In the study, conductive distributed Bragg reflectors (DBRs) fabricated at room temperature based on porous indium tin oxide (ITO) on dense ITO bilayers were proposed for resonant cavity organic light emitting diodes (RCOLEDs). In the fabrication of the ITO DBRs, the low refractive index porous ITO films were obtained by applying supercritical CO2 treatment at different temperature and pressures on the spin-coated sol-gel ITO films. On the other hand, the high refractive index ITO films were grown at room temperature by long-throw reactive ratio-frequency magnetron sputtering. The refractive index of the porous ITO film and ITO films were 1.54 and 2.0, respectively. For the DBR with 4 pairs ITO bilayers, the optical reflectance of more than 70 % was achieved. The stop band and the average resistivity is 140 nm and 2.2¡Ñ10-3 £[-cm, respectively. Finally, electrical and optical characteristics of the RCOLEDs fabricated on the ITO DBR were investigated and compared with those of the conventional OLEDs. The maximum luminous efficiency of 3.79 cd/A was obtained at 347 mA/cm2 for the RCOLED. This luminous efficiency was 26 % higher than that of the conventional OLED.

This dissertation focuses on characterizing the nanoscale and surface averaged electrical properties of transparent conducting oxide (TCO) electrodes such as indium tin oxide (ITO) and transparent metal-oxide (MO) electron selective interlayers (ESLs), such as zinc oxide (ZnO), the ability of these materials to rapidly extract photogenerated charges from organic semiconductors (OSCs) used in organic photovoltaic (OPV) cells, and evaluating their impact on the power conversion efficiency (PCE) of OPV devices. In Chapter 1, we will introduce the fundamental principles regarding the need for low cost power generation, the benefits of OPV technologies, as well as the key principles that govern the operation of OPV devices and the key innovations that have advanced this technology. In Chapter 2 of this dissertation, we demonstrate an innovative application of conductive probe atomic force microscopy (CAFM) to map the nanoscale electrical heterogeneity at the interface between an electrode, such as ITO, and an OSC such as the p-type OSC copper phthalocyanine (CuPc).(MacDonald et al. (2012) ACS Nano, 6, p. 9623) In this work we collected arrays of J-V curves, using a CAFM probe as the top contact of CuPc/ITO systems, to map the local J-V responses. By comparing J-V responses to known models for charge transport, we were able to determine if the local rate-limiting step for charge transport is through the OSC (ohmic) or the CuPc/ITO interface (nonohmic). These results strongly correlate with device PCE, as demonstrated through the controlled addition of insulating alkylphosphonic acid self-assembled monolayers (SAMs) at the ITO/CuPc interface. Subsequent chapters focus on the electrical property characterization of RF-magnetron sputtered ZnO (sp-ZnO) ESL films on ITO substrates. We have shown that the energetic alignment of ESLs and the organic semiconducting (OSC) active materials plays a critical role in determining the PCE of OPV devices and the appearance of, or lack thereof, UV light soaking sensitivity. For ZnO and fullerene interfaces, we have shown that either minimizing the oxygen partial pressure during ZnO deposition or exposure of ZnO to UV light minimizes the energetic offset at this interface and maximizes device PCE. We have used a combination of device testing, device modeling, and impedance spectroscopy to fully characterize the effects that energetic alignment has on the charge carrier transport and charge carrier distribution within the OPV device. This work can be found in Chapter 3 of this dissertation and is in preparation for publication. We have also shown that the local properties of sp-ZnO films varies as a function of the underlying ITO crystal face. We show that the local ITO crystal face determines the local nucleation and growth of the sp-ZnO films. We demonstrate that this effects the morphology, the chemical resistance to etching as well as the surface electrical properties of the sp-ZnO films. This is likely due to differences in the surface mobility of sputtered Zn and O atoms on these crystal faces during film nucleation. This affects the nanoscale distribution of electrical and chemical properties. As a result we demonstrate that the PCE, and UV sensitivity of the J-V response of OPVs using sp-ZnO ESLs are strongly impacted by the distribution of ITO crystal faces at the surface of the substrate. This work can be found in Chapter 4 of this dissertation and is in preparation for publication. These studies have contributed to a detailed understanding of the role that electrical heterogeneity, insulating barriers and energetic alignment at the MO/OSC interface play in OPV PCE.

Photovoltaics manufactured using organic materials as a substitute for inorganic materials may provide for cheaper production of solar cells if their efficiencies can be made comparable to existing technologies. Photovoltaic devices are comprised of layered structures where the electrical, chemical, and physical properties at the multiple interfaces play a significant role in the operation of the completed device. This thesis attempts to establish a relationship between interfacial properties and overall device performance by investigation of both the organic/organic heterojunction interface, as well as the interface between the inorganic substrate and the first organic layer with useful insights towards enhancing the efficiency of organic solar cells.It has been proposed that residual chemical species may act as barriers to charge transfer at the interface between the transparent conductor (TCO) and the first organic layer, possibly causing a large contact resistance and leading to reduced device performance. Previous work has investigated the surface of the TCO but no baseline characterization of carbon-free surfaces has previously been given. In this work clean surfaces are investigated to develop a fundamental understanding of the intrinsic spectra such that further analyses of contaminated surfaces can be presented systematically and reproducibly to develop a chemical model of the TCO surface.The energy level offset at the organic/organic heterojunction has been proposed to relate to the maximum potential achievable for a solar cell under illumination, however, few experimental observations have been made where both the interface characterization and device performance are presented. Photovoltaic properties are examined in this work with titanyl phthalocyanine used as a novel donor material for enhancement of spectral absorption and optimization of the open-circuit potential. Characterization of the interface between TiOPc and C60 coupled with characterization of the interface between copper phthalocyanine and C60 shows that the higher ionization potential of TiOPc does correlate to greater open circuit potentials.Examination of photovoltaic behavior using equivalent circuit modeling relates the importance of series resistance and recombination to the homogeneity of the solar cell structure.

The research presented in this dissertation explores the possibility of using colloidal indium tin oxide (ITO) nanoparticle solutions to direct write transparent conducting coatings (TCCs), as an alternative route for TCC fabrication. ITO nanoparticles with narrow size distribution of 5-7 nm were synthesized using a non-aqueous synthesis technique, and fabricated into films using spin coating on substrates made from glass and fused quartz. The as-coated films were very transparent (>95% transmittance), but highly resistive, with sheet resistances around 10¹³ Ω/sq . Pre-annealing plasma treatments were investigated in order to improve the electrical properties while avoiding high temperature treatments. Composite RIE treatment recipes consisting of alternating RIE treatments in O₂ plasma and in Ar plasma were able to reduce the sheet resistance of as spin coated ITO films by 4-5 orders of magnitude, from about 10¹³ Ω/sq in as-coated films to about 3 x 10⁸ Ω/sq without any annealing. Plasma treatment, in combination with annealing treatments were able to decrease the sheet resistance by 8-9 orders of magnitude down to almost 10 kΩ/sq , equivalent to bulk resistivity of ~0.67 Ω.cm. Investigation into effectiveness of various RIE parameters in removing residual organics and in reducing the sheet resistance of colloidal ITO films suggested that while reactive ion annealing (RIE) pressure is an important parameter; parameters like plasma power, number of alternating O₂-Ar RIE cycles were also effective in reducing the residual organic content. Impedance spectroscopy analysis of the colloidal ITO films indicated the dominance of the various interfaces, such as grain boundaries, insulating secondary phases, charge traps, and others in determining the observed electrical properties.

Il y a un intérêt croissant concernant la découverte de nouvelles méthodes commercialement viables pour réaliser des analyses spectro-électrochimiques — combinant des techniques électrochimiques et spectrales. Pour ce faire, nous avons préparé un matériau d'électrode transparent et conducteur, l’oxyde d’indium dopé à l’étain. Nous avons utilisé la technique d’électrofilage conduisant à la formation de fibres très fines avec une surface spécifique élevée. Ces électrodes ont ensuite été recouvertes d'une couche supplémentaire de silice poreuse et fonctionnalisée pour maximiser la surface spécifique et introduire des propriétés de détection supplémentaires. Le dispositif a été utilisé dans la détection du bleu de méthylène qui est un colorant industriel mais également un polluant environnemental. il a été mis en évidence qu’avec l'utilisation d'une telle électrode, il était possible de détecter des concentrations inférieures aux niveaux environnementaux nocifs. Enfin, les électrodes fonctionnalisées ont également été utilisées avec succès pour générer une luminescence plus intense et plus stable, ce qui ouvre de nouvelles perspectives pour la conception de capteurs spectroélectrochimiques / There is a growing interest in finding new and commercially viable methods of performing a spectroelectrochemical analysis which combines electrochemical and spectral techniques. For this purpose, an electrode material that is transparent and conductive needs to be prepared. In this work, such electrode was prepared by electrospinning which is a technique capable of forming very thin fibers with high surface area. Those electrodes were also covered with additional layer of porous and functionalized silica to maximize the surface area and introduced additional sensing properties. This material was used in the detection of methylene blue which is an industrial dye and an environmental pollutant. It was found that using such electrode it was possible to detect concentrations that are smaller than the harmful environmental levels. Finally, the layers were also used with success to generate luminescence which is opening new prospects for the design of spectroelectrochemical sensors

Made available in DSpace on 2016-06-02T20:36:45Z (GMT). No. of bitstreams: 1 5302.pdf: 3050627 bytes, checksum: d14aad95482698e7d39d217f3b9d0922 (MD5) Previous issue date: 2011-11-22 / Universidade Federal de Sao Carlos / In this study, in the first step, Indium tin oxide nanoparticles were synthesized via a non-aqueous route involving the solvothermal treatment of indium (III) acetylacetonate and tin (IV) chloride in polyethylene glycol Mw=1000. The use of microwave heating reduced the reaction time considerably when compared to traditional heating methods. An analysis by transmission electron microscopy (TEM) revealed particles of relatively uniform sizes and shapes. The high crystallinity of the material was observed by high resolution transmission electron microscopy (HRTEM). The nanocristal size founded by count was 5,1nm. A powder X-ray diffraction analysis indicated that all the materials were crystalline. Infrared spectra confirmed the presence of organic material on the nanoparticle surface. By thermogravimetric analysis (TGA) determined that 11.3% of the total mass corresponds to the polymer. Resistivity values below 10-1 Ω.cm were obtained in thin films and pellets, and semiconductor behavior. In the second step, a model to control the covered area of graphene oxide (GO) sheets by ITO nanoparticles was proposed. The method used was add graphene oxide at the synthetic route to obtain pure ITO. The composites were characterized by XRD, FT-IR, TGA and TEM. XRD results for the synthesized materials confirmed the diffraction patterns of ITO in the different composites synthesized. Through the analysis of FT-IR was possible confirm the presence of the polymer formed on the surface of the oxide nanoparticle and functional groups of graphene oxide sheets. The polymer attached on the oxide surface is responsible for the strong interaction between the ITO and graphen oxide sheets. TEM images for the samples with different cover percentage showed the controller achieved with the synthesis proposed. The composite with 100 or 10% of metal oxides covering the sheets surface did not show the presence of nanocrystals out sheets. The percent value of the covered area obtained of 15% founded by image J analisys is near to the calculated value. From this value we can say that the model works well to control the covered area of GO by nanocristals. The electrical resistivity values found are comparable to the pure ITO, however, with a smaller amount of ITO. / Neste estudo, na primeira etapa, nanopartículas de óxido de índio dopado com estanho foram sintetizadas por uma rota não aquosa envolvendo o tratamento solvotermal de acetilacetonato de índio (III) e cloreto de estanho (IV) em polietilenoglicol de massa molecular 1000. O uso de aquecimento auxiliado por microondas reduziu o tempo de reação quando comparado aos métodos tradicionais de aquecimento. A análise por microscopia eletrônica de transmissão (TEM) mostrou partículas com tamanho e forma relativamente uniformes. A alta cristalinidade do material foi observada por microscopia eletrônica de alta resolução (HRTEM). O tamanho dos nanocristais obtidos por contagem foi de 5,1 nm. A análise de difração de Raios-X (DRX) indicou a cristalinidade do material. O espectro de infravermelho (FT-IR) confirmou a presença do material orgânico na superfície das nanopartículas. Pela análise termogravimétrica (TGA) determinou que 11,3% da massa total corresponde ao polímero. Resistividade abaixo de 10-1 Ω.cm foi obtido no filme e na pastilha, com comportamento semicondutor do óxido. Na segunda etapa, um modelo de controlar a área das folhas de óxido de grafeno (OG) coberta por nanocristais foi proposto. O método usado foi adicionar óxido de grafeno à rota de síntese do ITO puro. Os compósitos foram caracterizados por DRX, FT-IR, TGA e TEM. Os resultados de difração de Raios-X confirmaram o padrão de difração do ITO nos diferentes compósitos. Pela análise de FT-IR foi possível confirmar a presença do polímero na superfície das nanoparticulas e os grupos funcionais das folhas de óxido de grafeno. O polímero ligado na superfície do óxido e responsável pela forte interação entre o ITO e as folhas de óxido de grafeno. As imagens de TEM para as amostras com porcentagens de cobertura diferente mostraram o controle alcançado com o método de síntese proposto. Os compósitos com 100% e 10% de óxido metálico cobrindo a superfície das folhas mostraram que não ocorreu a formação de nanopartículas fora das folhas. O valor de 15 % de porcentagem de área coberta obtido é próximo ao valor calculado. A partir deste valor, pode-se dizer que o modelo funciona bem para controlar a área de OG coberta por nanocristais. Os valores de resistividade elétrica encontrados são comparáveis ao ITO puro, no entanto, com uma quantidade menor de ITO.

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<p>The development of nanocomposite materials has had significant influence on modern material design. Novel properties can be achieved and controlled for a diverse range of applications. The work presented here focused on characterization of polyurethane based coatings with ITO nano-inclusions. The coatings displayed high transparency in the visible range, and UV/IR shielding properties when studied with UV-Vis spectroscopy. UV/IR shielding improved with greater ITO density, with minor affect on visible transmittance. The effective medium approximation was successfully applied to ellipsometry modeling. Coatings with varying fractions of nanoparticles were analyzed. The modeled volume percent of the nanoparticles followed a strong linear trend with the known weight percentages. SEM and TEM imaging determined that majority of the particles existed in clusters. The nanoparticles were oblong shaped, 10-20nm big, randomly distributed, with no segregation to interfaces. Agglomerates varied in size, with the largest observed agglomerate being 250nm.</p> <p>Thermal stability was studied by TGA and DSC. No degradation occurred until 238°C. DSC revealed that the matrix continued to undergo modifications with consecutive runs. It was inconclusive whether the changes were from the polyurethane or dispersive agents in the system. Electron micrographs showed that segregation did not occur post-annealing. Average surface roughness increased from 3.5nm to 5nm after annealing at 120°C for several weeks. Ellipsometry results showed that film thickness decreased 20nm and 50nm before equilibriating for the 80°C and 120°C anneals respectively. The optical and thermal measurements demonstrated that the coating has great potential for improving the PV performance.</p> / Master of Applied Science (MASc)

Chemisorption of an organic monolayer to tune the surface properties of a transparent conductive oxide (TCO) electrode can improve the performance of organic electronic devices that rely on efficient charge transfer between an organic active layer and a TCO contact. Here, a series of perylene diimides (PDIs) was synthesized and used to study relationships between monolayer structure/properties and electron transfer kinetics at PDI-modified indium-tin oxide (ITO) electrodes. In these PDI molecules, one of the imide substituents is a benzene ring bearing a phosphonic acid (PA) and the other is a bulky aryl group that is twisted out of the plane of the PDI core. The size of the bulky aryl group and the substitution of the benzene ring bearing the PA were both varied, which altered the extent of aggregation when these molecules were absorbed as monolayer films (MLs) on ITO, as revealed by both attenuated total reflectance (ATR) and total internal reflection fluorescence spectra. Polarized ATR measurements indicate that, in these MLs, the long axis of the PDI core is tilted at an angle of 33-42 degrees relative to the surface normal; the tilt angle increased as the degree of bulky substitution increased. Rate constants for electron transfer (k(s,opt)) between these redox-active modifiers and ITO were determined by potential-modulated ATR spectroscopy. As the degree of PDI aggregation was reduced, k(s,opt) declined, which is attributed to a reduction in the lateral electron self-exchange rate between adsorbed PDI molecules, as well as the heterogeneous conductivity of the ITO electrode surface. Photoelectrochemical measurements using a dissolved aluminum phthalocyanine as an electron donor showed that ITO modified with any of these PDIs is a more effective electron-collecting electrode than bare ITO.