Estudio de células solares flexibles de última generación: aplicaciones y limitaciones

El presente trabajo tiene como objetivo realizar un estado del arte de las células solares flexibles y un análisis de sus aplicaciones y limitaciones. El proyecto se divide en tres capítulos principales. En el capítulo 1, se realiza un estudio sobre la célula solar como dispositivo, historia de las células solares, tipos, estructura, parámetros. El capítulo 2, engloba todo lo referente a los distintos tipos de elementos que puede tener una célula flexible y a como estos influyen en su funcionamiento como dispositivo. En el capítulo 3, se realiza un estudio de células flexibles reales, concretamente analizando mediante simulaciones en MATLAB la densidad de corriente de 3 capas de contacto distintas, con datos proporcionados por el CIEMAT.The present work aims to provide a state of the art of flexible solar cells and an analysis of their applications and limitations. The project is divided into three main chapters. In chapter 1, a study is made on the solar cell as a device, history of solar cells, types, structure, parameters. Chapter 2 covers everything related to the different types of elements that a flexible cell can have and how they influence its performance as a device. In chapter 3, a study of real flexible cells is carried out, specifically analyzing by means of MATLAB simulations the current density of 3 different contact layers, with data provided by CIEMAT.Grado en Ingeniería en Electrónica y Automática Industria

Cu(In,Ga)Se2 based ultrathin solar cells: the pathway from lab rigid to large scale flexible technology

For the first time, the incorporation of interface passivation structures in ultrathin Cu(In,Ga)Se2 (CIGS) based solar cells is shown in a flexible lightweight stainless-steel substrate. The fabrication was based on an industry scalable lithography technique - nanoimprint lithography (NIL) - for a 15x15 cm2 dielectric layer patterning, needed to reduce optoelectronic losses at the rear interface. The nanopatterning schemes are usually developed by lithographic techniques or by processes with limited scalability and reproducibility (nanoparticle lift-off, spin-coating, etc). However, in this work the dielectric layer is patterned using NIL, a low cost, large area, high resolution, and high throughput technique. To assess the NIL performance, devices with a NIL nanopatterned dielectric layer are benchmarked against electron-beam lithography (EBL) patterning, using rigid substrates. Up to now, EBL is considered the most reliable technique for patterning laboratory samples. The device patterned by NIL shows similar light to power conversion efficiency average values compared to the EBL patterned device - 12.6 % vs 12.3 %, respectively - highlighting the NIL potential for application in the solar cell sector. Moreover, the impact of the lithographic processes, such as different etch by-products, in the rigid solar cells’ figures of merit were evaluated from an elemental point of view via X-ray Photoelectron Spectroscopy and electrically through a Solar Cell Capacitance Simulator (SCAPS) fitting procedure. After an optimised NIL process, the device on stainless-steel achieved an average power conversion efficiency value of 11.7 % - a slightly lower value than the one obtained for the rigid approach, due to additional challenges raised by processing and handling steel substrates, even though scanning transmission electron microscopy did not show any clear evidence of impurity diffusion towards the absorber. Notwithstanding, time-resolved photoluminescence results strongly suggested the presence of additional non-radiative recombination mechanisms in the stainless-steel absorber, which were not detected in the rigid solar cells, and are compatible with elemental diffusion from the substrate. Nevertheless, bending tests on the stainless-steel device demonstrated the mechanical stability of the CIGS-based device up to 500 bending cycles.This work was funded in part by the Fundação para a Ciência e a Tecnologia (FCT) under Grants 2020.04564.BD, IF/00133/2015, PD/BD/142780/2018, SFRH/BD/146776/2019, UIDB/04564/2020 and UIDP/04564/2020, 2020.07073.BD, as well as through the projects NovaCell (PTDC/CTMCTM/28075/2017), CASOLEM (028917) “Correlated Analysis of Inorganic Solar Cells in and outside an Electron Microscope”, and InovSolarCells (PTDC/FISMAC/29696/2017) co-funded by FCT and the ERDF through COMPETE2020. And by the European Union's Horizon 2020 research and innovation 15 programme under the grants agreements N°. 720887 (ARCIGS-M project) and grand agreement N°.715027 (Uniting PV). The Special Research Fund (BOF) of Hasselt University is also acknowledged. P. Salomé and P. A. Fernandes would like to acknowledge FCT for the support of the project FCT UIDB/04730/2020. This work was developed within the scope of the project i3N, UIDB/50025/2020 & UIDP/50025/2020, financed by national funds through the FCT/MEC. The authors also acknowledge the support of Carlos Calaza in the fabrication for the 200 mm Si point contact stamp.info:eu-repo/semantics/publishedVersio

Transparent Silver Nanowire Bottom Electrodes in Organic Solar Cells

Organic solar cells (OSCs) is an emerging photovoltaic technology that opens up new application areas where common inorganic techniques are not able to score. Some of those key features are flexibility, light weight, semitransparency, and low cost processing. The current industry-standard for the transparent electrode, indium tin oxide (ITO), cannot provide these properties because it is brittle and expensive. This thesis aims to investigate an alternative type of promising transparent electrode: silver nanowire (AgNW) networks. They exhibit similar or even better optical and electrical performance than ITO down to a sheet resistance of 12 Ohm/sq at 84% transmission (including the glass substrate). Furthermore, AgNWs are more flexible, solution-processable, and more cost-effective than ITO. However, two challenges occur during implementation as bottom electrode in OSCs. First, their inherently high roughness causes devices to shunt. Second, the AgNW network structure exhibits – in contrast to the continuous ITO – µm²-sized voids that have to be bridged electrically by the organic layers. In the first part of this thesis, solution-processed small molecule charge transport layers are investigated. In the case of hole transport layers (HTL), the host BF-DPB and the dopant NDP9 are investigated using tetrahydrofuran as a solvent. It is shown that BF-DPB is already doped by NDP9 in solution via the formation of a hybrid molecule complex. Solution-processed layers exhibit similar conductivities as compared to the reference deposition, which is thermal evaporation in high vacuum. The layers sufficiently smoothen the AgNW electrode such that DCV5T-Me:C60 organic solar cells with an efficiency up to 4.4% are obtained. Moreover, the influence of the square micrometer large network voids is investigated using HTLs of varying conductivity. As a result, a minimum conductivity of 1e−4 S/cm is needed to avoid macroscopic performance losses. Equivalent circuit simulations are performed to confirm these results. As a second planarization method, the AgNWs are buried in an insulating polymer that serves concurrently as flexible and ultrathin substrate. Out of three different polymers tested, the optical adhesive ’NOA63’ gives the best results. The roughness is strongly reduced from 30 nm down to (2 ± 1) nm. Two different OSC types are employed as testing devices with fully-flexible alumina encapsulation against moisture ingress. Maximum power conversion efficiencies of 5.0% and 5.6% are achieved with a fullerene-free cascade layer architecture and a DCV5T-Me:C60 OSC, respectively. To evaluate the applicability of these fully-flexible and encapsulated devices, degradation studies are performed under continuous illumination and a humid climate. Although employing the intrinsically stable DCV5T-Me:C60 stack design, within one day a fast degradation of the fully-flexible solar cells is observed. The degradation is attributed to AgNW electrode failure that results from photo-oxidation and -sulfurization, photo-migration, and electromigration. It is further shown that the cascade organic solar cell lacks intrinsic stability. In summary, efficient, fully-flexible, and encapsulated devices are shown. However, in terms of competitive OSCs, the low stability of AgNW electrodes is a challenge to be taken care of. In current research, this issue needs to be addressed more frequently.Organische Solarzellen (OSZ) sind ein junges Forschungsgebiet der Photovoltaik, welches neue Anwendungsgebiete erschließt, für die herkömmliche anorganische Solarzellen nicht einsetzbar sind. Einige der Haupteigenschaften sind Flexibilität, niedriges Gewicht, Teiltransparenz und geringe Herstellungskosten. Indiumzinnoxid (ITO), der aktuelle Industriestandard transparenter Elektrodentechnologie, ist nicht in der Lage, diese Eigenschaften zu gewährleisten. Dies liegt vor allem an der Brüchigkeit von ITO und der begrenzten Verfügbarkeit von Indium, welche mit einem hohen Preis einhergeht. Das Ziel dieser Dissertation ist die Integration einer alternativen und vielversprechenden Elektrodentechnologie: Netzwerke aus Silbernanodrähten (AgNWs). Mit einem Schichtwiderstand von 12 Ohm/sq bei einer Transmission von 84% (inklusive Glassubstrat) besitzen sie ähnliche oder sogar bessere optische und elektrische Eigenschaften als ITO. Des Weiteren sind AgNW-Elektroden flexibler und kostengünstiger als ITO und aus flüssiger Phase prozessierbar. Es gibt allerdings zwei Herausforderungen, welche die Integration als Grundelektrode in OSZ erschweren. Zum einen sind AgNW-Netzwerke sehr rauh, sodass organische Bauteile kurzgeschlossen werden. Zum anderen weisen AgNW-Elektroden, im Gegensatz zu einer vollflächigen ITO-Schicht, Lücken zwischen den einzelnen Drähten auf. Diese Lücken müssen von den organischen Schichten der OSZ elektrisch überbrückt werden. Im ersten Teil der Arbeit werden daher flüssigprozessierte Ladungsträgertransportschichten aus kleinen Molekülen untersucht, welche die AgNW-Elektroden glätten und die verhältnismäßig großen Lücken füllen sollen. Im Falle von Lochleitschichten (HTL) wird BF-DPB als Matrix und NDP9 als Dotand in Tetrahydrofuran gelöst und zur Anwendung gebracht. BF-DPB wird dabei schon in Lösung von NDP9 dotiert, wobei sich ein Hybridmolekülkomplex ausbildet. Die Leitfähigkeit der entstehenden Schichten ist ähnlich zu Referenzschichten, die durch thermisches Verdampfen im Hochvakuum hergestellt wurden. Die erhaltenen HTLs glätten die AgNW-Elektroden, sodass DCV5T-Me:C60-Solarzellen mit einer Effizienz von maximal 4.4% hergestellt werden können. Weiterhin wird der Einfluss der quadratmikrometergroßen Löcher auf die makroskopische Effizienz der Solarzelle in Abhängigkeit der HTL Leitfähigkeit untersucht. Um signifikante Effizienzverluste zu verhindern, muss der HTL eine minimale Leitfähigkeit von etwa 1e−4 S/cm aufweisen. Simulationen eines Ersatzschaltkreises bestätigen hierbei die experimentellen Ergebnisse. Im zweiten Teil der Arbeit wird eine Planarisierungsmethode untersucht, in welcher die AgNWs in nichtleitfähigen Polymeren eingebettet werden. Diese Polymere fungieren anschließend als flexibles Substrat. Der optische Kleber ”NOA63” erzielt hierbei die besten Ergebnisse. Die Rauheit der AgNW-Elektroden wird von etwa 30 nm auf 1 bis 3 nm stark reduziert. Anschließend werden diese AgNW-Elektroden in zwei unterschiedlichen OSZ Konfigurationen getestet und mit einer vollflexiblen Schicht aus Aluminiumoxid gegen Wasserdampfpermeation verkapselt. Somit können maximale Effizienzen von 5% mithilfe einer organischen Kaskadenstruktur und 5.6% mit DCV5T-Me:C60 OSZ erreicht werden. Um die Anwendbarkeit dieser vollflexiblen und verkapselten OSZ zu bewerten, werden Alterungsstudien unter konstanter Beleuchtung und feuchtem Klima durchgeführt. Es wird gezeigt, dass die in das Polymer eingebettete AgNW-Elektrode aufgrund von Photooxidation und -schwefelung und Photo- und Elektromigration instabil ist. Dieser Sachverhalt ist für die Anwendung von AgNW-Elektroden in kommerziellen OSZ von großer Bedeutung und wurde in der Forschung bisher nicht ausreichend thematisiert

n-Alkyl Methacrylate Polymeric Memristors for Synaptic Response Modeling: Organic and Biologically Relevant Thin Films

There is a strong interest in organic materials for electrical devices due to several advantages that organic systems have over their inorganic counterparts including ease of processability and lower toxicity. Many of these organic materials can be utilized in the creation of thin-film devices that can be formed in high-throughput processes and with a very small profile. One such device that has emerged in recent years is the memristor which can be used in new computational concept or as a synaptic model. This work studies the alternating current (AC) and direct current (DC) electrical response of a number n-alkyl methacrylate polymers with a charge transporting pendant carbazole ring. The electrical properties of the polymers were studied as a function of n-alkyl length with n ranging from 2 to 11. The DC current (I)-voltage (V) response of the polymers was characterized by an erratic and bistable response, while their AC I-V response was a pinched hysteresis loop when measured between 1-100 Hz. For polymers with n \u3c 9, their pinched hysteresis loop is characterized by jump transitions indicative of bistability, while polymers with n ≥ 9 had a pinched hysteresis loop that is smooth in appearance. Dielectric spectroscopy on the polymers indicates that as the n-alkyl length is increased, the rotation flexibility of the carbazole moiety is enhanced. The n-alkyl methacrylate polymers with a pendant carbazole ring spaced n ≥ 9 exhibited a lower activation energy and temperature for the onset of ring motion and resulted in polymer-based memristors that exhibit electrical characteristics, such as incrementally adjustable conductivity, that are potential candidates for mimicking synaptic plasticity. Further characterization was done on similar methacrylate systems with oxygen-substituted side chains and the addition of bulky phenyl groups to the carbazole moieties. From this work, the most promising candidate for synaptic modeling behavior was taken and further examined. It was shown that this polymer could be pulsed through a multitude of conductivity states and demonstrated behavior consistent with the Hebbian Learning Rule upon the application of pre- and post-synaptic pulses. The system was further characterized for the effects of different spike rates and voltages before being utilized in a flexible device. Other thin-film devices as well as novel processing methods were also demonstrated in this work including a biologically based reserve battery and a printed diode utilizing pentacene. The battery utilized standard alkaline chemistry where the zinc and manganese oxide electrodes are formed using stencil printing. Fish eggs are used to sequester the electrolyte out of the system until the application of force to the device. This application of force bursts the fish eggs and allows the battery to function by introducing the electrolyte into the system. A printed diode is also demonstrated through the use of a miniemulsion process that allows for the dispersion of the material into aqueous solution. This pentacene emulsion in water can then be used as the basis for the formation of diodes in a variety of fabrication processes

Development of novel coatings for dye-sensitized solar cell applications.

This research work was undertaken to solve an industrial problem related to roll-to- roll production of dye-sensitised solar cells (DSCs). It is possible to manufacture DSCs in a roll-to-roll production line on a sheet metal such as titanium. However, DSCs produced in such a way are not commercially viable due to the use of expensive titanium metal. Therefore, the intention behind this work was to utilize a cheap sheet metal such as ECCS (electro chrome coated steel) to manufacture DSCs in a roll-to-roll production facility of TATA steel Europe, as this project was funded by them. Unfortunately, ECCS corrodes in the I[-]/I[3-] redox electrolyte present in a DSC therefore, to protect ECCS from the corrosion whilst using it as a DSC substrate was the real challenging task in this research. In order to solve this problem high temperature resistant polyimide based coatings were developed which can be used to coat ECCS substrates whilst maintaining excellent dimensional stability at the DSC processing temperatures. Such coatings were electrically conducting which helped preserve the electrical conductivity of the underlying metallic substrate. Electrically conductive polyimides were developed by simply blending conductive fillers such as carbon materials and titanium nitride. It was initially thought that carbon/polyimide based coatings would be suitable for this application. However, severe interfacial charge recombination and poor reflectivity made carbon/PI coatings inferior compared to the TiN/PI coatings. TiN/PI coatings performed well but poor reflectivity produced low current outputs. Moreover, TiN/PI was found to reduce the catalytic activity of thermally deposited platinum therefore it was not useful as a counter electrode material. As a solution to these problems, TiN and carbon materials based hybrid coatings were developed. Hybrid coatings did perform efficiently in terms of overall PV performance but due to poor reflectivity, such coatings also produced low J[sc] values. However, counter electrodes prepared using hybrid coating demonstrated excellent PV performance with thermally deposited platinum. Furthermore, TCO (transparent conducting oxide) free glass substrates can also be used to manufacture low-cost PV devices when coated with these conductive coatings

Electrical Switching and Memory Effects in Electroactive Polymers Containing Electron-Donor and -Acceptor Moieties

Ph.DDOCTOR OF PHILOSOPH

Eurodisplay 2019

The collection includes abstracts of reports selected by the program by the conference committee

Laser Patterned N-doped Carbon: Preparation, Functionalization and Selective Chemical Sensors

Die kürzliche globale COVID-19-Pandemie hat deutlich gezeigt, dass hohe medizinische Kosten eine große Herausforderung für unser Gesundheitssystem darstellen. Daher besteht eine wachsende Nachfrage nach personalisierten tragbaren Geräten zur kontinuierlichen Überwachung des Gesundheitszustands von Menschen durch nicht-invasive Erfassung physiologischer Signale. Diese Dissertation fasst die Forschung zur Laserkarbonisierung als Werkzeug für die Synthese flexibler Gassensoren zusammen und präsentiert die Arbeit in vier Teilen. Der erste Teil stellt ein integriertes zweistufiges Verfahren zur Herstellung von laserstrukturiertem (Stickstoff-dotiertem) Kohlenstoff (LP-NC) ausgehend von molekularen Vorstufen vor. Der zweite Teil demonstriert die Herstellung eines flexiblen Sensors für die Kohlendioxid Erfassung basierend auf der Laserumwandlung einer Adenin-basierten Primärtinte. Die unidirektionale Energieeinwirkung kombiniert mit der tiefenabhängigen Abschwächung des Laserstrahls ergibt eine neuartige geschichtete Sensorheterostruktur mit porösen Transducer- und aktiven Sensorschichten. Dieser auf molekularen Vorläufern basierende Laserkarbonisierungsprozess ermöglicht eine selektive Modifikation der Eigenschaften von gedruckten Kohlenstoffmaterialien. Im dritten Teil wird gezeigt, dass die Imprägnierung von LP-NC mit Molybdäncarbid Nanopartikeln die Ladungsträgerdichte verändert, was wiederum die Empfindlichkeit von LP-NC gegenüber gasförmigen Analyten erhöht. Der letzte Teil erläutert, dass die Leitfähigkeit und die Oberflächeneigenschaften von LP-NC verändert werden können, indem der Originaltinte unterschiedliche Konzentrationen von Zinknitrat zugesetzt werden, um die selektiven Elemente des Sensormaterials zu verändern. Basierend auf diesen Faktoren zeigte die hergestellte LP-NC-basierte Sensorplattform in dieser Studie eine hohe Empfindlichkeit und Selektivität für verschiedene flüchtige organische Verbindungen.The recent global COVID-19 pandemic clearly displayed that the high costs of medical care on top of an aging population bring great challenges to our health systems. As a result, the demand for personalized wearable devices to continuously monitor the health status of individuals by non-invasive detection of physiological signals, thereby providing sufficient information for health monitoring and even preliminary medical diagnosis, is growing. This dissertation summarizes my research on laser-carbonization as a tool for the synthesis of functional materials for flexible gas sensors. The whole work is divided into four parts. The first part presents an integrated two-step approach starting from molecular precursor to prepare laser-patterned (nitrogen-doped) carbon (LP-NC). The second part shows the fabrication of a flexible LP-NC sensor architecture for room-temperature sensing of carbon dioxide via laser conversion of an adenine-based primary ink. By the unidirectional energy impact in conjunction with depth-dependent attenuation of the laser beam, a novel layered sensor heterostructure with a porous transducer and an active sensor layer is formed. This molecular precursor-based laser carbonization method enables the modification of printed carbon materials. In the third part, it is shown that impregnation of LP-NC with molybdenum carbide nanoparticle alters the charge carrier density, which, in turn, increases the sensitivity of LP-NC towards gaseous analytes. The last part explains that the electrical conductivity and surface properties of LP-NC can be modified by adding different concentrations of zinc nitrate into the primary ink to add selectivity elements to the sensor materials. Based on these factors, the LP-NC-based sensor platforms prepared in this study exhibited high sensitivity and selectivity for different volatile organic compounds

Ultra thin films of Oligoimide, polyimide and polyimide composites on amine-terminated substrates through covalent molecular assembly

Ph.DDOCTOR OF PHILOSOPH

Gallium nitride on low temperature cofired ceramic templates for Schottky junctions

In this work aluminum, silicon and zinc oxide were used as intermediate layers for thin film growth on cofired glass ceramic substrates. The motivation behind this work is a direct deposition of nitride thin films on the surface of the ceramic substrate, eliminating the die and attach techniques. Ceramics have unique applications due too the nature of their mechanical processing, and their physical resilience and chemical inertness. The low melting of the glass ceramics from a device processing perspective and their rough, inhomogeneous surface presents a challenge for device fabrication. Oxide materials can be applied by a variety of techniques compatible with large device areas and arbitrary shapes to apply a surface texture to improve thin film properties for device fabrication. Ideally these techniques could be applied to any substrate that meets the thermal budget of the thin film process. Solution coating was found to be a good candidate for applying coatings since it can deposit many different oxide materials over large areas, for relatively low cost, and surface tension of the liquid phase helps to planarize the surface. Several (>7) microns of coating materials were found to be needed to reduce the appearance of the ceramic surface features. Deposition of GaN on the surface of the oxide coatings was performed using a Flow Modulation Epitaxy (FME) style deposition in conjunction with a unique hollow cathode plasma source. These features are designed to lower the overall temperature requirements for GaN growth by providing additional Ga migration time during growth and by using nitrogen plasma as an alternative to thermal decomposition of ammonia. Ni/Au Schottky junctions fabricated on sapphire using ceramic compatible temperatures and FME show leaky characteristics with high ideality factors, indicating tunneling is a significant contributor to carrier transport through the junction. The same Ni/Au GaN devices fabricated on ZnO coated ceramics was found to produce ohmic junctions. The density of surface states is a likely candidate for this behaviour