# Emergence of White Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence
## 2 Fundamental Concepts of TADF  
### The Evolution of OLED Emitters  
Due to the effect of spin statistics, when holes injected from the anode meet electrons injected from the cathode, singlet and triplet excitons will be formed with a ratio of 1:3 [67; 68; 69]. The singlet excitons produced decay rapidly, yielding prompt electroluminescence (fluorescence with lifetimes in nanoseconds). For conventional fluorescent materials, only the singlet excitons (25%) can emit light since the radiative decay of triplet excitons (75%) is spin forbidden [70; 71; 72; 73], as shown in Figure 1a. The conventional fluorescent material is also called the first-generation OLED emitter. By using this kind of emitters, the EQE of fluorescence WOLEDs is usually below 5%, considering that the outcoupling factor is \(\sim\)20%. In 1998, [PERSON] et al. first demonstrated electron-generated phosphorescence from heavy-metal osmium(II) complexes [74]. In the same year, [PERSON] et al. reported high-efficiency OLED by using a heavy-metal platinum(II) complex of PtOEP as the phosphor dopant [75]. For the phosphorescent emitter (direct radiative decay of triplet excitons results in phosphorescence with lifetimes in the microsecond to second regime), it can not only harvest triplets via the triplet-triplet energy transfer but also harvest singlets via the singlet-triplet ISC due to the heavy-atom effect, leading to a maximum IQE of 100% [76; 77; 78; 79], as shown in Figure 1b. The phosphorescent material is usually referred as the second-generation OLED emitter. By using this kind of emitters, the EQE of phosphorescence WOLEDs can be as high as 20%.  
In terms of the TADF material (the third-generation OLED emitter), a small energy gap (\(\Delta\)EST) between singlet (S\({}_{1}\)) and triplet (T\({}_{1}\)) excited states is required and can be attained by carefully designing organic molecules [43]. Generally, the S\({}_{1}\) level is considerably higher in energy than the T\({}_{1}\) level by 0.5-1.0 eV, because of the electron exchange energy between these levels [60]. However, to enhance thermal up-conversion (i.e., T\({}_{1}\)\(\rightarrow\) S\({}_{1}\) RISC), the molecular design of TADF materials requires small \(\Delta\)EST, typically less than 0.2 eV, to overcome competitive non-radiative decay pathways, leading to highly luminescent TADF materials [80]. In addition, to enhance the photoluminescence efficiency of TADF materials, the geometrical change in molecular conformation between its ground state (S\({}_{0}\)) and S\({}_{1}\) states should be restrained to suppress non-radiative decay. In TADF emitters, triplets are harvested as delayed fluorescence through RISC, as shown in Figure 1c. As a result, the maximum theoretical IQE of TADF emitters can be 100%. On the other hand, although triplet-triplet annihilation (TTA) makes dark triplet states accessible to emit p-type delayed fluorescence, the maximum theoretical IQE of TTA emitter is only 62.5% [81; 82; 83; 84], as shown in Figure 1d. As a result, the further development of TTA emitter may be limited. Herein, we mainly focused on the TADF emitters. By virtue of TADF, the IQE of WOLEDs is possible to be 100%.  
Figure 1: Exciton energy diagram and the possible decay ways of singlet and triplet excitons. Emissions from conventional fluorescent emitters (**a**); phosphorescent emitters (**b**); TADF emitters(**c**) and TTA based emitters (**d**). Reproduced with permission from [42].