In this doctoral thesis the two dimensional material graphene has been studied in depth with particular respect to Zener tunnelling devices. From the hexagonal structure the Hamiltonian at a Dirac point was derived with the option of including an energy gap. This Hamiltonian was then used to obtain the tunnelling properties of various graphene nano-devices; the devices studied include Zener tunnelling potential barriers such as single and double graphene potential steps. A form of the Landauer formalism was obtained for graphene devices. Combined with the scattering properties of potential barriers the current and conductance was found for a wide range of graphene nano-devices. These results were then compared to recently obtained experimental results for graphene nano-ribbons, showing many similarities between nano-ribbons and infinite sheet graphene. The methods studied were then applied to materials which have been shown to possess three dimensional Dirac cones known as topological insulators. In the case of Cd3As2 the Dirac cone is asymmetrical with respect to the z-direction, the effect of this asymmetry has been discussed with comparison to the symmetrical case.

The diploma thesis deals with the study of perovskite solar cells with a regular n-i-p architecture. The theoretical part of this work is mainly focused on the stability of perovskite solar cells, i.e. thermal stability and the influence of UV radiation on final perovskite solar cell stability. Furthermore, the deposition methods, the architecture of solar cells and the materials used for the preparation of electron and hole transport layers were described in more detail. The experimental part deals with the optimization of the preparation of perovskite solar cells (especially in terms of resulting photovoltaic conversion efficiency), with a description of the structure preparation process of the final photovoltaic cell and the interpretation of the measured results.

This thesis is focused on the photochromic molecule of spiropyran, which changes its structure as well as physical and chemical properties after UV irradiation. These changes are reversible, the molecule thermally restore its initial structure. For the study, the molecule SP1 with the systematic name 1',3'-dihydro-1',3',3'-trimethyl-6-nitrospiro[2H-1-benzopyran-2,2'-(2H)-indole] was used. This molecule was incorporated within polymers PVK, Tg PPV, PCBTDPP and PCDTBT and the method of UV-VIS spectroscopy was used to observe the photochromic activity within these matrices. The influence of matrices to the ability to undergo UV induced photochromic conversion as well as to the reverse conversion to the initial structure induced by heat was monitored. Furthermore, the influence of spiropyran to the electrical properties of individual matrices was studied. The effect of photochromic conversion to both, the mobility of charge carriers and to the photogeneration was observed. For this purpose, the method of current-voltage measurement was used.

The aim of this project is to model current volt-age characteristic curves for different plasma conditions (i.eelectron density, electron temperature, ion temperature andplasma potential) that can be found in active auroras. This isdone by simulating the charging of a FFU with a connectedLangmuir probe in the software SPIS. These I-V curves wereused to determine the plasma properties of the auroras in whichthe sounding rockets SPIDER-1 and SPIDER-2 were launched.Through the simulations it was also studied how the differentparameters effects the I-V curves.The results showed that the plasma SPIDER-1 was launchedin most likely had properties close to nominal conditions and forSPIDER-2 there was colder electrons in the plasma. A conclusionthat only the electron temperature affects the shape of the I-Vcurves for the values simulated in this project can be drawnas well as the conclusion that the geometry of the probe doesnot affect the shape of the I-V curves. Another result showsthat electron temperature also affect how the hull of the FFUcharges. A higher electron temperature gives the hull a morenegative charge. / Syftet med detta projekt är att modellera ström-pänningskaraktäristisk kurvor för olika plasmatillstånd som finns i aktiva auroror. Detta görs genom att simulera laddning på en FFU med en ansluten Langmuir-prob i SPIS. Dessa I-V-kurvor används för att bestämma plasma egenskaperna för aurororna sondraketerna SPIDER-1 och SPIDER- 2 skjöts upp i. I-V kurvorna används också för att bestämma hur plasma parametrarna elektron temperatur, jon temperatur och elektrondensitet samt hur probens geometri påverkar I-V- kurvornas utseende. Resultaten visade att den plasma SPIDER-1 blev uppskjuten i troligen hade nominella förhållanden och att den SPIDER-2 blev uppskjuten i troligtvis hade kallare elektroner. En slutsats om att endast elektron temperaturen påverkar formen på IV- kurvorna kan dras, såväl som slutsatsen att probens geometri inte påverkar formen på IV-kurvorna. Ett annat resultat visar att elektron temperaturen också påverkar ytpotentialen på FFUn. En högre elektron temperatur ger FFUn en mer negativ laddning. / Kandidatexjobb i elektroteknik 2020, KTH, Stockholm

The dissertation is focused on the study of electric and especially dielectric properties of thin film organic materials with their huge potential for optoelectronics and other industrial sectors. The theoretical part deals with the use of organic materials in organic photovoltaic cells, the methods of deposition techniques and characterization. The theoretical knowledge of the dielectric spectroscopy, including methods for determining the physical properties and evaluation of experimental data are also described. The experimental part is devoted to the study of small molecule organic solar cells with bulk heterojunction composed of electron donor molecule of DPP(TBFu)2 and electron acceptor fullerene derivatives, such as PC60BM, PC70BM and TC60BM. The experimental part is divided into two main parts. The first part deals with the study of processes occurring at the interface between the active layer and the contact; the second part is focused on transport processes inside the structure of photovoltaic cells and also contains a study of perovskite solar cells.

This diploma´s thesis is focused on a study of electrical and dielectric properties of gas sensors based on ionic liquids. Measurements were done on two different types of OECT substrates (0099 and 0160). Three ionic liquids and physiological solution PBS were chosen as electrolytes. Direct current current-voltage characteristic was measured. Switching rations of transistors with this electrolyte were determined by current-voltage characteristic. Alternating resistivity dependence on frequency were measured as well. Better properties showed OECT 0099 substrates.

This work concerns with today's challenges of photoelectrical characterization methods in the research and development of thin film silicon solar cells. Relevant results are obtained only when photocurrent spectroscopy and measurement of current-voltage characteristics, are applied on the real structures that can however be multi-layered, multi-junction devices with nanostructured interfaces. Analytical and numerical optical models comprising light scattering are used for analysis of light absorption and for evaluation of optical absorption coefficient of silicon layers in sub-gap region. The slope of absorption edge and residual absorption in mid-gap indicate material disorder and defect density. Based on the investigation of electrical interaction between sub-cells in the dual-junction solar cell we developed new methods of evaluation of photocurrent spectra and current-voltage characteristics individually for each sub-cell with no need to contact them directly. Usability of Fourier Transform Photocurrent Spectroscopy as a robust method for photocurrent spectroscopy of amorphous silicon is thoroughly analyzed here. The issues of frequency dependence are addressed in detail and comparison with photothermal deflection spectroscopy is made.

Parmi les nitrures III-V, le matériau InGaN a été intensément étudié depuis les années 2000 pour des applications photovoltaïques, en particulier pour des cellules multi-jonctions, grâce à son large gap modulable pouvant couvrir quasiment tout le spectre solaire. On pourrait alors atteindre de hauts rendements tout en assurant de bas coûts. Cependant, l’un des problèmes de l’InGaN est l’absence de substrat accordé en maille provoquant une grande densité de défauts et limitant ainsi les performances des composants. Nous avons donc étudié la faisabilité de cellules solaires simples jonctions à base d’InGaN sur des substrats alternatifs comme le silicium et le verre afin de baisser les coûts et d’avoir de larges applications. Afin d’adapter l’InGaN sur ces substrats alternatifs, nous avons utilisé une couche tampon en ZnO. Ce travail a été réalisé dans le cadre du projet ANR NewPVonGlass. Plus particulièrement, dans ce projet, mon travail avait pour objectifs de réaliser des caractérisations électriques et optiques des matériaux et des composants. Les deux premières parties de cette thèse introduisent le matériau InGaN et l’effet photovoltaïque. Les techniques de caractérisation utilisées sont expliquées dans le troisième chapitre. Ensuite, les résultats obtenus lors de la caractérisation cristalline du matériau InGaN sont présentés en fonction du substrat, de la concentration d’indium et de l’épaisseur de la couche. Puis, la cinquième partie développe les caractérisations des premières cellules à base d’InGaN sur saphir. Enfin, dans le dernier chapitre, des simulations de cellules solaires à base d’InGaN ont été réalisées. Le modèle développé nous a permis d’optimiser la structure et le dopage du composant et de déterminer les paramètres critiques. Nous montrons donc, dans ce travail, le développement d’une cellule solaire à base d’InGaN : des caractérisations des matériaux de base à celles des cellules solaires, en passant par la modélisation. / Among III-V nitrides, the InGaN material has intensively been studied since the year 2000 for photovoltaic applications, in particular for multi-junction solar cells, thanks to its large tunable band gap covering almost the entire solar spectrum. Then, it will be possible to reach high efficiency and low cost. However, one of the problems of InGaN material is the absence of lattice-matched substrate leading to high defect density which limits device performances. We have thus studied the feasibility of single junction InGaN based solar cells on alternative substrate such as silicon and glass in order to lower the price and to benefit from their wide application fields. To adapt InGaN material on these new substrates, we have utilized ZnO buffer layer. This work has been carried out within the framework of the ANR project NewPVonGlass. More particularly, in this project, I was in charge of the electrical and optical characterizations of the materials and devices. In the two first parts of this manuscript, the InGaN material and the photovoltaic effect are introduced. Then, the characterization techniques are explained in the third chapter. In the fourth part, the results obtained during crystalline characterization of the InGaN materials are presented depending on the substrate, the indium percentage and the InGaN layer thickness. Then, the fifth chapter presents the first InGaN-based solar cell characteristics on sapphire substrate. Finally, in the last part, simulations of InGaN-based solar cell have been performed. The developed model was able to optimize the structure and to determine the critical parameters. Thus, we have shown in this work the development of an InGaN-based solar cell from the base material characterizations to the device tests, through modeling.

<p>Dynamic properties of type-II superconductors have been experimentally studied in zero and low magnetic fields using SQUID magnetometry and <i>I–V</i> measurements.</p><p>In zero magnetic field close to the critical temperature, the physical properties of type-II superconductors are dominated by spontaneously created vortices. In three dimensions (3D) such vortices take the form of vortex loops and in two dimensions (2D) as vortex-antivortex pairs.</p><p>The 2D vortex dynamics has been probed using mutual inductance and flux noise measurements on YBa₂Cu₃O₇ (YBCO) and MgB₂ thin films in zero and low magnetic fields. In such measurements, information about vortex correlations is obtained through a temperature dependent characteristic frequency, below (above) which the vortex movements are uncorrelated (correlated). The results obtained in zero magnetic field indicate that sample heterogeneities influence the vortex physics and hinder the divergence of the vortex-antivortex correlation length.</p><p>In low magnetic fields the vortex dynamics is strongly dependent on the applied magnetic field and a power law dependence of the characteristic frequency with respect to the magnetic field is observed. The results indicate that there is a co-existence of thermally and field generated vortices.</p><p>The <i>I–V</i> characteristics of untwinned YBCO single crystals show that only a small broadening of the transition region influences the length scale over which the vortex movements are correlated. The dynamic and static critical exponents therefore exhibit values being larger in magnitude as compared to values predicted by relevant theoretical models. The results also suggest that the copper oxide planes in YBCO decouple slightly below the mean field critical temperature and hence, the system has a crossover from 3D to 2D behaviour as the temperature is increased. </p><p>From temperature dependent DC-magnetisation measurements performed on untwinned YBCO single crystals in weak applied fields, detailed information about the critical current density and the irreversibility line is obtained.</p>

Dynamic properties of type-II superconductors have been experimentally studied in zero and low magnetic fields using SQUID magnetometry and I–V measurements. In zero magnetic field close to the critical temperature, the physical properties of type-II superconductors are dominated by spontaneously created vortices. In three dimensions (3D) such vortices take the form of vortex loops and in two dimensions (2D) as vortex-antivortex pairs. The 2D vortex dynamics has been probed using mutual inductance and flux noise measurements on YBa2Cu3O7 (YBCO) and MgB2 thin films in zero and low magnetic fields. In such measurements, information about vortex correlations is obtained through a temperature dependent characteristic frequency, below (above) which the vortex movements are uncorrelated (correlated). The results obtained in zero magnetic field indicate that sample heterogeneities influence the vortex physics and hinder the divergence of the vortex-antivortex correlation length. In low magnetic fields the vortex dynamics is strongly dependent on the applied magnetic field and a power law dependence of the characteristic frequency with respect to the magnetic field is observed. The results indicate that there is a co-existence of thermally and field generated vortices. The I–V characteristics of untwinned YBCO single crystals show that only a small broadening of the transition region influences the length scale over which the vortex movements are correlated. The dynamic and static critical exponents therefore exhibit values being larger in magnitude as compared to values predicted by relevant theoretical models. The results also suggest that the copper oxide planes in YBCO decouple slightly below the mean field critical temperature and hence, the system has a crossover from 3D to 2D behaviour as the temperature is increased. From temperature dependent DC-magnetisation measurements performed on untwinned YBCO single crystals in weak applied fields, detailed information about the critical current density and the irreversibility line is obtained.