Because of its potential application for flat panel displays, solid-state lighting and 1.5 µm emitter for fiber optical communications, organic light-emitting diodes (OLEDs) have been intensively researched. One of the major problems with current OLED technology relates to inefficient electron injection at the cathode interface, which causes high driving voltage and poor device stability. Making a low resistance cathode contact for electron injection is critical to device performance. This work mainly focuses on cathode interface design and engineering. The Ohmic contact using a structure of C60/LiF/Al has been developed in electron only devices. It is found that application of the C60/LiF/Al contact to Alq based OLEDs leads to a dramatic reduction in driving voltages, a significant improvement in power efficiency, and a much slower aging process. A new cathode structure based on metal-organic-metal (MOM) tri-layer films has been developed. It is found that MOM cathodes reduce reflection by deconstructive optical interference from two metal films. The absolute reflectance from the MOM tri-layer films can be reduced to as low as 7% in the visible light spectrum. In actual working devices, the reflectance can be reduced from ~80% to ~ 20%. MOM cathodes provide a potential low-cost solution for high contrast full-color OLED displays. Low voltage Erq based OLEDs at 1.5 µm emission have been developed. The Erq/Ag cathode interface has been found to be efficient for electron injection. Dramatic improvement in driving voltage and power efficiency has been realized by implementing Bphen and C60 into Erq devices as an electron transport layer. Integration of Erq devices on Si wafers has also been demonstrated.

Organic semiconductors have the advantage over traditional inorganic semiconductors, such as Si or GaAs, in that they do not require perfect single crystal films to operate in real devices. Complicated multi-layer structures with nanometer scale thicknesses can thus be easily fabricated from organic materials using low-cost roll-to-roll manufacturing techniques. However, the discrete nature of organic semiconductors also implies that they typically contain almost no intrinsic charge carriers (i.e., electrons or holes), and thus act as insulators until electrical charges are injected into them. In electrical device applications this means that all of the holes and electrons within a device must be injected from the anode and cathode respectively. As a result, device stability, performance, and lifetime are greatly influenced by the interface between the organic materials and the electrode contacts. Despite the fundamental importance of the electrode/organic contacts, much of the basic physical understanding of these interfaces remains unclear. As a result, the current design of state-of-the-art organic optoelectronic devices tends to be based on trial and error experimentation, resulting in overly complicated structures that are less than optimal. In the present thesis, various electrode/organic interfaces relevant to device applications are studied using a variety of different techniques, including photoelectron spectroscopy and the iii temperature dependent current-voltage characteristics of single carrier devices. The fundamental understanding gleaned from these studies has been used to develop new strategies for controlling the energy-level alignment at electrode/organic interfaces. A universal method for tuning the work function of electrode materials using a halogenated organic solvent and UV light has been developed. Application of this technique in organic light emitting diodes enabled the first highly simplified two-layer device with a state-of-the-art record breaking efficiency.

Benzobisazoles are a class of molecules that initially found their use in high-performance materials as high tensile strength fibers. Recent modifications to the syntheses of benzobisazoles have allowed for the materials to be studied as an n-type material to be used in organic semiconductors, more specifically organic light-emitting diodes (OLEDs). The high molecular stability required to produce blue light gives an opportunity for benzobisazoles to fulfill the requirement. Prior work on benzobisazoles, more specifically, the oxygen analog benzobisoxazole, has been used to try to achieve blue (<450 nm) but fell short in terms of efficiency due to molecular design choices. The following describes new design strategies such as utilizing single-bond linkage between the electron rich and deficient molecules, as well as transitioning from polymer to small molecules to fine-tune the properties of the materials for semiconductor applications. Utilizing a new design strategy, we demonstrate the ability to blue-shift the emission on two benzobisoxazole-based polymers by adopting single bond linkage between the benzobisoxazole and electron rich moieties fluorene and carbazole and achieve a usable brightness (> 1000 Cd/m2) when incorporated into OLEDs. With further modification of the benzobisoxazole core piece by adding dual conjugation along both axes to produce small molecules, we were able to achieve a deeper blue emission at higher efficiencies due to the reduced conjugation and aggregation than our previous systems experienced. Development of the small molecules led us to adopt a modular synthetic strategy for the high-efficiency material design of benzobisoxazole-based materials. In combination with Density Functional Theory calculations, we show the viability of performing computer-backed molecular design to develop materials to be used in all types of semiconductor applications. From calculations, we synthesize benzobisoxazole cruciforms that have both electron rich and electron deficient moieties. These products we then compared to experimental data to confirm the validity of computer-based rational design of molecules for not only blue OLEDs but for all semiconductor applications. The extremely high number of possible combinations of electron rich and electron deficient moieties allows for extensive future studies for the most optimal substituents for proper energy leveling tuning.

No description available.

This dissertation focuses on the design, synthesis, characterization of platinum (II) pyridylazolate complexes and develop high performance organic light emitting diodes (OLEDs) and design and execute high-sensitivity humidity sensors based on the luminescent metal-organic complexes of platinum. A majority of existing platinum compounds do not dissolve in organic solvents, making it difficult to analyze the photophysical characteristics of complexes in solution, a key part of understanding chemical photophysical properties. Furthermore, due to the poor quantum yield, it is inefficient for use in devices such as OLEDs. Chapter 2 reports the synthesis and characterization of a novel heteroleptic platinum(II) pyridylazolate complex with high solubility and quantum yield. The photochemistry of the complex is studied, including efficiency, emission profiles, and lifetimes at different temperatures. Chapter 3 reports the power efficiency (lm/W), current efficiency (cd/A), external quantum efficiency (EQE), luminance and operating voltage (V) of OLED devices made with the heteroleptic platinum(II) pyridylazolate complex. The relation between thickness of hole transport layer and electron transport layer on performance of devices has been studied through building a variety of devices. Chapter 4 includes application of a homoleptic platinum(II) pyridylazolate complex in humidity sensor. In many environments, the relationship between moisture content and emissive wavelength has been investigated. This research reveals that regardless of the humidity level, there is a link between increasing the temperature and decreasing the moisture absorption capacity of the complex.

abstract: White organic light emitting diodes (WOLEDs) are currently being developed as the next generation of solid state lighting sources. Although, there has been considerable improvements in device efficiency from the early days up until now, there are still major drawbacks for the implementation of WOLEDs to commercial markets. These drawbacks include short lifetimes associated with highly efficient and easier to fabricate device structures. Platinum (II) complexes are been explored as emitters for single emissive layer WOLEDs, due to their higher efficiencies and stability in device configurations. These properties have been attributed to their square planar nature. Tetradentate platinum (II) complexes in particular have been shown to be more rigid and thus more stable than their other multidentate counterparts. This thesis aims to explore the different pathways via molecular design of tetradentate platinum II complexes and in particular the percipient engineering of a highly efficient and stable device structure. Previous works have been able to obtain either highly efficient devices or stable devices in different device configurations. In this work, we demonstrate a device structure employing Pt2O2 as the emitter using mCBP as a host with EQE of above 20% and lifetime values (LT80) exceeding 6000hours at practical luminance of 100cd/m2. These results open up the pathway towards the commercialization of white organic light emitting diodes as a solid state lighting source. / Dissertation/Thesis / Masters Thesis Materials Science and Engineering 2016

Neste trabalho, foram realizados estudos de propriedades elétricas e de eletroluminescência em diodos emissores de luz (OLED) contendo modulação energética de poços de potencial para elétrons e buracos (tipo I), poços esses posicionados na região central da camada ativa. A camada ativa é composta por poços simples e duplos, de espessura de 5 e 10nm, de Poli (fenilenovinileno), PPV (Eg = 2,4 eV), dispostos entre duas barreiras de Polifluoreno ou PFO (Eg = 3,0 eV) de espessura 40 nm. Os filmes de PFO foram obtidos a partir de uma solução em Clorofórmio via spin coating e os de PPV a partir de um precursor solúvel em agua via spin assistant LbL, técnica essa que permitiu o crescimento alternado de filmes de PFO e filmes extremamente finos de PPV mesmo em vista da ortogonalidade de seus solventes. Camadas injetoras de polieletrólitos foram depositadas adjacentes ao catodo para diferenciar injeção eletrônica da injeção de buracos. Foram feitos dispositivos contendo somente uma camada de PFO de 80 nm, chamados referência, para comparação do efeito dos poços nos dispositivos com um e dois poços de potencial. Na caracterização foram utilizadas as técnicas de microscopia confocal, com o intuito de demonstrar o crescimento efetivo das camadas, e medidas elétricas de corrente (IxV) e eletroluminescência (LxV) por voltagem. Medidas do perfil de intensidade ao longo do filmes e espectros de fotoluminescência em três regiões distintas da área total do dispositivo mostraram que as camadas de PPV de aproximadamente 5 e 10 nm estavam homogêneas e que recobriam bem as camadas de PFO. Os espectros de eletroluminescência dos dispositivos mostraram que as diferenças energéticas entre os orbitais &#960; (&Delta;EHOMO= 0,54 eV) e &pi;* (&Delta;ELUMO = 0,37 eV) do PFO e PPV foram suficientes para causar o aprisionamento e recombinação dos portadores dentro do poço, resultando em emissões características do PPV com picos bem definidos próximos a 520 nm, bastante distintas das emissões dos dispositivos referência, contendo somente PFO (banda larga e não definida de emissão com &lambda; > 480 nm). A presença dos poços de potencial alterou significativamente as propriedades dos dispositivos levando a diminuição da voltagem de acendimento (Von) para 3,5 V mesmo para dispositivos contendo camada injetora que dificultava a injeção eletrônica. Quando há apenas um poço de potencial na camada ativa dos dispositivos, com ou sem camada injetora, o regime de corrente para voltagens abaixo de 3,5 V é ôhmico e unipolar, sendo ditado por buracos, mas quando a voltagem é maior do que 3,5 V o regime de corrente fica limitado pelo portador minoritário, o elétron. Surpreendentemente, quando são colocados dois poços na camada ativa, separando os portadores, tanto corrente como a formação excitônica e consequente recombinação, ficam sujeitas a um processo de tunelamento do portador majoritário, o buraco. / In this work, studies of electrical properties and electroluminescence in organic light emitting diodes (OLED) containing energetic modulation of potential wells for charge carriers (type I), positioned in the central region of active layer. The active layer is composed of single and double wells of Poly (phenylenevinylene), PPV (2.4 eV), arranged between two barriers of polyfluorene, PFO (3.0 eV), with 40 nm thickness. The PFO films were obtained from a chloroform solution by spin coating and PPV from a water soluble precursor via spin assistant LbL technique, a technique that has allowed the alternate growth of PFO films and extremely thin PPV films from a orthogonal solvent to chloroform, water. Injection layers of polyelectrolytes were deposited adjacent to the cathode to differentiate electronic injection from hole injection. Confocal microscopy measurements showed that the PPV layer of 5 to 10nm thickness were homogeneous and covered PFO layers entirely. Electroluminescence measurements of the devices showed that the energetic difference between &#960; (&Delta;EHOMO = 0.54 eV) and &pi;* (&Delta;ELUMO = 0.37 eV) orbitals from PFO and PPV were enough to cause the charge carriers efficient trapping and recombination in the well, resulting in PPV characteristic emission peaks near to 520 nm, quite different from the reference device emission containing only PFO (broad emission band in the lower energy range). The current measurements showed that the presence of potential wells in the middle of the active layer is responsible for effective change in electrical properties of devices such as carrier density n, &mu; the mobility and conductivity. When there is only one potential well in the active layer, with or without injection layer, the current regime for voltages below 3.5 V is ohmic and unipolar, being dictated by holes, but when the voltage is greater than 3.5 V current regime is limited by the minority carrier, the electron. Surprisingly, when two wells are placed in the active layer, separating the carriers, both current as the excitonic formation and subsequent recombination are subject to a tunneling process by the majority carrier, the hole.

Neste trabalho, foram realizados estudos de propriedades elétricas e de eletroluminescência em diodos emissores de luz (OLED) contendo modulação energética de poços de potencial para elétrons e buracos (tipo I), poços esses posicionados na região central da camada ativa. A camada ativa é composta por poços simples e duplos, de espessura de 5 e 10nm, de Poli (fenilenovinileno), PPV (Eg = 2,4 eV), dispostos entre duas barreiras de Polifluoreno ou PFO (Eg = 3,0 eV) de espessura 40 nm. Os filmes de PFO foram obtidos a partir de uma solução em Clorofórmio via spin coating e os de PPV a partir de um precursor solúvel em agua via spin assistant LbL, técnica essa que permitiu o crescimento alternado de filmes de PFO e filmes extremamente finos de PPV mesmo em vista da ortogonalidade de seus solventes. Camadas injetoras de polieletrólitos foram depositadas adjacentes ao catodo para diferenciar injeção eletrônica da injeção de buracos. Foram feitos dispositivos contendo somente uma camada de PFO de 80 nm, chamados referência, para comparação do efeito dos poços nos dispositivos com um e dois poços de potencial. Na caracterização foram utilizadas as técnicas de microscopia confocal, com o intuito de demonstrar o crescimento efetivo das camadas, e medidas elétricas de corrente (IxV) e eletroluminescência (LxV) por voltagem. Medidas do perfil de intensidade ao longo do filmes e espectros de fotoluminescência em três regiões distintas da área total do dispositivo mostraram que as camadas de PPV de aproximadamente 5 e 10 nm estavam homogêneas e que recobriam bem as camadas de PFO. Os espectros de eletroluminescência dos dispositivos mostraram que as diferenças energéticas entre os orbitais &#960; (&Delta;EHOMO= 0,54 eV) e &pi;* (&Delta;ELUMO = 0,37 eV) do PFO e PPV foram suficientes para causar o aprisionamento e recombinação dos portadores dentro do poço, resultando em emissões características do PPV com picos bem definidos próximos a 520 nm, bastante distintas das emissões dos dispositivos referência, contendo somente PFO (banda larga e não definida de emissão com &lambda; > 480 nm). A presença dos poços de potencial alterou significativamente as propriedades dos dispositivos levando a diminuição da voltagem de acendimento (Von) para 3,5 V mesmo para dispositivos contendo camada injetora que dificultava a injeção eletrônica. Quando há apenas um poço de potencial na camada ativa dos dispositivos, com ou sem camada injetora, o regime de corrente para voltagens abaixo de 3,5 V é ôhmico e unipolar, sendo ditado por buracos, mas quando a voltagem é maior do que 3,5 V o regime de corrente fica limitado pelo portador minoritário, o elétron. Surpreendentemente, quando são colocados dois poços na camada ativa, separando os portadores, tanto corrente como a formação excitônica e consequente recombinação, ficam sujeitas a um processo de tunelamento do portador majoritário, o buraco. / In this work, studies of electrical properties and electroluminescence in organic light emitting diodes (OLED) containing energetic modulation of potential wells for charge carriers (type I), positioned in the central region of active layer. The active layer is composed of single and double wells of Poly (phenylenevinylene), PPV (2.4 eV), arranged between two barriers of polyfluorene, PFO (3.0 eV), with 40 nm thickness. The PFO films were obtained from a chloroform solution by spin coating and PPV from a water soluble precursor via spin assistant LbL technique, a technique that has allowed the alternate growth of PFO films and extremely thin PPV films from a orthogonal solvent to chloroform, water. Injection layers of polyelectrolytes were deposited adjacent to the cathode to differentiate electronic injection from hole injection. Confocal microscopy measurements showed that the PPV layer of 5 to 10nm thickness were homogeneous and covered PFO layers entirely. Electroluminescence measurements of the devices showed that the energetic difference between &#960; (&Delta;EHOMO = 0.54 eV) and &pi;* (&Delta;ELUMO = 0.37 eV) orbitals from PFO and PPV were enough to cause the charge carriers efficient trapping and recombination in the well, resulting in PPV characteristic emission peaks near to 520 nm, quite different from the reference device emission containing only PFO (broad emission band in the lower energy range). The current measurements showed that the presence of potential wells in the middle of the active layer is responsible for effective change in electrical properties of devices such as carrier density n, &mu; the mobility and conductivity. When there is only one potential well in the active layer, with or without injection layer, the current regime for voltages below 3.5 V is ohmic and unipolar, being dictated by holes, but when the voltage is greater than 3.5 V current regime is limited by the minority carrier, the electron. Surprisingly, when two wells are placed in the active layer, separating the carriers, both current as the excitonic formation and subsequent recombination are subject to a tunneling process by the majority carrier, the hole.

The synthesis and optoelectronic properties of biphenyl based conjugated copolymers with varying acceptor units in the polymer backbone were investigated. The well known Donor-Acceptor Theory was used to establish the synthetic pathway for the structural modifications. Solubility issues regarding biphenyl polymer was solved by copolymerizing with soluble units. For this purpose / poly 4-(biphenyl-4-yl)- 4&rsquo / -tert butylspiro[benzo[d]imidazole-2,1&rsquo / -cyclohexane] (P1), poly 4-(biphenyl-4-yl)- 2- dodecyl-2H-benzo[d][1,2,3]triazole (P2) and poly(4-(5-(biphenyl-4-yl)-4-hexylthiophen- 2-yl)-2-dodecyl-7-(4-hexylthiophen-2-yl)-2H-benzo[d][1,2,3]triazole (P3) were synthetized using Suzuki coupling process. Electrochemical properties of these polymers were examined by cyclic voltammetry, spectroelectrochemistry and kinetic studies. Polymers P2 and P3 showed both p- and n-doping behaviors and multicolored electrochromic states. Optical studies revealed that emission color of biphenyl is tuned from blue to orange and the polymers are good candidates for light emitting diode applications. OLED application of P3 was established and outputs of the device were increased by energy transfer studies. The preliminary investigation indicated that P3 possesses promising efficiencies.

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.