# 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
The key strategy is the utilization of blue-green-red cascade energy transfer structure, the regulation of the doping concentration and the manipulation of charges and excitons. For the blue emission, it is originated from the TADF emitter DMAC-DPS. In the case of green emission, it is ensured by the energy transfer process between DMACDPS and Ir(pppy)\({}_{2}\)(acac). In terms of red emission, it is resulted from not only the energy transfer between Ir(pppy)\({}_{2}\)(acac) and Ir(PPQ)\({}_{2}\)(acac) but the self-charge-trapping effect, as shown in Figure 10. As a result, an efficient white emission is produced. Although high-performance hybrid WOLEDs based on TADF materials have been demonstrated, there are still some problems, even for these state-of-the-art devices [142; 154]. For example, (i) the driving voltages are somewhat high (e.g., 3.2 V at 1 cd m\({}^{-2}\)[154]); (ii) the luminances are very low (e.g., only \(\sim\)10000 cd m\({}^{-2}\)[154]); iii) the efficiency at high luminance is not high (e.g., \(<\)6 Im W  
Figure 10: Proposed energy level diagram of the WOLED and chemical structures of emitters. R, G and B represent Ir(PPQ)\({}_{2}\)(acac), Ir(pppy)\({}_{2}\)(acac) and DMAC-DPS, respectively. Solid and dashed lines refer to HOMO and LUMO energy levels; circles and diamonds correspond to the exciton energies (S\({}_{0}\), S\({}_{1}\) and T\({}_{1}\)), respectively. 1 presents the energy transfer process and 2 is the carrier trapping process. PF is the prompt fluorescence, DF is the delayed fluorescence. Reproduced with permission from [154], John Wiley and Sons, 2016.  
at 10000 cd m\({}^{-2}\)[154]); (iv) the CRI is not high enough; (v) negligible attention has been paid to the lifetime of TADF-based hybrid WOLED. To solve the issues, [PERSON] et al. recently reported high-performance two-color and three-color hybrid WOLEDs [155]. The two-color WOLED exhibits (i) low voltage (i.e., 2.9 V at 1 cd m\({}^{-2}\)); (ii) high luminance (103756 cd m\({}^{-2}\)); (iii) maximum total EQE and PE of 23.5% and 70.92 Im W\({}^{-1}\), respectively; (iv) 21.59 Im W\({}^{-1}\) at 10000 cd m\({}^{-2}\). The three-color WOLED exhibits (i) low voltage and high luminance (51514 cd m\({}^{-2}\)); (ii) superior CRI of 94; (iii) EQE and PE of 17.3% and 46.09 Im W\({}^{-1}\), respectively. The configuration of the two-color WOLEDs is ITO/HAT-CN (100 nm)/TAPC (20 nm)/mCP: 9,9\({}^{\prime}\),9\({}^{\prime\prime}\),9\({}^{\prime\prime\prime}\)/((6-phenyl-1,3,5-triazine-2,4-diyl)bis(benzene-5,3,1-triyl))tetrakis (9\(H\)-carbazole) (DDCzTrz, 10 nm, 20%)/interlayers (3 nm)/bis[2-(2-hydroxyethyl)-pyridine] beryllium (Bepp\({}_{2}\)): bis(2-phenyl-4,5-dimethylpyridinato)[2-(biphenyl-3-yl)pyridinato] iridium(III) (Ir(dmpppy)\({}_{2}\)(dpp), 15 nm, 1:2%)/Bepp\({}_{2}\) (35 nm)/LiF (1 nm)/Al (160 nm), where interlayers are none, mCP and 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26 DCzPPy) for device W11, W12 and W13, respectively. The configuration of the two-color WOLED is ITO/HAT-CN (100 nm)/TAPC (20 nm)/mCP: DDCzTrz (10 nm, 20%)/26 DCzPPy (interlayer, 3 nm)/Bepp\({}_{2}\): Ir(dmpppy)\({}_{2}\)(dpp): Ir(piq)\({}_{3}\) (15 nm, 1:2%:1.3%)/Bepp\({}_{2}\) (35 nm)/LiF (1 nm)/Al (160 nm). Unlike previous TADF-based hybrid WOLEDs, the bipolar interlayer is demonstrated to enhance the performance. Particularly, it is demonstrated that the use of interlayer can enhance the lifetime (2.3 times). The working mechanism of the two-color WOLED can be described as follows, which is beneficial to comprehend the reason why the bipolar interlayer can enhance the performance. For W11, since mCP and Bepp\({}_{2}\) are p-type and n-type materials, respectively, holes and electrons are easily accumulated at the mCP/Bepp\({}_{2}\) interface, forming singlet and triplet excitons, as shown in Figure 11a.