Over the last three decades, the performance of OLEDs has been improved rapidly, however, as an important assessment for OLED, the EQE data are still quite low. As outlined in the theoretical background, the EQE is the product of out-coupling efficiency and internal quantum efficiency (IQE). Therefore, this thesis focuses on designing two types of polymers with different optical functionalities, to increase the EQE addressing the two aforementioned determining factors.
Thus, the first part of the thesis addresses the light out-coupling efficiency in OLED devices. Here high refractive index (HRI) polymers are aimed for as potential material for the out-coupling layer, which are so far scarily reported for application in OLED devices, due to existing limitations, such as limited transparency, extra fluorescence, tedious synthesis, poor thermal stability and low solubility. However, if suitable polymers are becoming easily available, they will offer the unique advantage, compared to low molar mass HRI compounds, of being able to using cost-effective solution based technology for large area film preparation. In addition, polymeric materials will allow to introducing fully new concepts for increasing the light-out-coupling efficiency, like patterning allowing micro-lens preparation, or the incorporation of light scattering particles into the out-coupling layer.
The approach described in this thesis is based on a previous work where HRI polymers were prepared via metal-free thiol-yne “A2+B3” polyaddition reaction, which led in an easy one-pot synthesis to hyperbranched polyvinylsulfides of high solubility and already reasonable high RI. For further increasing RI, in this work B3 as well as finally A2 monomers with high naphthalene content were chosen which should, in addition to the positive effect of the sulfur-containing units, render polymers with even higher RI, and hopefully also of high solubility due to the branching. A challenging aspect of this work was to find reaction conditions which allow the preparation of high molar mass as well as highly soluble, highly aromatic polymers by that A2+B3 approach, even so very sterically demanding monomers are used. In addition, the material properties should be fine-tuned by careful selection of the monomer ratio.
It was expected that these new, easily available HRI polymers will be of high potential in OLED application. Thus, the work in this part of the thesis comprises on the one hand monomer and polymer synthesis as well as detailed characterization of the structure and the solution and thermal properties of the new materials. But on the other hand, the elucidation of the thin film preparation and the quality and optical properties of the resulting polymer films are major objectives. Finally, evaluation of the performance of the polymer films in an OLED device compared to conventionally low molar mass our-coupling layers was aimed for, which could be realized with the help of partners from the Institute of Applied Photophysics at TU Dresden.
For increasing the IQE in OLEDs, this thesis focuses on the development of a new type of polymeric thermally activated delayed fluorescence (TADF) material. TADF materials have the potential of theoretical 100% IQE and are considered as key materials for the next generation of OLEDs. So far, a significant amount of low molar mass TADF molecules has been developed, however, only a limited number of design rules are reported so far for polymers, even though polymers would offer, as already outlined above, significant advantages with regard to processing cost-effectively more efficient OLEDs for large area application.
The new concept described in this thesis for TADF polymers is based on a new monomer which exhibits individual promising structural units for achieving TAFD properties but does not emit TADF itself due to its large ΔEST (the energy gap between singlet S1 and triple T1 state). However, it has to be expected that once the monomer is polymerized, the resulting polymeric product will have reduced ΔEST due to the increased conjugation length and thus can be expected to emit TADF. This new concept has the potential to significantly increase the scope of polymeric materials with TADF properties.
Thus the second part on the thesis focuses on the design of a new monomer based on carbazole units with a pendant benzophenone moiety and its polymerization and full structural elucidation with the help of model compounds involving intensive NMR and MALDI-TOF analysis. In addition to the expect TADF properties, the benzophenone unit will also provide the possibility for film stabilization and even photopatterning due to photo-crosslinking. Thus the study of film formation and photo-crosslinking of the new TADF polymers was a further objective of this thesis. Finally, first theoretical as well as experimental studies of the photo physical properties of the monomer, a low molar mass model compound and the polymer, again together with the partners from the Institute of Applied Photophysics, should provide evidence on the suitability of the new polymer design principle.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:29831 |
| Date | 06 September 2016 |
| Creators | WEI, Qiang |
| Contributors | Voit, Brigitte, Reineke, Sebastian, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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