticles. The effect of temperature and pH on the growth and morphology of nanoparticles was investigated. X-ray diffraction patterns confirms the structure, single cubic and hexagonal phase for the synthesized nanoparticles. TEM studies of metal selenide nanoparticle show that particle size increases with the increase in reaction temperature. The vibrating sample magnetometer (VSM) shows almost linear without any hysteresis loop for copper selenide, which indicated the absence of magnetism and exhibits paramagnetic nature than diamagnetic properties while iron selenide revealed twofold ferromagnetic behavior in low fields and paramagnetic behavior in up fields.

The performance of electrical and optical devices based on two-dimensional semiconductors (2D) such as molybdenum disulfide is critically influenced due to very poor light absorption in the atomically thin layers. In this study, the phonon mediated optical absorption and emission properties in single atomic layers of MoS2 have been investigated. The electronic transitions in MoS2 due to near-field optical interaction and the influence of interface phonons due to the dielectric substrate GaN on the relaxation of optically generated carriers will be described. The near-field interaction can be induced in the presence of metal plasmons deposited on the surface of MoS2 monolayers. A hybrid metal-semiconductor system was realized by the deposition of silver (Ag) NPs on MoS2 layer and the localized plasmon modes were selectively chosen to interact with quasiparticles such as excitons and phonons. These quasiparticles are confined within the single atomic layer of MoS2 and are stable at room temperatures due to high binding energy. The lattice vibrational modes in MoS2 can be optically excited with the pulses from a femtosecond laser. These phonon modes can be optically dressed due to near-field interaction in the hybrid Ag-MoS2 system under an optical excitation resonant to localized plasmon modes. The coherent dynamics of the carriers in MoS2 were manipulated by the generation of dressed phonons. The driving field creates a coherence between the ground levels in the presence of optical near-field. A strong coupling between the exciton and plasmon modes forming a plexciton band is observed at room temperature within the coherence lifetime of the system. A significant enhancement of photoluminescent (PL) emission from MoS2 monolayer occurs due to carrier density modulation in the presence near-field interactions. The absorption and emission properties of MoS2 are influenced due to the interactions with the semiconducting substrate. The coupling of carriers in MoS2 with the interfacial phonons, and the charge and energy transfer across the interface in 2D MoS2-GaN (0001) significantly change the UF absorption properties and the relaxation of carriers from the excitonic absorption states. An increased light absorption and enhanced PL emission from the single atomic layer of MoS2 was observed. The phonon-assisted processes can activate the dipole forbidden transitions and hence can explain the interaction of incident light in single atomic layer of MoS2. The MoS2-GaN heterostructure provides a platform to exploit strong coupling between the free carriers or excitons, plasmons and phonons. The gold (Au) NPs have a plasmon energy resonant to MoS2 and hence results in the strong exciton-plasmon coupling due to near-field interaction. In the meantime, the localized plasmon energy of platinum (Pt) NPs is selected to be in resonance to GaN bandedge emission and resonant to C excitonic state in MoS2. The localized plasmons in Pt can actively interact with carriers in MoS2 near Γ-point. The non-equilibrium absorption characteristics of MoS2 nanosheets on GaN hybridized with Au and Pt NPs are influenced due to activation of the defect levels of GaN induced due to interband optical excitation.

This thesis discusses modern methods for fast defect detection of solar cells. For the means of the defect classification, the technological production process of crystalline silicon solar cells is described. The defects are researched by the electroluminescence and prohtoluminescence methods. The Photoluminiscence method is improved by the posibility of using different wavelenght of excitacion sources. The range of using in industrial production is determined for the discussed methods.

This work deals with the modern diagnostic methods of defects detection of solar cells. Defects are detected by electroluminescence and photoluminescence methods. The main focus of this work is an innovation workplace for measuring solar cell defects at very low temperatures using photoluminescence. Liquid nitrogen is used to cooling solar cells. By using low temperatures can be detection methods to obtain more objektive results of diagnostic defects.

No description available.

Die vorliegende Arbeit befasst sich mit dem vertieften Verständnis der Rekombinationsdynamik von polaren ZnO/MgZnO-Quantengraben(QW)-Strukturen zur exakten Bestimmung des unabgeschirmten Grundzustandes und der Analyse der zugrundeliegenden Emissionsprozesse. Dafür werden ausgehend von Beobachtungen an ZnO-Dünnschichten die Eigenschaften von mittels PLD hergestellten QWs unter dem Einfluss des internen elektrischen Feldes mit Hilfe der zeitintegrierten (TI-) und zeitaufgelösten (TR-) Photolumineszenz(PL)-Spektroskopie untersucht. Die Differenz der spontanen und piezoelektrischen Polarisation zwischen ZnO und MgZnO führt zur Ausbildung eines internen elektrischen Feldes und damit zum Auftreten des quantum-confined Stark effect (QCSE). Es wird gezeigt, dass der QCSE durch eine Durchmischung der Grenzflächen stark vermindert wird. Für QWs mit schwachem QCSE ist die Übergangsenergie und Zerfallszeit des Grundzustandes experimentell gut bestimmbar. Bei starkem QCSE müssen jedoch bereits bei geringen Anregungsdichten (1E10 /cm²) Abschirmeffekte berücksichtigt werden. Dadurch ist es sehr schwierig, den unabgeschirmten Grundzustand mittels herkömmlicher experimenteller Methoden mit einem aussagekräftigen Signal-Rausch-Verhältnis zu bestimmen. Es wird gezeigt, dass für QWs mit einer Dicke > 4 nm die Übergangsenergie des unabgeschirmten Grundzustandes nicht durch TI-PL-Messungen bestimmt werden kann. TR-PL-Messungen zeigen energetisch tiefere Übergangsenergien, jedoch ebenfalls nicht den unabgeschirmten Grundzustand. Mit einem eingeführten Modell zur Beschreibung der zeitabhängigen Abschirmung des Grundzustandsniveaus wird die Zerfallszeit für QW-Dicken in einem Bereich von 1 - 10 nm bestimmt. Durch die selbstkonsistente Lösung von Schrödinger- und Poissongleichung werden die Übergangsenergie und Zerfallszeit der Exzitonen im QW in Abhängigkeit der Feldstärke und auch der Ladungsträgerdichte berechnet. Dadurch ist eine exaktere Bestimmung der Feldstärke möglich. Zusätzlich wird durch die vergleichende Untersuchung von QWs unterschiedlicher Dicke, Potentialhöhe und Wachstumsunterlage die spontane und piezoelektrische Polarisation der Materialien experimentell bestimmt. Mittels temperaturabhängiger Messungen wird der Ursprung der Lumineszenz für QW-Dicken > 2 nm der Rekombination freier Exzitonen im QW zugeschrieben. Für dünnere QWs ist der temperaturabhängige Verlauf des PL-Maximums durch Lokalisation der Exzitonen bestimmt.

Silicon nanocrystals (SiNCs) of sizes below approximately 5 nm are a material with an efficient room-temperature photoluminescence (PL) and optical gain. Optical gain is a pre- requisite for obtaining stimulated emission from a pumped material, and the achievement of stimulated emission (and lasing) from Si-based nanostructures is of particular interest of the field of silicon photonics. The aim of this work was (i) to investigate fundamental optical properties of SiNCs, (ii) to design and prepare a photonic crystal with enhanced light ex- traction efficiency and (iii) to explore a possibility of enhancing optical gain of light-emitting SiNCs by combining them with a two-dimensional photonic crystal. First, free-standing oxide (SiOx/SiO2)-passivated SiNCs were prepared by electrochemical etching of a Si wafer. Their optical properties were studied by employing time-resolved spectroscopy, also at cryogenic temperatures. The fast blue-green emission band of these SiNCs was linked with the quasi- direct recombination of hot electrons and holes in the vicinity of the Γ-point. Furthermore, the spectral shift of the slow orange-red band (of these SiNCs) as a function of temperature was explained on the basis of an interplay between tensile strain and bulk Si temperature-induced indirect bandgap shift. The...