The increasing demand for renewable energy has led to substantial research on different solar cell technologies. The dye-sensitized solar cell (DSC) is a technology utilizing dye molecules for light absorption. Dye molecules are adsorbed to a mesoporous semiconductor surface and after light absorption in the dye, charge separation occurs at this interface. Traditionally, DSCs have used layers of single dye species, but in recent efforts to enhance power conversion efficiency, more complex molecular layers have been designed to increase the light absorption. For example, the most efficient DSCs use a combination of two dye molecules, and such dye co-adsorption is studied in this thesis. A key to highly efficient DSCs is to understand the dye/semiconductor interface from a molecular perspective. One way of gaining this understanding is by using an element specific, surface sensitive technique, such as photoelectron spectroscopy (PES). In this thesis, PES is used to understand new complex dye/semiconductor interfaces. Dyes adsorbed to semiconductor surfaces are analyzed using PES in terms of geometric and electronic surface structure.  The investigations ultimately target the effects of co-adsorbing dyes with other dyes or co-adsorbents. PES shows that Ru dyes can adsorb in mixed configurations to TiO2. Co-adsorption with an organic dye affects the configuration of the Ru dyes. As a consequence, shifts in energy level alignment and increased dye coverage are observed. The dyes are affected at a molecular level in ways beneficial for solar cell performance. This is called collaborative sensitization and is also observed in todays most efficient DSC. Dye molecules are generally sensitive to high temperatures and the substantial decrease in power conversion efficiency after heat-treatment can be understood using PES. Furthermore, comparing two mesoscopic TiO2 morphologies used in DSCs show differences in trap state density in the band gap, explaining the photovoltage difference in DSCs comprising these morphologies. Using mixed molecular layers on NiO results in significant improvements of p-type DSC power conversion efficiency. PES shows that changed adsorption configuration contribute to this effect. This thesis shows that PES studies can be used to obtain insight into functional properties of complex DSC interfaces at a molecular level.

A measurement system is to be presented, which is used to obtain the I-V characteristics of a solar cell and to track its temperature during irra-diation before mounting it into a complete array/module. This project presents both the design and implementation of an Electronic load for testing the solar cell under field conditions of 10000 W/m^2, which is able to provide current versus voltage and power versus voltage charac-teristics of a solar cell using a software based model developed in Lab-VIEW. An efficient water cooling method which includes a heat pipe array system is also suggested. This thesis presents the maximum power tracking of a solar cell and the corresponding voltage and current values. In addition, the design of the clamp system provides an easy means of replacing the solar cell during testing.Keywords: Solar cell, Metal Oxide Semiconductor Field Effect Transistor (MOSFET), I-V characteristics, cooling system, solar cell clamp system, LabVIEW, Graphical User Interface (GUI).

Interest in research concerning renewable energy sources has grown in recent decades. In this context, the study of the physical processes that are important in the conversion of solar energy radiation via the photovoltaic effect has increasingly been the topic of theoretical and experimental research. In this thesis, we discuss the photovoltaic effect in pn junctions found in Si solar cells, as well as the photoelectrochemical effect at semiconductor-electrolyte interfaces. Specifically, we propose a new linearizable model for the photocurrent-voltage characteristics of nanocrystalline TiO2 dye sensitized solar cells, as well as ZnO solar cells. We also report predicted values for fill factors. The model uses the Butler-Volmer and Nernst equations to describe the photocurrent-voltage characteristics. A single free parameter of the model controls the fill factor. Upon renormalization, diverse experimental photocurrent-voltage data collapse onto a single universal function. These advances allow the estimation of the complete current-voltage curve and the fill factor from any three experimental data points, e. g., the open circuit voltage, the short circuit current and one intermediate measurement. / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / O interesse em pesquisas sobre fontes de energia renováveis tem crescido nas últimas décadas. Nesse contexto, cada vez mais o estudo dos processos físicos relevantes na conversão da energia da radiação solar, através do efeito fotovoltaico, tem sido alvo de pesquisas experimentais e teóricas. Nesta tese, discutimos o efeito fotovoltaico em junções pn que ocorrem nas células solares de Si, bem como o efeito fotoeletroquímico em interfaces semicondutor-eletrólito. Especificamente, propomos um novo modelo linearizável para as curvas características de fotocorrente-voltagem de células solares de TiO2 nanocristalino sensibilizado por corante, assim como células solares de ZnO. Descrevemos também valores previstos para os fatores de preenchimento. O modelo utiliza as equações de Butler-Volmer e de Nernst para descrever as curvas características de fotocorrente-voltagem. Um único parâmetro livre do modelo controla o fator de preenchimento. Após a renormalização, as curvas com os dados experimentais de fotocorrente-voltagem colapsam em uma única função universal. Estes avanços permitem a estimativa completa da curva de fotocorrentevoltagem e o fator de preenchimento, usando apenas três pontos experimentais, tais como, a voltagem de circuito aberto, a corrente de curto-circuito e uma medida intermediária.

Au cours de ces travaux de thèse, deux axes d’études ont été développés. Ils consistent d’une part à la synthèse et la caractérisation de nouveaux complexes de bore, visant à être utilisés dans des cellules solaires organiques, ainsi qu’au développement de plateformes d’empilement π, permettant la modulation des propriétés structurales de semi-conducteurs organiques. Ainsi, le rendement de conversion énergétique de cellules solaires utilisant un BODIPY a été amélioré par l’addition de triazatruxènes sur ses positions β-pyrroliques. Le motif BOPHY, complexe di-nucléaire de bore, a été fonctionnalisé puis étudié optoélectroniquement. Le premier exemple d’utilisation d’un BOPHY dans une cellule solaire organique a également été effectué. Deux familles de BODIPYs ont été préparées en vue d’une utilisation dans des cellules solaires à colorant. Leur utilisation au sein d’une même structure a permis d’établir un nouveau record de conversion pour une telle utilisation d’un BODIPY. / During this thesis, boron complexes and π-stacking mediator planar moieties have been synthetized for photovoltaic applications. Thanks to the use of triazatruxene units on β-pyrrolic positions of a BODIPY core, the solar cells photoconversion efficiency has been increased. The BOPHY moiety, a di-nuclear boron complex, has been functionalized and optoelectronically studied. The first example of the use of BOPHY in organic solar cells has been performed. Two families of BODIPYs have also been prepared to be used in dye-sensitized solar cells. A new record of photoconversion efficiency for BODIPY based solar cells has been reached using a co-adsorption of two dyes having complementary absorptions within a unique solar cell.

The master‘s thesis is focused on the modification of measurement workplace for solar cells. In the first part of the thesis are introduced general problems of the solar energy and possible use in the international power supply, the details about the photovoltaic effect, processing of solar cells and their subsequent parameters. More further in the text the autor is concerned with causes of solar cells defects formation and representation of the most important defects. For defects detection are known the detection methods of solar cells, which are generally described in the text. The second part of the thesis includes detailed description of the LBIC method and workplace both the VUT Brno, and in Solartec, Ltd. The most important point of this part is project and description of innovated workplace from hardware and software realization. There is out of print the function and the description of the user interface and subsequently there are mentioned results gained from original and innovated workplace. In the conclusion are summarized whole activities of the master’s thesis, described and evaluated achieved results and outlined the direction, where would the other development of the LBIC workplace on VUT Brno be proceeding.

Doctor of Philosophy / Department of Physics / Hongxing Jiang / III-nitride photonic devices such as photodetectors (PDs), light emitting diode (LEDs), solar cells and optical waveguide amplifiers were designed, fabricated and characterized. High quality AlN epilayers were grown on sapphire and n-SiC substrates by metal organic chemical vapor deposition and utilized as active DUV photonic materials for the demonstration of metal-semiconductor-metal (MSM) detectors, Schottky barrier detectors, and avalanche photodetectors (APDs). AlN DUV PDs exhibited peak responsivity at 200 nm with a very sharp cutoff wavelength at 207 nm and extremely low dark current (<10 fA), very high breakdown voltages, high responsivity, and more than four orders of DUV to UV/visible rejection ratio. AlN Schottky PDs grown on n-SiC substrates exhibited high zero bias responsivity and a thermal energy limited detectivity of about 1.0 x 1015 cm Hz1/2 W-1. The linear mode operation of AlN APDs with the shortest cutoff wavelength (210 nm) and a photocurrent multiplication of 1200 was demonstrated. A linear relationship between device size and breakdown field was observed for AlN APDs. Photovoltaic operation of InGaN solar cells in wavelengths longer than that of previous attainments was demonstrated by utilizing InxGa1−xN/GaN MQWs as the active layer. InxGa1-xN/GaN MQWs solar cells with x =0.3 exhibited open circuit voltage of about 2 V, a fill factor of about 60% and external quantum efficiency of 40% at 420 nm and 10% at 450 nm. The performance of InxGa1-xN/GaN MQWs solar cell was found to be highly correlated with the crystalline quality of the InxGa1-xN active layer. The possible causes of poorer PV characteristics for higher In content in InGaN active layer were explained. Photoluminescence excitation studies of GaN:Er and In0.06Ga0.94N:Er epilayers showed that Er emission intensity at 1.54 µm increases significantly as the excitation energy is tuned from below to above the energy bandgap of these epilayers. Current-injected 1.54 µm LEDs based on heterogeneous integration of Er-doped III-nitride epilayers with III-nitride UV LEDs were demonstrated. Optical waveguide amplifiers based on AlGaN/GaN:Er/AlGaN heterostructures was designed, fabricated, and characterized. The measured optical loss of the devices was ~3.5 cm−1 at 1.54 µm. A relative signal enhancement of about 8 dB/cm under the excitation of a broadband 365 nm nitride LED was achieved. The advantages and possible applications of 1.54 µm emitters and optical amplifiers based on Er doped III-nitrides in optical communications have been discussed.

The tandem solar cell concept is a promising approach to improve the efficiency of photovoltaic devices. However, characterization of tandem solar cell devices is challenging since correct efficiency determination demands special experimental infrastructure as well as suitable characterization procedures. Even though the appropriate IEC and ASTM measurement standards define all that very precisely, they cannot be applied without special care to organic photovoltaics (OPV) because they were originally developed for inorganic devices. As a consequence, nowadays almost all tandem organic solar cell publications are not using correct characterization procedures, often resulting in questionable efficiency values. The aim of this work is developing a measurement procedure for tandem organic solar cells assuring their correct characterization. Therefore, at first the existing standards and measurement procedures for tandem solar cells are reviewed and challenges when applying these standards to organic solar cells are identified. As main challenges the relatively low fill factors and distinct nonlinearities of organic solar cells are identified. As preliminary experiments, single junction organic solar cells are investigated to analyze the influence of measurement parameters like bias irradiance, bias voltage, and chopper frequency on the external quantum efficiency (EQE) of organic solar cells. This results in parameter sets assuring minimized artifacts for the subsequent EQE determination of the subcells of tandem organic solar cells. The main part of this thesis presents the detailed characterization of a tandem OPV example device. First, EQE is measured and validated by two independent institutes. The EQE results are used to calculate the illumination conditions to reach AM1.5g conditions for both subcells with a multi-source sun simulator. The resulting efficiency value under standard reporting conditions (SRC) is found to be 5% lower than the efficiency measured with a single-source sun simulator. A full spectrometric characterization shows that differing fill factors of the subcells are the reason for this behavior. To overcome the main reason for the complicated measurement procedure of tandem solar cells, the inaccessibility of the individual subcells, three different approaches for the jV-characteristics determination of the subcells are presented. The so-called Bias Voltage Approach is based on EQE-measurements under varying bias voltage and needs no additional electrical contacts. Therefore, it can be applied to existing devices. The Voltage Contact Approach as well as the Current Contact Approach require in changed stack designs. Therefore, they cannot be applied to existing devices but give more accurate results. Finally, a procedure for characterizing tandem organic solar cells is formulated. This procedures aims at giving practical advice how to characterize tandem organic solar cells to achieve results conforming to the measurement standards and being as accurate and reproducible as possible. Hence, this thesis attempts to establish standards for a correct measurement of tandem organic solar cells of which other emerging solar cell technologies can profit as well.

The huge environmental awareness emerging last years by reason of global warming and greenhouse effect, on one hand, and the need of finding other sources of energy production and conversion due to the declining of fossil resources and the increasing cost of this kind of energy resource, on the other hand, both have led position renewable energies as a powerful alternative on the energy production and conversion. PV-systems have emerged at an exponential rate in recent year as the main candidate and a satisfactory possibility with respect to environmental and economic sustainability. Nowadays, the large volume on photovoltaic market is currently dominated by four types of solar cells, divided by the semiconductor material used to absorb light and convert the energy into electricity: (1) crystalline silicon (monocrystalline and polycrystalline), (2) amorphous silicon, (3) CIGS and (4) cadmium telluride; and among them, monocrystalline silicon and CIGS technologies are installed on the building 45 of the University of Gävle, at the south face of the laboratory. In this context and with the motivation to contribute knowledge on PV field, a comparison between single crystal solar technology and thin film CIGS technology has carried out through f ratio and performance ratio procedures in order to perform an assessment of the energy conversion of each one under field conditions. A logger monitors the power conversion from the PV modules since June 2014 while two pyranometers monitor global and diffuse solar radiation since March 2016. It must take into account that only clear sunny days have been considered during a period from 8:00 to 14:00 in order to avoid shadows effect on the PV systems. The results come to conclude that single crystal silicon modules present a better behavior with respect to energy conversion under no shadows effect conditions by two reason: (1) f ratio, relationship of PV conversion per kW (PV yield) between CIGS and single crystal silicon, is about 87.25% with some variations along a day due to ambient temperature, cell temperature and incidence angle; (2) PV module's performance ratio of monocrystalline silicon modules is higher than thin film CIGS ones during a sunny day about 87.56% and 76.38%, respectively; and they are consistent with usual performance ratio values between 80% and 90% since 2010 onwards. In light of the outcome and in order to confirm these conclusions, it intends to launch a project with the objective of evaluating the data collected and compare the performance of the module after a year of measurements outdoors by the PV module's performance ratio procedure. Along the same lines, the next step of the University of Gävle will be to launch a project with the objective of evaluating the potential to be self-sufficient.

Selectively-transparent and conducting photonic crystals (STCPCs) made of alternating layers of sputtered indium-tin oxide (ITO) and spin-coated silica (SiO2) nanoparticle films have potential applications in micromorph solar cells and building integrated photovoltaics (BIPVs). In this work, theoretical calculations have been performed to show performance enhancement of the micromorph solar cell upon integration of the STCPC an intermediate reflector. Thin semi-transparent hydrogenated amorphous silicon (a-Si:H) solar cells with STCPC rear contacts are demonstrated in proof-of-concept devices. A 10% efficiency increase in a 135nm thick a-Si:H cell on an STCPC reflector with Bragg peak at 620nm was observed, while the transmitted solar irradiance and illuminance are determined to be 295W/m2 and 3480 lux, respectively. The STCPC with proper Bragg peak positioning can boost the a-Si:H cell performance while transmitting photons that can be used as heat and lighting sources in building integrated photovoltaic applications.

Today there is no mobile application that simultaneously indicates the electricity consumption of a household and the solar cell's electricity production. Our expectation of the mobile application Watt-sun is to achieve an increased level of interest, usage and understanding of solar electricity. The solar cell’s electricity production depends on the season, time of the day and weather, thus making it difficult to estimate the amount of electricity the solar cells produce. Watt-sun visualizes and compares the solar cell’s electricity production and the user’s consumed electricity using graphs. Watt-sun helps the user to distribute the electricity consumption over the day such that an increased proportion of the produced solar electricity is used. The result of the performed user tests indicates that the mobile application helps the user to increase the proportion of solar electricity out of the total electricity consumption. By utilizing the electricity of the solar cells more efficiently, it will in turn contribute to a reduced usage of other electrical sources. Accordingly, the emission's of greenhouse gases can be reduced. / Idag finns det ingen mobilapplikation som samtidigt indikerar elförbrukningen i ett hushåll och elproduktionen från en solcell. Vår förhoppning med mobilapplikationen Watt-sun är att öka intresset, användningen och förståelsen av solceller. Solcellens elproduktion beror på årstid, tid på dygnet samt väder, vilket gör det svårt att uppskatta mängden el solcellerna producerar. Watt-sun visualiserar och jämför solcellens elproduktion och användares elkonsumtion med hjälp av grafer. Watt-sun underlättar för användaren att förflytta sin elkonsumtionen över dygnet så att en större andel av den producerade solelen används. Resultatet av de utförda användartesterna indikerar att mobilapplikationen underlättar för användaren att öka andelen solel av den totala elkonsumtionen. Genom att solelen utnyttjas bättre, kommer det i sin tur att bidra till ett minskat behov av andra elkällor och kan därmed minska utsläppen av växthusgaser.