| 1. |
- Lindell, Linda, 1976-
(författare)
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Interface Engineering in Organic Electronics
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Organic electronics is a field covering all applications and devices where one or several of the active components are made of organic material, such as organic light emitting diodes, organic solar cells, organic thin film transistors, organic magnets for spintronics etc. In all of the applications mentioned above, transport of charges across both inorganic/organic and organic/organic interfaces play a key role for device performance. In order to achieve high efficiencies and longer life-times, proper matching of the electronic energy levels of the different materials is needed.The aim of the research presented in this thesis has been to explore different routes to optimize interface energetics and gain deeper knowledge of the mechanisms that govern charge transport over the interface. Photoelectron spectroscopy (PES) is a method well suited to study both interactions between different materials taking place at surfaces as well as interface energetics.One way to achieve proper matching of interfaces energy levels is by adding a dipole layer. In the three first papers presented in the thesis, the method of adding a monolayer of small organic molecules to change the work function of the surface is investigated. We start with a model system consisting of a nickel surface and PPDA molecules where we have strong interaction and mixing of orbitals between the molecule and the metal surface. The second system consists of a gold surface and TDAE molecules with weaker interaction with integer electron transfer and finally in the third paper an organic surface VPP-PEDOT-Tos is modified, with TDAE, to create a transparent low work function organic electrode. In the fourth paper, we focus on gaining deeper understanding of the Integer Charge Transfer (ICT) model and the mechanisms governing the alignment of energy levels at organic/(in)organic interfaces and in the fifth paper we continue to challenge this model by using it to predict the behavior of a bilayer device, in terms of energy level alignment.
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| 2. |
- Hansson (f.d. Wadeasa), Amal, 1977-
(författare)
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Heterojunctions between zinc oxide nanostructures and organic semiconductor
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Lighting is a big business, lighting consumes considerable amount of the electricity. These facts motivate for the search of new illumination technologies that are efficient. Semiconductor light emitting diodes (LEDs) have huge potential to replace the traditional primary incandescent lighting sources. They are two basic types of semiconductor LEDs being explored: inorganic and organic semiconductor light emitting diodes. While electroluminescence from p-n junctions was discovered more than a century ago, it is only from the 1960s that their development has accelerated as indicated by an exponential increase of their efficiency and light output, with a doubling occurring about every 36 months, in a similar way to Moore's law in electronics. These advances are generally attributed to the parallel development of semiconductor technologies, optics and material science. Organic light emitting diodes (OLEDs) have rapidly matured during the last 30 years driven by the possibility to create large area light-emitting diodes and displays. Another driving force to specifically use semiconducting polymers is the possibility to build the OLED on conventional flexible substrates via low-cost manufacturing techniques such as printing techniques, which open the way for large area productions.This thesis deals with the demonstration and investigation of heterojunction LEDs based on p-organic semiconductor and n-ZnO nanostructures. The ZnOorganic heterojunctions are fabricated using low cost and simple solution process without the need for sophisticated vacuum equipments. Both ZnO-nanostructures and the organic materials were grown on variety of substrates (i.e. silicon, glass and plastic substrates) using low temperature methods. The growth mechanism of the ZnO nanostructures has been systematically investigated with major focus in ZnO nanorods/nanowires. Different organic semiconductor materials and device configurations are explored starting with single polymer emissive layer ending up with separate emissive and blocking layers, or even blends. Interestingly, the photoluminescence and electroluminescence spectra of the hybrid LEDs provided a broad emission band covering entirely the visible spectrum [∼400-∼800nm]. The hybrid light emitting diode has a white emission attributed to ZnO intrinsic defects and impurities in combination with the electroluminescence from the conjugated polymers. The ZnO nanostructures in contact with a high workfunction electrode constitute an air stable electron injecting contact for the organic semiconductor. Hence, we have shown that a white light emission can be achieved in a ZnO-organic hybrid light emitting diode using cheap and low temperature growth techniques for both organic and inorganic materials.
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| 3. |
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| 4. |
- Larsson, Oscar, 1978-
(författare)
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Polarization characteristics in polyelectrolyte thin film capacitors : Targeting field-effect transistors and sensors
- 2009
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Polymers are very attractive materials that can be tailored for specific needs and functionality. They can for instance be made electrically insulating or (semi)conducting, with specific mechanical properties. Polymers are often processable from a solution, which enables the use of low-cost manufacturing techniques to fabricate polymer devices. Polymer-based electronic and electrochemical devices and sensors have been developed.This thesis is related to the polarization characteristics in polyelectrolyte thin film capacitor structures. The polarization characteristics have been analyzed at various humidity levels for polyelectrolyte capacitors alone and when incorporated as the gate-insulating material in polyelectrolyte-gated organic field-effect transistors. Both limitations and possibilities of this class of transistors have been identified. Also, a concept for wireless readout of a passively operated humidity sensor circuit is demonstrated. The sensing mechanism of this sensor is related to the polarization in a polyelectrolyte thin film capacitor. This sensor circuit can be manufactured entirely with common printing technologies of today and can be integrated into a low-cost passive sensor label.
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| 5. |
- Ullah, Zia
(författare)
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Thermoelectric Devices with Electronic and Ionic Conducting Polymers
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The energy consumption in the world is continuously growing and the sources of energy are largely dominated by fossil fuels. However, the resources of oil, gas and coal are diminishing in capacity. Moreover the CO2 emissions arising from their combustion is a great concern because it induces climate changes that threaten our habitat. There is a dire need to look for alternative sources of energies and to minimize losses of energy in our surroundings. Heat engines and turbines typically running with fossil energy have efficiencies of about 35%, i.e. 65% of the energy is lost in the form of heat. Low temperature heat (<200 ᴼC) is almost always wasted in power plants, industries, automobiles and household appliances. This is a huge resource that can be directly converted to electricity through the concept of thermoelectricity. Major challenges for heat to electricity conversion include finding the abundant materials with efficient thermoelectric (TE) conversion that can be mass produced at low cost.This thesis presents an investigation of the TE properties of electronic and ionic conducting polymers, as well as their implementation in thermoelectric devices. This is a journey from thin solid films on a substrate to wet and liquid media and towards bulk structures utilizing the same core concept of thermoelectricity. The TE device concepts introduced here are suitable for various heat sources i.e. continuous, intermittent and instantaneous. The thesis has three major parts as follows:Conducting polymers (CPs) have been studied mainly as thin films. They have been synthesized in different ways and their properties have been compared to propose the most efficient amongst them for thermoelectricity. Simple methods of exposure to certain gases or liquids have been used to tune their TE properties and demonstrated its applications in thermoelectric generator (TEGs).Ionic materials have also been studied as potential candidates for thermoelectricity. Polyelectrolytes constitute a special class of electrolytes with dissimilar sizes of ions; a polymeric ion and a small counter ion. The movement of the small sodium (Na+) cation under heat gradient was explored in wet films and in solution. Because the ions could not cross the electrolyte-electrode junction, we propose the idea of ionic thermoelectric supercapacitor (ITESC), suitable for intermittent heat source.Nanofibrillated cellulose (NFC) has been used along with conducting polymers to realize the three dimensional conducting bulks as a TEG leg. NFC bulks were coated with conducting polymers as a first approach and later the mixture of (NFC & CP) was freeze-dried. The later approach resulted in mechanically flexible structures that were used as dual sensors for pressure and temperature based on the TE properties of the CP which can be utilized for electronic skin applications.The thesis shows new ways of utilizing waste heat using polymeric materials and points to a sensory application area, broadening the horizons of thermoelectricity.
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| 6. |
- Wang, Xiaodong, 1981-
(författare)
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Controlling ion transport in organic devices
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Organic electronics and printed electronics have been attracting more and more research interest in the past decades. Polymers constitute an important class of materials within the field organic electronics due to their unique physical and chemical properties. One great benefit of the polymers is their solution processability, which provides us the possibility to utilize conventional printing techniques to fabricate devices on flexible substrates.This thesis focuses on utilizing and controlling the ion transport in polyelectrolytes in electronic devices for different applications. A polyelectrolyte is a polymer in which the polymeric backbone includes ionic sites compensated by counter ions.Firstly, we have used a specific property of the polyelectrolyte: its electric polarization is strongly dependent on the humidity level. The ions are screened by water molecules; this improves the mobility and dissociation of ions. A polyelectrolyte-based capacitor is thus ideal to sense humidity. Such a capacitor is integrated into an LC resonant circuit possessing a specific resonant frequency. The wirelessly detected resonant frequencies of the sensing circuit indicate the corresponding humidity levels. With the appropriate choice of materials, the complete sensing circuit (resistor, capacitor, capacitor-like sensor head) can be screen-printed on an antenna manufactured using a roll-to-roll dry phase patterning technique.Secondly, we have modified the polarization characteristics of ions in a polyelectrolyte layer by trapping the ions in molecular macrocycles dispersed in a polymer overlayer. The resulting remanent polarization is read out as a hysteresis loop in the capacitance-voltage characteristic of a capacitor. The strategy is further implemented in an electrolyte-gated organic transistor to control its threshold voltage by applying defined programming voltages. Although the lifetime of the “remanent” polarization is rather short, the concept might be further improved to fit those of memory applications.Finally, we take use of the ionic selectivity of a polyelectrolyte to stabilize the operation of a water-gated organic field-effect transistor. The polyanionic membrane is added onto the semiconductor channel to prevent small anions of the aqueous electrolyte to penetrate into the p-channel semiconductor. Moreover, the polyelectrolyte layer protects the semiconductor and thus strongly stabilizes the shelf lifetime of those transistors. This improved version of the water-gated organic transistor is a candidate for developing transistor-based sensors working in, for instance, biological media.
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| 7. |
- Wijeratne, Kosala, 1983-
(författare)
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Conducting Polymer Electrodes for Thermogalvanic Cells
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Fossil fuels are still the dominant (ca. 80%) energy source in our society. A significant fraction is used to generate electricity with a heat engine possessing an efficiency of approximately 35%. Therefore, about 65% of fossil fuel energy is wasted in heat. Other primary heat sources include solar and geothermal energies that can heat up solid and fluids up to 150°C. The growing demand and severe environmental impact of energy systems provide an impetus for effective management and harvesting solutions dealing with waste heat. A promising way to use waste heat is to directly convert thermal energy into electrical energy by thermoelectric generators (TEGs). Solid state TEGs are electronic devices that generate electrical power due to the thermo-diffusion of electronic charge carriers in the semiconductor upon application of the thermal field. However, there is another type of thermoelectric device that has been much less investigated; this is the thermogalvanic cell (TGCs). The TGC is an electrochemical device that consists of the electrolyte solution including a reversible redox couple sandwiched between two electrodes. In our study, we focus on iron-based organometallic molecules in aqueous electrolyte. A temperature difference (Δ?) between the electrodes promotes a difference in the electrode potentials [Δ?(?)]. Since the electrolyte contains a redox couple acting like electronic shuttle between the two electrodes, power can be generated when the two electrodes are submitted to a temperature difference. The focus of this thesis is (i) to investigate the possibility to use conducting polymer electrodes for thermogalvanic cells as an alternative to platinum and carbon-based electrodes, (ii) to investigate the role of viscosity of the electrolyte in order to consider polymer electrolytes, (iii) to understand the mechanisms limiting the electrical power output in TGCs; and (iv) to understand the fundamentals of the electron transfer taking place at the interface between the polymer electrode and the redox molecule in the electrolyte. These findings provide an essential toolbox for further improvement in conducting polymer thermogalvanic cells and various other emerging electrochemical technologies such as fuel cells, redox flow battery, dye-sensitized solar cells and industrial electrochemical synthesis.
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