Highly efficient organic light-emitting diodes from thermally activated delayed fluorescence
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Organic light-emitting diodes (OLEDs) are emerging as a technology that advances the performance of display and lighting applications. This thesis presents recent progress made in the design, fabrication, modeling, testing, and application of state-of-the-art highly efficient OLEDs employing emitters exhibiting thermally activated delayed fluorescence (TADF). Challenging the conventional emissive layer (EML) design strategy, the work shows that high-efficiency performance can be achieved in devices employing heavily doped or host-free EMLs. A systematic study on the influence of host/guest ratio in the EML on the external quantum efficiency (EQE) performance was conducted by characterizing a series of OLEDs doped with oBFCzTrz, a blue-emitting TADF emitter, at various concentrations. Results showed that aggregation-induced fluorescence quenching in heavily doped EMLs is small. A time-resolved electroluminescent decay experiment was conducted, and an analysis based on a Correlated-Charge-Pair model reveals significant differences in charge trapping and recombination in devices as a function of emitter concentration. After optimizing the concentration of oBFCzTrz at 50 wt.% in the EML, devices yielded a maximum EQE of 25.5% at 10 cd/m2; host-free devices achieved a high EQE of 14.0% with zero efficiency roll-off at luminance value of 1,000 cd/m2. Employing the same strategy, a yellow-green-emitting TADF emitter, TCZPBOX, was used as a host-free EML in devices and achieved a state-of-the-art maximum EQE of 21.2% at 10 cd/m2, and retained a value of 13% at 10,000 cd /m2. A simplified single-stack structure for white OLEDs using an EML only consisted of blue and yellow TADF emitters was studied. TADF OLEDs fabricated on polymer substrates were successfully demonstrated.