This thesis consists of three main studies: magnetic field effects in thermally activated delayed fluorescent (TADF) organic light emitting diodes (OLEDs), magnetic field effects in bipolar and unipolar polythiophene (P3HT) devices and a study of hybrid organic/inorganic perovskite devices. Spin-dependent transport and recombination processes of spin-pair species have been detected by magnetic field effect (MFE) technique in carbon-based semi- conductor devices. Magneto-electroluminescence (MEL) and magneto-conductivity have been measured as a function of the applied magnetic field, B, in light emitting diodes. TADF materials have been used instead of simple fluorescent materials in OLEDs. We have observed very large magnetic response with TADF materials. The second study is magnetic field effects of regio-regular P3HT based OLED devices. P3HT is a well known semiconducting polymer, and its electrical properties such as magneto-conductance can be affected by an applied magnetic field. P3HT was chosen because it exhibits a sign change in magnetoresistance (MR) as the bias is increased. Unipolar and bipolar devices have been fabricated with different electrode materials to understand which model can be best to explain organic magnetoresistance effect, possibly depending on the operating regime of the device. Transport and luminescence spectroscopies were studied to isolate the different mechanisms and identify their fingerprints. The third study is on hybrid organic-inorganic perovskite devices. With the potential of achieving very high efficiencies and the very low production costs, perovskite solar cells have become commercially attractive. Scanning electron microscopy (SEM) images and absorption spectrum of the films were compared in single-step solution, two-step solution and solution-assisted vapor deposition techniques. Grain size, morphology and thickness parameters of perovskite films were studied within these techniques. Perovskite solar cells were fabricated and their efficiencies were measured.

Organic semiconductors (OSCs) have already been shown to have great potential to play an important role in the future of clean energy generation (organic solar cells) and provide energy efficient lighting (organic light-emitting diodes, OLED). Prior research has found that the light-emission efficiency of OLED is severely limited by the magnetic state (technically the spin-configuration) of the light-emission process. In this thesis, we work on the processes using external magnetic fields that can overcome these magnetic limitations. A major focus of this research is to enhance the performance of OLED, while at the same time to unravel the scientific mechanisms by which magnetic fields act on OSCs devices. Thermally activated delayed fluorescence (TADF) is a next-generation OLED emission technology which enables nearly 100% light-emission efficiency without using heavy precious metals. TADF characteristics depend on the probability of reverse intersystem crossing (RISC) from the triplet excited states (T1) to singlet excited states (S1). The conversion (T1 to S1) process depends strongly on spin dynamics, thus we predict a dramatic magnetic field effects (MFEs) in such TADF OLED devices. In subsequent experiments we observed that changes in TADF devices due to various forms of electrical stress can lead to enormous increases in magnetic field effects (MFEs) on the current (> 1400%) and electroluminescence (> 4000%). Our work provides a flexible and inexpensive pathway towards magnetic functionality and field sensitivity in current organic devices. Such OLED pave the way for novel magnetic sensitive OSCs devices with integrated optical, electronic and magnetic characteristics. Organic magnetoresistance (OMAR) has been observed to alter the current and efficiency of OLED without any ferromagnetic components. Here we utilizes slight alterations to the device properties, the addition of a radical-doped functional layer, in which the spin-relaxing effects of localized nuclear spins and electronic spins interfere, to address the assumption about the importance of the hyperfine interaction and to attempt to differentiate between the different models for OMAR. A feature where the magnitude of OMAR exhibits a plateau over a wide range of doping fraction was observed at all temperatures investigated. This phenomenon is well explained by a theory in which a single dopant spin strongly interacts, by exchange, with one of the bottleneck sites. A similar can be used to explain the efficiency increases observed in organic solar cells for certain doping fractions.

This thesis focuses mainly on the techniques to achieve high-performance top-emitting OLEDs, regarding device efficiency and lifetime for both non-inverted and inverted structures. It is thus organized as follows: In Chapter 2, the basic physics of organic semiconductor materials are reviewed, including the electronic properties of organic semiconductor materials, molecular excitations and their electronic transitions etc., which are believed to be critical for understanding of the work. Then, the general device physics of OLEDs are reviewed in detail, which includes almost every important electrical and optical process involved in the device. Finally, techniques and methods used to improve the device performance are summarized, which includes electrical doping of charge carrier transport layers. In Chapter 3, all organic materials, experimental techniques, and characterization methods used in this study are briefly described. In the following Chapter 4, techniques that are used for device optimization of non-inverted top-emitting OLEDs are discussed. Also, the mechanism of light outcoupling enhancement by a capping layer is discussed there. In the last part of Chapter 4, the influence of the optical device structure on the intrinsic quantum yield of the emitters is studied. Chapter 5 is focused on inverted top-emitting OLEDs, which are believed to be better applicable with current mainstream n-type amorphous silicon thin film transistor (TFT) technology. In this Chapter, the organic/metal and metal/organic interfaces are investigated in detail and their influence on device performance is discussed. In Chapter 6, the degradation of top-emitting OLEDs is studied, with a focus on the influence of electrode material and electrode thickness on the lifetime of top-emitting devices.

The electronics industry today is one that stands as a multi-billion dollar industry that is increasingly incorporating more and more products that have ever escalating applications in our everyday life. One of the main sectors of this industry, and one that is likely to continue expanding for a considerable number of years are flat-panel displays. Traditionally, the displays market has been dominated by cathode ray tube (CRT) and liquid crystal displays (LCDs) display types. The drawback of such display displays is that they can be bulky, heavy and/or expensive and so there is considerable room for an alternative and superior technology. One possibility is organic semiconductor displays where light-emitting molecules can be dissolved in common solvents before being inkjet printed, spin-coated or even painted onto any surface giving the benefits of simple and cost effective processing. Organic light-emitting diodes (OLEDs) have recently become ever more evident as a major display type. This thesis focuses on the advancement of light-emitting dendrimers towards flat-panel display applications. The particular interest in dendrimers arises because it has been found they are capable of giving solution-processed phosphorescent devices with high efficiency. Throughout the thesis the benefits of the dendrimer concept are repeatedly shown revealing why this could become the ideal organic material for display applications. The thesis introduces various techniques of electroluminescence and photoluminescence measurements before applying such methods to study a large number of light-emitting dendrimers in order to explore the role of intermolecular interactions, how they are related to molecular structure, and how this determines photophysical and charge transporting properties of the dendrimers. By such studies a number of highly efficient solution-processed phosphorescent light-emitting dendrimers have been identified while the efficiency of devices made from these dendrimers has been improved. This has been demonstrated in each of the three primary display colours of red, green and blue. The work detailed thus brings closer the prospect of dendrimer light-emitting diodes being the future flat-panel display type of choice.

Orientador: Teresa Dib Zambon Atvars / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Química / Made available in DSpace on 2018-08-27T20:25:11Z (GMT). No. of bitstreams: 1 Germino_JoseCarlos_M.pdf: 14479907 bytes, checksum: c9f86b7bc1995a755d797d3af0c41cfc (MD5) Previous issue date: 2014 / Resumo: Nesta dissertação foram estudadas as propriedades fotoluminescentes de alguns compostos de coordenação de Zn(II) com N,N'-bis(salicilidenos) em soluções de THF e DMSO e em estado sólido: N,N'-bis(salicilideno)-1,2-fenilenodiamino - salofeno (KG-15) e seu composto de coordenação de Zn(II) aquo[N,N'-bis(salicilideno)-1,2- fenilenodimino]zinco(II)([Zn(salofeno)(H2O)]; (KG-15/Zn)); N,N'-bis(salicilideno)-4,5- diaminopirimidina - sal-4,5-pym (KG-17) e seu composto de coordenação de Zn(II) aquo[N,N'-bis(salicilideno)-4,5-diaminopirimidina]zinco(II) ([Zn(sal-4,5-pym)(H2O)]; (KG- 17/Zn). Os N,N'-bis(salicilidenos) e seus compostos de coordenação de Zn(II) foram sintetizados, observando-se aumento de cerca de 30 % dos rendimentos reacionais sob irradiação ultrassônica. Foram caracterizados por FTIR, ressonância magnética nuclear 1D de hidrogênio (1H) e de carbono (13C), caracterização estrutural por CHN, no caso dos ligantes realizou-se a caracterização estrutural por difração de raios-X de monocristal, foram também determinados os potenciais de óxido-redução do [Zn(salofeno)(H2O)] por voltametria cíclica e os valores de energia das bandas foram comparados com dados de espectroscopia eletrônica de absorção. As espectroscopias de fluorescência estacionária e resolvida no tempo em solução diluída de THF e de DMSO (apenas KG-17) e no estado sólido mostraram evidências da ocorrência da transferência de prótons no estado eletrônico excitado nos ligantes livres e coordendados. O composto [Zn(salofeno)(H2O)] apresentou eletroluminescência muito fraca em um dispositivo formado por vidro/ITO/PEDOT:PSS/composto de coordenação/Ca/Al muito baixa. Um diodo montado por vidro/ITO/PEDOT:PSS/PVK:(PFOFPen:[Zn(salofeno)(H2O)])/Ca/Al apresentou luminescência do composto de coordenação de Zn(II) devido à processos de transferência de energia entre o PFOFPen (doador) e o [Zn(salofeno)(H2O)] (receptor), além da eletroluminescência do PFOFPen / Abstract: In this dissertation the photoluminescent properties of some coordination compounds of Zn(II) with N,N'-bis(salicylidenes) in THF and DMSO solutions and solid state were studied: N,N'-bis(salicylidene)-1,2-phenylenediamine - salophen (KG-15) and its Zn(II) coordination compounds aquo[N,N'-bis(salicylidene)-1,2- phenylenediaminate]zinc(II) - [Zn(salophen)(H2O)] (KG-15/Zn) , N,N'-bis(salicylidene)- 4,5-diaminepyrimidine - sal-4,5-pym (KG-17) and its Zn(II) coordination compounds aquo[N,N'-bis(salicylidene)-4,5-diaminate]zinc(II) - [Zn(sal-4,5-pym)(H2O)] (KG-17/Zn). The ligands N,N'-bis(salicylidenes) and their Zn(II) coordination compounds were synthesized, observing an increase of about 30 % of the reaction proceeds under ultrasonic irradiation. The ligands and Zn(II) coordination compounds were characterized by FTIR,1D nuclear magnetic resonance of hydrogen (1H) and carbon (13C), structural characterization by CHN elemental analysis, in the case of the ligands the structural characterization was performed by monocrystal X-ray diffraction. The oxyreduction potential of [Zn(salophen)(H2O)] were determined by cyclic voltammetry and the values of the electrochemical energy band gap were compared with optical data. The stationary and time resolved fluorescence spectroscopy of the compounds in dilute solutions of THF and of DMSO (only KG-17) and in solid state showed proton transfer in the electronic excited state locaded on the ligands. The [Zn(salophen)(H2O)] compound exhibit a very low electroluminescence in a device consisting by glass/ITO/PEDOT:PSS/PVK:[Zn(salophen)(H2O)]/Ca/Al. A diode assembled by glass/ITO/PEDOT:PSS/PVK:(PFOFPen:[Zn(salophen)(H2O)])/Ca/Al showed luminescence of Zn(II) coordination compound due to energy transfer processes between PFOFPen (donor) and [Zn(salophen)(H2O)] (acceptor), beyond the electroluminescence of the PFOFPen / Mestrado / Físico-Química / Mestre em Química

Widespread use of organic light emitting diodes (OLEDs) in solid state lighting and display technologies require efficiency and lifetime improvements, as well as cost reductions, inclusive of the transparent conducting oxide (TCO). Indium tin oxide (ITO) is the standard TCO anode in OLEDs, but indium is expensive and the Earth's reserve of this element is limited. Zinc oxide (ZnO) and its variants such as aluminum-doped ZnO (AZO) exhibit comparable electrical conductivity and transmissivity to ITO, and are of interest for TCO applications. However, the workfunction of ZnO and AZO is smaller compared to ITO. The smaller workfunction of AZO results in a higher hole injection barrier at the anode/organic interface, and methods of tuning its workfunction are required. This dissertation tested the hypothesis that workfunction tuning of AZO films could be achieved by surface modification with electronegative oxygen and fluorine plasmas, or, via use of nanoscale transition metal oxide layers (MoOx, VOx and WOx). Extensive UPS, XPS and optical spectroscopy studies indicate that O2 and CFx plasma treatment results in an electronegative surface, surface charge redistribution, and a surface dipole moment which reinforces the original surface dipole leading to workfunction increases. Donor-like gap states associated with partially occupied d-bands due to non-stoichiometry determine the effective increased workfunction of the AZO/transition-metal oxide stacks. Reduced hole injection barriers were engineered by ensuring that the surface ad-layers were sufficiently thin to facilitate Fowler-Nordheim tunneling. Improved band alignments resulted in improved hole injection from the surface modified AZO anodes, as demonstrated by I-V characterization of hole only structures. Energy band alignments are proposed based on the aforementioned spectroscopies. Simple bilayer OLEDs employing the surface modified AZO anodes were fabricated and characterized to compare their performance with standard ITO. Anodes consisting of AZO with MoOx or VOx interfacial layers exhibited 50% and 71% improvement in power efficiency (PE) and external quantum efficiency (EQE), respectively, compared to ITO at a working voltage of 9 V. The efficiencies of dipole reinforced AZO (O2/CFx plasma treated) anodes were comparable to ITO. The improved performance of the surface modified AZO anodes compared to as-deposited AZO is ascribed to improved hole injection, improved charge balance, and improved radiative recombination kinetics. The results suggest that surface modified AZO anodes are a promising alternative to ITO, given the lower cost and Earth abundance of Al and Zn.

By using phosphorescent and thermally activated delayed fluorescence emitters, the internal quantum efficiency of organic light-emitting diodes (OLEDs) can now reach 100%. However, a major fraction of generated photons is trapped inside the device, because of the intrinsic multi-layer device structure and the mismatch of refractive indices. This thesis comprises different approaches for the efficiency enhancement of planar OLEDs. In particular, outcoupling strategies to extract trapped photons to obtain highly efficient OLEDs are investigated.

Over the last decades organic light-emitting diodes (OLEDs) and organic solar cells (OSCs) have gained considerable attention as efficient, flexible, lightweight, and potentially low-cost technology for lighting and display applications or as a renewable energy source, respectively. However, achieving long-term stability remains challenging. Revealing and understanding aging processes is therefore of great interest. This work presents fundamental investigations to understand and circumvent organic device degradation. In the first part, single materials used in organic devices were investigated. By tailoring an attenuated total reflection infrared (ATR-IR) spectrometer to the specific needs and subsequent measurements, it is shown that the tris(8-hydroxyquinoline)aluminum (Alq3) molecule, a well known fluorescent green emitter, degrades during air exposure by the formation of carbonyl groups. By using a laser desorption/ionization time of flight mass spectrometer (LDI-TOF-MS) it was shown that a,w-bis-(dicyanovinylen)-sexithiophen (DCV6T-Bu4), a well known small-molecule material which is used as part of the active layer, reacts with oxygen during ultraviolet (UV) irradiation. By using climate boxes and a sun simulator the impact of dry and humid air as well as sunlight on C60, a widely-used acceptor molecule in organic solar cells, was investigated. The breaking of the C60 cage to C58 and C56 and the further reaction of these components with oxygen as well as the dimerization of C58 and C56 molecules were found. The degradation products such as C58O increase with air exposure time but they are independent of the humidity level of the ambient air as well as sunlight irradiation. Subsequent annealing leads to a decrease of the C58O concentration. Many efficient n-dopants are prone to degradation in air, due to the low ionization potentials, thereby limiting the processing conditions. It was found that the air exposure of the highly efficient n-dopant tetrakis(1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinato)ditungsten(II) (W2(hpp)4) leads to oxidation reactions of the molecule to [W(hpp)2 + O] and other degradation products. The decay constant of W2(hpp)4 and the matching mean growth time of the [W(hpp)2 + O] degradation as well as a second very quick degradation of the dopant could be determined. The two decay constants can be explained by the assumption that W2(hpp)4 molecules, which are involved in the charge transfer, do degrade slower due to the fact that the charge transfer leads to a downshift of the energy levels of the W2(hpp)4 molecule. Apart from the properties of the organic materials, other effects such as the impact of different purification systems on the material purity as well as the dependence of material purity on the OLED lifetime has been investigated. No correlations between the purification grade and the amount of impurities were found. OLEDs which contain N,N\'-di(naphthalen-1-yl)-N,N\'-diphenyl-benzidine (alpha-NPD) purified in a vertically interlaced stainless steel sublimation systems shows slightly higher external quantum efficiencies compared to tube-based vacuum sublimation systems. The devices which contain alpha-NPD purified by a sublimation system have an extended lifetime. Finally, the impact of residual gases during device fabrication on OLED lifetime and electrical characteristics was investigated. It was found that water vapor introduces an additional series resistance to the OLED, while the other gases do not influence the electric characteristics. The presence of nitrogen or oxygen impacts the lifetime of the OLEDs by the same amount. Nitrogen is non-reactive, this leads to the conclusion that the influence of nitrogen and oxygen on the OLED lifetime is of non-chemical nature, such as changes in the morphology of the organic layers. Water vapor introduces an additional, even faster degradation process within the first hours of OLED operation. As major sources of device degradation, the dimerization of 4,7-diphenyl-1,10-phenanthroline (BPhen) as well as the complexation reaction of alpha-NPD with a bis(1-phenylisoquinoline)iridium(III) (Ir(piq)2) fragment was identified.

Organic electronics is a rapidly evolving field with vast number of applications having high potential for commercial success. Although a great progress has been made, many organic electronic applications: organic light-emitting diodes (OLEDs), organic fieldeffect transistors (OFETs), organic solar cells, etc; still require further optimization to fulfill the requirements for successful commercialization. For many applications, available at this time organic materials do not provide satisfactory performance and stability, which hinders the possibility of a large-scale production. Therefore, the key ingredient needed for a successful improvement in performance and stability of organic electronic devices is in-depth knowledge of physical and chemical properties of molecular and polymeric materials. Since many applications encompass several thin film layers made of organics, and often also inorganic materials, the understanding of both organic-organic and hybrid interfaces is yet another important issue necessary for the successful development of organic electronics. The research presented in this thesis is based mainly on photoelectron spectroscopy, which is an experimental technique especially suited to study both surfaces and interfaces of materials. In the thesis, the properties of one of the most successful polymeric materials, poly(3,4-ethylenedioxythiophene), often abbreviated as PEDOT, have been extensively studied. The research was done in close cooperation with an industrial partner – AGFA Gevaert, Belgium. The study was focused on the exploration of the intrinsic properties of the material, such as stability, morphology and conductivity. In addition, however, a possibility of alternation of these properties was also explored. This thesis reports also about investigations of the properties of various organic-organic and hybrid interfaces. The energy level alignment at such interfaces plays important role in charge injection and performance of the thin film organic-based devices. The conditions for different energy level alignment regimes at the various interfaces have been studied. The studies on interfaces were performed in close collaboration with the R&D division of DuPont Corporation, USA. This work led to the significant advances in understanding of the interface energetics and properties of industryrelevant organic materials, as represented not only by published scientific papers, but also patent applications.

We demonstrate a new approach for fabricating alternating current driven organic electroluminescent devices using the concept of doping in organic semiconductors. Doped charge transport layers are used for generation of charge carriers within the device, hence eliminating the need for injecting charge carriers from external electrodes. The device is an organic-inorganic hybrid: We exploit the mechanical strength and chemical stability of inorganic semiconductors and combine it with better optical properties of organic materials whose emission color can be chemically tuned so that it covers the entire visible spectrum. The device consists of an organic electroluminescence (EL) layer composed of unipolar/ambipolar charge transport materials doped with organic dyes (10 wt% ) as well as molecularly doped charge generation layers enclosed between a pair of transparent insulating metal oxide layers. A transparent indium doped tin oxide (ITO) layer acts as bottom electrode for light outcoupling and Aluminium (Al) as top reflective electrode. The electrodes are for applying field across the device and to charge the device, instead of injection of charge carriers in case of direct current (DC) devices. Bright luminance of up to 5000 cd m-2 is observed when the device is driven with an alternating current (AC) bias. The luminance observed is attributed to charge carrier generation and recombination, leading to formation of excitons within the device, without injection of charge carriers through external electrodes.