Polymer-based photovoltaics have the potential to contribute to boosting photovoltaic energy conversion overall. Besides allowing large-area inexpensive processing, polymeric materials have the added benefit of opening new market applications for photovoltaics due to their low-weight and interesting mechanical properties. The energy conversion efficiency values of polymer photovoltaics have reached new record values over the past years. It is however crucial that stability issues are addressed together with efficiency optimization. Understanding fundamental materials aspects is key in both areas. In the work presented in this thesis, the morphology of polymer:fullerene films and its influence on device performance was studied, as well as the effect of light exposure on the surface of fullerene films. Several polyfluorene copolymers were used for the morphology studies, where the effects of changing spin-coating solvent and of side chain engineering were investigated with dynamic secondary ion mass spectrometry (dSIMS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Polymer-enriched surfaces were found in all blend films, even in the cases with homogeneous distributions in the bulk. Side chain engineering of the polymer led to gradual changes in the compositional variations perpendicular to the surface, and to slight variations in the photocurrent. The electronic structure of the fullerene derivative PCBM was studied in detail and the spectroscopic fingerprint of the materials was analysed by comparison with theoretically simulated spectra. Photo-stability studies done in air showed that the surface of fullerene films underwent severe damages at the molecular level, which is evident from changes in the valence band and X-ray absorption spectra. These changes were explained by transitions from sp2-type to sp3 hybridization of the carbon atoms in the cage that resulted in the destruction of the fullerene cage.

Solar electricity is a promising energy technology for the future, and by using reflectors for concentrating solar radiation onto photovoltaic cells, the cost per produced kWh can be significantly reduced. The optical efficiency of a concentrating system determines the fraction of the incident energy that is transferred to the cells and depends on the optical properties of the system components. In this thesis, low-concentrating photovoltaic and photovoltaic-thermal systems with two-dimensional parabolic reflectors were studied and optimised, and a new biaxial model for the incidence angle dependence of the optical efficiency was proposed. Concentration of light generally results in high cell temperatures, and the uneven irradiance distribution on cells with parabolic reflectors leads to high local currents and temperatures, which reduce fill-factor and voltage. Cooling the cells by means of water increases the voltage and makes it possible to utilize the thermal energy. The performance of a 4X concentrating photovoltaic-thermal system was evaluated. If operated at 50°C, this system would produce 250 kWhelectrical and 800 kWhthermal per m2 cell area and year. Optical performance can be increased by 20% by using better reflectors and anti-reflectance glazing. Low-concentrating photovoltaic systems for façade-integration were studied and optimised for maximum annual electricity production. The optimisation was based on measured short-circuit currents versus solar altitude. Measurements were performed outdoors and in a solar simulator. It was found that the use of 3X parabolic reflectors increases the annual electricity production by more than 40%. High solar reflectance is crucial to system performance but by using a low-angle scattering reflector, the fill-factor and power are increased due to a more even irradiance on the modules. Long-term system performance depends on the durability of the components. The optical properties and degradation of reflector materials were assessed using spectrophotometry, angular resolved scatterometry, Fresnel modelling, optical microscopy, and surface profilometry before and after ageing. The degradation of reflectors was found to be strongly dependent on material composition and environmental conditions. Back surface mirrors, all-metal reflectors, and polymer-metal laminates degraded in different ways, and therefore accelerated ageing must be tailored for testing of different types of reflector materials. However, new types of reflector laminates showed a potential for increasing the cost-effectiveness of low-concentrating solar energy systems.

The objective of this thesis is to develop low-cost high-efficiency crystalline silicon solar cells which are at the right intersection of cost and performance to make photovoltaics (PV) affordable. The goal was addressed by improving the optical and electrical performance of silicon solar cells through process optimization, device modeling, clever cell design, fundamental understanding, and minimization of loss mechanisms. To define the right intersection of cost and performance, analytical models to assess the premium or value associated with efficiency, temperature coefficient, balance of system cost, and solar insolation were developed and detailed cost analysis was performed to quantify the impact of key system and financial parameters in the levelized cost of electricity from PV.

No description available.

The efficiency of an organic photovoltaic cell depends mainly on its morphology where an exciton has to migrate to a p-n junction to create a photocurrent. Therefore the distance from the bulk of the cell to a junction interface should not exceed the diffusion length of the exciton. In this thesis, two novel lithographical methods, to produce specific polymer morphologies, were developed and evaluated. In the first method, called embedded annealing, self-assembled polystyrene colloids were embedded in a polydimethylsiloxane (PDMS) film and annealed under an electric field to produce a bi-polymer structure consisting of polymer columns in a thin film of PDMS. Polymer colloids were successfully assembled into two dimensional hexagonally close packed arrays. However, the annealing process was unsuccessful. The second method, imprint annealing, aimed to increase the aspect ratio (height/width) of thermally imprinted micrometer sized polystyrene features by annealing them in uniform electric fields. The results showed that the aspect ratio of imprinted features can be significantly increased, 21-fold, while maintaining the periodicity of the original imprint. This is in contrast to previous results where smooth polymer films annealed in uniform fields where the periodicity of the resulting structures cannot be independently controlled, and are highly sensitive to the electrode spacing. Feature sizes down to 1 µm and aspect ratios up to 4.5 were achieved using imprint annealing. / Verkningsgraden hos en hos en solcell beror, för givna material, framförallt på dess uppbyggnad. För att bidra till fotoströmmen måste en genererad exciton vandra till en pn-övergång. På grund av detta bör det längsta avståndet till närmaste pn-övergång i solcellen inte vara längre än excitonens diffusionslängd. I detta examensarbete testas två olika litografiska metoder för att åstadkomma en specifik filmgeometri lämpad för organiska solceller. Den första metoden, kallad embedded annealing, går ut på att bädda in spontant ordnade sfäriska polystyrenkolloider i en polydimetylsiloxan (PDMS) -film för att sedan vid förhöjd temperatur applicera ett elektiskt fält över filmen. Förhoppningen var att på detta sätt töja ut kolloiderna till pelare genom PDMS-filmen. I det första steget ordnades kolloiderna sponant i tätpackade hexagonala tvådimensionella gitter på kiselsubstrat. Experimenten lyckades inte med hjälp av elektriska fält töja ut kolloiderna. Den andra metoden, imprint annealing, syftar till att öka höjd/bredd -förhållandet och minska diametern hos präglade polystyrenstrukturer. Dessa ursprungliga topografiska stukturer skapas med hjälp av en tryckpressmetod kallad nanoimprinting. Dessa strukturer värmdes upp, och ett uniformt elekrisk fält applicerades över dem. Mina resultat visar att man med elektriska fält avsevärt kan öka höjd-breddförhållandet hos polymerstrukturer och samtidigt bevara periodiciteten hos de ursprungliga strukturerna. Detta står i kontrast mot tidigare resultat på släta filmer, där periodiciteten inte kan kontrolleras oberonde av andra parametrar. Med imprint annealing ökades höjd-breddförhållandet hos enskilda strukturer upp till 21 gånger. Diametrar ner till 1 µm och höjd/breddförhållanden upp till 4,5 uppnåddes.

The declining cost of photovoltaic (PV) modules has enabled the vision of ubiquitous photovoltaic (PV) power to become feasible. Emerging PV technologies are facilitating the creation of intentionally non-flat PV modules, which create new applications for this sustainable energy generation currently not possible with the traditional rigid, flat silicon-glass modules. However, since the photovoltaic cells are no longer coplanar, there are significant new requirements for the power electronics necessary to convert the native form of electricity into a usable form and ensure maximum energy harvest. Non-uniform insolation from cell-to-cell gives rise to non-uniform current density in the PV material, which limits the ability to create series-connected cells without bypass diode or other ways to shunt current, which is well known in the maximum power tracking literature. This thesis presents a modeling approach to determine and quantify the variations in generation of energy due to intentionally non-flat PV geometries. This will enable the power electronics circuitry to be optimized to harvest maximum energy from PV pixel elements – clusters of PV cells with similar operating characteristics. This thesis systematically compares different geometries with identical two-dimensional projection "footprints" for energy harvest throughout the day. The results show that for the same footprint, a semi-cylindrical surface harvests more energy over a typical day than a flat plate. The modeling approach is then extended to demonstrate that by using non flat geometries for PV panel, the availability of a remotely located stand-alone power system can be increased when compared to a flat panel of same footprint. These results have broad application to a variety of energy scavenging scenarios in which either total energy harvested needs to be maximized or unusual geometries for the PV active surfaces are required, including building-integrated PV. This thesis develops the analysis of the potential energy harvest gain for advanced non-planar PV collectors as a necessary first step towards the design of the power electronics circuits and control algorithms to take advantage of the new opportunities of conformal and non-flat PV collectors.

Includes vita. / Title from PDF of title page. / Document formatted into pages; contains 204 pages. / Thesis (Ph.D.)--University of South Florida, 2003. / Includes bibliographical references. / Text (Electronic thesis) in PDF format. / ABSTRACT: The goal of this research project was to contribute to the understanding of CuGaSe2/CdS photovoltaic devices, and to improve the performance of these devices. The initial part of the research dealt with the optimization of a Sequential Deposition process for CuIn(Ga)Se2 absorber formation. As an extension of this, a recipe (Type I Process) for CuGaSe2 absorber layer fabrication was developed, and the deposition parameters were optimized. Electrical characterization of the thin films and completed devices was carried out using techniques such as Two-Probe and Three-Probe Current-Voltage, Capacitance-Frequency, Capacitance-Voltage, and Spectral Response measurements. Structural/chemical characterization was done using XRD and EDS analysis. Current densities of up to 15.2 mA/cm2, and Fill Factors of up to 58% were obtained using the Type I CuGaSe2 Process. VOC's, however, were limited to less than 700 mV. / ABSTRACT: Several process variations, such as changes in the rate/order/temperature of depositions and changes in the thickness of layers, resulted in little improvement. With the aim of breaking through this VOC performance ceiling, a new absorber recipe (Type II Process) was developed. VOC's of up to 735 mV without annealing, and those of up to 775 mV after annealing, were observed. Fill Factors were comparable to those obtained with Type I Process, whereas the Current Densities were found to be reduced (typically, 10-12 mA/cm2, with the best value of 12.6 mA/cm2). This performance of Type II devices was correlated to a better intermixing of the elements during the absorber formation. To gain an understanding of the performance limitations, two simulation techniques, viz. SCAPS and AMPS, were used to model our devices. / ABSTRACT: Several processing experiments and SCAPS modeling indicate that a defective interface between CuGaSe2 and CdS, and perhaps a defective absorber layer, are the cause of the VOC limitation. AMPS simulation studies, on the other hand, suggest that the back contact is limiting the performance. Attempts to change the physical back contact, by changes in the absorber processing, were unsuccessful. Processing experiments and simulations also suggest that the CuGaSe2/CdS solar cell involves a true heterojunction between these two layers. / System requirements: World Wide Web browser and PDF reader. / Mode of access: World Wide Web.

Στα πλαίσια της έλλειψης της μελέτης του φαινομένου της ΑΘΝ σε περιφερειακές Ελληνικές πόλεις, η παρούσα διδακτορική διατριβή παρουσιάζει και αναλύει τα αποτελέσματα της μελέτης του φαινομένου της ΑΘΝ σε δύο μικρές πόλεις της περιφέρειας, το Αγρίνιο και τα Ιωάννινα. Στόχος της εργασίας είναι να μελετήσει την ύπαρξη της ΑΘΝ, καθώς και να προσδιορίσει την ένταση και τη μορφή της. Για το σκοπό αυτό ένα δίκτυο αισθητήρων θερμοκρασίας σε συνδυασμό με μετεωρολογικούς σταθμούς αναπτύχθηκε στις δύο πόλεις και θερμοκρασιακά δεδομένα συλλέχθηκαν για το σχηματισμό του θερμικού τους προφίλ. Τα αποτελέσματα δείχνουν έντονη παρουσία του φαινομένου κυρίως κατά τις βραδινές ώρες και ένταση θερμικής νησίδας που φτάνει και τη μέση τιμή των 3.8 οC το μήνα Αύγουστο στο Αγρίνιο ενώ στα Ιωάννινα για την ίδια περίοδο το φαινόμενο έχει πιο ήπια μορφή (1.5 οC). Για την αντιμετώπιση του φαινομένου, την εξοικονόμηση ενέργειας καθώς και τη βελτίωση της θερμικής άνεσης των κτιρίων πόλεων που αντιμετωπίζουν πρόβλημα με την ΑΘΝ, μελετήθηκε η περίπτωση χρήσης υδρόφιλων ενεργειακών υλικών ως επικάλυψη οροφών, για μείωση της ροής θερμότητας από την οροφή με εξατμιστικό δροσισμό καθώς και τα δευτερεύοντα ενεργειακά κέρδη από μια πιθανή εγκατάσταση φωτοβολταϊκών οροφής. Η μελέτη περιλάμβανε την ανάπτυξη πειραματικής διάταξης αεροσήραγγας για τη δοκιμή των υλικών, με προσομοίωση των καιρικών συνθηκών της πόλης κατά του καλοκαιρινούς μήνες, όπου και το φαινόμενο της αστικής θερμικής νησίδας είναι πολύ έντονο, καθώς και ανάπτυξη φωτοβολταϊκών σε ταράτσα πανεπιστημιακού κτιρίου. Τα αποτελέσματα δείχνουν ότι ο εξατμιστικός δροσισμός μπορεί να αποτελέσει μια πολλά υποσχόμενη μέθοδο για την αντιμετώπιση της ΑΘΝ, ενώ η εγκατάσταση φωτοβολταϊκών μπορεί να συνεισφέρει επίσης στην εξοικονόμηση ενέργειας για δροσισμό τους καλοκαιρινούς μήνες, επιπλέον της παραγωγής ενέργειας. / Due to the lack of research on the field of the UHI effect in minor Greek cities, this thesis presents and analyzes the results of studying the UHI phenomenon in two small cities of the Greek province, Agrinio and Ioannina. Our aim was to study the existence of the UHI phenomenon as well as to determine its intensity and its type. In order to achieve that, a network of thermal sensors and meteorological stations was set on the two cities and temperature data were collected in order to define their thermal profile. The results point severe presence of the phenomenon especially during the nocturnal hours while the intensity of the phenomenon reaches up to 3.8 οC during August in Agrinio while in Ioannina at the same period the phenomenon is softer (1.5 οC). To mitigate the problem, save energy and improve the thermal capacity of the urban buildings in cities where the UHI occurs, the use of hydrophilic materials as a layer sheathing the roof was developed, in order to control the thermal flow from the roof by evaporative cooling. Furthermore, secondary energy benefits from a possible photovoltaics roof installation were investigated. Our research included the developing of an experimental set-up of a wind tunnel where sample materials were tested, by simulating the weather conditions of the city during the summer months, exactly the period of the high UHI intensity, as well as the installation of photovoltaics on a terrace of the university. The results indicate that evaporative cooling might be a very promising method in the field of the UHI phenomenon confrontation while the installation of photovoltaics can contribute in power saving for cooling during the summer months, in addition to electricity production.

Solar irradiation is a vast and plentiful source of energy. The use of photovoltaic (PV) devices to convert solar energy directly to electrical energy is an elegant way of sustainable power generation which can be distributed or in large PV plants based on the need. Solar cells are the small building blocks of photovoltaics and when connected together they form PV modules. Thin film solar cells require significantly less energy and raw materials to be produced, as compared to the dominant Si wafer technologies. CIGS thin film solar cells are considered to be the most promising thin film alternative due to its proven high efficiency. Most thin film PV modules utilise monolithic integration, whereby thin film patterning steps are included between film deposition steps, to create interconnection of individual cells within the layered structure. The state of the art is that CIGS thin film modules are made using one laser patterning step (P1) and two mechanical patterning steps (P2 and P3). Here we present work which successfully demonstrates the replacement of mechanical patterning by laser patterning methods. The use of laser ablation promises such advantages as increased active cell area and reduced maintenance and downtime required for regular replacement of mechanical tools. The laser tool can also be used to transform CIGS into a conducting compound along a patterned line. We have shown that this process can be performed after all semiconductor layers are deposited using a technique we call laser micro-welding. By performing patterning at the end of the process flow P2 and P3 patterning could be performed simultaneously. Such solutions will further reduce manufacturing times and may offer increased control of semiconductor interfaces. While showing promising performance on par with reference processes there are still open questions of importance for these novel techniques, particularly that of long term stability. Thin film modules are inherently sensitive to moisture and require reliable encapsulation. Before the techniques introduced here can be seen industrially they must have achieved proven stability. In this work we present a proof of existence of stable micro-welded interconnections. / Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 731

Organic semiconductors functionalized nanostructures are becoming as promising materials for electronic device applications including organic photovoltaics (OPVs). Perylenediimide (PDI) derivatives have also been known as one of the best n-type organic semiconductors. PDI derivatives can form bulk materials, which are both photochemically and thermally stable and have been widely used in various optoelectronic devices. Due to the formation of high electron mobility of crystalline domains, they prefer to incorporate into a silsesquioxane network. Here, we describe the potential applicability of perylenediimide functionalized silsesquioxane nanoribbons (PDI-dimethyl nanoribbons) as an acceptor for optoelectronic devices. We have developed synthetic procedures to make the PDI-dimethyl nanoribbons by the substitution reaction and the modified Stöber method. The PDI-dimethylethoxy silane precursor was produced in high yield by substituting 3-aminopropyldimethylethoxysilane on perylene-3,4,9,10-tetracarboxylicdianhydride as side chains. The optically active PDI-dimethyl nanoribbons were then formed upon hydrolysis with the certain concentration of ammonium hydroxide as a base. These nanoribbons were characterized using transmission electron microscopy (TEM), elemental analysis, and polarized optical microscopy. The photophysical properties in solution phase were also studied. The synthesis procedure developed here will have a great promise in large-scale manufacturing. Different shapes of PDI-dimethyl nanostructures, such as nanorods, nanochains, and nanoparticles, were discovered while varying the base concentrations. Also the morphologies of these PDI nanostructures were studied using TEM. Future studies will focus on optimizing procedures of PDI-dimethyl nanostructures and exploring new derivatives like perylenediimide dimer functionalized silsesquioxane polymers.