Designing stable deep-blue thermally activated delayed fluorescence emitters for organic light-emitting diodes : degradation analysis of emitter lifetime

Abstract

Current and future full-color display applications demand highly efficient and stable organic light emitting diodes (OLEDs) because they require better color properties with higher brightness and lower power consumption than conventional technologies. Phosphorescence based on Ir(III)- or Pt(II)-complexes is the most successful technology which has fulfilled such requirements. The transition metal complexes have realized 100% internal quantum efficiency through harvesting not only singlet-excited states but also triplet-excited states by means of fast intersystem crossing provided by large spin-orbit coupling , which is not expected in the traditional fluorescent OLEDs. However, the electro-phosphorescence technology inevitably leads to high fabrication cost because of using the expensive Ir and Pt. An alternative OLED technology based on all-organic thermally activated delayed fluorescent (TADF) emitters has thus attracted great attention because of its ability to harvest triplet-excited states and shown tremendous potential in terms of efficiency, cost and environmental issues . In TADF materials, their non-emissive triplet-excited states can be harvested via repopulation of the emissive singlet-excited states through reverse intersystem crossing (RISC). In TADF materials, RISC can be induced by ambient thermal energy due to the small energy gap (EST) between the lowest singlet (S1) and triplet (T1) excited states. However, it is still unusual to find stable TADF devices, especially for blue color, in spite of their remarkable improvement in efficiency. We can find out the reason from not only the well-known properties of TADF materials such as long-lived triplet excitons but also the structural features of TADF materials. In chapter2, we demonstrate high efficiency TADF OLEDs which are attributed to employing triazine acceptor type TADF compounds having high RISC rates(kRISC). Triphenyltriazine (or a dibenzothiophene) carbazole derivatives are employed as the acceptor (A) and the donor (D) moieties (D) respectively. We can design four D-A type blue TADF compounds through the combination of the moieties and adjust the energy gap between the lowest excited singlet and triplet states and accordingly kRISC for the compounds. A blue OLED using one of the TADF molecules, 9-(9H-carbazol-9-yl)-12-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-5-phenyl-5,12-dihydroindolo[3,2-a]carbazole exhibits high maximum external quantum efficiency (22.3%) and external quantum efficiency (16.8 %) with low roll-off (27.3%) at a practical brightness (500 Cd/m2). due to its short exciton lifetime (6.1 s) induced by its high kRISC (1.825  106 s-1). In chapter3, we show that using a carbazole donor and a triazine acceptor with varied functional groups the position of the triplet exciton is playing a major role in enhancing operational stability, and thus device lifetime. Interestingly, repositioning the local character of triplet exciton distant from the C–N bond alters the dissociation pathway from smooth transition state to more abrupt conical intersection, the latter of which imposes higher energy barrier. With judicious selection of functional group within the acceptor, we have achieved 2.3-fold increase in device lifetime without compromising the more traditional design factors such as singlet-triplet energy gap ∆EST.