TADF technologies have been developed that change the performance of future OLEDs to enable their practical application greatly. Furthermore, in the view of photochemistry and material chemistry, TADF technologies have given rise to a new category of luminescent materials, contributing to the academic progress in this field. [1]

Since the pioneering work of Tang and Vanslyke in 1980s[2], using organic materials for OLEDs has thrived for decades because of the enormous potential of OLEDs for smart phones, flatpanel displays, and solid-light emitting applications. The extensive research in this field to gain insight into the fundamental processes that determine the operation of the devices provides numerous interesting challenges to material design and fundamental questions to generate and transport charges and excitons.
The exciton formation under electrical excitation typically results in 25% singlet excitons and 75% triplet excitons is well known, in spite of that, 75% of the electrically generated energy is dissipated as heat by triplet excitons in the fluorescence materials, leading to the theoretically highest external quantum efficiency (EQE) of 5% after considering a light outcoupling efficiency of ~20% in device.
Many efforts to utilize the non-emissive triplet excitons have been devoted to breaking through the 5% limitation of the OLED device in order to increase the efficiency of the OLEDs. The most successful one is by incorporating heavy metals into the organic aromatic frameworks to increase spin–orbit interactions, which facilitate the lowest triplet excited state (T1) to the ground state (S0) transition (T1→S0) for phosphorescence luminescence. This approach harvests light from both triplet and singlet excitons, allowing the internal quantum efficiency of the device to reach nearly 100%. After delicately dispersed in carefully selected host materials, the phosphorescent metal complex exhibits very high external quantum efficiency (EQE), which has been over 30% in doped phosphorescent OLEDs(PhOLEDs) recently.

Nevertheless, the heavy metals Iridium (Ir) and Platinum (Pt) are confined for phosphorescence generally, which are rather expensive and relying on limited global resources. So that, several other strategies to avoid the use of those expensive metals in practical applications, such as triplet–triplet annihilation (TTA), tuning spin–orbit coupling by side-stepping Kasha’s rule, hybridized local and charge transfer (HLCT) and thermally activated delayed fluorescence (TADF)[3][4][5] have also been proposed to harvest the 75% triplet excitons for luminescence. In those researches, TADF was found to have the most rapid progress in recent investigations. The TADF strongly depends on HOMO–LUMO separation in a single molecule. TADF materials have a sufficiently small energy gap between (S1) and (T1) (⊿EST) to enable up-conversion of the triplet exciton from (T1) to (S1). This small ⊿EST enables TADF materials to realize 100% of the exciton formation generated by electrical excitation at (S1).

According to a professor Chihaya Adachi at Kyushu University with“Hyper-Fluorescence" idea , the new 3.5 -generation TADF element is the common use of a TADF and fluorescence characteristics of co-dopant material , more to further improve efficiency.[6]

The success in theoretical and technical challenges of TADF may prepare for the future of organoelectronics.

[1] Japanese Journal of Applied Physics 53, 060101 (2014)
[2] C. W. Tang , S. A. Vanslyke , Appl. Phys. Lett. 1987 , 51 , 913 .
[3] Q. Zhang , Q. Zhou , Y. Cheng , L. Wang , D. Ma , X. Jing , F. Wang ,Adv. Mater. 2004 , 16 , 432 .[4] H. Uoyama , K. Goushi , K. Shizu , H. Nomura , C. Adachi , Nature 2012 , 492 , 234 .
[5] A. Chihaya , Jpn. J. Appl. Phys. 2014 , 53 , 60101 .

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