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 π (ΔEHOMO= 0,54 eV) e π* (ΔELUMO = 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 λ > 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 π (ΔEHOMO = 0.54 eV) and π* (ΔELUMO = 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, μ 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 π (ΔEHOMO= 0,54 eV) e π* (ΔELUMO = 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 λ > 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 π (ΔEHOMO = 0.54 eV) and π* (ΔELUMO = 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, μ 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.

InAs/GaSb superlattices are an attractive material system for infrared light emitting diodes, due to the ability to tune the band gap throughout most of the infrared regime. A key consideration in the epitaxial growth of these heterostructures is crystalline material quality. In developing thick layers of epitaxially grown material, there are moderate amounts of elastic strain that can be incorporated into a heterostructure, beyond which deformations will form that will alleviate the lattice mismatch. This thesis investigates the optical and electronic properties of lattice-mismatched and strained materials through the study of thick dual-color light emitting diodes, broadband light emitting diodes, and InAs/GaSb superlattice devices developed on GaAs substrates and GaAs integrated circuits. A dual-color infrared light emitting diode is demonstrated emitting in the mid-wave infrared band at 3.81 μm and 4.72 μm. The design of the device stacks two independently operable InAs/GaSb superlattices structures on top of one another, so that 10 μm of material is grown with molecular beam epitaxy. Each layer is lattice-matched to a GaSb substrate. At quasi-continuous operation, radiances of 5.48 W/cm2-sr and 2.67 W/cm2-sr are obtained. A broadband light emitting diode spanning the mid-wave infrared is demonstrated with eight stages of InAs/GaSb superlattices individually tuned to a different color. The performance of the device is compared with an identical eight stage device emitting in the middle of the mid-wave infrared. The emission of the fabricated broadband device spans from 3.2 μm to 6 μm with peak radiance of 137.1 mW/cm2-sr. Growth of antimonide-based devices on GaAs is desirable to the relative transparency of semi-insulating substrates throughout the infrared, and as semi-insulating GaSb substrates are not available. The growth of bulk GaSb on GaAs is explored through different techniques in order to confine relaxation due to lattice mismatch strain to the GaSb/GaAs interface. A low temperature nucleation technique with a thin GaSb wetting layer is found to have the best overall surface morphology, although screw dislocations are a prominent feature on all samples. The dislocations and overall surface roughness are not found to destructively impact the overall device quality, as four stage InAs/GaSb superlattice devices grown on GaAs substrates are found to have superior electroluminescent emission and external quantum efficiency compared to an identical device grown on a GaSb substrate due to the higher substrate transparency and superior thermal properties. Epitaxy on electronics growth techniques on GaAs integrated circuits are developed to bypass the hybridization process in light emitting diode development. Chips obtained from Quorvo, Inc. are found to endure ultra-high vacuum molecular beam epitaxy environment at higher temperatures with silicon nitride encapsulation, and a low temperature oxide removal technique is developed using an atomic hydrogen source. Chemical-mechanical polishing techniques are developed to create an “epi-ready” substrate surface. Ultimately, no photoluminescent emission is observed from InAs/GaSb superlattices grown on GaAs integrated circuits, although electroluminescent emission is still possible.

Photovoltaics (PV) is fast emerging as an attractive renewable energy technology due to concerns of global warming, pollution and scarcity of fossil fuel supplies. However to compete in the global energy market, solar cells need to be cheaper and more energy efficient. Silicon is the favorite semiconductor used in solar photovoltaic cells because of its ubiquity and established technology, but due to its indirect bandgap silicon is a poor absorber and light emitter. Thin film cells play an important role in low cost photovoltaics, but at the cost of reduced efficiencies when compared to wafer based cells. There remains much untapped potential in thin-film solar cells which this work has attempted to exploit through exploring novel approaches of enhancing the efficiency of thin film cells using the optical properties of sub-wavelength metal nanoparticles. Metals are considered as strong absorbers of light because of their large free-electron density. How can metals improve light trapping in solar cells? This question has raised several eyebrows and this thesis is an attempt to show that metal nanoparticles can be useful in producing efficient solar cells. Subwavelength metal particles support surface modes called surface plasmons when light is incident on them, which cause the particles to strongly scatter light into the underlying waveguide or substrate, enhancing absorption. The process of coupling thin film silicon waveguide modes to plasmonic metals using unpolarised light at normal incidence is applied to silicon-based solar cells and light emitting diodes, and enhanced photocurrent and electroluminescence is realized with potential for further optimisation and improvement. The results from this study correspond to a current increase of up to 19% from planar wafer based cells and up to 33% increase from 1.25 micron thin-film silicon-on-insulator structures for the AM1.5 global spectrum. We also report for the first time an up to twelve fold increase in electroluminescence signal from 95nm thick light-emitting diodes. From the results we conclude that this method which involves simple techniques of nanoparticle deposition and characterization could hold important implications in the improvement of thin-film silicon cell absorption / emission efficiencies where conventional methods of light trapping are not feasible, resulting in promising near-term applications of surface plasmons in photovoltaics and optoelectronics.

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.

AlₓGa₁₋ₓN (x>0.6) based Ultraviolet Light Emitting Diodes (UV LEDs) emit in the UV C range of 200 - 290 nm and suffer from low external quantum efficiencies (EQEs) of less than 3%. This low EQE is representative of a large number of non-radiative recombination events in the multiple quantum well (MQW) layers, which leads to high device temperatures due to self-heating at the device junction. Knowledge of the device temperature is essential to implement and evaluate appropriate thermal management techniques, in order to mitigate optical degradation and lifetime reduction due to thermal overstress. The micro-scale nature of these devices and the presence of large temperature gradients in the multilayered device structure merit the use of several indirect temperature measurement techniques to resolve device temperatures. This work will study UV LEDs with AlₓGa₁₋ₓN active layers, grown on sapphire or AlN growth substrates, and flip-chip mounted onto submounts and package configurations with different thermal properties. Thermal metrology results will be presented for devices with different electrode geometries (i.e., interdigitated and micropixel), for bulk and thinned growth substrates. The body of this work will present a comparative study of optical techniques such as Infrared (IR), micro-Raman and Electroluminescence (EL) spectroscopy for the thermal metrology of UV LEDs. The presence of horizontal and vertical temperature gradients within the device layers will be studied using micro-Raman spectroscopy, while the occurrence of thermal anomalies such as hotspots and shorting paths will be studied using IR spectroscopy. The Forward Voltage (Vf) method, an electrical junction temperature measurement technique, will also be investigated. The Vf method will be applied to the Thermal Resistance Analysis by Induced Transient (TRAIT) procedure, whereby electrical data at short time scales from an operational device will be used to discretize the junction-to- package thermal resistance pathway from the total junction- to-ambient heat path. The TRAIT procedure will be conducted on several LEDs, for comparison. The scope and applicability of each thermal metrology technique will be examined, and the merits and demerits of each technique will be exhibited.

Light emitting diodes (LEDs) and laser diodes (LDs) have many advantages over conventional light sources. Current commercial LEDs span the spectrum from IR to near- UV. There are a variety of applications for devices that extend into the deep-UV, including biological agent detection and optical storage. The nitride material system is a set of semiconducting compounds that have wavelengths that span a broad range, from yellow to deep-UV. AlGaN has a direct bandgap that extends into the deep-UV range; we will try to grow device-quality material, deposited epitaxially using metalorganic chemical vapor deposition on sapphire substrates.

Light emitting diodes (LEDs) and laser diodes (LDs) have many advantages over conventional light sources. Current commercial LEDs span the spectrum from IR to near- UV. There are a variety of applications for devices that extend into the deep-UV, including biological agent detection and optical storage. The nitride material system is a set of semiconducting compounds that have wavelengths that span a broad range, from yellow to deep-UV. AlGaN has a direct bandgap that extends into the deep-UV range; we will try to grow device-quality material, deposited epitaxially using metalorganic chemical vapor deposition on sapphire substrates.

The field of organic electronics is recently emerging in modern electrical applications. Organic light emitting diodes have been developed and are implemented in commercially available products. The novel materials are also used in sensor applications, utilizing their intrinsic physical, chemical and electrical characteristics. Poly(3,4-ethylenedioxythiophene): poly(styrene-sulfonic acid) (PEDOT:PSS) is one of the most successful organic conductive materials. Developed as antistatic coating, it is now used in other fields as well such as in electro-optical devices as transparent electrodes. One of the reasons for its widely spread use is that water-based dispersions in high quality are available. In addition, it is considered highly stable, resisting degradation under typical ambient conditions. For this work, the usability of PEDOT:PSS as active layer for electromechanical sensor applications was investigated. The electrical properties of the material were characterized including temperature dependencies and environmental influences. A piezoresistive effect with negative sign was found. It is small in magnitude and of the same order as the change in resistance due to geometrical effects. The piezoresistive effect is temperature dependent and increasing in magnitude with higher temperatures. An average longitudinal piezoresistive coefficient pi_l of -5.6x10-10 Pa-1 at room temperature has been evaluated. The transverse effect under the same conditions is opposite in sign and two thirds in magnitude of the lateral effect. The hole mobility of PEDOT:PSS follows an Arrhenius function and thus the resistivity has a negative temperature coefficient. Some other thermally induced effects have been observed such as de-doping of the material resulting in an irreversibly lowered conductivity. Due to the low thermal conductivity of the substrate material used, Joule heating of the samples played an important role during the characterization and was utilized to investigate the temperature dependencies. The change of resistance caused by an applied stress to the sample is small, with a gage factor smaller than one.

The use of polymers in electro-optical devices, especially light-emitting diodes (OLEDs), has become very popular in recent years, due to their ease of processability. The major drawback of using polymers in these systems is their time-consuming synthesis when trying to improve upon their physical properties. For example, each time a new color or better conducting properties are desired, a new monomer must be synthesized. To circumvent these problems, the system described in this work is designed to connect the well-known chromophore aluminum tris(8-hydroxyquinoline) (Alq3) to a norbornene monomer unit, followed by polymerization using ring-opening metathesis polymerization (ROMP), thus allowing for the processability of a polymer while maintaining the fluorescent properties of the metalloquinolate. The benefit of this system is that the monomers can be easily altered in order to tune color emission or to enhance the polymer properties. Some of the alterations include changing the metal center from aluminum to zinc in order to improve electron injection, adding substituents to the 8-hydroxyquinoline ligand in order to tune the emission color, and copolymerizing the Alq3-monomer with other norbornene monomers containing either a hole- or an electron-transport material side-chain to improve conductivity. These alterations lead to improved device performance and, more importantly, to a new method of designing polymeric systems for use in electronic devices.