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.
|Qualification||Doctor of Philosophy|
|Award date||26 Apr 2012|
|Place of Publication||Eindhoven|
|Publication status||Published - 2012|