Obtenção e caracterização de filmes SnO2 depositados em vidro borosilicato por silk-screen modificado: SnCl2.2H2O como precursor

O presente trabalho tem como objetivo investigar as características de filmes de SnO2 depositados em substrato de vidro borosilicato por um processo de silk-screen modificado para obtenção de espessura fina compatível com a aplicação em células solares policristalinas de baixo custo. O filme de SnO2 é um dos mais apropriados para obtenção de vidro TCO (transparent conductive oxide) para uso em células solares devido a sua baixa resistividade elétrica e alta transmitância, sendo quimicamente inerte, mecanicamente duro e tem resistência a altas temperaturas, o que facilita então a calcinação das amostras entre 500º C a 550º C. Os filmes foram obtidos a partir de uma solução precursora básica, preparada pela dissolução de SnCl2.2H2O em Etanol (99,5 %). Foi realizado um planejamento fatorial 2(3-1) para analisar a influência dos parâmetros concentração da solução precursora (CETN), temperatura de calcinação (TC) e taxa de aquecimento (tX) na calcinação, sendo a concentração CETN o parâmetro que apresentou maior efeito sobre os parâmetros de respostas investigados: espessura do filme (ω), resistividade de superfície (ρ) e a transmitância relativa (θ). Foi possível obter com a metodologia utilizada, filmes com espessuras da ordem de 1 Nm com resistividade de superfície de 10 / e transmitância relativa entre 70 e 80 %.

Deposition of controlled thickness ultrathin SnO2 : Sb films by spin-coating

The technological interest in transparent conductive oxide films (TCOs) has motivated several works in processing techniques, in order to obtain adequate routes to application. In this way, this work describes a new route to obtain antimony-doped tin oxide (ATO) films, based in colloidal dispersions of oxide nanocrystals. The nanoparticles were obtained by a hydrolisis method, using SnCl2 and SbCl3 in ethanolic solutions. The residual halides were removed by dyalisis, obtaining a limpid and transparent colloidal suspension. By this, the method offers the advantage of producing ultrathin films without organic contaminants. This route was employed to produce films with 5, 10, 14, and 18 mol% Sb doping, with thickness ranging from 40 to 70 nm. The physical characterization of the samples showed a uniform layer deposition, resulting in good packing density and high transmittance. A preliminar electrical study confirmed the low electrical resistivity even in the ultrathin films, in such level similar of reported data. The method described is similar in some aspects to layer-by-layer (LbL) techniques, allowing fine control of thickness and interesting properties for ultrathin films, however, with low cost when compared to similar routes.

Interconexión monolítica de dispositivos fotovoltaicos basados en a-Si mediante ablación directa con fuentes láser ultravioleta

Resumen En la última década la tecnología láser se ha convertido en una herramienta imprescindible en la fabricación de dispositivos fotovoltaicos, muy especial¬mente en aquellos basados en tecnología de lámina delgada. Independiente¬mente de crisis coyunturales en el sector, la evolución en los próximos años de estas tecnologías seguirá aprovechándose de la flexibilidad y calidad de proceso de la herramienta láser para la consecución de los dos objetivos básicos que harán de la fotovoltaica una opción energética económicamente viable: la reducción de costes de fabricación y el aumento de eficiencia de los dispositivos. Dentro de las tecnologías fotovoltaicas de lámina delgada, la tecnología de dispositivos basados en silicio amorfo ha tenido un gran desarrollo en sistemas estándar en configuración de superestrato, pero su limitada efi¬ciencia hace que su supervivencia futura pase por el desarrollo de formatos en configuración de substrato sobre materiales flexibles de bajo coste. En esta aproximación, las soluciones industriales basadas en láser actualmente disponibles para la interconexión monolítica de dispositivos no son aplica¬bles, y desde hace años se viene investigando en la búsqueda de soluciones apropiadas para el desarrollo de dichos procesos de interconexión de forma que sean transferibles a la industria. En este contexto, esta Tesis propone una aproximación completamente orig¬inal, demostrando la posibilidad de ejecutar una interconexión completa de estos dispositivos irradiando por el lado de la lámina (es decir de forma com¬patible con la opción de configuración de substrato y, valga la redundancia, con el substrato del dispositivo opaco), y con fuentes láser emitiendo en UV. Este resultado, obtenido por primera vez a nivel internacional con este trabajo, aporta un conocimiento revelador del verdadero potencial de estas fuentes en el desarrollo industrial futuro de estas tecnologías. Si bien muy posiblemente la solución industrial final requiera de una solución mixta con el empleo de fuentes en UV y, posiblemente, en otras longitudes de onda, esta Tesis y su planteamiento novedoso aportan un conocimiento de gran valor a la comunidad internacional por la originalidad del planteamiento seguido, los resultados parciales encontrados en su desarrollo (un número importante de los cuales han aparecido en revistas del JCR que recogen en la actualidad un número muy significativo de citas) y porque saca además a la luz, con las consideraciones físicas pertinentes, las limitaciones intrínsecas que el desarrollo de procesos de ablación directa selectiva con láseres UV en parte de los materiales utilizados presenta en el rango temporal de in¬teracción de ns y ps. En este trabajo se han desarrollado y optimizado los tres pasos estándar de interconexión (los habitualmente denominados Pl, P2 y P3 en la industria fotovoltaica) demostrando las ventajas y limitaciones del uso de fuentes en UV tanto con ancho temporal de ns como de ps. En particular destaca, por el éxito en los resultados obtenidos, el estudio de procesos de ablación selectiva de óxidos conductores transparentes (en este trabajo utilizados tanto como contacto frontal así como posterior en los módulos) que ha generado resultados, de excelente acogida científica a nivel internacional, cuya aplicación trasciende el ámbito de las tecnologías de silicio amorfo en lámina delgada. Además en este trabajo de Tesis, en el desarrollo del objetivo citado, se han puesto a punto técnicas de análisis de los procesos láser, basadas en métodos avanzados de caracterización de materiales (como el uso combi¬nado de la espectroscopia dispersiva de rayos X y la microscopía confocal de barrido) que se presentan como auténticos avances en el desarrollo de técnicas específicas de caracterización para el estudio de los procesos con láser de ablación selectiva de materiales en lámina delgada, procesos que no solo tienen impacto en el ámbito de la fotovoltaica, sino también en la microelectrónica, la biotecnología, la microfabricación, etc. Como resultado adicional, parte de los resultados de este trabajo, han sido aplicados exi¬tosamente por el grupo de investigaci´on en la que la autora desarrolla su labor para conseguir desarrollar procesos de enorme inter´es en otras tec-nolog´ıas fotovoltaicas, como las tecnolog´ıas est´andar de silicio amorfo sobre vidrio en configuraci´on de superestrato o el procesado de capas delgadas en tecnolog´ıas convencionales de silicio cristalino. Por u´ltimo decir que este trabajo ha sido posible por una colaboraci´on muy estrecha entre el Centro L´aser de la UPM, en el que la autora de¬sarrolla su labor, y el Grupo de Silicio Depositado del Centro de Inves¬tigaciones Energ´eticas, Medioambientales y Tecnol´ogicas, CIEMAT, que, junto al Grupo de Energ´ıa Fotovoltaica de la Universidad de Barcelona, han preparado la mayor parte de las muestras utilizadas en este estudio. Dichas colaboraciones se han desarrollado en el marco de varios proyectos de investigaci´on aplicada con subvenci´on pu´blica, tales como el proyecto singular estrat´egico PSE-MICROSIL08 (PSE-120000-2006-6), el proyecto INNDISOL (IPT-420000-2010-6), ambos financiados porel Fondo Europeo de Desarrollo Regional FEDER (UE) ”Una manera de hacer Europa y el MICINN, y los proyectos de Plan Nacional AMIC (ENE2010-21384-C04-´ 02) y CLASICO (ENE2007-6772-C04-04), cuya financiaci´on ha permitido en gran parte llevar a t´ermino este trabajo Abstract In the last decade, the laser technology has turned into an indispensable tool in the production of photovoltaic devices, especially of those based on thin film technology. Regardless the current crisis in the sector, the evolution of these technologies in the upcoming years will keep taking advantage of the flexibility and process quality of the laser tool for the accomplishment of the two basic goals that will convert the photovoltaic energy into economically viable: the manufacture cost reduction and the increase in the efficiency of the devices. Amongst the thin film laser technologies, the technology of devices based on amorphous silicon has had a great development in standard systems of superstrate configuration, but its limited efficiency makes its survival de¬pendant on the development of formats in substrate configuration with low cost flexible materials. In this approach, the laser industrial solutions cur¬rently available for the monolithic interconnection are not applicable, and in the last few years the investigations have been focused on the search of appropriate solutions for the development of such interconnection processes in a way that the same are transferable to the industry. In this context, this Thesis proposes a totally original approach, proving the possibility of executing a full interconnection of these devices by means of irradiation from the film side, i.e., compatible with the substrate con¬figuration, and with UV laser sources. This result, obtained for the first time at international level in this work, provides a revealing knowledge of the true potential of these sources in the future industrial development of these technologies. Even though very probably the final industrial solution will require a combination of the use of UV sources along with other wave¬lengths, this Thesis and its novel approach contribute with a high value to the international community because of the originality of the approach, the partial results found throughout its development (out of which, a large number has appeared in JCR journals that currently accumulate a signifi¬cant number of citations) and brings to light, with the pertinent scientific considerations, the intrinsic limitations that the selective direct ablation processes with UV laser present in the temporal range of interaction of ns and ps for part of the materials used in this study. More particularly, the three standard steps of interconnection (usually de¬nominated P1, P2 and P3 in the photovoltaic industry) have been developed and optimized, showing the advantages as well as the limitations of the use of UV sources in both the ns and ps pulse-width ranges. It is highly remark¬able, because of the success in the obtained results, the study of selective ablation processes in transparent conductive oxide (in this work used as a front and back contact), that has generated results, of excellent interna¬tional scientific reception, whose applications go beyond the scope of thin film photovoltaic technologies based on amorphous silicon. Moreover, in this Thesis, with the development of the mentioned goal, differ¬ent techniques of analysis of laser processes have been fine-tuned, basing the same in advanced methods for material characterization (like the combined use of EDX Analysis and Confocal Laser Scanning Microscopy) that can be presented as true breakthroughs in the development of specific techniques for characterization in the study of laser processes of selective ablation of materials in thin film technologies, processes that not only have impact in the photovoltaic field, but also in those of microelectronics, biotechnology, micro-fabrication, etc. As an additional outcome, part of the results of this work has been suc¬cessfully applied, by the investigation group to which the author belongs, to the development of processes of enormous interest within other photo¬voltaic technologies, such as the standard technologies on amorphous silicon over glass in superstrate configuration or the processing of thin layers in conventional technologies using crystalline silicon. Lastly, it is important to mention that this work has been possible thanks to the close cooperation between the Centro L´aser of the UPM, in which the author develops her work, and the Grupo de Silicio Depositado of Centro de Investigaciones Energ´eticas, Medioambientales y Tecnol´ogicas, CIEMAT, which, along with the Grupo de Energ´ıa Fotovoltaica of Univer¬sidad de Barcelona, has prepared the largest part of the samples utilized in this study. Such collaborations have been carried out in the context of several projects of applied investigation with public funding, like Proyecto Singular Estrat´egico PSE-MICROSIL08 (PSE-120000-2006-6), Proyecto IN-NDISOL (IPT-420000-2010-6), both funded by the European Regional De¬velopment Fund (ERDF), ”Una manera de hacer Europa” and MICINN, and the projects of Plan Nacional AMIC (ENE2010-21384-C04-02) and ´ CLASICO (ENE2007-6772-C04-04), whose funds have enabled the devel-opment of large part of this work.

New strategies in laser processing of TCOs for light management improvement in thin-film silicon solar cells

Light confinement strategies play a crucial role in the performance of thin-film (TF) silicon solar cells. One way to reduce the optical losses is the texturing of the transparent conductive oxide (TCO) that acts as the front contact. Other losses arise from the mismatch between the incident light spectrum and the spectral properties of the absorbent material that imply that low energy photons (below the bandgap value) are not absorbed, and therefore can not generate photocurrent. Up-conversion techniques, in which two sub-bandgap photons are combined to give one photon with a better matching with the bandgap, were proposed to overcome this problem. In particular, this work studies two strategies to improve light management in thin film silicon solar cells using laser technology. The first one addresses the problem of TCO surface texturing using fully commercial fast and ultrafast solid state laser sources. Aluminum doped Zinc Oxide (AZO) samples were laser processed and the results were optically evaluated by measuring the haze factor of the treated samples. As a second strategy, laser annealing experiments of TCOs doped with rare earth ions are presented as a potential process to produce layers with up-conversion properties, opening the possibility of its potential use in high efficiency solar cells.

Highly conductive and transparent ZnO thin film using Chemical Spray Pyrolysis technique: Effect of doping and deposition parameters

In the present work we report the preparation details studies on ZnO thin films. ZnO thin films are prepared using cost effective deposition technique viz., Chemical Spray Pyrolysis (CSP). The method is very effective for large area preparation of the ZnO thin film. A new post-deposition process could also be developed to avoid the adsorption of oxygen that usually occurs after the spraying process i.e., while cooling. Studies were done by changing the various deposition parameters for optimizing the properties of ZnO thin film. Moreover, different methods of doping using various elements are also tried to enhance the conductivity and transparency of the film to make these suitable for various optoelectronic applications.

Electrical and optical properties of Zn-In-Sn-O transparent conducting thin films

Indium tin oxide (ITO) is one of the widely used transparent conductive oxides (TCO) for application as transparent electrode in thin film silicon solar cells or thin film transistors owing to its low resistivity and high transparency. Nevertheless, indium is a scarce and expensive element and ITO films require high deposition temperature to achieve good electrical and optical properties. On the other hand, although not competing as ITO, doped Zinc Oxide (ZnO) is a promising and cheaper alternative. Therefore, our strategy has been to deposit ITO and ZnO multicomponent thin films at room temperature by radiofrequency (RF) magnetron co-sputtering in order to achieve TCOs with reduced indium content. Thin films of the quaternary system Zn-In-Sn-O (ZITO) with improved electrical and optical properties have been achieved. The samples were deposited by applying different RF powers to ZnO target while keeping a constant RF power to ITO target. This led to ZITO films with zinc content ratio varying between 0 and 67%. The optical, electrical and morphological properties have been thoroughly studied. The film composition was analysed by X-ray Photoelectron Spectroscopy. The films with 17% zinc content ratio showed the lowest resistivity (6.6 × 10 - 4 Ω cm) and the highest transmittance (above 80% in the visible range). Though X-ray Diffraction studies showed amorphous nature for the films, using High Resolution Transmission Electron Microscopy we found that the microstructure of the films consisted of nanometric crystals embedded in a compact amorphous matrix. The effect of post deposition annealing on the films in both reducing and oxidizing atmospheres were studied. The changes were found to strongly depend on the zinc content ratio in the films.

Oxide Semiconductors For Organic Opto-electronic Devices

Development of transparent oxide semiconductors (TOS) from Earth-abundant materials is of great interest for cost-effective thin film device applications, such as solar cells, light emitting diodes (LEDs), touch-sensitive displays, electronic paper, and transparent thin film transistors. The need of inexpensive or high performance electrode might be even greater for organic photovoltaic (OPV), with the goal to harvest renewable energy with inexpensive, lightweight, and cost competitive materials. The natural abundance of zinc and the wide bandgap ($sim$3.3 eV) of its oxide make it an ideal candidate. In this dissertation, I have introduced various concepts on the modulations of various surface, interface and bulk opto-electronic properties of ZnO based semiconductor for charge transport, charge selectivity and optimal device performance. I have categorized transparent semiconductors into two sub groups depending upon their role in a device. Electrodes, usually 200 to 500 nm thick, optimized for good transparency and transporting the charges to the external circuit. Here, the electrical conductivity in parallel direction to thin film, i.e bulk conductivity is important. And contacts, usually 5 to 50 nm thick, are optimized in case of solar cells for providing charge selectivity and asymmetry to manipulate the built in field inside the device for charge separation and collection. Whereas in Organic LEDs (OLEDs), contacts provide optimum energy level alignment at organic oxide interface for improved charge injections. For an optimal solar cell performance, transparent electrodes are designed with maximum transparency in the region of interest to maximize the light to pass through to the absorber layer for photo-generation, plus they are designed for minimum sheet resistance for efficient charge collection and transport. As such there is need for material with high conductivity and transparency. Doping ZnO with some common elements such as B, Al, Ga, In, Ge, Si, and F result in n-type doping with increase in carriers resulting in high conductivity electrode, with better or comparable opto-electronic properties compared to current industry-standard indium tin oxide (ITO). Furthermore, improvement in mobility due to improvement on crystallographic structure also provide alternative path for high conductivity ZnO TCOs. Implementing these two aspects, various studies were done on gallium doped zinc oxide (GZO) transparent electrode, a very promising indium free electrode. The dynamics of the superimposed RF and DC power sputtering was utilized to improve the microstructure during the thin films growth, resulting in GZO electrode with conductivity greater than 4000 S/cm and transparency greater than 90 %. Similarly, various studies on research and development of Indium Zinc Tin Oxide and Indium Zinc Oxide thin films which can be applied to flexible substrates for next generation solar cells application is presented. In these new TCO systems, understanding the role of crystallographic structure ranging from poly-crystalline to amorphous phase and the influence on the charge transport and optical transparency as well as important surface passivation and surface charge transport properties. Implementation of these electrode based on ZnO on opto-electronics devices such as OLED and OPV is complicated due to chemical interaction over time with the organic layer or with ambient. The problem of inefficient charge collection/injection due to poor understanding of interface and/or bulk property of oxide electrode exists at several oxide-organic interfaces. The surface conductivity, the work function, the formation of dipoles and the band-bending at the interfacial sites can positively or negatively impact the device performance. Detailed characterization of the surface composition both before and after various chemicals treatment of various oxide electrode can therefore provide insight into optimization of device performance. Some of the work related to controlling the interfacial chemistry associated with charge transport of transparent electrodes are discussed. Thus, the role of various pre-treatment on poly-crystalline GZO electrode and amorphous indium zinc oxide (IZO) electrode is compared and contrasted. From the study, we have found that removal of defects and self passivating defects caused by accumulation of hydroxides in the surface of both poly-crystalline GZO and amorphous IZO, are critical for improving the surface conductivity and charge transport. Further insight on how these insulating and self-passivating defects cause charge accumulation and recombination in an device is discussed. With recent rapid development of bulk-heterojunction organic photovoltaics active materials, devices employing ZnO and ZnO based electrode provide air stable and cost-competitive alternatives to traditional inorganic photovoltaics. The organic light emitting diodes (OLEDs) have already been commercialized, thus to follow in the footsteps of this technology, OPV devices need further improvement in power conversion efficiency and stable materials resulting in long device lifetimes. Use of low work function metals such as Ca/Al in standard geometry do provide good electrode for electron collection, but serious problems using low work-function metal electrodes originates from the formation of non-conductive metal oxide due to oxidation resulting in rapid device failure. Hence, using low work-function, air stable, conductive metal oxides such as ZnO as electrons collecting electrode and high work-function, air stable metals such as silver for harvesting holes, has been on the rise. Devices with degenerately doped ZnO functioning as transparent conductive electrode, or as charge selective layer in a polymer/fullerene based heterojunction, present useful device structures for investigating the functional mechanisms within OPV devices and a possible pathway towards improved air-stable high efficiency devices. Furthermore, analysis of the physical properties of the ZnO layers with varying thickness, crystallographic structure, surface chemistry and grain size deposited via various techniques such as atomic layer deposition, sputtering and solution-processed ZnO with their respective OPV device performance is discussed. We find similarity and differences in electrode property for good charge injection in OLEDs and good charge collection in OPV devices very insightful in understanding physics behind device failures and successes. In general, self-passivating surface of amorphous TCOs IZO, ZTO and IZTO forms insulating layer that hinders the charge collection. Similarly, we find modulation of the carrier concentration and the mobility in electron transport layer, namely zinc oxide thin films, very important for optimizing device performance.

Enhanced zinc oxide and graphene nanostructures for electronics and sensing applications

Zinc oxide and graphene nanostructures are important technological materials because of their unique properties and potential applications in future generation of electronic and sensing devices. This dissertation investigates a brief account of the strategies to grow zinc oxide nanostructures (thin film and nanowire) and graphene, and their applications as enhanced field effect transistors, chemical sensors and transparent flexible electrodes. Nanostructured zinc oxide (ZnO) and low-gallium doped zinc oxide (GZO) thin films were synthesized by a magnetron sputtering process. Zinc oxide nanowires (ZNWs) were grown by a chemical vapor deposition method. Field effect transistors (FETs) of ZnO and GZO thin films and ZNWs were fabricated by standard photo and electron beam lithography processes. Electrical characteristics of these devices were investigated by nondestructive surface cleaning, ultraviolet irradiation treatment at high temperature and under vacuum. GZO thin film transistors showed a mobility of ∼5.7 cm2/V·s at low operation voltage of <5 V and a low turn-on voltage of ∼0.5 V with a sub threshold swing of ∼85 mV/decade. Bottom gated FET fabricated from ZNWs exhibit a very high on-to-off ratio (∼106) and mobility (∼28 cm2/V·s). A bottom gated FET showed large hysteresis of ∼5.0 to 8.0 V which was significantly reduced to ∼1.0 V by the surface treatment process. The results demonstrate charge transport in ZnO nanostructures strongly depends on its surface environmental conditions and can be explained by formation of depletion layer at the surface by various surface states. A nitric oxide (NO) gas sensor using single ZNW, functionalized with Cr nanoparticles was developed. The sensor exhibited average sensitivity of ∼46% and a minimum detection limit of ∼1.5 ppm for NO gas. The sensor also is selective towards NO gas as demonstrated by a cross sensitivity test with N2, CO and CO2 gases. Graphene film on copper foil was synthesized by chemical vapor deposition method. A hot press lamination process was developed for transferring graphene film to flexible polymer substrate. The graphene/polymer film exhibited a high quality, flexible transparent conductive structure with unique electrical-mechanical properties; ∼88.80% light transmittance and ∼1.1742Ω/sq k sheet resistance. The application of a graphene/polymer film as a flexible and transparent electrode for field emission displays was demonstrated.

Printed electronics for ubiquitous computing applications

Dissertação para obtenção do Grau de Doutor em Química

ELETRODOS DE FTO MODIFICADOS POR ELETRODEPOSIÇÃO DIRETA DE OURO: PRODUÇÃO, CARACTERIZAÇÃO E APLICAÇÃO COMO SENSOR ELETROQUÍMICO

Chemically modified electrodes have been studied to obtain new and better electrochemical sensors. Transparent conductive oxides, such as fluorine-doped tin-oxide (FTO), shows electrical conductivity comparable to metals and are potential candidates for new sensors. In this work, FTO was modified by gold electrodeposition from chlorine-auric acid solution using cyclic voltammetry (CV) technique. A set of different materials were produced, varying the scan number. Scanning electron microscopy and electrochemical impedance spectroscopy were performed for the characterization of electrodes surfaces. From this analysis was possible to observe the resistive, capacitive and difusional aspects from all kind of modified electrodes produced, establishing a relationship between this parameters and the scan number. The electrode with 100 scans of CV presented better characteristics for an electrochemical sensor; it has the lowest global impedance and rising of capacitive behavior (related to electrical double layer formation) at lower frequencies. This electrode was tested for paracetamol and caffeine detection. The results showed a high specificity, decreased oxidation potential (0.58 V and 0.97 Vvs. SCE, for paracetamol and caffeine, respectively) and low detection limits (0.82 and 0.052 µmol L-1).

Development of p-type transparent semiconducting oxides for thin film transistor applications

Semiconductor physics has developed significantly in the field of re- search and industry in the past few decades due to it’s numerous practical applications. One of the relevant fields of current interest in material science is the fundamental aspects and applications of semi- conducting transparent thin films. Transparent conductors show the properties of transparency and conductivity simultaneously. As far as the band structure is concerned, the combination of the these two properties in the same material is contradictory. Generally a trans- parent material is an insulator having completely filled valence and empty conduction bands. Metallic conductivity come out when the Fermi level lies within a band with a large density of states to provide high carrier concentration. Effective transparent conductors must nec- essarily represent a compromise between a better transmission within the visible spectral range and a controlled but useful electrical con- ductivity [1–6]. Generally oxides like In2O3, SnO2, ZnO, CdO etc, show such a combination. These materials without any doping are insulators with optical band gap of about 3 eV. To become a trans- parent conductor, these materials must be degenerately doped to lift the Fermi level up into the conduction band. Degenerate doping pro- vides high mobility of extra carriers and low optical absorption. The increase in conductivity involves an increase in either carrier concen- tration or mobility. Increase in carrier concentration will enhance the absorption in the visible region while increase in mobility has no re- verse effect on optical properties. Therefore the focus of research for new transparent conducting oxide (TCO) materials is on developing materials with higher carrier mobilities.

Dye-sensitized solar cell architecture based on indium-tin oxide nanowires coated with titanium dioxide

A new architecture for dye-sensitized solar cells is employed, based on a nanostructured transparent conducting oxide protruding from the substrate, covered with a separate active oxide layer. The objective is to decrease electron-hole recombination. The concept was tested by growing branched indium-tin oxide nanowires on glass using pulsed laser deposition followed by deposition of a sputtered titanium dioxide layer covering the wires. The separation of charge generation and charge transport functions opens many possibilities for dye-sensitized solar cell optimization. (c) 2007 Acta Materialia. Inc. Published by Elsevier Ltd. All rights reserved.

Construção e caracterização de células solares de filmes finos de CdS e CdTe

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Inorganic-organic hybrid polymers: Solution-processible coating materials for defined surface functionalization

A new class of inorganic-organic hybrid polymers could successfully been prepared by the combination of different polymerization techniques. The access to a broad range of organic polymers incorporated into the hybrid polymer was realized using two independent approaches.rnIn the first approach a functional poly(silsesquioxane) (PSSQ) network was pre-formed, which was capable to initiate a controlled radical polymerization to graft organic vinyl-type monomers from the PSSQ precursor. As controlled radical polymerization techniques atom transfer radical polymerization (ATRP), as well as reversible addition fragmentation chain transfer (RAFT) polymerization could be used after defined tuning of the PSSQ precursor either toward a PSSQ macro-initiator or to a PSSQ macro-chain-transfer-agent. The polymerization pathway, consisting of polycondensation of trialkoxy-silanes followed by grafting-from polymerization of different monomers, allowed synthesis of various functional hybrid polymers. A controlled synthesis of the PSSQ precursors could successfully be performed using a microreactor setup; the molecular weight could be adjusted easily while the polydispersity index could be decreased well below 2.rnThe second approach aimed to incorporate differently derived organic polymers. As examples, polycarbonate and poly(ethylene glycol) were end-group-modified using trialkoxysilanes. After end-group-functionalization these organic polymers could be incorporated into a PSSQ network.rnThese different hybrid polymers showed extraordinary coating abilities. All polymers could be processed from solution by spin-coating or dip-coating. The high amount of reactive silanol moieties in the PSSQ part could be cross-linked after application by annealing at 130° for 1h. Not only cross-linking of the whole film was achieved, which resulted in mechanical interlocking with the substrate, also chemical bonds to metal or metal oxide surfaces were formed. All coating materials showed high stability and adhesion onto various underlying materials, reaching from metals (like steel or gold) and metal oxides (like glass) to plastics (like polycarbonate or polytetrafluoroethylene).rnAs the material and the synthetic pathway were very tolerant toward different functionalities, various functional monomers could be incorporated in the final coating material. The incorporation of N-isopropylacrylamide yielded in temperature-responsive surface coatings, whereas the incorporation of redox-active monomers allowed the preparation of semi-conductive coatings, capable to produce smooth hole-injection layers on transparent conductive electrodes used in optoelectronic devices.rnThe range of possible applications could be increased tremendously by incorporation of reactive monomers, capable to undergo fast and quantitative conversions by polymer-analogous reactions. For example, grafting active esters from a PSSQ precursor yielded a reactive surface coating after application onto numerous substrates. Just by dipping the coated substrate into a solution of a functionalized amine, the desired function could be immobilized at the interface as well as throughout the whole film. The obtained reactive surface coatings could be used as basis for different functional coatings for various applications. The conversion with specifically tuned amines yielded in surfaces with adjustable wetting behaviors, switchable wetting behaviors or as recognition element for surface-oriented bio-analytical devices. The combination of hybrid materials with orthogonal reactivities allowed for the first time the preparation of multi-reactive surfaces which could be functionalized sequentially with defined fractions of different groups at the interface. rnThe introduced concept to synthesis functional hybrid polymers unifies the main requirements on an ideal coating material. Strong adhesion on a wide range of underlying materials was achieved by secondary condensation of the PSSQ part, whereas the organic part allowed incorporation of various functionalities. Thus, a flexible platform to create functional and reactive surface coatings was achieved, which could be applied to different substrates. rn

Novel transparent and flexible transistors for radiation harsh environment

La scoperta dei semiconduttori amorfi ha segnato l’era della microelettronica su larga scala rendendo possibile il loro impiego nelle celle solari o nei display a matrice attiva. Infatti, mentre i semiconduttori a cristalli singoli non sono consoni a questo tipo di applicazioni e i s. policristallini presentano il problema dei bordi di grano, i film amorfi possono essere creati su larga scala (>1 m^2) a basse temperature (ad es. <400 °C) ottenendo performance soddisfacenti sia su substrati rigidi che flessibili. Di recente la ricerca sta compiendo un grande sforzo per estendere l’utilizzo di questa nuova elettronica flessibile e su larga scala ad ambienti soggetti a radiazioni ionizzanti, come lo sono i detector di radiazioni o l’elettronica usata in applicazioni spaziali (satelliti). A questa ricerca volge anche la mia tesi, che si confronta con la fabbricazione e la caratterizzazione di transistor a film sottili basati su ossidi semiconduttori ad alta mobilità e lo studio della loro resistenza ai raggi X. La micro-fabbricazione, ottimizzazione e caratterizzazione dei dispositivi è stata realizzata nei laboratori CENIMAT e CEMOP dell’Università Nova di Lisbona durante quattro mesi di permanenza. Tutti i dispositivi sono stati creati con un canale n di ossido di Indio-Gallio-Zinco (IGZO). Durante questo periodo è stato realizzato un dispositivo dalle ottime performance e con interessanti caratteristiche, una delle quali è la non variazione del comportamento capacitivo in funzione della frequenza e la formidabile resistenza alle radiazioni. Questo dispositivo presenta 114 nm di dielettrico, realizzato con sette strati alternati di SiO2/ Ta2O5. L’attività di ricerca svolta al Dipartimento di Fisica e Astronomia di Bologna riguarda prevalentemente lo studio degli effetti delle radiazioni ionizzanti su TFTs. Gli esperimenti hanno rivelato che i dispositivi godono di una buona stabilità anche se soggetti alle radiazioni. Infatti hanno mostrato performance pressoché inalterate anche dopo un’esposizione a 1 kGy di dose cumulativa di raggi X mantenendo circa costanti parametri fondamentali come la mobilità, il threshold voltage e la sub-threshold slope. Inoltre gli effetti dei raggi X sui dispositivi, così come parametri fondamentali quali la mobilità, si sono rivelati essere notevolmente influenzati dallo spessore del dielettrico.