Development of carbazole based donor acceptor typed fluorescent materials for oled applications

Abstract

Organic light-emitting diodes (OLEDs) are a growing area for display and lighting technologies due to their numerous applications from both scientific and industry perspectives. Typically, an OLED device structure is formed within multi-organic layers placed between positive and negative electrodes. The most crucial part of the structure is the emissive layer, which emits the three primary colors red, green, and blue via the photoluminescence process. Especially blue-emissive materials suffer from several problems due to having a wide band gap (Eg), through the red & green ones. The main strategy to state of the art device structure is high photoluminescence (PL) and electroluminescence (EL) efficiency. The historical development of OLEDs is divided into three generations by their emission mechanism: fluorescence, phosphorescence, and thermally activated delayed fluorescence (TADF). While 1st generation devices rely on PL emission pathway by fluorescence and 2nd ones are based on phosphorescence. Both of them have their cons and pros but 3rd generation devices have more superior features than others. These last-generation devices, TADF emitters, can meet the requirements of the device's performance efficiency, lifetime, and long-term stability. In this thesis, we aimed to design, synthesize, and characterize carbazole-based organic materials that emit blue light with the intent to show the TADF mechanism. In the first chapter, new well-defined structurally different 9,10-bis(9-hexyl-6-((E)-styryl)-9H-carbazol-3-yl)anthracene derivatives, shortly named "6a, 6b and 6c" with side groups having electron-donating and withdrawing were systematically synthesized using Suzuki cross-coupling and Vilsmeier-Haack reactions. The impact of electron-donating and withdrawing groups and their influence on the molecule's photophysical properties has been investigated. The materials showed sky-blue emissions with high internal quantum efficiencies. Based on photophysical investigations the most promising molecule (6a) has been selected and high-efficiency OLEDs with external quantum efficiency at very low current efficiency (~1 mA/cm2) reaching 5 % (doped) were obtained. In the second chapter, we report the synthesis and photophysical properties of deep-blue emitting donor-acceptor (D-A) and donor-acceptor-donor (D-A-D) thermally activated delayed fluorescence (TADF) molecules using carbazole as a donor (D) and a pyridyl (a)-sulfonyl (A) based bifunctional group as an acceptor. The work reveals how structural changes favor reverse intersystem crossing (rISC) by forming an emissive charge transfer (CT) state, which is thoroughly investigated in different donor and asymmetric acceptor positions. Three comparison sets of regioisomers are investigated. 2,5-substituted pyridine derivatives in Set-1 are D-Aa, D-aA, and D-Aa-D structures with asymmetric acceptor systems, revealing that the donor nearer to the pyridine group substantially controls the TADF properties. In Set-2, modified the D-Aa-D structures reveal how ortho and meta positioned relative to A (keeping the carbazole at meta to the A) affects the emission properties, deactivating TADF, and promotion triplet-triplet annihilation. In the final set, 2,4-substituted pyridyl-sulfonyl derivatives show that the positioning of the donor far from the pyridine group has minimal influence. This final set of molecules shows superior optical and physical properties though, indicating the importance of correct positioning between D, a, and A. In the third chapter, we have designed and synthesized a pair of highly asymmetric D-aA-D' type pyridyl-sulfonyl based isomers comprising phenothiazine (PTZ) and carbazole (Cz) donor units, which can emit thermally activated delayed fluorescence. PTZ-pS4-Py-2Cz and PTZ-mS4-Py-2Cz both possess spatial separation of HOMO/LUMO on the donor and acceptor moieties, resulting in small calculated singlet–triplet energy gaps (~0.25 eV). Both isomers exhibit dual emission, which is attributed to charge transfer states associated with the Cz and PTZ moieties at higher and lower energies, respectively. Photoluminescence quantum yields and time-resolved emission decays show significant differences for the two isomers, with the para-isomer exhibiting more efficient emission and stronger delayed fluorescence than the meta-isomer in strong contrast to recently reported analogous Cz-Cz D-aA-D isomers. The findings clearly show that the interconversion of triplets via the rISC mechanism is promoted when parallel Cz and PTZ charge transfer states are allowed to interact, explaining the improved performance of the Cz-PTZ materials compared to the previous Cz-Cz ones. Finally, moderate device performance was achieved in warm-yellowish (CIE; 0.41; 0.53 & 0.49; 0.48) non-doped OLEDs, which exhibited 0.5% & 1.9% maximum external quantum efficiencies for the meta- and para-isomers, respectively.