# Emergence of White Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence
## 3 Approaches for WOLEDs Based on TADF
### WOLEDs Based on TADF and Conventional Fluorescence
#### 3.3.2 WOLEDs Employing Yellow TADF and Complementary Fluorescence Materials  
Aside from the use of above-mentioned blue TADF emitter system, the utilization of other-color TADF emitters combined the complementary fluorescence materials is also an effective way to organize WOLEDs. In principle, the TADF molecule also acts as a triplet harvester for fluorescent emitters, ensuring the high efficiency. For example, [PERSON] et al. reported high-efficiency and high CRI WOLEDs with the chromaticity-adjustable yellow TADF emitter 2-(4-phenoxazinephenyl) thianthrene-9,9\({}^{\prime}\), 10,10\({}^{\prime}\)-tetraoxide (PXZDSO2) [56]. By combining the conventional deep-blue fluorescence emitter NI-1-PhTPA  
Figure 4: Schematic illustrations of a conceptual energy transfer mechanism from DMACDPS (common blue TADF emitter) to TTPA and DBP under electrical excitation. Reproduced with permission from [126], John Wiley and Sons, 2015.  
and PXZDSO2, the two-color WOLED showed a maximum EQE of 15.8% (device W3). Then, since the chromaticity of the EML containing PXZDSO2 could be tuned to yellowish green, they introduced a deep-red-fluorescence emitter DBP (dibenzo[[f,f']-4,4',7',7'-tetraphenyl]diindeno [1,2,3-cd:1',2',3'-lm]perylene) subtly to fabricate three-color WOLED, achieving the most efficient ever EQE of 19.2% with CRI of 68 (device W4) and the highest ever CRI of 95 with EQE of 15.6% (device W6). The configurations are ITO/hexaatriphenylene hexacarbonitrile (HATCN)/TAPC/EMLs/TmPyPB/LiF/Al, in which device W3 has the EML of CBP: 8 wt. % NI-1-PhTPA (10 nm)/CBP (3 nm)/CBP: 6 wt. % PXZDSO2 (15 nm)/CBP: 8 wt. % NI-1-PhTPA (10 nm), device W4 has the EML of CBP: 7 wt. % NI-1-PhTPA (10 nm)/CBP (3 nm)/CBP: 3 wt. % PXZDSO2 (5 nm)/CBP: 5 wt. % PXZDSO2: 0.3 wt. % DBP (5 nm)/CBP: 3 wt. % PXZDSO2 (5 nm)/CBP: 7 wt. % NI-1-PhTPA (10 nm), device W6 has the EML of CBP: 10 wt. % NI-1-PhTPA (10 nm)/CBP (3 nm)/CBP: 5 wt.% PXZDSO2: 0.35 wt. % DBP (15 nm)/CBP (3 nm)/CBP: 10 wt. % NI-1-PhTPA (10 nm). The device working mechanisms can be described as follows. For device W3, (i) since NI-1-PhTPA is a deep-blue-fluorescence emitter and CBP: 6 wt. % PXZDSO2 emit yellow light with broad spectrum, high-performance two-color WOLEDs were realized; (ii) given the almost equal T\({}_{1}\) level of NI-1-PhTPA and PXZDSO2, the efficiency roll-off occurs if they are directly contact due to the triplet exciton quenching by NI-1-PhTPA; (iii) the efficiency roll-off can be further induced as the formed triplet excitons of NI-1-PhTPA cannot be utilized by PXZDSO2; (iv) to stabilize the recombination zone which occurs in whole EMLs since NI-1-PhTPA/CBP are bipolar and avoid triplet exciton quenching by NI-1-PhTPA, two 3 nm CBP were inserted between the blue and yellow EMLs, restraining the inevitable Forster energy transfer from NI-1-PhTPA to PXZDSO2; (v) to reduce the triplet exciton energy loss via nonradiative transition process, blue-fluorescence emitter was dispersed in CBP for blue emission, leading to most excitons being generated at CBP; (vi) triplet energy transferred from CBP gives most of triplet excitons of PXZDSO2 since triplet excitons typically have long diffusion lengths (\(\approx\)100 nm), as shown in Figure 5a. Hence, an EQE of 15.8% was achieved for device W3. For device W4, (i) a deep-red-fluorescence emitter DBP was conceived to be used; (ii) PXZDSO2 was an assistant host for DBP to realize red light emission due to efficient energy transfer from the S\({}_{1}\) of PXZDSO2; (iii) the doping concentration of PXZDSO2 was decreased to reduce intermolecular aggregation and thus blue shifted emission (20 nm), achieving green emission, complementary to emissions of NI-1-PhTPA and DBP; (iv) a red EML of CBP: 5 wt. % PXZDSO2: 0.3 wt. % DBP was inserted between two green EMLs of CBP: 3 wt.