Thermally activated delayed fluorescence (TADF) has been proposed as a pathway to achieve high efficiency organic light-emitting diodes (OLEDs) without the use of heavy metal atoms in molecular structures. Many different factors can be decisive for efficient light emission from TADF emitters. However, a complete picture of the working mechanisms behind TADF is still missing and further research exploring novel material and device ideas is required. This thesis aims to extend the understanding of TADF emitter and OLED design considerations by investigating photophysical properties of novel materials as well as fabricating, optimizing and characterizing devices. TADF emitters have great potential of being used in OLEDs because they allow for high quantum efficiencies by utilizing triplet states via reverse intersystem crossing (RISC). In small molecules this is done by spatially separating the frontier orbitals, forming an intramolecular charge-transfer state (iCT) and leading to a small energy difference between lowest excited singlet and triplet states (Δ𝐸ST).
In polymer emitters, sufficient frontier orbital separation is harder to achieve, and typical strategies usually include adding known TADF units as sidechains onto a polymer backbone. In this thesis, a novel pathway of TADF polymer design is explored. A shift from a non-TADF monomer to TADF oligomers is explored. The monomer shows non-TADF emission and the delayed emission is shown to be of triplet-triplet annihilation (TTA) origin. An iCT state is formed already in the dimer, leading to a much more efficient TADF emission. This is confirmed by an almost two-fold increase of photoluminescence quantum yield (PLQY), a decrease in the delayed luminescence lifetime and the respective spectral line shapes of the molecules.
Recently, intermolecular effects between small-molecule TADF emitters have been given more attention, revealing strong solid-state solvation or aggregation induced changes of sample performance. Implications of this on device performance are not yet fully covered. A thorough investigation of a novel TADF emitter 5CzCO2Me is conducted. Steady-state emission spectra reveal a luminescence redshift with increasing emitter concentration in a small molecule host. In all investigated concentrations, the emission profile remains the same, thus the redshift is attributed to the solid-state solvation effect. The highest photoluminescence quantum yield (PLQY) is achieved in the 20 wt% sample, reaching 66 %. The best OLED in terms of current-voltage-luminance and external quantum efficiency parameters is the device with 60 wt% emitter concentration, reaching maximal EQE values of 7.5 %. It is shown that the emitter transports holes and that charge carrier recombination does not take place on the bandgap of the host, but rather, a mixed host-guest concentration dependent recombination is seen. The hole transporting properties of 5CzCO2Me allows for a new dimension in tuning the device performance by controlling the emitter concentration.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:78625 |
| Date | 29 March 2022 |
| Creators | Imbrasas, Paulius |
| Contributors | Reineke, Sebastian, Zysman-Colman, Eli, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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