Development of a charge transport model for white OLEDs

Organic light-emitting diodes (OLEDs) are promising candidates for future lighting applications since they can be made ultra-thin, color-tunable and exible. Improving their e??ciency is an important challenge in the ??eld of organic electronics. The availability of a device model can make the rational design of more e??cient devices possible. State-of-the-art models give insights on processes at the molecular scale, for instance by yielding physically interpretable parameters. This brings the molecular-scale design of more suitable materials one step closer. In this thesis, crucial elements towards the development of a multilayer white OLED model are presented. The white OLEDs on which this study ??nally focusses are introduced in Chapter 1. They contain evaporation deposited electron and hole transport layers but also a blue-emitting uorescent layer and red- and green-emitting phosphorescent layers. An overview is given of how one can analyze current-density (J) versus voltage (V ) characteristics of single layer devices. Also the recently developed \extended Gaussian disorder model" (EGDM) and the \extended correlated disorder model" (ECDM) are introduced. In this thesis, these models are used to analyze the measured electrical characteristics of various small- molecule based single-layer devices. This was done to explore the necessary steps towards multilayer device modeling, and to obtain the charge carrier mobility in the layers. In addition, single layer polymer devices are investigated in order to develop and validate additional methods for studying the charge transport in disordered organic semiconductors. The interpretation of the characteristics is found to require detailed knowledge of the materials and the interfaces. Several parameters must be determined to achieve an accurate description. Firstly, this led to the question how accurately materials parameters which determine the mobility can be determined based on J(V ) characteristics of single layer devices. In Chapter 2, an extraction method based on a Gauss-Newton algorithm is developed and thoroughly tested on both hole-only and electron-only characteristics to investigate the accuracy but also the limitations of parameter extraction. It is concluded that for hole transport the extraction is straightforward, in contrast to the case of electron transport where the extraction often requires additional knowledge about the system. Obviously, if one could use an independent experimental technique to determine one of the parameters involved this would improve the accuracy of electrical characterization. One of the important parameters in single layer devices is the built-in voltage. To give insight into this parameter, an electroabsorption setup was built with which poly uorene based co-polymer hole-only devices were studied. These devices were already found to be well described using the EGDM, which makes it possible to interpret the voltage at which the electroabsorption signal goes to zero in relation to the built-in voltage. A signi??cant diff??erence between both voltages was found. This is explained by charge carrier di??usion in the co-polymer layer. This work is described in Chapter 3. As mentioned, the poly uorene-based co-polymer devices were found to be well described using the EGDM. Now the question arises how well the ECDM can describe the transport in these devices. This was investigated in Chapter 4, using the extraction method introduced in Chapter 2. It was found that a similarly good ??t to the data could be obtained but with an unrealistically high hopping site density. We view the unrealistically high site density as evidence for the absence of correlated disorder in this material. Hence, one may discriminate between both models by making a comparison with the experimental site density. Analyzing electron transport is often more di??cult due to an increased experimental uncertainty (possibly caused by charge carrier relaxation e??ects) and due to the presence of additional parameters describing a density of trap states in such materials. In Chapter 5, we re-analyzed published electrical characteristics of electron-only devices based on the well-studied small-molecule material Alq3 using the EGDM and the ECDM. A consistent set of parameters was used for all devices studied. In contrast to the earlier analysis of these characteristics using a conventional model, a strong indication for the presence of a signi??cant injection barrier was found. A good ??t quality was found using a realistic site density with the ECDM. In studying charge transport, it has often been noticed that charging e??ects can signi??cantly a??ect the measurement results, in particular for electron transport. For this reason, an extension of the model in which also time-dependent relaxation e??ects are described, is valuable. Chapter 6 describes to what extent the EGDM can explain measured dark injection (DI) transients. For the polymer based hole-only devices studied already in Chapter 3 and 4, a strong indication for the presence of the (time-dependent) relaxation e??ect was found from a thorough comparison between the experimental DI peak position with modeling results. In Chapter 7 parameter extraction is applied to measured characteristics of two small-molecule based materials (Spiro-DPVBi and NET-5) which are employed in the white multilayer stack mentioned. Subsequently, these experimental results on the mobility have been employed in a charge transport model based on 3D Monte Carlo simulations, developed by the Eindhoven University of Technology. A comparison between the predicted and the experimental current-density versus voltage curve is presented. This shows that it is now possible to make a multilayer OLED model based on physically interpretable pa- rameters and experimentally determined mobilities. With this model, further experimental validation is possible and scenario studies can be done that can bring the rational design of more e??cient OLEDs one step closer

Polymer induced condensation of dna supercoils

Macromolecular crowding is thought to be a significant factor driving DNA condensation in prokaryotic cells. Whereas DNA in prokaryotes is supercoiled, studies on crowding-induced DNA condensation have so far focused on linear DNA. Here we compare DNA condensation by poly(ethylene oxide) for supercoiled and linearized pUC18 plasmid DNA. It is found that supercoiling has only a limited influence on the critical amount of PEO needed to condense plasmid DNA. In order to pack DNA supercoils in condensates, it seems inevitable that they must be deformed in one way or another, to facilitate dense packing of DNA. Analytical estimates and Monte Carlo simulations indicate that packing of DNA supercoils in condensates is most likely facilitated by a decrease of the superhelical diameter rather than by unwinding of the supercoil

Monte Carlo simulations of flexible polyanions complexing with whey proteins at their isoelectric point

The complexation of globular proteins with flexible polyelectrolytes with homogeneous, oppositely charged spheres was discussed using Monte Carlo simulations. The proteins were considered at their respective isoelectric points. A coarse-grained model of the protein shape was also considered in order to take into account the protein excluded volume. A simple statistical analysis of the surface charge density was found sufficient for identifying potential polyelectrolyte binding regions. The protein-only approach identified only possible regions of polyelectrolyte binding, and did not account for any of the polyelectrolyte properties tha influenced binding

From specification to measurement: the bottleneck in analog industrial testing

The translation of the specification of an analog device into the necessary set of measurements to be carried out by an industrial test facility is discussed. Algorithms are developed to compute the number of test vectors needed to guarantee a certain parameter and to compare several possible test methods based on accuracy. It is shown that the specification of a circuit can be transformed into single-parameter measurements, to be carried out by an industrial test facility. There is a tradeoff between the number of measurements and the accuracy of the specified parameter. A computationally efficient tradeoff between test methods based on maximum accuracy can be made. One of the aspects needed to make these tradeoffs, the measurement error, can be predicted using one of the proposed experiments. There is a tradeoff between the complexity of the experiment and the accuracy of the error prediction

Hoogveen en klimaatverandering in Nederland

Voor de instandhouding en ontwikkeling van hoogveen zijn het neerslagoverschot, de temperatuur en de positie in het landschap belangrijk. Gunstige ontwikkelingen doen zich voor in gebieden waar het (actieve) hoogveen water uit zijn omgeving ontvangt. De landelijke instandhoudingsdoelen voor Natura 2000-habitattype Actieve hoogvenen kunnen waarschijnlijk ook onder het klimaatscenario W+ worden gerealiseerd: behoud van kwaliteit en oppervlakte zijn kansrijk en verbetering van kwaliteit en uitbreiding van oppervlakte zijn mogelijk. Voorwaarden hierbij zijn een optimale waterhuishouding. Dat wil zeggen voldoende hoge grondwaterstanden in de zandondergrond en de veenbasis in combinatie met een waterondoorlatende (veen)laag en/of de toevoer van lokaal grondwater. Om hoogvenen op de lange termijn in Nederland te behouden onder het W+- scenario zijn waterhuishoudkundige maatregelen nodig, zoals de aanleg en inrichting van bufferzones en compartimenten en/of door het bevorderen van kwel

Blauwdruk voor een grensoverschrijdende fiscale eenheid in de vennootschapsbelasting

Het huidige fiscale-eenheidsregime in de vennootschapsbelasting ligt onder “EU-spervuur”. Indien de bij het Hof van Justitie EU aanhangige zaken negatief uitpakken voor de Nederlandse staat, is er een gerede kans dat het fiscaleeenheidsregime zal worden afgeschaft en vervangen door een minder vergaande regeling, bijvoorbeeld een group reliefregeling die (louter) verliesoverdracht binnen een groep mogelijk maakt. Om veel redenen zou het te betreuren zijn indien het huidige fiscale-eenheidsregime wordt afgeschaft, want het biedt niet alleen de belastingplichtigen maar ook de fiscus belangrijke voordelen. Na een inventarisatie van deze voordelen wordt een lans gebroken voor de grensoverschrijdende fiscale eenheid. [...