Dissertations / Theses on the topic 'Perovskite photovoltaic cells'

Many researchers have studied conjugated polymer-based photovoltaic cells and perovskite-based photovoltaic cells. They have shown lower efficiencies than inorganic photovoltaic cells so far. However, they are attractive because of their potential low cost and easy process. In order to fabricate organic photovoltaic cells, organic solvents are typically used, which results in significant waste solvent being produced. These are moderately expensive and many are toxic. Perovskite photovoltaics commonly incorporate lead, which is toxic and may hinder their adoption. This thesis aims reduce the need for organic solvents during organic photovoltaic cell manufacture by employing water-soluble conjugated polymers as an alternative. It also seeks to improve the efficiency of the devices such the less solvents are required per Watt produced. Reducing the usage of organic solvents would reduce fabrication and solvent treatment costs. Bismuth perovskites are also studied for use in perovskite photovoltaic cells to replace the toxic lead with a less toxic material. The poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole transporting layer used in both types of solar cells has been characterised in order to understand the influence of moisture and oxygen in air on the layer. Eight different thermally annealed PEDOT:PSS films were fabricated namely: as cast, 50, 75, 100, 125, 150, 175, and 200 °C. UV-vis absorption and conductvity were measured. Absorption intensity increased very slightly as thickness increased. In order to develop fabrication skills and understand the principles of these devices, P3HT bulk-heterojunction photovoltaic cells were prepared. The devices were fabricated with different blend ratios both in air and in an oxygen free glovebox. P3HT:PCBM blend ratios of 1:0.6 and 1:0.8 showed the best efficiencies. In this thesis, the synthesis of a new low band gap polyelectrolyte based on fluorene and dithiano-benzothiadiazole is described. Poly[(9,9-bis(4-sulfonatobutyl sodium) fluorene-alt-phenylen)-ran-(4,7-di-2-thienyl-2,1,3-benzothiadiazole-alt-phenylene)] is an anionic charged conjugated polyelectrolyte and was synthesised via Suzuki-cross coupling. Sulfonate groups were introduced to help the low band gap polyelectrolyte to dissolve in water. The aim was a new bulk-heterojunction material to be applied in organic photovoltaic cells. It has a strong absorption peak at 372 nm, a weaker one at 530 nm and a photoluminescence emission peak at 647 nm. Although the conjugated polyelectrolyte did not show any photovoltaic effects as an active layer, it resulted in an improvement of efficiency when used as an additive in the PEDOT:PSS hole transporting layer in the devices. There is an efficiency gain as a result of improved carrier generation and charge transport across the interface into the hole transporting layer which is optimised at a CPE concentration close to 5 mg/ml. Improving the efficiency will improve the sustainability of the devices by reducing the materials required and waste produced per Watt of power produced. Although lead-based perovskites have shown high performance in photovoltaic cells, they have led to concerns regarding their toxicity. Hybrid perovskites with reduced lead content are currently being investigated as a strategy to overcome this issue and to this end we evaluate the use of bismuth as a possible candidate for lead substitution. A series of hybrid perovskite films with the general composition MA(PbyBi1-y)I3-xClx were characterised by their basic optical and structural properties using UV-vis spectroscopy, scanning electron microscopy and grazing incidence wide angle X-ray scattering. The bismuth perovskite precursors form a perovskite crystal structure upon annealing, with a corresponding optical bandgap, for MABiI3, of around 2 eV. Whilst the structural and optical characterisation is promising, preliminary photovoltaic cell tests show power conversion efficiencies below 0.01% with a maximum VOC of 0.78 V. It was suggested that such low overall efficiencies reflect a competition between precursor conversion and material de-wetting from the substrate that occurs during perovskite formation, the overall outcome of which is severely limited photocurrent. In the context of current processing methods, these factors may limit the general applicability of hybrid bismuth perovskites in photovoltaic applications. A blend ratio of 3:1 MAI:BiCl3 used to make a perovskite based photovoltaic cell and annealed at 90 °C showed the best results in this research but it was very low efficiency.

This thesis focuses on the study of hybrid perovskites properties for the purposes of photovoltaic applications. During the almost four years PhD project that has lead to this thesis the record photovoltaic efficiency for this technology has in- creased from 10.9% to 22.1%. Such a significant pace of development can be com- pared with few other materials. It is for this reason that hybrid perovsites have at- tracted impressive research efforts. We approached the study of such unique ma- terials using computational ab-initio techniques, and in particular Density Func- tional Theory. We considered different materials, but most of the attention was concentrated on MAPI (CH 3 NH 3 PbI 3 ). The results are divided in three chapters, each exploring a different material prop- erty. The first chapter reports the electronic structure of the material bulk, sur- faces, and other electronic-related properties such as the rotation barrier for the organic component and the Berry phase polarization. The second chapter focuses on the vibrational properties primary employing the harmonic approximation but also extends to the quasi-harmonic approximation. The outcome of these calculations permitted us to calculate theoretical IR and Ra- man spectra which are in good agreement with different experimental measure- ments. The quasi-harmonic approximation was used to calculate temperature dependent properties, such as the Grüneisen parameter, the thermal dependence of heat capacity and the thermal volumetric expansion. The third and last chapter reviews the thermodynamic properties of binary halide compounds. The cobination of ab-initio calculations with the generalised quasi- chemical approximation has allowed to study the stability of mixed composition perovskites. The results certified a set of stable structures that could stand at the base of observed phenomena of photo-degradation of hybrid perovskite based devices. All three chapters have been written to understand the chemical and physical behaviour of hybrid perovskites and to extended and contribute to the under- standing of experimental work.

Recent advances in third generation photovoltaics, particularly the rapid increase in perovskite power conversion efficiencies, may provide a cheap alternative to silicon solar cells in the near future. A key component to these devices is the transparent front electrode, and in the case of Dye Sensitized Solar Cells, it is the most expensive part. A lightweight, cost-effective, robust, and easy-to-fabricate new generation TCE is required to enable competition with silicon. Indium Tin Oxide, commonly used in touchscreen devices, Organic Light Emitting Diodes (OLEDs), and thin film photovoltaics, is widely used and commonly referred to as the industry standard. As the global supply of indium decreases and the demand for this TCE increases, a similar alternative TCE is required to accompany the next generation solar cells that promise energy with lighter and significantly cheaper modules. This alternative TCE needs to provide similar sheet resistance and optical transmittance to ITO, while also being mechanically and chemically robust. The work in this thesis begins with an exploration of several synthesized ITO replacement materials, such as copper nanowires, conductive polymer PEDOT:PSS, zinc oxide thin films, reduced graphene oxide and combinations of the above. A guiding philosophy to this work was prioritizing cheap, easy deposition methods and overall scalability. Shortcomings of these TCEs were investigated and different materials were hybridized to take advantage of each layers strengths for development of an ideal ITO replacement. For CuNW-based composite electrodes, ~85% optical transmittance and ~25 Ω/sq were observed and characterized to understand the underlying mechanisms for optimization. The second half of this work is an examination of many different perovskite synthesis methods first to achieve highest performance, and then to integrate compatible methods with our CuNW TCEs. Several literature methods investigated were irreproducible, and those that were successful posed difficulties integrating with CuNW-based TCEs. Those shortcomings are discussed, and how future work might skirt the issues revealed here to produce a very low cost, high performance perovskite solar cell.

The aim of this thesis is to develop a deeper understanding and the technology in the nascent field of solid-state organic-inorganic perovskite solar cells. In recent years, perovskite materials have emerged as a low-cost, thin-film technology with efficiencies exceeding 16% challenging the quasi-paradigm that high efficiency photovoltaics must come at high costs. This thesis investigates perovskite solar cells in more detail with a focus on incorporating plasmonic nanostructures and perovskite film formation. Chapter 1 motivates the present work further followed by Chapter 2 which offers a brief background for solar cell fabrication and characterisation, perovskites in general, perovskite solar cells in specific, and plasmonics. Chapter 3 presents the field of plasmonics including simulation methods for various core-shell nanostructures such as gold-silica and silver-titania nanoparticles. The following Chapters 4 and 5 analyze plasmonic core-shell metal-dielectric nanoparticles embedded in perovskite solar cells. It is shown that using gold@silica or silver@titania NPs results in enhanced photocurrent and thus increased efficiency. After photoluminescence studies, this effect was attributed to an unexpected phenomenon in solar cells in which a lowered exciton binding energy generates a higher fraction of free charge. Embedding thermally unstable silver NPs required a low-temperature fabrication method which would not melt the Ag NPs. This work offers a new general direction for temperature sensitive elements. In Chapters 6 and 7, perovskite film formation is studied. Chapter 6 shows the existence of a previously unknown crystalline precursor state and an improved surface coverage by introducing a ramped annealing procedure. Based on this, Chapter 7 investigates different perovskite annealing protocols. The main finding was that an additional 130°C flash annealing step changed the film crystallinity dramatically and yielded a higher orientation of the perovskite crystals. The according solar cells showed an increased photocurrent attributed to a decrease in charge carrier recombination at the grain boundaries. Chapter 8 presents on-going work showing noteworthy first results for silica scaffolds, and layered, 2D perovskite structures for application in solar cells.

In the present work, anomalous distortions occurring in the current-voltage characteristic of perovskite solar cells (PSCs), usually called J-V curve hysteresis, are studied by several methods. This includes dynamic direct current (DC) mode J-V experiments and impedance spectroscopy (IS) analyses in dark and under illumination. The J-V curves of PSCs were measured under different conditions showing capacitive hysteretic currents. This is related with low frequency excess capacitance in the IS spectra. These two features are correlated with the response of mobile ions in space charge regions close to the interfaces. The large values of capacitance under illumination in the sub-Hz regime were explained in terms of mobile ions space charges and chemical capacitances assuming a proportionality between the number of ionized/activated mobile ions and the concentration of charge carriers and photon fluence.
En el presente trabajo se estudian por varios métodos las distorsiones anómalas en la característica de corriente-voltaje de las celdas solares de perovskita (PSC), típicamente llamada histéresis de J-V. Esto incluye experimentos dinámicos de J-V en modo de corriente continua (DC) y análisis de espectroscopía de impedancia (IS) en oscuridad y bajo iluminación. Las curvas J-V en oscuridad de las PSCs exhiben corrientes capacitivas, relacionadas con un exceso de capacitancia de baja frecuencia en los espectros de IS. Estas dos características están correlacionadas con la respuesta de iones móviles en regiones espaciales de carga hacia las interfaces. Los grandes valores de capacitancia bajo iluminación a frecuencias por debajo de las unidades de Hz se explicaron en términos de regiones de cargas espaciales de iones móviles y capacitancias químicas, suponiendo una proporcionalidad entre el número de iones móviles ionizados/activados y la concentración de portadores de carga y flujo de fotones.

Ce travail de thèse a pour sujet la conception des cellules solaires photovoltaïques à base de pérovskite hybride par le biais de la technologie d’impression jet d’encre. Les deux premiers chapitres font la présentation du contexte de la thèse, à savoir l’alimentation d’un réseau autonome de capteurs, et passent en revue les aspects scientifiques des technologies jet d’encre et photovoltaïque de nouvelle génération. Le troisième chapitre présente la mise au point d’une cellule photovoltaïque à l’état de l’art et son évolution vers une architecture imprimable à basse température de recuit. La problématique de la stabilité des cellules photovoltaïques à pérovskite est aussi abordée. La dernière partie présente les différents aspects et problématiques de l’impression par jet d’encre des trois couches internes d’une cellule solaire pérovskite. Au terme de ce travail la possibilité d’imprimer des cellules solaires pérovskites avec des rendements supérieurs à 10 % a été démontrée, le tout en condition ambiante et à basse température
This thesis is about the design of photovoltaic solar cells based on hybrid perovskite using inkjet printing technology. The first two chapters present the context of the thesis, namely the powering of an autonomous sensor network, and review the scientific aspects of inkjet and photovoltaic technologies. The third chapter presents the development of a state-of-the-art photovoltaic cell and its evolution towards a printable architecture at low annealing temperatures. The problem of the stability of photovoltaic cells with perovskite is also discussed. The last part presents the different aspects and problems of the inkjet printing of the three inner layers of a perovskite solar cell. At the end of this work the possibility of printing perovskite solar cells with efficiencies higher than 10% has been demonstrated, all in ambient conditions and at low temperature

Perovskite solar cells (PSCs) have come to the forefront of photovoltaic technology due to their impressive power conversion efficiency (PCE) of up to 25%. This high efficiency comes together with great advances regarding large-scale deposition methods and a critical enhancement of device stabilities. However, important challenges remain in the shadows for commercialization of this technology. his thesis addresses issues related with the stability under real operation condicions and those associated with the interfacial interactions. For both purposes, two main perovskite materials based in methylammonium lead halides (MAPbX3) were optimized: MAPbI3 and MAPbBr3. Coupled with a wide number of instrumentak techniques for bulk and interfacial characterization, a robust method to fabricate PSCs under moisture conditions was developed using iodide derivatives. On the other han, interfacial engineering with lithium additives in MAPbBr3 devices promoted a decrease in recombination mechanisms allowing to achieve a record open circuit poetential approaching 1.6 V.
Las celdas solares de perovskita han alcanzado la primera línea de la tecnología fotovoltaica debido a las impresionantes eficiencias energéticas conseguidas, superando el 25% en la actualidad. Estos valores vienen acompañados de grandes avances como los métodos de depósito de los films a gran escala y a una mejora considerable en la estabilidad de estos dispositivos. Sin embargo, aún existen numerosas cuestiones que deben solucionarse para conseguir una comercialización real de esta tecnología. sta tesis doctoral aborda las cuestiones relacionadas precisamente con la estabilidad de los dispositivos bajo condiciones reales de operación, así como aquellas cuestiones relacionadas con las interacciones interfaciales. Para la consecución de ambos objetivos, dos formulaciones de perovskita han sido optimizadas con éxito: MAPbI3 y MAPbBr3. Junto con una amplia variedad de técnicas instrumentales de caracterización, tanto del bulk como de las regiones interfaciales, se ha desarrollado un método para la obtención de altas eficiencias bajo condiciones de humedad, así como la reducción de procesos de recombinación interfaciales que han permitido la obtención de valores récord de fotovoltage, alcanzanco los 1.6 V.

Les pérovskites hybrides célèbrent cette année leurs 10e anniversaire dans le domaine du photovoltaïque. En plus de la progression inégalée des rendements des cellules solaires, les pérovskites ont des propriétés optoélectroniques ajustables et peuvent être fabriquées par des procédés bas coûts, ce qui en fait de sérieuses candidates pour les cellules solaires multijunctions. Le réseau cristallin caractéristique des pérovskites hybrides offre une certaine liberté, supportant l’introduction partielle de cations et d’ions halogénures multiples. L’ajustement de la composition d’un matériau pérovskite se traduit par un ajustement de ces propriétés électroniques dont notamment sa structure de bandes. En adaptant la composition il est possible d’obtenir un matériau pérovskite avec une bande interdite de 1,7 eV qui serait parfaitement adapté pour une cellule tandem à base de Silicium cristallin. Les films minces de pérovskites peuvent être fabriqués par une grande diversité de techniques de dépôt, à partir de précurseurs ‘bon marché’ (CH3NH3I et PbI2 par exemple), par des procédés à basse température. Même si la grande majorité des films de pérovskites sont obtenus par la méthode d’enduction centrifuge, celle-ci ne permet pas l’obtention de films homogènes, sur grandes surfaces et de façon répétable. Etant donné l’enjeu industriel qui attend les pérovskites et l’intérêt croissant pour les structures tandems Silicium/Pérovskite, les méthodes sans solvant semblent plus adaptées. Déjà très largement utilisé dans l’industrie des OLEDs, le procédé de coévaporation thermique semble constituer une solution commercialement viable. Publiée pour la première fois en 2013, la synthèse par coévaporation des pérovskites est pour le moment encore étudiée par peu de groupes, car nécessitant des équipements plus coûteux. La présente thèse vise à mettre en place et développer la technique de coévaporation pour la fabrication de films de pérovskites hybrides pour des applications en cellules solaires.Afin d’évaluer la faisabilité du procédé, nous avons commencé notre travail sur un réacteur de démonstration, ce qui nous a permis d’appréhender la réponse à la sublimation des deux précurseurs. Nous avons très vite identifié le comportement du sel organique CH3NH3I comme étant problématique car difficilement contrôlable (s’évaporant sous forme de « nuage »), comme nous l’avions lu dans la littérature. En six mois d’utilisation de ce réacteur, nous avons fabriqué des films de pérovskites ayant permis d’atteindre des rendements de 9% sur des cellules solaires, malheureusement avec une faible reproductibilité (que nous expliquons en partie par le caractère aléatoire de l’évaporation du composé organique CH3NH3I). Nous nous sommes trouvés dans l’incapacité de comprendre plus en profondeur le procédé à cause d’un manque de fonctionnalités de l’équipement. Grâce à ces différents retours d’expérience nous avons pu concevoir, en étroite collaboration avec l’équipementier, un réacteur semi-industriel dédié à la fabrication de films de perovskites par coévaporation. Suite à sa mise en place, nous nous somme focalisé sur la problématique de la reproductibilité dans nos expériences en essayant de diminuer l’impact du nuage organique. Bien que les efficacités atteintes en cellules solaires pour des films coévaporés fussent moindres que pour des films déposés par la technique classique d’enduction centrifuge, nous soupçonnions néanmoins une meilleure homogénéité des films obtenus par voie sèche. Nous avons ainsi intégré à cette thèse une étude comparative voie liquide/voie sèche par le biais d’une technique de spectromicroscopie rayons X en Synchrotron
Hybrid perovskites celebrate this year their 10-year anniversary in the photovoltaic field. Besides the unprecedented rise in solar cells efficiencies, perovskite materials have tunable optical properties and can be manufactured at low cost, making them very promising candidates for the high efficiency, multijunction solar cells strategy. Perovskite crystal structure offers a relative degree of freedom, allowing the partial integration of multiple cations and halide ions. This chemical composition tuning translates into a bandgap tuning. Through fine chemical engineering, the 1.7 eV requirement for a c-Si-based tandem device can be achieved. Perovskite thin films can be prepared by a large variety of deposition techniques, from low cost precursors (CH3NH3I and PbI2 for instance), through low-temperature processes. While most of the reported works on perovskite thin films are based on the basic wet-process spincoating technique, this latter hardly allows large scale, homogeneous and reproducible deposition. With the future challenge of industrialization and the increasing interest for the Silicon/Perovskite tandem approach, solvent-free methods appear more suitable. Already widely implemented in the OLED industry, coevaporation stands as a viable option for perovskites’ future. Reported for the first time in 2013, coevaporated perovskites are still scarcely studied compared to wet-based techniques, requiring more expensive set ups. In the present thesis, we implemented and developed the coevaporation process to fabricate perovskite thin films for solar cells applications.Starting off on a proof-of-concept reactor to assess the feasibility of the technique, we got accustomed to the perovskite precursors behaviour and identify very early on the organic precursor to be hardly manageable, as reported in the literature. In six months, we were nonetheless able to obtain nice perovskite films leading to 9% efficient photovoltaic devices, unfortunately with a poor reproducibility that we think to be partially due to the cloud vapour behaviour of CH3NH3I. We eventually found ourselves missing some features on the equipment, preventing us from accurately get a grasp on the process. From this feedback we then designed, hand in hand with the manufacturer, a dedicated semi-industrial equipment for perovskite coevaporation. Following its implementation, we then focused on establishing the reproducibility of the method, trying to mitigate the parasitic effect of the organic compound. Even though the efficiencies in solar cells were still slightly lower for coevaporated perovskites, with respect to classical spincoated ones, we expected the material homogeneity to be in favour of the vacuum-based process. We then eventually integrated to this thesis a comparative study between wet- and dry-processed perovskite films using a Synchrotron-based X-ray spectromicroscopy technique

[ES] Desde hace una década se esta investigando intensamente la forma de mejorar la eficiencia de conversión de energía (PCE) de las células solares de silicio (Si) y reducir sus precios. Sin embargo, a pesar de las mejoras obtenidas, la fabricación de células solares de Si sigue siendo costosa y puede rebajarse usando materiales en forma de capa fina. Por ello la búsqueda de materiales absorbentes alternativos, no tóxicos, abundantes en la naturaleza y con buenos rendimientos de conversión se ha intensificado en los últimos años. Entre los diferentes materiales absorbentes el sulfuro de estaño (SnS), con una banda prohibida de 1.3 eV cercana a la óptima, es un candidato adecuado para la conversión fotovoltaica. Pero para células experimentales de SnS el rendimiento alcanzado hasta ahora es de 4.6%, que es mucho menos que el PCE para dispositivos de silicio, mientras que entre otras células híbridas (orgánicas-no orgánicas) como la perovskita de metilamonio de plomo y yodo (MAPbI3) se demuestra que es un candidato adecuado con PCE que alcanza un valor del 23%. Aparte de la estabilidad, uno de los problemas para la comercialización de células de MAPbI3 es la naturaleza tóxica del plomo (Pb). Por este motivo, se ha utilizado el análisis numérico para revisar los parámetros de diseño de las células solares de perovskita híbrida sustituyendo el absorbente MAPbI3 por MASnI3 y estudiar el efecto del resto de parámetros de diseño en el rendimiento de estas células solares. Hay varios softwares de simulación disponibles que se utilizan para el análisis numérico de células solares. En este trabajo hemos usamos un software llamado "A Solar Cell Capacitance Simulator" (SCAPS), está disponible de forma gratuita y es muy popular entre la comunidad científica y tecnológica. Para lograr un diseño efectivo para una célula solar eficiente, se propuso una aproximación numérica basada en la mejora de la PCE de una célula solar experimental. Esto se hizo reproduciendo los resultados para la célula solar diseñada experimentalmente en un entorno SCAPS con estructura p-SnS / n-CdS con una eficiencia de conversión del 1,5%. Después de la reproducción de los resultados experimentales, el rendimiento del dispositivo se optimizó ajustando el grosor de la capa absorbente y la capa tampón, la el tiempo de vida de los portadores minoritarios, la concentración del dopado en las capas absorbente, tampón y en la capa de la ventana. Mediante la optimización gradual de los parámetros del dispositivo, se alcanzó un valor de 14.01% en PCE de células solares diseñadas con SCAPS con arquitectura p-SnS / n-CdS / n-ZnO. A partir del análisis, se encontró que la PCE de una célula solar depende en gran medida de la concentración de dopaje de la capa absorbente, el espesor de la capa absorbente y los defectos de la interfaz. Sobre la base de los resultados obtenidos, se realizó un análisis para determinar el efecto de la recombinación de la interfaz en el rendimiento de las células solares y cómo se puede controlar. Para realizar esta tarea, se realizó un análisis para la selección de la capa tampón adecuada para la célula solar de perovskita metilamonio de estaño y yodo (MASnI3) y se encontró que el PCE de la célula solar también depende de la alineación de la banda entre el absorbedor y la capa de tampón. Por otra parte, se ha propuesto una nueva estructura para la célula solar de perovskita libre de Pb (contacto posterior / MASnBr3 / MASnI3 /CdZnS / FTO) con un PCE de 18.71% para un espesor del absorbedor de 500 nm y una concentración de dopado en el aceptor de 1x1016 cm-3. Los resultados obtenidos en esta tesis proporcionarán una guía para que los investigadores experimentales puedan construir células solares más eficientes.
[CAT] Des de fa una dècada s'està investigant intensament la forma de millorar l'eficiència de conversió d'energia (PCE) de les cèl·lules solars de silici (Si) i reduir els seus preus. No obstant això, tot i les millores obtingudes, la fabricació de cèl·lules solars de Si segueix sent costosa i pot rebaixar-se usant materials en forma de capa fina. Per això la recerca de materials absorbents alternatius, no tòxics, abundants en la naturalesa i amb bons rendiments de conversió s'ha intensificat en els últims anys. Entre els diferents materials absorbents, el sulfur d'estany (SnS), amb una banda prohibida de 1.3 eV propera a l'òptima, és un candidat adequat per a la conversió fotovoltaica. Però per a cèl·lules experimentals de SnS el rendiment assolit fins ara és de 4.6%, que és molt menor que el PCE per a dispositius de silici, mentre que entre altres cèl·lules híbrides (orgàniques-no orgàniques) com la perovskita de metilamonio de plom i iode ( MAPbI3) es demostra que és un candidat adequat amb PCE que arriba a un valor del 23%. A part de l'estabilitat, un dels problemes per a la comercialització de cèl·lules de MAPbI3 és la naturalesa tòxica del plom (Pb). Per aquest motiu, s'ha utilitzat l'anàlisi numèrica per revisar els paràmetres de disseny de les cèl·lules solars de perovskita híbrida substituint l'absorbent MAPbI3 per MASnI3 i estudiar l'efecte de la resta de paràmetres de disseny en el rendiment d'estes cèl·lules solars. Hi ha diversos programaris de simulació disponibles que s'utilitzen per a l'anàlisi numèric de cèl·lules solars. En aquest treball hem fem servir un programari anomenat "A Solar Cell Capacitance Simulator" (SCAPS), està disponible de forma gratuïta i és molt popular entre la comunitat científica i tecnològica. Per aconseguir un disseny efectiu per a una cèl·lula solar eficient, es va proposar una aproximació numèrica basada en la millora de la PCE d'una cèl·lula solar experimental. Això es va fer reproduint els resultats per a la cèl·lula solar dissenyada experimentalment en un entorn SCAPS amb estructura p-SnS / n-CdS amb una eficiència de conversió de l'1,5%. Després de reproduir els resultats experimentals, el rendiment del dispositiu es va optimitzar ajustant el gruix de la capa absorbent y de la capa tampó, el temps de vida dels portadors minoritaris, la concentració del dopatge en les capes absorbent, tampó i en la capa finestra. Mitjançant l'optimització gradual dels paràmetres del dispositiu, es va assolir un valor de 14.01% en PCE de cèl·lules solars dissenyades experimentalment en SCAPS amb arquitectura p-SnS / n-CdS / n-ZnO. A partir de l'anàlisi, es va trobar que la PCE d'una cèl·lula solar depèn en gran mesura de la concentració de dopatge de la capa absorbent, el gruix de la capa absorbent i els defectes de la interfície. D'altra banda, es va realitzar una anàlisi per determinar l'efecte de la recombinació de la interfície en el rendiment de les cèl·lules solars i com es pot controlar. Per realitzar aquesta tasca, es va realitzar una anàlisi per a la selecció de la capa tampó adequada per a la cèl·lula solar de perovskita de metilamoni d'estany i iode (MASnI3) i es va trobar que el PCE de la cèl·lula solar també depèn de l'alineació de la banda entre l'absorbidor i la capa de tampó.
[EN] A decade of extensive research has been conducted to enhance the power conversion efficiency (PCE) of silicon (Si) solar cells and to cut their prices short. But still, the fabrication of Si solar cells are costly. So, to reduce the fabrication cost of the solar cell search for alternate earth abundant and non-toxic absorber materials is thriving. Among different absorber materials tin sulfide (SnS) is found to be a suitable candidate for the non-organic solar cell with a band gap of 1.3 eV. But the PCE achieved for SnS is 4.6% that is far less from the PCE of (Si), whereas among other organic non-organic solar cells like methylammonium lead halide perovskite ({\rm MAPbI}_3) is proven to be a suitable candidate with PCE reaching to a value of 23%. The problem with the commercialization of {\rm MAPbI}_3 is due to the toxic nature of lead (Pb). So, in dealing with these issues of solar cell numerical analysis can play a key role as numerical analysis allows flexibility in the design of realistic problem and experimentation with different hypotheses can easily be performed. Complete set of device characteristic can often be easily generated by consuming less amount of time and effort. Because of this reason numerical analysis was used to revisit solar cells design parameters and the effect of solar cell physical parameters on solar cell performance. There are various simulation software's available that are used for solar cell numerical analysis. Here in this work, we used Solar cell capacitance simulator (SCAPS) software, it is freely available and is most popular among the research community. To achieve effective design for efficient solar cell a numerical guide was proposed based on which PCE of an experimental designed solar cell can be enhanced. This was done by reproducing results for the experimentally designed solar cell in SCAPS environment with structure p-SnS/n-CdS having a conversion efficiency of 1.5%. After reproduction of experimental results device performance was optimized by varying thickness of (absorber layer, buffer layer), minority carrier lifetime, doping concentration (absorber, buffer), and adding window layer. By stepwise optimization of device parameters, PCE of an experimental designed solar cell in SCAPS with architecture p-SnS/n-CdS/n-ZnO was reached to a value of 14.01%. From the analysis, it was found that PCE of a solar cell is highly depended upon doping concentration of the absorber layer, the thickness of the absorber layer and interface defects. Based on the results evaluated an analysis was performed for tin based organic non-organic methylammonium tin halide perovskite solar cell ({\rm MASnI}_3) to find the effect of interface recombination on solar cell performance and how it can be governed. The reason for this transition from SnS to {\rm MASnI}_3 was because {\rm MASnI}_3 can be fabricated simply by spin-coating methylammonium iodide (MAI) over SnS layer. To perform this task analysis was performed for the selection of suitable buffer layer for Pb free methylammonium tin halide perovskite solar cell ({\rm MASnI}_3) and it was found that PCE of the solar cell is also depended upon band alignment between absorber and buffer layer. Based on the results a new structure was proposed for Pb free perovskite solar cell (Back\ contact/{\rm MASnBr}_3/{\rm MASnI}_3/CdZnS/FTO) with PCE of 18.71% for absorber thickness of 500 nm and acceptor doping concentration of 1x10^{16}\ {\rm cm}^3. The results achieved in this thesis will provide an imperative guideline for researchers to design efficient solar cells.
Baig, F. (2019). Numerical analysis for efficiency enhancement of thin film solar cells [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/118801
TESIS

Novel photovoltaic cells receive considerable attention from researchers as evidenced by high numbers of published articles. Different types of materials are currently being examined in order to reduce the cost and improve the efficiency of solar cells. Essentially, solar cells are constructed by placing layers of light absorber between electron and hole transport materials. Electricity generation by solar cells involves multiple processes. These processes require an understanding of the physical properties of the surfaces and interfaces of the materials. In this thesis, materials for novel photovoltaic cells are studied by X ray photoelectron spectroscopy (XPS), a surface and interface characterisation technique. The materials studied in this thesis are colloidal quantum dots (CQDs) of the core/shell systems CdTe/CdSe and PbS/CdS, and CQDs that have been surface passivated using Cl- (CdTe/Cl) and CdS (CdTe/CdSe/CdS and PbS/CdS). Moreover, CsSnI3, a perovskite material, is also studied in both bulk and thin film form. CQDs can be used as light absorbers in solar cells while CsSnI3 can be employed as the hole transport material. The role of the core shell structure and surface passivation treatment is to improve or maintain charge transport as well as acting as a protective layer to the CQDs. Depth profiling synchrotron radiation XPS is used to determine these structures. In the CdTe/CdSe samples, the elemental ratio between Se (shell) and Te (core) increases with decreasing sampling depth, demonstrating the presence of a CdSe shell located at the surface of the CQDs. The shell thicknesses of the core-shell systems are estimated from XPS and show that the addition of the third thin shell (of CdS) protects the CQD during ligand exchange. Cl- passivation is shown to reduce the energy the valence band maximum and the energy gap of CdTe CQDs. This is associated with the passivation of midgap trap states due to the removal of dangling bonds at the surface of CQDs. Surface passivation is shown to improve the stability of CQDs to air exposure. This is indicated by a significant reduction of the surface oxide species in the passivated PbS/CdS samples. In the unpassivated core-only PbS samples, however, oxidation rapidly occurs which affects the electronic states required for charge transport in solar cells. XPS studies of CsSnI3 show that this material is reactive to air exposure. Surface preparation techniques are performed to remove the contamination layer and reveal the physical properties of the perovskite itself. This is confirmed by the elemental ratios from XPS. The metallic character of CsSnI3 is also observed in the valence band spectra as evidenced by the appearance of the Fermi edge.

Le soleil offre une ressource abondante et inépuisable d’énergie. Le photovoltaïque est la technologie la plus importante pour rendre l'énergie solaire utilisable car les cellules solaires photovoltaïques recueillent le rayonnement solaire et le convertissent en énergie électrique. Les cellules solaires à colorant (DSSC) ont été très étudiées en raison de leur faible coût, d’une technique de fabrication facile et une grande versatilité. Un dispositif classique DSSC comprend une photo-anode à colorant, une contre-électrode et un électrolyte contenant un couple redox et des additifs. Pour améliorer la stabilité de ces dispositifs, le remplacement de l'électrolyte liquide par des matériaux solides transporteur de trous a été étudié pour donner ce que l’on appelle des cellules solaires à colorant solides (ssDSSCs). Récemment, les pérovskites hybrides organique/inorganiques ont été introduites dans les systèmes ssDSSCs comme absorbeur de lumière. Les cellules correspondantes, appelées cellules solaires à pérovskite (PSC) ont ouvert une nouvelle ère en photovoltaïque en raison du faible coût de ce matériau et la grande efficacité de ces cellules. L'efficacité de conversion de puissance a augmenté de 3,8% en 2009 à un rendement certifié de 20,1% fin 2014. Les composants des cellules solaires à pérovskite comprennent: une couche compacte d'oxyde jouant le rôle de barrière pour les trous photogénérés, une couche de transport des électrons (un semiconducteur de type n), la couche de l’absorbeur de lumière à base de pérovskite d’halogénure de plomb, la couche de transport des trous et le contact arrière. Dans cette thèse, nous nous sommes concentrés sur la préparation et l’amélioration des propriétés de la couche de transport d'électrons et la couche de pérovskite
Solar energy is one of the most important resources in our modern life. Photovoltaic is the most important technology to render the solar energy usable since photovoltaic solar cells harvest light coming from sun and convert sunlight into electrical energy. Dye sensitized solar cells have gained widespread attention due to their low cost, easy fabrication technique and tunable choice for the device. A traditional DSSC device includes a dye-sensitized photo-anode, a counter electrode and an electrolyte containing a redox couple system and additives. To improve the device stability, the liquid electrolyte replacement by a solid state hole transport material has been studied in so-called solid-state dye sensitized solar cells (ssDSSCs). Recently, an amazing light perovskite absorber was introduced into the ssDSSC system to replace the dye, opening the new field of research. Perovskite solar cells (PSCs) open a new era in photovoltaic due to the low cost of this material and the high efficiency of these cells. The power conversion efficiency has risen from 3.8% to a certified 20.1% within a few years. The components in the perovskite solar cell include: the compact metal oxide blocking layer, the electron transport layer, the lead halide perovskite layer, the hole transport layer and the back contact. In this thesis, we focused on the preparation and improving the properties of the electron transport layer and the perovskite layer

This dissertation describes our study on different physical properties of passivated and chemically modified hybrid metal halide perovskite materials and development of highly efficient charge transport layers for perovskite solar cells. We first developed an efficient electron transport layer via modification of titanium dioxide nanostructure followed by a unique chemical treatment in order to have clean interface with fast electron injection form the absorber layer in the perovskite solar cells. We then explored monovalent cation doping of lead halide perovskites using sodium, copper and silver with similar ionic radii to lead to enhance structural and optoelectronic properties leading to higher photovoltaic performance of the resulting perovskite solar cells. We also performed thorough experimental characterizations together with modeling to further understand the chemical distribution and local structure of perovskite films upon monovalent cation doping. Then, we demonstrate a novel passivation approach in alloyed perovskite films to inhibit the ion segregation and parasitic non-radiative losses, which are key barriers against the continuous bandgap tunability and potential for high-performance of metal halide perovskites in device applications, by decorating the surfaces and grain boundaries with potassium halides. This leads to luminescence quantum yields approaching unity while maintaining high charge mobilities along with the inhibition of transient photo-induced ion migration processes even in mixed halide perovskites that otherwise show bandgap instabilities. We demonstrate a wide range of bandgaps stabilized against photo-induced ion migration, leading to solar cell power conversion efficiencies of 21.6% for a 1.56 eV absorber and 18.3% for a 1.78 eV absorber ideally suited for tandem solar cells. We then systematically compare the optoelectronic properties and moisture stability of the two developed passivation routes for alloyed perovskites with rubidium and potassium where the latter passivation route showed higher stability and loading capacity leading to achieve substantially higher photoluminescence quantum yield. Finally, we explored the possibility of singlet exciton fission between low bandgap perovskites and tetracene as the triplet sensitizer finding no significant energy transfer between the two. We then used tetracene as an efficient dopant-free hole transport layer providing clean interfaces with perovskite layer leading to high photoluminescence yield (e.g. ~18%). To enhance the poor ohmic contact between tetracene and the metal electrode, we added capping layer of a second hole transport layer which is extrinsically doped leading to 21.5% power conversion efficiency for the subsequent solar cells and stabilised power output over 550 hours continuous illumination.

An increasing concentration of greenhouse gases makes necessary changing the energy model to one in which photovoltaics play an important role. This thesis aims to assist the development of a novel photovoltaic technology based on halide perovskite towards commercialisation, in terms of sustainability. Life Cycle Assessment is used as tool for this purpose. In Chapter 3, the environmental performance of the initial developments of perovskite solar cells through four different devices is assessed through life cycle assessment. Improved stability, reproducibility and efficiency is achieved by combining methylammonium with formamidinium and caesium. Four perovskites combining these cations are compared in Chapter 4 with a plain methylammonium lead iodide perovskite. Further progression of the technology entails improving perovskite photovoltaics manufacturing at pilot scale. A pilot-scale manufacturing process producing perovskite solar modules in a carbon stack configuration is evaluated in Chapter 5. Finally, the conclusions of the thesis are presented in Chapter 6.
Programa de Doctorat en Tecnologies Industrials i Materials

[ES] El objetivo principal de esta tesis es contribuir al avance de nuevas técnicas de elaboración con bajo coste, utilizando materiales tipo de cobre, indio, galio y selenio CIGS y Perovskita para aplicaciones en energía solar fotovoltaica. CIGS parecen ser adecuadas ya que son de bajo costo de producción y se han reportado eficiencias de conversión del 23,35%. Por otro lado, las perovskitas híbridas de haluros de plomo orgánicos-inorgánicos han aparecido como nuevos materiales excepcionales para celdas solares, especialmente porque la eficiencia de las celdas solares basadas en perovskita ha aumentado del 3.8% al 22.7% en menos de un lustro. Este trabajo se ha dedicado a experimentar sobre la elaboración y caracterización de CIGS y los perovskitas de metilamonio de yoduro de plomo de (MAPbI3) y formamidinio de yoduro de plomo (FAPbI3), que se utilizo tanto en la aplicación a las células solares de perovskitas y en las células Tándem CIGS-perovskita. Las películas se caracterizaron por difracción de rayos X, espectroscopía Raman, microscopía electrónica de barrido, análisis de espectroscopía de energía dispersiva, microscopía de fuerza atómica, transmisión electrónica microscopía, fotoluminiscencia y espectroscopia UV-Vis. En las capas de CIGS depositadas por electrodeposición se investigó el efecto de diferentes parámetros, También investigamos en detalle el efecto del contacto posterior en las propiedades estructurales y ópticas de CIGS. Constatamos que el tipo de contacto posterior tiene un efecto significativo en el rendimiento posterior de las películas delgadas CIGS. Además, estudiamos la técnica de espray pirólisis para producir películas CIGS. Se estudió el proceso de recocido, que es el factor clave para mejorar el rendimiento de las células solares. Se elaboraron diferentes películas delgadas constituidas de nuestro dispositivo CdZnS/CdS/CIGS/Mo eso utilizó una capa conductora transparente de CdZnS para minimizar la alineación de la interfaz. Por otro lado, se analizó el proceso de cristalización y la estabilidad de las capas MAPbI3. Las capas de MAPbI3 se trataron añadiendo antisolvente a diferentes velocidades. Durante el tratamiento se producen intercambios complejos que influencian muchas propiedades fisicoquímicas. Se investigaron las propiedades ópticas y eléctricas de las películas de MAPbI3. Para mejorar la estabilidad de MAPbI3, se incorporó tetrabutilamonio (TBA), observando una mejora en la formación de la estructura perovskita que crece en la dirección preferente (110). La fase cristalina de MAPbI3 dopada con TBA presenta mejor cristalinidad, gran tamaño de grano, morfología superficial sin poros lo que es adecuado para la fabricación de dispositivos optoelectrónicas con mayor rendimiento. Además, hemos identificado el impacto de TBA en las propiedades foto físicas de MAPbI3. En las muestras de TBA:MAPbI3 aumenta la intensidad de la fotoluminiscencia al reducir la densidad de los estados de trampa y la absorción óptica muestra un cambio significativo hacia longitudes de onda más largas y la banda prohibida óptica varió de 1.8 a 1.52 eV. Finalmente, las muestras dopadas con 5% TBA mejoraron su estabilidad y se encontró que después de 15 días la estabilidad permanecía excelente en una humedad de ~ 60%. Por otra parte, investigamos el efecto de guanidinio (GA) sobre las propiedades estructurales y ópticas de FAPbI3. La relación entre la fase a de perovskita deseable y la fase indeseable y se ha estudiado en función del contenido de GA. Se comprobó que el dopaje con GA es eficaz en el control de la relación de fases a/y y luego en la estabilización de la fase a. Los resultados muestran que añadiendo una cantidad adecuada del 10% GA conduce a una mejora de película de perovskita que se evidencia en la homogeneidad de la fase a estable, granos de mayor tamaño y capas libres de poros. Además, 10% GA:FaPbI3 demostraron una excelente estabilidad después de ser envejecidas durante 15 días en un ambiente con humedad relativa del 60%.
[EN] The thesis work presented is part of the work in the Laboratory of New Materials for Photovoltaic Energy in the main target to use low cost techniques for elaboration of Perovskite and Copper, indium, gallium, and selenium CIGS materials for photovoltaic application. Organic-inorganic lead halides perovskites have currently and exceptionally appeared as new materials for low cost thin film solar cells specially that the efficiency of perovskite based solar cell have jumped from 3.8% to 22.7% in short time.in other hand, CIGS solar cells record 23.35% efficiency and still can be boosted. Here, we report the elaboration and characterization of CIGS as well as methylammonium lead iodide perovskites MAPbI3 and formamidinuim iodide lead iodide perovskites FAPbI3 absorbers for perovskite-based solar cells and Tandem Perovskites/ CIGS. The thin films prepared were characterized by X-ray diffraction (XRD), Raman spectroscopy (RS), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis, atomic force microscopy (AFM), transmission electron microscopy (TEM), Photoluminescence analysis (PL) and UV-Vis spectroscopy. The first stage was devoted for the effect of different parameters on the growth of CIGS by electrodeposition and we investigate the impact of different back contact in structural and optical proprieties. In a second stage, we report the growth of CIGS films by spray pyrolysis, we studied the effect of experimental parameter also the annealing process which is the key factor for improving the performance of solar cells,subsequently we elaborated different films constituted CdZnS/CdS/CIGS/Mo solar cells, the approach is to change the toxic ZnO by using a transparent, conductive CdZnS layer. In other hand, MAPbI3 film was investigated in order to optimize the chemical composition and to study the crystallization process also to get sight about the stability of perovskite materials to meet the requirement of their application as an active layer in perovskite solar cell. For this purpose. the MAPbI3 film surface was treated by adding diethyl ether antisolvent with different rates. during the treatment complex exchanges are appearing at the same time under the influence of quite a lot of physicochemical properties. A whole understanding of this topic is critically important for improving solar cell performance. MAPbI3 doped by the tetrabutylammonium TBA is boosting the formation of perovskite structure, leading to a higher orientation along the (110) and shows better crystallinity, large grain size, pinhole-free, which is suitable for the manufacturing of the optoelectronic devices with higher performance. Also, we have identified the impact of TBA in the photo-physical properties, we have noticed that the TBA improve the photoluminescence emission by reducing the density of trap states and the optical absorption indicates a significant shift to the lower wavelength and optical bandgap varied from 1.8 to 1.52 eV. Finally, the stability was explored for 5% TBA, it found that after 15 days the stability remained excellent in relative humidity of ~60%. These results would be helpful for realizing stable and high performance MAPbI3-based devices. Furthermore, we inspect the effect of monovalent cation substitution of Guanidinium (GA) on the structural and optical properties of FAPbI3 thin films perovskites. The ratio between the desirable a-phase and the undesirable y yellow phase is studied as a function of GA content. GA doping is shown to be efficient in the control of a/y phases ratio and then in the stabilization of the a-FaPbI3 phase. We qualitatively evaluate the impact of 10% of guanidinium on the phase composition and microstructure of films. The results show that an adequate amount of 10% GA:FaPbI3 leads to a homogeneous perovskite film with stable a phase, large grains, and free pinholes. 10% GA: FaPbI3 films demonstrate excellent stability after aging for 15 days in relative humidity of~60%.
[CA] L'objectiu principal d'aquesta tesi és contribuir a l'avanç de noves tècniques d'elaboració de baix cost, fent servir materials d'aliatges del tipus de coure, indi, gal·li i seleni (CIGS) i perovskites, per a aplicacions en energia solar fotovoltaica. El CIGS sembla ser adequat ja que són de baix cost de producció i s'han reportat eficiències de conversió del 23,35%. D'altra banda, les perovskites híbrides d'halurs de plom orgànics-inorgànics han aparegut com a nous materials excepcionals per cel·les solars, especialment perquè l'eficiència de les cel·les solars basades en perovskites ha augmentat del 3.8% al 22.7% en menys d'un lustre. En el present treball, reportem l'elaboració i caracterització de CIGS y de perovskitas de iodur de plom de metilamoni (MAPbI3) i de iodur de plom de formamidini (FaPbI3) per a les cèl·lules solars de CIGS i tàndem Perovskites/CIGS. En les capes de CIGS dipositades per electrodeposició es va investigar l'efecte dels diferents paràmetres sobre el procés d'electrodeposició, així com l'efecte del contacte posterior sobre les propietats estructurals i òptiques del CIGS. Ens trobem que el tipus de contacte posterior té un efecte significatiu en la posterior interpretació de pel·lícules primes CIGS. A més, vam estudiar la tècnica de polvorització de la piròlisi per produir pel·lícules de CIGS. Es va estudiar el procés de recuit, que és el factor clau per millorar el rendiment de les cèl·lules solars. Es van produir diferents pel·lícules fines formades pel nostre dispositiu CdZnS/CdS/CIGS/Mo que utilitzaven una capa conductiva CdZnS transparent per minimitzar l'alineació de la interfície. D'altra banda, es van investigar perovskites MAPbI3, amb la finalitat d'optimitzar la composició química i estudiar el procés de cristal·lització també per a conèixer l'estabilitat dels materials de perovskita. la cristal·lització s'aconsegueix alentint la solubilitat en una solució saturada mitjançant l'addició d'una quantitat diferent de l'antisolvent d'èter dietílic. Durant el tractament apareixen al mateix temps intercanvis complexos sota la influència de moltes propietats fisicoquímiques. Una comprensió completa d'aquest tema és de vital importància per a millorar el rendiment. Amb l'objectiu principal d'augmentar l'estabilitat de MAPbI3, el tetrabutilamoni (TBA) es pot incorporar a MAPbI3, impulsant la formació de l'estructura de perovskita, la qual cosa porta a una major orientació al llarg de (110). MAPbI3 dopades amb TBA presenten una millora de la cristalinitat, major grandària, la qual cosa és adequada per a la fabricació de dispositius optoelectròniques de major rendiment. A més, hem identificat l'impacte de TBA en les propietats foto físiques de MAPbI3. Hem notat que el dopatge amb TBA millora tant l'emissió de la fotoluminiscència en reduir la densitat dels estats de trampes com l'absorció òptica on apareix un canvi significatiu de la banda òptica prohibida cap a longituds d'ona més llargues que significa disminuir l'energia del gap, que va variar de 1.8 a 1.52 eV. Finalment, es va explorar l'estabilitat per les perovsquites dopades amb 5%TBA. Es va trobar que després de 15 dies l'estabilitat romania excel·lent en un humitat de 60%. A més, hem estudiat FAPbI3 com un dels materials de perovskita més atractius. Hem investigat l'efecte de la substitució de guanidini (GA) sobre les propietats estructurals i òptiques de FAPbI3. La relació entre la fase a de perovskita desitjable i la fase indesitjable y es va estudiar en funció del contingut de GA. Es mostra que el dopatge amb GA és eficaç en el control de la relació de fases a /y i després en l'estabilització de la fase a-FaPbI3. Els resultats mostren que una quantitat adequada de 10% GA condueix a una pel·lícula homogènia amb fase a estable, grans grans lliures de porus i forats. Les pel·lícules de 10% GA:FaPbI3 demostraren una excel·lent estabilitat després de l'envelliment durant 15 dies en un ambient humit (humitat relativa de 60%).
Bouich, A. (2020). Study and Characterization of Hybrid Perovskites and Copper-Indium-Gallium Selenide thin films for Tandem Solar Cells [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/160621
TESIS

Perovskite solar cells are part of third generation of solar cells and thanks to their atractive properties they are potencional candidates for replacement of silicon solar cells. Inverted planar structure of perovskite solar cells is possible to prepare by low temperature methods and by solution processing techniques. Main aim of this thesis was to prepare and characterize inverted planar perovskite solr cells with a different modification of structure. Perovskite solar cells was prepared with following structures: reference perovskite solar cells, perovskite solar cells with only a perovskite layer, perovskite solar cell without electron transport and perovskite solar cell without hole transport layer. They were prepared simultaneously by spin coating. Methods used for a characterisation of a parameters of this cells were current-voltage characteristic, impedance spektroskopy, meassure of external quantum efficiency and electrolumiscence. Reference perovskite solar cell reached best results folowed by solar cell without hole transport layer. Lowest results were gained by perovskite solar cell without electron transport layer and perovskite solar cell with only perovskite layer, where increased recombination and acumulation of charge were observed,

Le matériau pérovskite présente de nombreux avantages : excellentes propriétés optoélectroniques, faibles coûts des matières premières et vaste éventail de techniques de dépôt. Ceci en fait un candidat prometteur pour les applications photovoltaïques. Les cellules solaires en pérovskite ont ainsi vu leurs rendements de conversion progresser de 6 % à 25 % en moins de 10 ans. Cependant les pérovskites présentent un gap direct, et leur absorption chute brusquement lorsque la longueur d’onde passe au-dessus de λgap. Les cristaux photoniques sont des structures périodiques permettant, en fonction de leurs paramètres, d’améliorer l’absorption aux alentours du gap du matériau. L’objectif de ce travail de thèse est d’étudier l’intégration d’un cristal photonique dans une cellule solaire pérovskite. Nous avons dans un premier temps optimisé le dépôt de pérovskite pour obtenir une couche uniforme et continue. Nous avons ensuite réalisé différentes étapes d’impression sur les couches. Ce procédé, grâce à l’application d’une pression, permet de structurer un matériau suivant la forme inversée d’un moule. Deux types d’impression ont été étudiés et optimisés : la compression plane, permet d’aplanir la pérovskite et d’obtenir une couche de très faible rugosité tout en améliorant la cristallisation de la pérovskite ; la nanoimpression, qui s’effectue avec un moule structuré. Les paramètres des deux procédés ont été optimisés, nous avons démontré qu’elle est applicable à une large gamme de moules, avec des motifs de tailles et de dimensions différentes. Nous avons pu réaliser un cristal photonique dans la couche de pérovskite permettant d’améliorer l’absorption de celle-ci. Finalement, nous avons réalisé des cellules solaires en utilisant le procédé d’impression (compression plan et nanoimpression) sur la couche de pérovskite. En obtenant un rendement de conversion pour les deux types de cellules, nous avons montré la faisabilité de telles cellules
Perovskite material has many advantages: excellent optoelectronic properties, low raw material costs and a wide range of deposition techniques. This makes it a promising candidate for photovoltaic applications. Perovskite solar cells have seen their efficiency increase from 6% to 25% in less than 10 years. However, the perovskites present a direct gap, and the absorption in this material drops suddenly when the wavelength passes above λgap. Using photonic crystals based on periodic structures allow, to improve the absorption around the gap of the material. The objective of this thesis is to study the integration of a photonic crystal in a perovskite solar cell.We have first optimized the perovskite deposition process in order to obtain a uniform and continuous layer. Then, we have carried out different imprint tests on the layers. This process, by applying a pressure, makes it possible to structure a material according to the inverted shape of a mold. Two types of imprint have been studied and optimized: flat compression, which, using a flat mold, makes the perovskite less rough and allows to obtain a better crystallization; and nanoimprint, using a nano-structured mold. An increase in grain size compared to simple annealing of perovskite without imprint has also been noticed. For nanoimprint, we have tested and optimized the impact of pressure and temperature on the perovskite structuration. After development and optimization of this technique on perovskite, we have shown that it is applicable to a wide range of molds, with patterns of different sizes and dimensions. We were able to make a photonic crystal in the perovskite layer to improve its absorption. Finally, we have realized solar cells using the imprint process (flat compression and nanoimprint) applied on the perovskite layer. By obtaining a conversion efficiency for the two types of cells, we have demonstrated the feasibility of such cells

Ce travail de thèse se concentre sur l'étude des modèles de type drift-diffusion. Des approches sont développées pour la modélisation de la Microscopie à sonde de Kelvin, des cellules solaires à base de matériaux pérovskites (PSCs), des cellules solaires tandem de type pérovskite/silicium et des îlots quantiques lll-V/GaP. Tout d'abord, l'approche de la modélisation de la sonde de Kelvin est examinée pour la surface de TiOx et l'absorbeur pérovskite MAPbI3 Ensuite, des mesures avec sonde de Kelvin et des simulations sont proposées pour des jonctions diffuses à base de silicium et pour des PSCs à base de TiOx mésa poreux. Les variations du potentiel interne sont étudiées ouvrant la voie à une amélioration supplémentaire des dispositifs. L'influence de l'état de surface des couches wo. sur des mesures à sonde de Kelvin est étudiée théoriquement. Différents facteurs à l'origine des pertes de tension de circuit ouvert (Voc) des PSCs sont discutés. L'effet anormal d'hystérésis dans les PSCs est également simulé, en tenant compte des étals de pièges d'interface et des ions mobiles. En outre, le design de cellules solaires tandem 2T pérovskite/silicium est étudié en détails. Une jonction tunnel à base de silicium entre les deux sous-cellules supérieure et inférieure est proposée pour assurer le bon fonctionnement des cellules en série. L'influence du profil de dopage dans la jonction tunnel est discutée. Au final, le transport des porteurs dans les îlots quantiques III-V/GaP est étudié dans le cadre plus général de l'intégration d'émetteurs lll-V sur silicium. Les caractéristiques électroluminescentes et électriques de ces structures sont simulées dans une approximation cylindrique
This PhD work focuses on optoelectronic device simulations based on drift-diffusion models. Approaches are developed for the modelling of Kelvin Probe Force Microscopy (KPFM), perovskite-based solar cells (PSCs), perovskite/silicon tandem solar cells and lll-V/GaP quantum dots (ODs). Firstly, a new approach for the modelling of KPFM is applied to TiOx slabs and to the MAPbI3 perovskite absorber. Secondly, KPFM measurements and simulations are proposed for silicon-based diffused junctions and mesoporous TiOx based PSCs. The built-in potential is investigated, and this study paves the way toward fu rther device improvements. In addition, the influence of the surface of WO. slabs on KPFM measurements is studied theoretically. Various facto rs influencing open circuit voltage (Voe) losses in PSCs are discussed. The abnormal hysteresis effect in the PSCs is simulated as well, considering interface trap states and mobile ions. The design of two-terminal perovskite/silicon tandem solar cells is studied in detail. A siliconbased tunnel junction between the top and the bottom subcells is proposed for serial current matching. The influence of the doping profile in the tunnel junction is discussed. At the end of the manuscript, the carrier transport in III-V/GaP QDs is investigated, for the integration of III-V emitters on silicon. The electroluminescence and electrical characteristics of these III -V light emitting devices are simulated by using a cylindrical approximation

Ces onze dernières années ont vu apparaitre les pérovskites organiques inorganiques hybrides (HOIPs) comme un passionnant domaine de recherche pour leur application potentielle dans les technologies du photovoltaïque (PV) en raison de leurs exceptionnelles propriétés optoélectroniques et de leur facilité de mise en oeuvre. Cependant, les matériaux HOIPs ont plusieurs inconvénients dont leur manque de stabilité en conditions opérationnelles. Améliorer celle-ci est l'un des plus grands défis à relever avant commercialisation. La formule générale est (A1,A2,A3,A4)Pb(X1,X2)3, où les sites A occupés par une distribution de 1 à 4 cations métalliques/organiques et les sites X par celle d’anions halogénures. Les défauts lacunaires natifs sont considérés comme une cause possible de dégradation des cellules solaires HOIPs. L'objectif de ce travail est de comprendre le rôle des défauts dans la stabilité à long terme des matériaux PV HOIPs. A cette fin, des défauts primaires ont été introduits de manière contrôlée par irradiation avec des électrons de haute énergie (1MeV) dans des lots de couches et cellules solaires (SCs) à base de divers composés HOIPs. Il s'agit notamment du prototype PV HOIPs, MAPbI3 (A1PbX13), et de nouveaux composés mixtes d’halogénures à triple ou quadruple cations, (CsMAFA)Pb(I1-xBrx)3 (A3PbX23) ou (GACsMAFA)Pb(I1-yBry)3 (A4PbX23). Les couches sont fabriquées selon la même procédure que les couches actives SCs et, ensuite, traitées dans des conditions similaires. Pour A1PbX13/A3PbX23, la structure SC est de type p-i-n avec des couches organiques pour le transport des trous et des électrons (HTL/ETL). Les couches sont déposées sur le substrat verre/ITO/HTL (PEDOT:PSS) sans ou avec couche supérieure ETL (PCBM). Pour A4PbX23, la structure SC est de type n-i-p avec des couches ETL inorganiques (TiO2) et HTL organiques (Spiro-OMeTAD). Les couches sont directement déposées sur du verre.La spectroscopie d'annihilation de positons donne une évidence directe de l'existence de défauts lacunaires natifs et induits par irradiation dans chaque composé. Les spectres d’absorbance en fonction de l’énergie montrent que le vieillissement naturel et après irradiation génère différentes populations de défauts dans chaque composé. De plus, celles-ci pour A1PbX13 et A3PbX23 diffèrent selon l'absence ou la présence de la couche supérieure ETL. Les populations de défauts évoluent pendant au moins 3 mois. Le vieillissement modifie (i) la bande interdite, (ii) les queues de bande de conduction/valence et (iii) l'absorption optique via des niveaux électroniques profonds. Les effets d’illumination sous laser varient aussi en fonction du vieillissement. L’asymétrie des pics de photoluminescence (PL) dans chaque composé sous illumination laser continue reflète une superposition de raies d’émission gaussiennes à énergie, FWHM et hauteur évoluant avec le temps d'illumination. Les transitions d'émission impliquent des niveaux électroniques localisés peu profonds dans A3PbX23/A4PbX23 et résonnants dans A1PbX13. De tels effets durent au moins 3 mois dans A4PbX23. Ces niveaux électroniques sont attribués à des populations de défauts spécifiquement induits par illumination. Le vieillissement naturel et après irradiation donne des spectres PL à décroissance temporelle résolue en une ou deux exponentielles. Le nombre et la durée de vie sont fortement influencés par l’irradiation initiale et la composition. Une amélioration frappante du fonctionnement PV pour le type SC p-i-n est induite par le vieillissement dû à l'irradiation. Le rendement quantique externe et les performances PVs ont des valeurs plus élevées pour l’état irradié que de référence durant 6 à 12 mois de vieillissement. Cela prouve que l'ingénierie des défauts par irradiation d'électrons à haute énergie a le potentiel de fournir des voies de traitement innovantes pour améliorer la stabilité à long terme des performances photovoltaïques HOIPs
During the last eleven years, Hybrid Organic Inorganic Perovskites (HOIPs) materials have emerged as an exciting topic of research for potential application in solar cell technologies due to their outstanding optoelectronic properties and processing advantages. However, HOIPs materials suffer from several drawbacks with, in peculiar, their lack of stability under operational conditions (light, bias, environment…). To improve this stability is one of the biggest challenges to be addressed before commercialization. The general formula for HOIPs is (A1,A2,A3,A4)Pb(X1,X2)3, where the A sites can be occupied by a distribution of 1 to 4 metallic/organic cations and X sites with halide anions. The role of native vacancy defects has been questioned as a possible cause for HOIPs solar cells degradation. The aim of this work is to understand the defect role in long term stability of HOIPs materials for photovoltaics. For this reason, primary defects were introduced in a controlled way via high energy electron irradiation (1MeV) in sets of layers and solar cells (SCs) fabricated using various HOIPs compounds. Those include the photovoltaic HOIPs prototype, MAPbI3 (A1PbX13), and emergent triple or quadruple cation mixed halide HOIPs, (CsMAFA)Pb(I1-xBrx)3 (A3PbX23) or (GACsMAFA)Pb(I1-yBry)3 (A4PbX23). The HOIPs layers are fabricated according to the same procedure as the HOIPs active SC layers and, subsequently, treated in similar conditions. For A1PbX13 and A3PbX23, the solar cells are of the p-i-n structure with organic hole and electron transport layer (HTL/ETL). The HOIPs layers are deposited on the glass/ITO/HTL (PEDOT:PSS) substrate without or with the top ETL layer (PCBM). For A4PbX23, the solar cells are of the n-i-p type with inorganic ETL (TiO2) and organic HTL (Spiro-OMeTAD) layers. The layers are directly deposited on glass without the ETL layer.Positron Annihilation Spectroscopy (PAS) gives direct evidence for native vacancy-type defects and irradiation induced ones in layers of each HOIP compound. The energy dependence of absorbance shows that natural and after irradiation ageing generates different defect populations in each HOIP compound. These populations strikingly also differ depending on the absence or presence of the top ETL layer for the A1PbX13 and A3PbX23 compounds. The defect populations evolve over ageing duration as long as 3 months. The prominent effects of ageing include (i) band gap modification, (ii) tailing of conduction/valence band extrema and (iii) optical absorption via deep subgap electronic levels. Illumination effects under laser also vary with ageing for each HOIP compound. Asymmetric photoluminescence (PL) peaks in each compound under continuous laser illumination reflect that radiative emission involves Gaussian emission rays with energy, FWHM and height evolving with illumination time. The emission transitions involve shallow localized electronic levels in A3PbX23 and A4PbX23 and resonant ones in A1PbX13. These electronic levels are attributed to specifically illumination-induced defect populations. Natural and after irradiation ageing result in PL decay lifetime spectra resolved into one or two exponential decay components. The decay components number and lifetime are strongly affected by the initial production of irradiation defects and HOIPs composition. Such effects last over 3 months at least in A4PbX23. The p-i-n solar cells exhibit most striking irradiation ageing induced photovoltaics performance. The External Quantum Efficiency (EQE versus photon energy) and the photovoltaic performance (I-V under illumination) of the irradiated solar cells have higher values than those in the reference SCs after 6 to 12 months of ageing. This gives evidence that defect engineering via high energy electron irradiation has a potential for providing innovative processing pathways to enhance the long-term stability of HOIPs photovoltaic performance

Over the last 4 years, perovskite solar cells emerged as an attractive, highly efficient, and low-cost alternative to established, conventional photovoltaic technologies. The power conversion efficiency of these devices recorded an unprecedented rise, currently exceeding certified values of 20%. This thesis covers a number of technological advancements which lead to improved photovoltaic performance, as well as vital insight into some more fundamental aspects of the perovskite device operation. The focus of this body of work is primarily directed towards the electric contact in the PV stack which is responsible for electron collection. The motivation of the study presented here is given in Chapter 1, and includes a brief summary of the current energy landscape. Chapter 2 introduces the theoretical background of photovoltaic technology, starting from the basics of semiconductor physics, through to the principles of solar cell operation, as well as some characteristic properties of the perovskite materials. Details of the experimental methods used in this study are reported in Chapter 3. Chapter 4 reports the development of a low temperature process (sub-150 °C) for the manufacture of perovskite solar cells. Dispersions of pre-synthesised, highly crystalline TiO2 nanoparticles were used as an electron selective contact, which eliminated the high temperature sintering step. Chapters 5, 6 and 7, report the interface modification of an n-type contact, resulting in a substantially improved device operation and suppression of hysteresis phenomenon which is characteristic of perovskite photovoltaics. Fullerene-based materials have been found to make excellent electronic contact with halide perovskite materials, and are shown to be far superior to commonly used metal oxides. The facilitated electron collection allows enhancements in the photovoltaic performance of these devices. Furthermore, the organic layers used in this study can be processed at low temperatures. Finally, the development of transparent conductive electrodes based on silver nanowires is presented in Chapter 8. The fabricated electrodes exhibit low sheet resistance, high degree of transparency, and can be processed at low temperatures, allowing them to be compatible with processing on flexible substrates and multi-junction architectures. The application of silver nanowires in different perovskite solar cell architectures is also reported.

L’énergie solaire est une importante source d’énergie renouvelable. Depuis 10 ans, les cellules solaires pérovskite ont montré leur énorme potentiel avec des rendements supérieurs à 22%. Ce travail de thèse a consisté à élaborer de nouveaux verres moléculaires transporteurs de trous à base de carbazole pour remplacer le spiro-OMeTAD (matériau de référence) dans ces cellules pérovskite. Tout d’abord, la synthèse de semi-conducteurs de type p a été optimisée en réalisant, à partir d’un intermédiaire à base de carbazole, appelé « synthon », en une seule étape, 4 nouvelles familles de molécules constituées d’un, deux ou trois synthons. Des coeurs à base de dérivés de spirobifluorène, de thiophènes, de triazatruxène ou d’espaceur fluorés ont été utilisés. Les propriétés physico-chimiques de ces familles de composés ont ensuite été étudiées afin de vérifier leurs possible utilisation en dispositifs solaires et de trouver une relation structure/propriétés. Par exemple, il a été montré que les Tg des composés dépendent fortement de la rigidité du coeur. Les analyses thermiques, électroniques et énergétiques montrent que tous les composés synthétisés ont des propriétés en accord avec leur utilisation comme HTM dans les cellules pérovskite. Finalement, ces matériaux ont été intégrés dans des dispositifs solaires et montrent un potentiel intéressant autant en termes de rendement (entre 13% et 15%), comparable au matériau actuel, que de prix (coût au moins 2 fois moins élevé que la référence)
The sun is the most important source of renewable energy. Over the last 10 years, perovskite solar cells have shown a tremendous interest with efficiencies above 22%. This PhD work has consisted in elaborating new molecular glasses, hole transporting materials, based on carbazole moiety to replace spiro-OMeTAD (reference material) in perovskite solar cells. First, the elaboration of p type semiconductors has been optimized by synthesizing, from a carbazole based intermediate called “synthon”, in only one step, 4 new families of molecules constituted of one, two or three synthons. Cores based on spirobifluorene derivatives, thiophenes, triaxatruxenes or fluorinated spacer have been used. The physicochemical properties have then been studied in order to confirm that they can be used in solar devices and to find a structure/properties relationship. For example, we showed that the Tg of the materials are clearly depending on the rigidity of the chemical structure of the core. Thermal, electronic and energetic measurements are showing that the whole families possess suitable properties to be used as HTM in perovskite solar cells. Finally, these materials have been integrated in solar devices and have shown promising results either in terms of efficiencies (between 13% and 15%), similar to the commercially available material, or in terms of price (the cost is at least twice cheaper than the reference)

This work studies the subject of perovskite solar cells. The structure of perovskites is described along with a portion of photovoltaics history that led to the employment of perovskites as absorpsion layers. Further, methods of measurement for solar cells such as impedance spectroscopy, photospectroscopy and load characteristics are summarized. Lastly there is a description of already done analytics of perovskite solar cells with the summary of the results. In the practical part, there are measurements made to find out any effect of cell topology or light source wavelength on the cell performance.

>Magister Scientiae - MSc
Perovskite solar cells have attracted a tremendous amount of research interest in the scientific community recently, owing to their remarkable performance reaching up to 22% power conversion efficiency (PCE) in merely 6 to 7 years of development. Numerous advantages such as reduced price of raw materials, ease of fabrication and so on, have contributed to their increased popularity.

This thesis presents a study of hybrid solar cells, specifically looking at various methods which can be employed in order to increase the power conversion efficiency of these devices. The experiments and results contained herein also present a very accurate picture of how rapidly the field of hybrid solar cells has progressed within the past three years. Chapters 1 and 2 present the background and motivation for the investigations undertaken, as well as the relevant theory underpinning solar cell operation. Chapter 2 also gives a brief review of the literature pertinent to the main types of devices investigated in this thesis; dye-sensitised solar cells, semiconductor sensitized solar cells and perovskite solar cells. Descriptions of the synthetic procedures, as well as the details of device fabrication and any measurement techniques used are outlined in Chapter 3. The first set of experimental results is presented in Chapter 4. This chapter outlines the synthesis of mesoporous single crystals (MSCs) of anatase TiO₂ as well as an investigation of its electronic properties. Having shown that this material has superior electronic properties to the conventionally used nanoparticle films, they were then integrated into low temperature processed dye-sensitised solar cells and achieved power conversion efficiencies of > 3%, exhibiting electron transport rates which were orders of magnitude higher than those obtained for the high temperature processed control films. Chapter 5 further investigates the use of MSCs in photovoltaic devices, this time utilising a more strongly absorbing inorganic sensitiser, Sb₂S₃. Utilising the readily tunable pore size of MSCs, these Sb₂S₃ devices showed an increase in voltage and fill factor which can be attributed to a decrease in recombination within these devices. This chapter also presents the use of Sb₂S₃ in the meso-superstructured configuration. This device architecture showed consistently higher voltages suggesting that in this architecture, charge transport occurs through the absorber and not the mesoporous scaffold. Chapters 6 and 7 focus on the use of hybrid organic-inorganic perovskites in photovoltaic devices. In Chapter 6 the mixed halide, lead-based perovskite, CH₃NH₃PbI_3-xCl_x is employed in a planar heterojunction device architecture. The effects of Lewis base passivation on this material are investigated by determining the photoluminescence (PL) lifetimes and quantum efficiencies of treated and untreated films. It is found that passivating films of this material using Lewis bases causes an increase in the PLQE at low fluences as well as increasing the PL lifetime. By globally fitting these results to a model the trap densities are extracted and it is found that using these surface treatments decreases the trap density of the perovskite films. Finally, these treatments are used in complete solar cells resulting in increased power conversion efficiencies and an improvement in the stabilised power output of the devices. Chapter 7 describes the materials synthesis and characterisation of the tin-based perovskite CH₃NH₃SnI₃ and presents the first operational, lead-free perovskite solar cell. The work presented in this thesis describes significant advances in the field of hybrid solar cells, specifically with regards to improvements made to the nanostructured electrode, and the development and implementation of more highly absorbing sensitizers. The improvements discussed here will prove to be quite important in the drive towards exploiting solar power as a clean, affordable source of energy.

Halide perovskites have had a huge impact on different fields in the last decade due to their versatility and amazing optoelectronic properties. However, there are still some issues to improve towards their future utilization and commercialization. In this thesis, several topics associated with perovskite-based photovoltaic and optoelectronic devices are addressed. The different works contribute to enhance the optoelectronic properties of perovskite quantum dots, to replace the most employed hole transporting material in photovoltaic devices (spiro-OMeTAD) and, moreover, to study a methodology which can be incorporated in up-scaling procedures towards their future commercialization as solar cells.
Programa de Doctorat en Ciències

This thesis studies the degradation of perovskite photovoltaic cells placed in atmospheres of different moisture. Samples with inverse structure: ITO/ PEDOT:PSS/ CH3NH3PbI3–XClX / PC70BM/ Ca/ Al were prepared. Electrical characteristics were measured for 2 months and similar degradation trend was observed for all the samples. Perovskite cell efficiency PCE decreased to 20 % of the initial value in t80= 46±3 days in laboratory, t80=23±1$ days in nitrogen atmosphere and t80=25,7±0,6 dní days in dry atmosphere. For the initial 27 days of the experiment, a faster degradation linked with the decrease of ISC, FF and VOC was observed. After this period, the value of FF has stabilised at 0,777±0,009 % and the value of VOC at 0,70±0,02 % of their original value (in the laboratory atmosphere). Additional fall of PCE resulted only from the decrease of ISC and was slower than in the initial period. From the results acquired, it has emerged that during the experiment, the cell encapsulation provided a sufficient barrier against outer moisture. Residual moisture present in the sctructure, was labeled as the source of the degradation. The moisture is believed to enter with the hygroscopic material PEDOT:PSS during the samples preparation procedure. During the degradation, absorption measurements of photovoltaic cells were executed. The absorption spectra didn't change. That indicates, that the decrease of ISC is not caused by the reduction of light absorption. The thesis also studied the degradation of perovskite solar cells under illumination. The samples were exposed to UV radiation for 55,5 hours. The PCE time of decrease to 20 % of the initial value was t80 = 6±2 days. It was revealed, that UV radiation significantly accelerates the decrease of ISC.

The primary goal of this work is related to the extension of an analytic electro-optical model. It will be used to describe single-junction crystalline silicon solar cells and a silicon/perovskite tandem solar cell in the presence of light-trapping in order to calculate efficiency limits for such a device. In particular, our tandem system is composed by crystalline silicon and a perovskite structure material: metilammoniumleadtriiodide (MALI). Perovskite are among the most convenient materials for photovoltaics thanks to their reduced cost and increasing efficiencies. Solar cell efficiencies of devices using these materials increased from 3.8% in 2009 to a certified 20.1% in 2014 making this the fastest-advancing solar technology to date. Moreover, texturization increases the amount of light which can be absorbed through an active layer. Using Green’s formalism it is possible to calculate the photogeneration rate of a single-layer structure with Lambertian light trapping analytically. In this work we go further: we study the optical coupling between the two cells in our tandem system in order to calculate the photogeneration rate of the whole structure. We also model the electronic part of such a device by considering the perovskite top cell as an ideal diode and solving the drift-diffusion equation with appropriate boundary conditions for the silicon bottom cell. We have a four terminal structure, so our tandem system is totally unconstrained. Then we calculate the efficiency limits of our tandem including several recombination mechanisms such as Auger, SRH and surface recombination. We focus also on the dependence of the results on the band gap of the perovskite and we calculare an optimal band gap to optimize the tandem efficiency. The whole work has been continuously supported by a numerical validation of out analytic model against Silvaco ATLAS which solves drift-diffusion equations using a finite elements method. Our goal is to develop a simpler and cheaper, but accurate model to study such devices.

Hybrid lead halide perovskites, AMX 3 compounds in which A = CH 3 NH 3 (MA), CH(NH 2 ) 2(FA), Cs; M = Pb,Sn; X = I, Br, Cl, display remarkable performance in solution-processed optoelectronic devices, including > 22% efficient thin film photovoltaic cells. These compounds represent the first class of materials discovered to exhibit properties associated with high performance compound semiconductors, while being formed at or near room temperature using simple solution chemistry techniques. This thesis is focused on the synthesis, structural characterisation and phase behaviour of MAPbI 3 , FAPbI 3 , A-site solid solutions and novel organic metal halide framework materials. The complete atomic structure and phase behaviour of methylammonium lead iodide is elucidated for the first time, including hydrogen positions, using high flux, constant wave-length neutron powder diffraction. At 100 K an orthorhombic phase, space group Pnma, is observed, with the methylammonium cations ordered as the C–N bond direction alternates in adjacent inorganic cages. Above 165 K a first order phase transition to tetragonal, I4/mcm, occurs with the unlocking of cation rotation, which is disordered primarily in the ab plane. Above 327 K a cubic phase, space group Pm3m, is formed, with the cations isotropically disordered on the timescale of the crystallographic experiment. The high temperature phase of formamidinium lead iodide, α-FAPbI 3 is shown for the first time to be cubic, (Pm3m), at room temperature using time-of-flight, high resolution neutron powder diffraction. Polymorphism and the low temperature phase behaviour of FAPbI 3 have been further investigated using reactor and spallation neutron sources with high resolution in temperature. A tetragonal phase, P4/mbm, is confirmed in the temperature range 140-285 K.The composition, structural and optical parameters of ’A’ site solid solutions (MA/FA)PbI 3 have been investigated by single crystal X-ray diffraction, UV-vis spectroscopy and 1 H solution NMR. A composition-dependent transition in the crystal class from tetragonal to cubic(or pseudo-cubic) at room temperature is identified and correlated to trends in the optical absorption. Novel hybrid materials with inorganic frameworks of varying dimensionality from 0D to 2D, including imidazolium lead iodide and piperazinium lead iodide, have been synthesised using various templating organic cations and their atomic structures solved by single crystal X-ray diffraction.

This thesis details the development of a novel photovoltaic device based on organometal halide perovskites. The initial focus of this thesis begins with the study of lighttrapping strategies in solid-state dye-sensitised solar cells (detailed in chapter 3). While I report enhancement in device performance through the application of near and far-field light-trapping techniques, I find that improvements remain step-wise due to fundamental limitations currently employed in dye-sensitised solar cell technology— notably, the available light-sensitising materials. I found a promising yet under researched family of materials in the methyl ammonium tri-halide plumbate perovskite (detailed in chapter 4). The perovskite light-sensitiser was applied to the traditional mesoscopic sensitised solar cell device architecture as a replacement to conventional dye yielding world-record breaking photo-conversion e!ciencies for solid-state sensitised solar cells as high as 8.5%. The system was further developed leading to the conception of a novel device architecture, termed the mesoporous superstructured solar cell (MSSC), this new architecture replaces the conventional mesoporous titanium dioxide semiconductor with a porous insulating oxide in aluminium oxide, resulting in very low fundamental losses evidenced through high photo-generated open-circuit voltages of over 1.1 V. This development has delivered striking photo-conversion ef- ficiencies of 10.9% (detailed in chapter 6).

The thesis discusses the problematics of perovskite solar cells. The introduction deals with the history of the solar cells as the next part is dedicated to generations of them as well as to their principle and the variables measured in photovoltaic cells. The next part deals with perovskites, their properties and structure. Later the part about perovskite solar cells follows. The last part is dedicated to experiments which were executed on samples of perovskite solar cells.

The thesis is about the problematics of perovskite, production of perovskite structure and measurment of photovoltaic cells. The introduction deals with material properties and sctructure of perovskite. There is explained a problem of perovskit photovoltaic cells and production of perovskite structure. Then the measurement methods are explained, which are used for evaluation of photovoltaic cells properties.

La ricerca su materiali innovativi per applicazioni fotovoltaiche si è orientata negli ultimi anni verso lo studio delle perovskiti, per lo sviluppo di nuove tipologie di celle solari ad alta efficienza e basso costo; tuttavia, la commercializzazione di tali celle solari è ancora lontana, a causa della loro grande instabilità. L’ossido di lantanio-vanadio (LaVO3), che presenta la struttura cristallina della perovskite, è un materiale molto promettente per applicazioni fotovoltaiche, in quanto potrebbe risolvere il problema della stabilità. Gli obiettivi della tesi sono stati: lo studio delle proprietà ottiche ed elettriche del LaVO3; l’implementazione e l’ottimizzazione di un apparato sperimentale di surface photovoltage spectroscopy (SPS), insieme con lo sviluppo del relativo software di acquisizione dati. L’apparato per SPS è stato implementato con successo, il set-up sperimentale è stato ottimizzato ed è stato sviluppato un software per l’acquisizione dati.
 Lo studio delle proprietà morfologiche del LaVO3 alla nanoscala, condotto mediante microscopia a forza atomica, ha permesso l’identificazione delle condizioni di deposizione ottimali dei film sottili. Lo studio delle proprietà elettriche, condotto mediante scanning Kelvin probe microscopy, ha permesso la determinazione del valore dell’altezza barriera all’interfaccia LaVO3/ZnO e delle work function di ZnO e LaVO3. Si noti che il valore di work function del LaVO3 non era mai stato riportato prima in letteratura. Le misure SPV sul LaVO3 hanno generato un segnale minore del limite di rilevazione dell’apparato: ciò significa che le coppie elettrone-lacuna fotogenerate non vengono separate e raccolte in modo efficiente. In conclusione, il LaVO3 è noto per avere proprietà ottiche ottimali ed elevata stabilità, che sono vantaggi considerevoli per eventuali dispositivi fotovoltaici. Tuttavia, le misure di SPV hanno chiaramente dimostrato che questo materiale non è ottimale come mezzo per il trasporto di carica.

The recent development of perovskite-based solar cells have shown a remarkably fast increase in power conversion efficiency making them a promising low-cost alternative to conventional cells. The most successful class of materials however, the lead-halide perovskites, are held back due to toxicity and stability issues significantly limiting their use. Because of this, the investigation of new, lead-free, light-absorber materials as a replacement is an important step towards improved solar cells. The focus of this licentiate thesis is the study of bismuth-based materials and their photovoltaic properties through electronic structure calculations. Specifically, the cubic-phase AgBi2I7 under gradual substitution of either bromine or antimony is investigated using density functional theory under periodic boundary conditions. This enables calculations of the system's energy levels and band structure. Furthermore, the energy variance of the employed model of the system is sampled with respect to its level of ion disorder to obtain a better understanding of the distribution of ions within the crystal. The materials are found to have good optical properties but comparatively low efficiencies. The introduced substitutions allow fine-tuning of the system's band gap and is shown to increase the overall performance of the solar cells. In addition, spin-orbit coupling effects are demonstrated to be important when treating these bismuth-based systems. The crystal structure is found to have a significant preference for separating its silver ions and cation vacancies.

Dans ce travail, nous avons étudié la croissance de films d’oxydes Bi2FeCrO6 (BFCO) en utilisant les techniques de sol-gel et dépôt par laser pulsé (PLD). Dans le cas de la voie chimique, des précurseurs en solution ont été préparés, puis déposés par centrifugation sur des substrats de silicium (100) ou de quartz. Les nombreuses analyses structurelles (DRX) et d'imagerie (SEM, TEM) effectuées sur ces films BFCO ont montré que les films sont assez homogènes mais présentent de nombreuses phases parasites qui peuvent être éliminés partiellement par recuit thermique rapide. Des dispositifs tests à base de films BFCO par sol-gel ont été préparés et qui ont montré des propriétés électriques limitées à cause des nombreux défauts. Des films BFCO ont également été produits par la technique PLD sur des substrats STO et NbSTO. Les propriétés structurelles, optiques et électriques sont présentées. La diffusion épitaxiale de haute qualité et les films en phase pure sont démontrés par diffraction des rayons X. Nous avons étudié l'évolution de paramètres tels que la bande interdite en fonction des conditions de croissance, montrant qu'elle peut être ajustée de 1, 9 à 2,6 eV. Ce comportement a été corroboré par des calculs théoriques sur l’arrangement atomique dans la structure BFCO. Les propriétés ferroélectriques sont étudiées par microscopie à force piézoélectrique. La lumière s'est avérée avoir un effet sur la polarisation. Il a également été démontré que la mémoire de la polarisation affecte la réponse photovoltaïque. Enfin, des dispositifs basés sur BFCO sont fabriqués et leurs propriétés photovoltaïques sont analysées. Des valeurs de tension de circuit ouvert de 600mV sont encourageantes pour la nouvelle génération de cellules solaires
In this work, we have produced Bi2FeCrO6 oxides (BFCO) by sol-gel technique and pulsed laser deposition (PLD). By sol-gel, precursors in solution were prepared, which are then deposited by centrifugation on silicon or quartz substrates. The numerous structural (XRD) and optical images (SEM, TEM) analyses carried out on these BFCO films show that the films are fairly homogeneous but exhibit many parasitic phases, which they can be partly eliminated by rapid thermal annealing. Finally, we present the first results obtained on BFCO-SG perovskite devices. On the other hand, BFCO films were deposited on STO and Nb:STO substrates. Their structural, optical and electrical properties are presented. High-quality epitaxial growth and pure-phase films are demonstrated by X-ray diffraction. We show that the band gap of the PLD-BFCO films can be tuned from 1, 9 to 2.6 eV thanks to the variation of growth conditions. Theoretical calculations has confirmed the observed behavior and highlight the importance of the ordering phase. The ferroelectric properties of the PLD films are studied by the piezoresponse force microscopy. Illumination is shown to have a strong effect on polarization. We show that the polarization memory affects the photovoltaic response. Finally, devices based on BFCO are manufactured and their photovoltaic properties are analyzed

Magister Scientiae - MSc
In this research, novel hybrid perovskite materials were synthesized, characterized and applied in photovoltaic cells (PVCs) to enhance the performance of PVCs. Mixed-organic-cations tin halide perovskites (MOCTPs) were successfully synthesized using sol-gel method. These MOCTPs include guanidinium dimethylammonium tin iodide ([GA][(CH3)2NH2]SnI3) and guanidinium ethylmmonium tin iodide ([GA][CH3CH2NH3]SnI3). The MOCTPs were studied in comparison to their single-organic-cation tin perovskites (SOCTPs), which include guanidinium tin iodide (GASnI3), ethylammonium tin iodide ([CH3CH2NH3]SnI3) and dimethylammonium tin iodide [(CH3)2NH2]SnI3. High Resolution Scanning Electron Microscopy (HR SEM) of the five perovskite materials showed good crystallinity and tetragonal and hexagonal cubic shapes, characteristic of perovskites. These shapes were also confirmed from High Resolution Transmission Electron Microscopy (HR TEM), and the internal structure of the perovskites gave similar zone axes (ZAs) with those obtained from X-ray Diffraction (XRD). XRD showed tetragonal lattice shape for these perovskite materials. Fourier Transform Infrared (FTIR) demonstrated similar functional groups for both the SOCTPs and MOCTPs. FTIR bands that were observed are; N-H, C-H sp3, C-H aldehyde, N-H bend, C-N sp3 and N-H wag. From the 13C Nuclear Magnetic Resonance (NMR) results, the carbon atom of guanidinium iodide precursor shifts from downfield to upfield position, e.g. from 110.57 ppm to 38.49 ppm in GASnI3 SOCTP. This confirms a shift upfield of the carbon atom in guanidinium iodide precursor as it bonded to Sn metal in the perovskite chemical structure. Similar behavior was also observed for the NMR spectra of [GA][CH3CH2NH3]SnI3 MOCTP, where C-2 and C-3 atoms of ethylammonium iodide precursor shifted upfield from 37.03 ppm to 15.69 ppm and 16.06 ppm to 14.39 ppm respectively.

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.

In the last 10 years, the research interest has been drawn towards the hybrid organic-inorganic halide perovskites, an innovative material characterized by remarkable optoelectronic properties and by its simplicity of fabrication; hybrid halide perovskites are currently being employed as active material in solar cells, X-ray photodetectors and light emitting devices. The following thesis presents the characterization of two perovskite-based materials. The first is a methylammonium lead iodide (MAPbI3) thin film solar cell, which has been fabricated and characterized at the University of Konstanz (Germany), with the aim to optimize the deposition procedure. The second material is a methylammonium lead bromide (MAPbBr3) single crystal that have been characterized at the University of Bologna with surface photovoltage and photocurrent spectroscopies, as a function of the deposited dose of X-rays in order to monitor the induced effects of radiation. After the exposure to X-rays, the exciton binding energy, calculated from the surface photovoltage spectra, has been found to increase by 20 meV with respect to the not irradiated sample. A similar result has been found with the photocurrent spectroscopy. The reasons for the increase in binding energy is discussed and attributed to a change in polarizability of the single crystal. The recovery of the crystals has been registered as well and has shown that the material is able to return to the initial condition after just few hours from the last X-ray's deposition.

Dye-sensitized solar cells (DSSCs) and perovskite solar cells are emerging as promising potential low-cost alternatives to established crystalline silicon photovoltaics. Of the employed functional materials, however, many fundamental optoelectronic properties governing photovoltaic device operation are not sufficiently well understood. This thesis reports on a series of studies using ultrafast THz and photoluminescence spectroscopy on two classes of such materials, providing insight into the dynamics of charge-transport and recombination processes following photoexcitation. For TiO₂-nanotubes, which have been proposed as easy-to-fabricate electron transporters for DSSCs, fast, shallow electron trapping is identified as a limiting factor for efficient charge collection. Trapping lifetimes are found to be about an order of magnitude shorter than in the prevalently employed sintered nanoparticles under similar excitation conditions and trap saturation effects are not observed, even at very high excitation densities. In organo-lead halide perovskites - specifically CH₃NH₃PbI₃ and CH₃NH₃PbI_3-xCl_x, which have only recently emerged as highly efficient absorbers and charge transporters for thin-film solar cells, carrier mobilities and fundamental recombination dynamics are revealed. Extremely low bi-molecular recombination rates at least four orders of magnitude below the prediction of Langevin's model are found as well as relatively high charge-carrier mobilities in comparison to other solution-processable materials. Furthermore a very low influence of trap-mediated recombination channels was observed. Due to a combination of these factors, diffusion lengths reach hundreds of nanometres for CH₃NH₃PbI₃ and several microns for CH₃NH₃PbI_3-xCl_x. These results are shown to hold for both, solution processed and vapour-deposited CH₃NH₃PbI_3-xCl_x and underline the superb suitability of the materials as absorbers in solar cells, even in planar heterojunction architectures. The THz-frequency spectrum of the conductivity of the investigated perovskites is consistent with Drude-like charge transport additionally exhibiting weak signatures of phonon coupling. These coupling effects are also reflected in the luminescence of CH₃NH₃PbI_3-xCl_x, where they are believed to be the cause of the observed homogeneous spectral broadening. Further photoluminescence measurements were performed at temperatures between 4 K and room temperature to study the nature of recombination pathways in the material.

To design an efficient solar energy conversion device, theoretical input is extremely important to provide the basic guideline for experimental scientists, to fabricate the most efficient, cheap, and stable device with less efforts. This desire can be made possible if computational scientist use a proper theoretical protocol, design an energy material, then the experimentalist will only invest weeks or months on the synthetic effort. This thesis highlights my recent efforts in this direction. Monoclinic BiVO4 is has been using as a photocatalyst due to its stability, cheap, easily synthesizable, narrow band gap and ideal VB (-6.80 eV vs vacuum) but inappropriate CB (-4.56 eV vs vacuum) edge position, responsible for its low efficiency. We have carried out a comprehensive experimental and periodic density functional theory (DFT) simulations of the pristine, Oxygen defective (Ov), Se doped monoclinic BiVO4 and heterojunction with Selenium (Se-BiVO4), to improve not only its CB edge position but photocatalytic and charge carrier properties. It is found that Ov (1% Oxygen vacancy) and mild doped BiVO4 (1 to 2% Se) are thermodynamically stable, have ideal band edges ~ -4.30 eV), band gaps (~1.96 eV), and small effective masses of electrons and holes. We have also investigated the contribution of Se to higher performance by effecting morphology, light absorption and charge transfer properties in heterojunction. Finally, it is found that Se makes a direct Z-scheme (band alignments) with BiVO4 where the photoexcited electron of BiVO4 recombine with the VB of Se, consequences electron-hole separation at Se and BiVO4, respectively, as a result, enhanced photocurrent is obtained. Theoretical study of β-TaON in the form of primitive unit cell, supercell and its N, Ta, and O terminated surfaces are carried out with the help of periodic DFT. Optical and electronic properties of all these different species are simulated, which predict TaON as the best candidate for photocatalytic water splitting contrast to their Ta2O5 and Ta3N5 counterparts. The calculated bandgap, valence band, and conduction band edge positions predict that β-TaON should be an efficient photoanodic material. The valence band is made up of N 2p orbitals with a minor contribution from O 2p, while the conduction band is made up of Ta 5d. Turning to thin films, the valence band maximum; VBM (−6.4 eV vs. vacuum) and the conduction band minimum; CBM (−3.3 eV vs. vacuum) of (010)-O terminated surface are respectively well below and above the redox potentials of water as required for photocatalysis. Charge carriers have smaller effective masses than in the (001)-N terminated film (VBM −5.8 and CBM −3.7 eV vs. vacuum). However, due to wide band gap (3.0 eV) of (010)-O terminated surface, it cannot absorb visible wavelengths. On the other hand, the (001)-N terminated TaON thin film has a smaller band gap in the visible region (2.1 eV) but the bands are not aligned to the redox potential of water. Possibly a mixed phase material would produce an efficient photoanode for solar water splitting, where one phase performs the oxidation and the other reduction. Computational study of an optically transparent, near-infrared-absorbing low energy gap conjugated polymer, donor−acceptor−donor (D-A-D) with promising attributes for photovoltaic application is reported herein. The D and A moiety on the polymeric backbone have been found to be responsible for tuning the band gap, optical gap, open circuit (Voc) and short-circuit current density (Jsc) in the polymers solar cells (PSC). Reduction in the band gap, high charge transformation, and enhanced visible light absorption in the D-A-D system is because of strong overlapping of molecular orbitals of D and A. In addition, the enhanced planarity and weak steric hindrance between adjacent units of D-A-D, resulted in red-shifting of its onset of absorption. Finally, PSC properties of the designed D-A-D was modeled in the bulk heterojunction solar cell, which gives theoretical Voc of about 1.02 eV. DFT study has been carried out to design a new All-Solid-State dye-sensitized solar cell (SDSC), by applying a donor-acceptor conjugated polymer instead of liquid electrolyte. The typical redox mediator (I1−/I3−) is replaced with a narrow band gap, hole transporting material (HTM). A unique “upstairs” like band energy diagram is created by packing N3 between HTM and TiO2. Our theoretical simulations prove that the proposed configuration will be highly efficient as the HOMO level of HTM is 1.19 eV above the HOMO of sanitizer (dye); providing an efficient pathway for charge transfer. High short-circuit current density and power conversion efficiency is promised from the strong overlapping of molecular orbitals of HTM and sensitizer. A low reorganization energy of 0.21 eV and exciton binding energy of 0.55 eV, confirm the high efficiency of HTM. Theoretical and experimental studies of a series of four porphyrin-furan dyads were designed and synthesized, having anchoring groups, either at meso-phenyl or pyrrole-β position of a zinc porphyrin based on donor–π–acceptor (D–π–A) approach. The porphyrin macrocycle acts as donor, furan hetero cycle acts as π-spacer and either cyanoacetic acid or malonic acid group acts as acceptor. Optical bandgap, natural bonding, and molecular bonding orbital (HOMO–LUMO) analysis confirm the high efficiency pyrrole-β substituted zinc porphyrins contrast to meso-phenyl dyads. DFT study of polypyrrole-TiO2 composites has been carried out to explore their optical, electronic and charge transfer properties for the development of an efficient photocatalyst. Titanium dioxide (Ti16O32) was interacted with a range of pyrrole (Py) oligomers to predict the optimum composition of nPy-TiO2 composite with suitable band structure for efficient photocatalytic properties. The study has revealed that Py-Ti16O32 composites have narrow band gap and better visible light absorption capability compared to individual constituents. A red-shifting in λmax, narrowing band gap, and strong intermolecular interaction energy (-41 to −72 kcal/mol) of nPy-Ti16O32 composites confirm the existence of strong covalent type interactions. Electron−hole transferring phenomena are simulated with natural bonding orbital analysis where Py oligomers found as donor and Ti16O32 as an acceptor in nPy-Ti16O32 composites. Sensitivity and selectivity of polypyrrole (PPy) towards NH3, CO2 and CO have been studied at DFT. PPy oligomers are used both, in the doped (PPy+) and neutral (PPy) form, for their sensing abilities to realize the best state for gas sensing. Interaction energies and amount of charges (NBO and Mulliken charge analysis) are simulated which reveal the sensing ability of PPy towards these gases. PPy, both in doped and neutral state, is more sensitive to NH3 compared to CO2 and CO. More interestingly, NH3 causes doping of PPy and de-doping of PPy+, providing evidence that PPy/PPy+ is an excellent sensor for NH3 gas. UV-vis and UV-vis-near-IR spectra of nPy, nPy+, and nPy/nPy+-X complexes demonstrate strong interaction of PPy/PPy+ with these atmospheric gases. The applications of graphene (GR) and its derivatives in the field of composite materials for solar energy conversion, energy storage, environment purification and biosensor applications have been reviewed. The vast coverage of advancements in environmental applications of GR-based materials for photocatalytic degradation of organic pollutants, gas sensing and removal of heavy metal ions is presented. Additionally, the presences of graphene composites in the bio-sensing field have been also discussed in this review.