# Emergence of White Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence
## 3 Approaches for WOLEDs Based on TADF
### Hybrid WOLEDs Based on Blue TADF and Phosphorescence  
#### 3.4.1 Hybrid WOLEDs including Blue TADF Materials and Complementary Phosphorescence Materials  
Since both TADF and phosphorescence emitters can harvest singlet and triplet excitons, the mixture of TADF and phosphorescence emitters is a significant approach to construct WOLEDs [139; 140; 141]. By virtue  
Figure 8: Schematic diagram showing the emission process of blue fluorescent device with a blue fluorescent emitter doped in a TADF host. Reproduced with permission from [138], John Wiley and Sons, 2015.  
of their respective advantages, high performance can be realized. In particular, there are tremendous interest in mixing blue TADF emitters with green/red or complementary color phosphorescence emitters. This is because blue TADF materials (i) are naturally advantageous to achieve high triplet energies due to their reduced singlet-triplet splits; (ii) can possess high efficiency, (iii) can harvest the triplets [142]. For this kind of WOLED (i.e., mixing blue TADF and other-color phosphorescence emitters), it is also called hybrid WOLEDs [60; 61; 62; 63; 64; 65; 66]. Previously, hybrid WOLEDs based on conventional blue fluorescent emitters have been extensively studied, since this kind of WOLEDs can show high efficiency, stable color, low voltage and long lifetime [143; 144; 145; 146; 147; 148; 149; 150; 151; 152; 153]. By replacing the conventional blue fluorescent emitters with TADF materials, high performance can also be expected. [PERSON] et al. reported the first hybrid WOLEDs by using blue TADF materials, achieving a maximum forward-viewing PE of 47.6 lm/W [142]. The device structure is ITO/HATCN (5 nm)/NPB (40 nm)/TCTA (10 nm)/mCP: 4,5-bis(carbazol-9-yl)-1,2-dicyanobenzene (2 CzPN, 11 nm, blue EML)/3-(4-biphenyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ): 4 wt. % (acetylacetonato)bis[2-(thieno[3,2-c]ridine-4-yl]phenyl]ridium(III) (PO-01, 4 nm, orange EML)/TAZ (40 nm)/LiF (0.5 nm)/Al (150 nm), as shown in Figure 9. The factors for the high performance: (i) mCP is chosen to be the host for 2 CzPN due to the wide energy gap and high T\({}_{1}\) of 3.0 eV, since the host of the TADF material plays an important role in determining the efficiency; (ii) 2 CzPN is placed nearest to the main recombination zone, ensuring that excitons can diffuse throughout the emissive region to produce a desired color-balanced output; (iii) triplets formed on 2 CzPN can be harvested by either energy transfer to the low-lying triplet states of the phosphor PO-01 (2.2 eV) or thermal upconversion to the emissive singlet states, eliminating the energy loss; (iv) the recombination zone is fixed as the voltage increases by 2 CzPN due to its charge trapping ability, achieving stable white emission. Figure 9: Schematic diagrams of the working mechanisms. The gray filled rectangle represents the main exciton generation zone. PF is the prompt fluorescence while DF is the delayed fluorescence. RISC indicates the reverse ISC and ET denotes the energy transfer. Reproduced with permission from [142], Royal Society of Chemistry, 2014. To further enhance the performance of this type of hybrid WOLEDs, [PERSON] et al. developed a three-color WOLED WOLEDs employing an efficient TADF material as the blue emitter, combined with red and green phosphorescent emitters, achieving the superior efficiency/CRI/color stability and low efficiency roll-off [154]. The WOLED shows the maximum EQE of 23.0%, PE of 51.7 Im W\({}^{-1}\), CRI of 89 and stable colors. The device structure is ITO (180 nm)/HATCN (10 nm)/TCTA: 20% HATCN (50 nm)/TCTA (20 nm)/mCP (10 nm)/TCTA: 8% ridium(III)bis(2,4-diphenyl-quinoline) (acetylacetonate) (Ir(PPQ)\({}_{2}\)(acac), red EML, 7 nm)/TCTA: 10% bis(2-phenylpyridine)iridium acetylacetonate (Ir(pppy)\({}_{2}\)(acac), green EML, 2.5 nm)/DPEPO: 10% DMAC-DPS (3.5 nm)/DPEPO (10 nm)/1,3-bis(3,5-dipyridyl-3-yl-phenyl)benzene (BmPyPB): 3% Li\({}_{2}\)CO\({}_{3}\) (35 nm)/Li\({}_{2}\)CO\({}_{3}\) (1 nm)/Al (100 nm).