# 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
#### 3.4.2 Hybrid WOLEDs Possessing Blue TADF Exciplex and Complementary Phosphorescence Materials  
Similar to blue TADF materials, blue TADF exciplexes, formed by mixing donor and acceptor molecules, can also be as the emitter in hybrid WOLEDs. To realize high performance, the highly efficient blue TADF exciplex system is required [157; 158; 159; 160; 161]. Besides, the blue exciplex system can provide a \"barrier-free\" architecture and a bipolar EML, which is also beneficial to the performance. To accomplish this approach, the T\({}_{1}\) of the blue exciplex should be higher than that of phosphorescence materials. Figure 11: Schematic illustration of the working mechanism of (**a**) W11; (**b**) W12 and (**c**) W13. The gray filled rectangles are the main exciton generation zones. The Dexter energy transfer can occur in W11, while it is prohibited in both W12 and W13. Reproduced with permission from [155], Elsevier, 2017. Figure 12: Schematic diagram showing the energy transfer process of the white EML. Reproduced with permission from [156], IOP Publishing, 2015. [PERSON] et al. reported a single-EML hybrid WOLED by using a blue TADF exciplex CDBP: 50 wt. % PO-T2T, achieving the forward-viewing EQE of 25.5% and PE of 84.1 lm W\({}^{-1}\)[162]. The device structure is ITO/TAPC (30 nm)/CDBP (10 nm)/CDBP: 50 wt. % PO-T2T: 0.1 wt. % Ir(pppy)\({}_{2}\)acac: 0.3 wt. % bis(2-methyldibenzo [f,h]quinoxaline)(acetylacetonate)iridium (III) (Ir(MDQ)\({}_{2}\)acac, 30 nm)/PO-T2T (40 nm)/LiF (1 nm)/Al (100 nm). The emission mechanism of this device can be explained as follows. A system with low green/red phosphorescent dopants and conventional blue fluorescent host molecules is shown in Figure 13a. In Zone I, both singlets and triplets of the blue host can transfer energy to dopants, giving theoretically 100% IQE emission from dopant. In Zone II, only triplets of the host can reach the dopant due to the different diffusion length of singlets and triplets. In this case, blue emission from the host and green/red emission from dopant can be obtained with theoretically 100% IQE. In Zone III, no exciton from the host can reach the dopant, giving only blue emission with an IQE of 25%. Therefore, 100% IQE white emission can only be achieved in Zone II but with warm-white emission, while the white balance can be maintained in Zone III but with 25% IQE. These issues can be effectively addressed by using a blue TADF host, as shown in Figure 13b. For this system, both singlet and triplet excitons in Zone III can be harvested for blue emission with a theoretical 100% IQE. Thus, a TADF host-phosphor dopant WOLED can simultaneously achieve 100% IQE and white emission even with extremely low dopant concentrations. Besides, due to the extremely small \(\Delta\)E\({}_{\text{ST}}\), blue TADF emitters intrinsically have T\({}_{1}\)s higher than those of the green phosphors. Figure 13: Schematic diagrams for management of singlet and triplet excitons in single-EML hybrid WOLEDs based on (**a**) conventional blue fluorophores and (**b**) TADF blue emitters with low dopant concentrations. Green circle coverage represents diffusion zone of singlets (Zone I). Pink circle coverage represents diffusion zone of triplets (Zone II). The uncovered zone is Zone III; for conventional blue fluorophores, all singlets and triplets formed in this zone cannot reach the phosphor molecules and only singlets can be used for emission; for TADF blue emitters, triplets can be up-converted into S\({}_{1}\) for emission together with the directly generated singlets. R\({}_{\text{S}}\) and R\({}_{\text{T}}\) are respectively diffusion lengths of singlet and triplet excitons; compared with conventional host, R\({}_{\text{T}}\) of TADF host is smaller due to the fast competition process of RISC. F, fluorescence; P, phosphorescence; Nr, nonradiative decay. Reproduced with permission from [162], John Wiley and Sons, 2015.