[ES] Las energía solar fotovoltaica ha emergido como una fuente de energía nueva y sostenible, que es ecológica y rentable si la producción es a gran escala. En el escenario actual, los dispositivos fotovoltaicos económicos y de alta eficiencia de conversión sin que se degraden sus componentes están bien posicionados para la generación de electricidad. Las células solares basadas en silicio dominan este mercado desde hace muchos años. Para la fabricación y producción de células solares basadas en silicio, se requieren sofisticadas técnicas de fabricación que hacen que el panel solar sea costoso. Por otra parte estan las células solares de película delgada, las cuales, debido a la intensificación de las capacidades de fabricación están ganando importancia. La tecnología de película delgada es una de las tecnologías más rentables y eficientes para la fabricación de células solares, y es un tema de intensa investigación en la industria fotovoltaica. La tecnología de película delgada es más económica que otras tecnologías porque los dispositivos utilizan menos material y están basados en varios tipos de materiales semiconductores que absorben la luz. Entre estos materiales, las células solares de kesterita que utilizan CZTS, CZTSe y sus aleaciones CZTSSe pueden convertirse en el reemplazo óptimo a los absorbentes de calcopirita. Estos materiales presentan unas características ópticas y eléctricas sobresalientes y tienen un gap óptico directo con una banda prohibida que oscila entre 1,4\ eV\ y 1,5\ eV y un coeficiente de absorción, \alpha>{10}^4{cm}^{-1}. Estas características han propiciado que las kesteritas esten siendo muy investigadas por la comunidad fotovoltaica de películas delgadas. De acuerdo con el límite de Shockley-Queisser, la eficiencia de conversión para una célula solar basada en CZTS\ es alrededor del 28%. Esta eficiencia es teóricamente posible mediante el ajuste de la banda prohibida, pero aún así, todavia no se ha podido alcanzar experimentalmente, probablemente debido a la falta de comprensión de las características de los dispositivos.Para una mejor comprensión de las características de los dispositivos, la modelación numérica puede jugar un papel importante al perimitir estudiar diferentes estructuras de dispositivos que pueden ahorrar tiempo y costos a la comunidad científico-técnica. En este trabajo, se ha llevado a cabo una modelazación numérica para estimar y analizar el efecto de parámetros físicos como el espesor y la concentración de dopado de la capa absorbente, la capa tampón y las capas ventana, además de estudiar el efecto de la temperatura y el efecto de la potencia de iluminación del sol en el rendimiento del dispositivo. El análisis numérico de los dispositivos se realizó con el software de simulación denominado "Solar Cell Capacitance Simulator" (SCAPS-1D). Para ello se analizó una estructura simple p-n-n^+ usando molibdeno como contacto posterior y FTO como ventana óptica y contacto frontal y siguiendo la secuencia de materiales Mo/CZTS/CdS/ZnO/FTO. A través del análisis, se estudió el rendimiento de las células solares con la variación en el espesor del absorbente para encontrar el espesor óptimo de la capa absorbente. También se estudió el efecto de la concentración del dopado y de la función de trabajo del metal. Después de la visualización de una estructura de dispositivo básica en SCAPS-1D, se modelo una célula solar experimental basada en CZTS. Los resultados de las células solares CZTS diseñados experimentalmente se simularon por primera vez en el entorno SCAPS-1D. Los resultados simulados de SCAPS-1D se compararon con los resultados experimentales. Después de la optimización de los parámetros de la celda, se incrementó la eficiencia de conversión de un dispositivo optimizado y, a partir del modelado, se descubrió que el rendimiento del dispositivo mejora al aumentar el tiempo de vida de los porta / [CA] L'energia solar fotovoltaica ha emergit com una font d'energia nova i sostenible, que és ecològica i rendible si la producció és a gran escala. En l'escenari actual, els dispositius fotovoltaics econòmics i de gran eficiència de conversió estan ben posicionats per a la generació d'electricitat neta i sostenible. Les cèl·lules solars basades en silici dominen aquest mercat des de fa molts anys. Per a la fabricació i producció de cèl·lules solars basades en silici, es requereixen tècniques de fabricació sofisticades que fan que el panell solar sigui costós. Per altra banda estan les cel·les solars de capa fina, que estan guanyant importància a causa de l'intensificació de les capacitats de fabricació. La tecnologia de capa fina és una de les tecnologies més rentables i eficients per a la fabricació de cel solars, i és un tema d'intensa investigació en la fotovoltaica industrial. La tecnologia de capa fina és més econòmica que altres tecnologies perquè els dispositius utilitzen menys material i estan basats en diversos tipus de materials semiconductors que absorbeixen la llum. Entre aquests materials, les cèl·lules solars de kesterita que utilitzen CZTS, CZTSe i les seves aleacions CZTSSe poden convertir-se en el reemplaçament òptim als absorbents de calcopirita. Aquests materials presenten unes característiques òptiques i elèctriques sobresalientes i tenen un gap òptic directe amb una banda prohibida que oscil·la entre 1,4eV i 1,5eV i un coeficient d'absorció, \alpha>{10}^4{cm}^{-1}. Aquestes característiques han propiciat que les Les kesteritas estan sent molt investigades per la comunitat fotovoltaica de capes primes. D'acord amb el límit de Shockley-Queisser, l'eficiència de conversió per a una cel·la solar basada en CZTS és d'aproximadament 28%. Aquesta eficiència és teòricament possible a través de l'ajust de la banda prohibida, però tot i així, encara no s'ha pogut assolir experimentalment, probablement a causa de la incomprensió del funcionament dels dispositius. Per a una millor comprensió de les característiques i funcionament dels dispositius, la modelització numèrica pot jugar un paper important al permetre estudiar diferents estructures de sistemes que poden estalviar temps i costos a la comunitat científica-tècnica. En aquest treball, s'ha dut a terme una modelització numèrica per estimar i analitzar l'efecte de paràmetres físics com l'espessor i la concentració de dopatge de la capa absorbent, la capa tampó i la capa finestra, a més d'estudiar l'efecte de la temperatura i l'efecte de la potència d'il·luminació del sol en el rendiment del dispositiu. L'anàlisi numèrica dels dispositius es va realitzar amb el programari de simulació denominat "Solar Cell Capacitance Simulator" (SCAPS-1D). Per això es va analitzar una estructura senzilla p-n-n^+ utilitzant molibdé com contacte posterior i FTO com a finestra òptica i contacte frontal i seguint la seqüència de materials Mo/CZTS/CdS/ZnO/FTO. A través de l'anàlisi, es va estudiar el rendiment de les cel·les solars amb la variació en l'espessor de l'absorbent per trobar l'espessor òptim de la capa absorbent. També es va estudiar l'efecte de la concentració del dopatge i de la funció de treball del metall. Després de la visualització d'una estructura de dispositiu bàsic en SCAPS-1D, es model una cel·la solar experimental basada en CZTS. Els resultats de les cel·les solars CZTS dissenyats experimentalment es simularen per primera vegada en l'entorn SCAPS-1D. Els resultats simulats de SCAPS-1D es van comparar amb els resultats experimentals. Després de l'optimització dels paràmetres de la celda, es va incrementar l'eficiència de conversió d'un dispositiu optimitzat i, a partir del modelatge, es va descobrir que el rendiment del dispositiu es millora a l'augmentar la vida útil dels minoritaris, cosa que es aconsegueix amb la incorporació d'un camp elèctric a la superfície del con / [EN] The solar cell has emerged as a newer and a relatively sustainable energy source, that is eco-friendly and cost-effective if the production is on a larger scale. In the current scenario, the economic and high-power conversion efficiency photovoltaic devices without degradation of materials are designed for the generation of electricity. The silicon-based solar cells dominated the market for many years. For the manufacturing and production of silicon-based solar cells, sophisticated fabrication techniques are required that make the solar panel costly. Due to intensification in manufacturing capabilities, thin film solar cells are gaining significance. Thin film technology is one of the most cost-effective and efficient technologies for the manufacturing of solar cells, and it is an excellent subject of intense research in the photovoltaic industry. Thin film technology is economical than other technologies because devices have relatively less material and are based on various types of light absorbing semiconductor materials. Among these materials, kesterite solar cells utilizing CZTS, CZTSe and their alloys CZTSSe are emerging as the most auspicious replacement for the chalcopyrite absorbers. The outstanding electrical and optical features having direct optical band gap ranges among 1.4eV to 1.5eV and large absorption coefficient \alpha\ >{10}^4{cm}^{-1} of CZTS have made it very interesting in the thin film community. According to the Shockley-Queisser limit, the optimum conversion efficiency of around 28\ % is theoretically possible from a CZTS based solar cell by tuning the band gap, but still, it is not experimentally possible to achieve 28% conversion efficiency from a solar cell due to lack of understanding of device characteristics. For a better understanding of device characteristics, numerical modeling can play a significant role by modeling different device structures that can save time and cost of the research community. In this work, numerical modeling was carried out for estimating and analyzing the effect of physical parameters such as thickness and doping concentration of absorber, buffer and window layers, temperature effect and effect of illumination power of the sun on device performance. Device modeling had performed on the dedicated simulation software "Solar Cell Capacitance Simulator" (SCAPS-1D). To achieve this task first, a simple {p-n-n}^+ structure for Mo/CZTS/CdS/ZnO/FTO had been analyzed with molybdenum as back contact and FTO as a front contact. Through analysis, it had been found that solar cell performance was affected by variation in absorber thickness, doping concentration, and metal work function. After visualization of a basic device structure in SCAPS-1D, CZTS based experimental solar cell had been modeled. Experimentally designed CZTS solar cell results were first simulated in SCAPS-1D environment. The SCAPS-1D simulated results were then compared with experimental results. After optimization of cell parameters, the conversion efficiency of an optimized device was increased and from modeling, it had been found that device performance was improved by improving minority carrier lifetime and integration of back surface field at the back contact. Based on the results presented, it was found that recombination in a solar cell can greatly affect the performance of a solar cell. Therefore, a new structure (Back\ contact/CFTS/ZnS/Zn(O,S)/FTO) was modeled and analyzed in which interface recombination is reduced by optimizing the band gap of Zn(O,S) layer. Based on different device structure modeling, it was found that solar cell with structure CFTS/ZnS/Zn(O,S)/FTO can exhibit an efficiency of 26.11% with optimized physical parameters like absorber thickness layer of 4\mu m and acceptor concentration density of 2\times{10}^{18}\ {cm}^{-3}. The proposed results will give a valuable guideline for the feasible fabrication and designing of high-power conversion efficiency solar cells. / Khattak, YH. (2019). Modeling of High Power Conversion Efficiency Thin Film Solar Cells [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/118802

<p dir="ltr">This research presents a comprehensive study of a 3D Perovskite Solar Cell model using Finite Element Analysis (FEA) and Machine Learning (ML). The research aims (i) to understand how material properties impact solar cell’s performance by applying basic semiconductor physics principles (ii) to investigate how interfacial delamination affects the performance of Perovskite solar cells (iii) to determine the optimum thickness of different layers of the solar cell (iv) to determine the fatigue life cycle of Perovskite layer.</p>

Mesoporous semiconductors such as TiO2 nanoparticles, as well as transparent conducting oxides (TCOs) such as indium tin oxide films are typically employed for setting up the photo-electrode module in variety of photoelectrochemical cells including Dye-Sensitized Solar Cells (DSSCs). In order to exhibit a high performance efficiency, the photo-electrodes in such applications are required to be able to harvest the light and transport the generated electrons effectively. Accordingly mesoporous layers with high values of surface area and well-established pore structure along with highly transparent and conductive TCOs are deposited on suitable substrates through the physical or chemical vapor deposition methods. The processing facilities and materials required to fabricate such high-quality devices with high values of efficiency are complicated and expensive, whereas devices of lower quality do not fulfill the demands. This issue is of particular importance regarding the energy production and developing the solar cell technologies, as it is considered by the concept of “cost per watt”. Thus, a great deal of effort is being carried out globally to enhance the efficiency of affordably-produced solar cells such as low-cost DSSCs. Utilizing the wet chemical techniques such as sol-gel method which provide a considerably more affordable route to synthesize nanoparticles and deposit thin films without the need of applying high temperature or vacuum condition is a widely-used approach to decrease the processing expenses. However, to achieve an acceptable cost-per-watt ratio requires enhancing the obtained efficiency value as well, and therefore, modifying the processing procedures to improve the required features of the products are highly encouraged. This thesis focuses on two individual activities: synthesis of TiO2 nanoparticles, and also thin film deposition of a promising TCO called aluminum-doped zinc oxide (AZO); both obtained through the sol-gel route that is modified to contribute to nanostructures with suitable features for application in photoelectrochemical devices such as DSSC. In the first part, mesoporous anatase nanoparticles were synthesized through the surfactant-mediated sol–gel route. Through changing the refluxing time and water-to-surfactant molar ratio, as-prepared nanocrystals of high density and large and narrowly-distributed pore sizes were obtained, displaying surface area values up to 240 m2·g-1, much higher than the reported values for commercial TiO2-based catalysts. In the second part, sol–gel dip–coating of ZnO thin films doped with 2 at.% of aluminium ions was carried out. By altering the hydrolysis reaction and changing the thermal treatment procedure, thin films of highly c-axis preferred orientation were obtained with optical transmittance of around 80% and resistivity values down to 6 – 15 mΩ·cm, corresponding to sheet resistance of around Rsh ~ 500 Ω/sq. The obtained conductivity values, even though one order magnitude lower than those reported for the AZO thin film prepared via expensive techniques, are in the suitable range to improve the cost per watt ratio in applications such as inkjet printing of low-cost printed electronics and more affordable DSSC devices.

The dye-sensitized solar cell (DSC) is a promising efficient low-cost molecular photovoltaic device. One of the key components in DSCs is the dye, as it is responsible for the capture of sunlight. State-of-the-art DSC devices, based on ruthenium dyes, show record efficiencies of 10-12 %. During the last decade, metal-free organic dyes have been extensively explored as sensitizers for DSC application. The use of organic dyes is particularly attractive as it enables easy structural modifications, due to fairly short synthetic routes and reduced material cost. Novel dye should in addition to the light-harvesting properties also be compatible with the DSC components. In this thesis, a series of new organic dyes are investigated, both when integrated in the DSC device and as individual components. The evaluation methods consisted of different electrochemical and photoelectrochemical techniques. Whereas the light-harvesting properties of the dyes were fairly easily improved, the behavior of the dye integrated in the DSC showed less predictable photovoltaic results. The dye series studied in Papers II and IV revealed that their dye energetics limited vital electron-transfer processes, the dye regeneration (Paper II) and injection quantum yield (Paper IV). Further, in Papers III-VI, it was observed that different dye structures seemed to alter the interfacial electron recombination with the electrolyte. In addition to the dye structure sterics, some organic dyes appear to enhance the interfacial recombination, possibly due to specific dye-redox acceptor interaction (Paper V). The impact of dye sterical modifications versus the use of coadsorbent was explored in Paper VI. The dye layer properties in the presence and absence of various coadsorbents were further investigated in Paper VII. The core of this thesis is the identification of the processes and properties limiting the performance of the DSC device, aiming at an overall understanding of the compatibility between the DSC components and novel organic dyes. / QC 20100730

This thesis deals with the device physics of organic solar cells. Organic photovoltaics (OPV) is a field of applied research which has been growing rapidly in the last decade leading to a current record value of power-conversion efficiency of 10 percent. One major reason for this boom is a potentially low-cost production of solar modules on flexible (polymer) substrate. Furthermore, new application are expected by flexible or semitransparent organic solar cells. That is why several OPV startup companies were launched in the last decade. Organic solar cells consist of hydrocarbon compounds, deposited as ultrathin layers (some tens of nm) on a substrate. Absorption of light leads to molecular excited states (excitons) which are strongly bound due to the weak interactions and low dielectric constant in a molecular solid. The excitons have to be split into positive and negative charges, which are subsequently collected at different electrodes. An effective dissociation of excitons is provided by a heterojunction of two molecules with different frontier orbital energies, such that the electron is transfered to the (electron) acceptor and the positive charge (hole) remains on the donor molecule. This junction can be realized by two distinct layers forming a planar heterojunction or by an intermixed film of donor and acceptor, resulting in a bulk heterojunction. Electrodes are attached to the absorber to collect the charges by providing an ohmic contact in the optimum case. This work focuses on the electrical processes in organic solar cells developing and employing a one-dimensional drift-diffusion model. The electrical model developed here is combined with an optical model and covers the diffusion of excitons, their separation, and the subsequent transport of charges. In contrast to inorganics, charge-carrier mobilities are low in the investigated materials and charge transport is strongly affected by energy barriers at the electrodes. The current-voltage characteristics (J-V curve) of a solar cell reflect the electrical processes in the device. Therefore, the J-V curve is selected as means of comparison between systematic series of simulation and experimental data. This mainly qualitative approach allows for an identification of dominating processes and provides microscopic explanations. One crucial issue, as already mentioned, is the contact between absorber layer and electrode. Energy barriers lead to a reduction of the power-conversion efficiency due to a decrease in the open-circuit voltage or the fill factor by S-shaped J-V curve (S-kink), which are often observed for organic solar cells. It is shown by a systematic study that the introduction of deliberate barriers for charge-carrier extraction and injection can cause such S-kinks. It is explained by simulated electrical-field profiles why also injection barriers lead to a reduction of the probability for charge-carrier extraction. A pile-up of charge carriers at an extraction barrier is confirmed by measurements of transient photocurrents. In flat heterojunction solar cells an additional reason for S-kinks is found in an imbalance of electron and hole mobilities. Due to the variety of reasons for S-kinks, methods and criteria for a distinction are proposed. These include J-V measurements at different temperatures and of samples with varied layer thicknesses. Most of the studies of this this work are based on experimental data of solar cells comprisiing the donor dye zinc phthalocyanine and the acceptor fullerene C60. It is observed that the open-circuit voltage of these devices depends on the mixing ratio of ZnPc:C60. A comparison of experimental and simulation data indicates that the reason is a changed donor-acceptor energy gap caused by a shift of the ionization potential of ZnPc. A spatial gradient in the mixing ratio of a bulk heterojunction is also investigated as a donor(acceptor)-rich mixture at the hole(electron)-collecting contact is supposed to assist charge extraction. This effect is not observed, but a reduction of charge-carrier losses at the “wrong” electrode which is seen at an increase in the open-circuit voltage. The most important intrinsic loss mechanism of a solar cell is bulk recombination which is treated at the example of ZnPc:C60 devices in the last part of this work. An examination of the dependence of the open-circuit voltage on illumination intensity shows that the dominating recombination mechanism shifts from trap-assisted to direct recombination for higher intensities. A variation of the absorption profile within the blend layer shows that the probability of charge-carrier extraction depends on the locus of charge-carrier generation. This results in a fill factor dependent on the absorption profile. The reason is an imbalance in charge-carrier mobilities which can be influenced by the mixing ratio. The work is completed by a simulation study of the influence of charge-carrier mobilities and different recombination processes on the J-V curve and an identification of a photoshunt dominating the experimental linear photocurrent-voltage characteristics in reverse bias. / Diese Dissertation beschäftigt sich mit der Physik organischer Solarzellen. Die organische Photovoltaik ist ein Forschungsgebiet, dem in den letzten zehn Jahren enorme Aufmerksamkeit zu Teil wurde. Der Grund liegt darin, dass diese neuartigen Solarzellen, deren aktueller Rekordwirkungsgrad bei 10 Prozent liegt, ein Potential für eine kostengünstige Produktion auf flexiblem (Polymer)substrat aufweisen und aufgrund ihrer Vielfältigkeit neue Anwendungsbereiche für die Photovoltaik erschließen. Organische Solarzellen bestehen aus ultradünnen (einige 10 nm) Schichten aus Kohlenwasserstoffverbindungen. Damit der photovoltaische Effekt genutzt werden kann, müssen die durch Licht angeregten Molekülzustände zu freien Ladungsträgern führen, wobei positive und negative Ladung an unterschiedlichen Kontakten extrahiert werden. Für eine effektive Trennung dieser stark gebundenden lokalisierten angeregten Zustände (Exzitonen) ist eine Grenzfläche zwischen Molekülen mit unterschiedlichen Energieniveaus der Grenzorbitale erforderlich, sodass ein Elektron auf einem Akzeptor- und eine positive Ladung auf einem Donatormolekül entstehen. Diese Grenzschicht kann als planarer Heteroübergang durch zwei getrennte Schichten oder als Volumen-Heteroübergang in einer Mischschicht realisiert werden. Die Absorberschichten werden durch Elektroden kontaktiert, wobei es für effiziente Solarzellen erforderlich ist, dass diese einen ohmschen Kontakt ausbilden, da ansonsten Verluste zu erwarten sind. Diese Arbeit behandelt im Besonderen die elektrischen Prozesse einer organischen Solarzelle. Dafür wird ein eindimensionales Drift-Diffusionsmodell entwickelt, das den Transport von Exzitonen, deren Trennung an einer Grenzfläche und die Ladungsträgerdynamik beschreibt. Abgesehen von den Exzitonen gilt als weitere Besonderheit einer organischen Solarzelle, dass sie aus amorphen, intrinsischen und sehr schlecht leitfähigen Absorberschichten besteht. Elektrische Effekte sind an der Strom-Spannungskennlinie (I-U ) sichtbar, die in dieser Arbeit als Hauptvergleichspunkt zwischen experimentellen Solarzellendaten und den Simulationsergebnissen dient. Durch einen weitgehend qualitativen Vergleich können dominierende Prozesse bestimmt und mikroskopische Erklärungen gefunden werden. Ein wichtiger Punkt ist der schon erwähnte Kontakt zwischen Absorberschicht und Elektrode. Dort auftretende Energiebarrieren führen zu einem Einbruch im Solarzellenwirkungsgrad, der sich durch eine Verringerung der Leerlaufspanung und/oder S-förmigen Kennlinien (S-Knick) bemerkbar macht. Anhand einer systematischen Studie der Grenzfläche Lochleiter/Donator wird gezeigt, dass Energiebarrieren sowohl für die Ladungsträgerextraktion als auch für die -injektion zu S-Knicken führen können. Insbesondere die Tatsache, dass Injektionsbarrieren sich auch negativ auf den Photostrom auswirken, wird anhand von simulierten Ladungsträger- und elektrischen Feldprofilen erklärt. Das Aufstauen von Ladungsträgern an Extraktionsbarrieren wird durch Messungen transienter Photoströme bestätigt. Da S-Knicke in organischen Solarzellen im Allgemeinen häufig beobachtet werden, werden weitere Methoden vorgeschlagen, die die Identifikation der Ursachen ermöglichen. Dazu zählen I-U Messungen in Abhängigkeit von Temperatur und Schichtdicken. Als eine weitere Ursache von S-Knicken werden unausgeglichene Ladungsträgerbeweglichkeiten in einer Solarzelle mit flachem Übergang identifiziert und von den Barrierefällen unterschieden. Weiterer Forschungsgegenstand dieser Arbeit sind Mischschichtsolarzellen aus dem Donator-Farbstoff Zink-Phthalozyanin ZnPc und dem Akzeptor Fulleren C60. Dort wird beobachtet, dass die Leerlaufspannung vom Mischverhältnis abhängt. Ein Vergleich von Experiment und Simulation zeigt, dass sich das Ionisationspotenzial von ZnPc und dadurch die effektive Energielücke des Mischsystems ändern. Zusätzlich zu homogenen Mischschichten werden Solarzellen untersucht, die einen Gradienten im Mischungsverhältnis aufweisen. Die Vermutung liegt nahe, dass ein hoher Donatorgehalt am Löcherkontakt und ein hoher Akzeptorgehalt nahe des Elektronenkontakts die Ladungsträgerextraktion begünstigen. Dieser Effekt ist in dem hier untersuchten System allerdings vergleichsweise irrelevant gegenüber der Tatsache, dass der Gradient das Abfließen bzw. die Rekombination von Ladungsträgern am “falschen” Kontakt reduziert und somit die Leerlaufspannung erhöht. Der wichtigste intrinsische Verlustmechanismus einer Solarzelle ist die Rekombination von Ladungsträgern. Diese wird im letzten Teil der Arbeit anhand der ZnPc:C60 Solarzelle behandelt. Messungen der Leerlaufspannung in Abhängigkeit von der Beleuchtungsintensität zeigen, dass sich der dominierende Rekombinationsprozess mit zunehmender Intensität von Störstellenrekombination zu direkter Rekombination von freien Ladungsträgern verschiebt. Eine gezielte Variation des Absorptionsprofils in der Absorberschicht zeigt, dass die Ladungsträgerextraktionswahrscheinlickeit vom Ort der Ladungsträgergeneration abhängt. Dieser Effekt wird hervorgerufen durch unausgeglichene Elektronen- und Löcherbeweglichkeiten und äußert sich im Füllfaktor. Weitere Simulationsergebnisse bezüglich des Einflusses von Ladungsträgerbeweglichkeiten und verschiedener Rekombinationsmechanismen auf die I-U Kennlinie und die experimentelle Identifikation eines Photoshunts, der den Photostrom in Rückwärtsrichtung unter Beleuchtung dominiert, runden die Arbeit ab.

Le développement des énergies renouvelables est aujourd’hui devenu un enjeu mondial majeur comme alternative aux énergies fossiles dans la production d'énergie. Parmi elles, l’énergie solaire est considérée comme la source la plus prometteuse, permettant de couvrir l’ensemble des besoins énergétiques liés à l’activité humaine. Les cellules photovoltaïques les plus performantes aujourd’hui, entre 16 et 18 % en modules, sont composées de silicium, un semi-conducteur inorganique. Cependant, leur coût de production élevé a nécessité le développement de matériaux alternatifs moins couteux. Parmi les voies explorées, les cellules solaires organiques ont émergé comme une alternative prometteuse pour produire l’électricité à faible coût. Le sujet de cette thèse s’intègre dans ce contexte de recherche. Deux types de cellules solaires ont été étudiés : les cellules à hétérojonction en volume (BHJ) et sensibilisées au colorant (DSSCs). Le courant photogénéré repose généralement (i) dans les cellules BHJ, sur le transfert entre de charge entre un polymère donneur et un accepteur d’électrons (fullerène), tels que le couple poly(3-hexyl)thiophène (P3HT) et [6,6]-phényl-C61-butanoate de méthyle (PCBM), et (ii) dans les DSSCs, la sensibilisation de la surface d’un semi-conducteur inorganique tel que l’oxyde de titane par un colorant et la présence d’un électrolyte, jouant le rôle de médiateur redox. Bien qu’ayant atteint des rendements de photoconversion respectifs de 5 et 13 %, ces cellules nécessitent des améliorations pour une commercialisation à grande échelle. Tout d’abord, les performances des cellules BHJ à base de P3HT sont considérablement limitées par sa faible absorption, ne couvrant pas la globalité du spectre solaire. Afin de palier ce problème, nous avons combiné le P3HT avec des chromophores, i.e. des porphyrines, ayant une absorption plus étendue. Ensuite, pour assurer une meilleure extraction des charges au sein du dispositif, une couche interfaciale cathodique à base de polyélectrolytes pi-conjugués a été ajoutée. Enfin, des colorants extraits de la biomasse ont été préparés afin de remplacer les colorants coûteux à base de ruthénium. En outre, les électrolytes liquides étant volatils et corrosifs, ce qui limite considérablement la stabilité des DSSCs, des électrolytes solides à base de polymères ont été étudiés comme alternative. / Among renewable energies, the sunlight has by far the highest theoretical potential to meet the worldwide need in energy. Photovoltaic devices are thus currently the subject of intense research for low-cost conversion of sunlight into electrical power. In particular, organic photovoltaics have emerged as an interesting alternative to produce electricity due to their low manufacturing cost compared to silicon solar cells, their mechanical flexibility and the versatility of the possible chemical structures. In this dissertation, we focused our research on the development of new organic pi-conjugated materials for organic solar cells applications. Two types of solar cells have been studied during this work: bulk heterojunction and dye-sensitized solar cells. The charge transfer leading to the photocurrent is usually based on (i) a polymer donor and a fullerene acceptor in BHJ solar cells, such as the widely studied poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) materials and (ii) a metal oxide (titanium oxide) sensitized with a dye and an electrolyte in DSSCs. Despite power conversion efficiencies have reached 5 and 13 % respectively for these two types of devices, they still display several drawbacks that limit their commercialization. P3HT displays a narrow absorption of the solar spectrum thus limiting the conversion efficiency. To overcome this limitation, we combined P3HT with chromophores, i.e. porphyrins, having an extending absorption. Then, to ensure better charge transfer and extraction within the device, a cathode interfacial layer based on cationic pi-conjugated polyelectrolytes was added. Finally, dyes extracted from the biomass (chlorophyll a derivatives) were synthesized to replace the expensive ruthenium dyes in DSSCs. Since liquid electrolytes are volatile and corrosive, which considerably limit the DSSCs stability, solid polymer electrolytes were also developed as an alternative.

This thesis outlines investigations of metal oxide/organic interfaces in photo-voltaic devices. It focuses on device instabilities originating from the metal oxide layer surface sensitivity and it presents suggested mechanisms behind these in- stabilities. A simple sol-gel solution deposition technique for the fabrication of stable and highly performing transparent conducting mixed metal oxides (ZnMO) is presented. It is demonstrated that the use of amorphous, mixed metal oxides allows improving the performance and stability of interfacial charge extraction layers for organic solar cells. Two novel ternary metal oxides, zinc-strontrium- oxide (ZnSrO) and zinc-barium-oxide (ZnBaO), were fabricated and their use as electron extraction layers in inverted organic photovoltaics is investigated. We show that using these ternary oxides can lead to superior devices by: prevent- ing a dipole forming between the oxide and the active organic layer in a model ZnMO/P3HT:PCBM OPV as well as lead to improved surface coverage by a self assembled monolayer and promote a significantly improved charge separation efficiency in a ZnMO/P3HT hybrid device. Additionally a spectroscopic technique allowing a versatility of characterisa- tion for long-term stability investigations of organic solar cells is reported. A device instability under broadband light exposure in vacuum conditions for an inverted ZnSrO/PTB7:PC71BM OPV is observed. Direct spectroscopic evidence and electrical characterisation indicate the formation of the PC71BM radical an- ion associated with a loss in device performance. A charge transfer mechanism between a heavily doped oxide layer and the organic layers is suggested and dis- cussed.

Photovoltaic cells are a promising solution to the current energy crisis. Among the different photovoltaic cell technologies developed, dye-sensitised solar cells (DSSC) are emerging as viable low-cost alternatives to Si PV technology. This thesis presents studies on fabrication and characterisation of TiO2 based dye-sensitised solar cells. Chapter 1 gives an overview of different photovoltaic cell technologies and a review of the state-of-the art DSSC technology. Chapter 2 describes the techniques used for characterisation of DSSCs. Chapter 3 describes the fabrication of TiO2 based dye-sensitised solar cells. Chapter 4 presents the analysis of measurements obtained by the characterisation techniques. Finally chapter 5 summarises the work done and suggests directions for future work.

No description available.

The perceived battery capacity of battery-powered devices can be increased by harvesting energy from readily available sources. Third generation solar cells are a good candidate for this purpose since they can be integrated with these battery-powered devices and harvest power from diffused light. For a single third generation solar cell to be useful in the context of charging a Lithium based battery, the voltage must be increased tenfold. To increase this perceived battery capacity as much as possible, efficiency is crucial. In this thesis, DCDC converter topologies and designs are studied from a system design perspective. The specifications of a converter suitable for interfacing Dye-Sensitised Solar Cells with Lithium batteries are described and a market research is conducted based on those specifications. A comparison of the available commercial solutions is presented, highlighting the most suitable options. However, none of the commercial solutions met the specifications to the full extent. The design process of two DC-DC converters is presented, one is a Boost converter operating in Continuous Conduction Mode and the other is a Boost converter operating in Discontinuous Conduction Mode. A comparison of the two designs highlights the advantages of operating the Boost converter in Discontinuous Conduction Mode when interfaced with a Dye-Sensitised Solar Cell. The design with a Boost converter operating in Discontinuous Conduction Mode has an efficiency of 80.3 % and is capable of tracking the Maximum Power Point of the Dye-Sensitised Solar Cell. / Den uppfattade batterikapaciteten hos batteridrivna enheter kan ökas genom att skörda energi från lättillgängliga källor. Tredje generationens solceller är en bra kandidat för detta ändamål eftersom de kan integreras med dessa batteridrivna enheter och skörda ström från spritt ljus. För att en enda tredje generationens solcell ska vara användbar i samband med laddning av ett litiumbaserat batteri måste spänningen ökas tiofaldigt. För att öka denna uppfattade batterikapacitet så mycket som möjligt är effektiviteten avgörande. I denna avhandling studeras topologier och strategier för DC-DC-omvandlare från ett systemdesignperspektiv. Specifikationerna för en omvandlare som är lämplig för att ansluta Dye-sensitized solceller med litiumbatterier beskrivs och en marknadsundersökning utförs utifrån dessa specifikationer. En jämförelse av de tillgängliga kommersiella lösningarna presenteras och belyser de lämpligaste alternativen. Ingen av de kommersiella lösningarna uppfyllde emellertid specifikationerna i sin helhet. Designprocessen för två DC-DComvandlare presenteras, en Boost-omvandlare som arbetar i kontinuerligt ledande läge och en Boost-omvandlare som arbetar i diskontinuerligt ledande läge. En jämförelse av de två designerna belyser fördelarna med att driva Boost-omvandlaren i diskontinuerligt ledningsläge när den kopplats till en färgkänslig solcell. Konstruktionen med en Boostomvandlare som arbetar i diskontinuerlig ledningsläge har en effektivitet på 80.3 % och kan spåra den maximala effektpunkten för solcellen.