# Emergence of White Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence
## 3 Approaches for WOLEDs Based on TADF
### WOLEDs Based on TADF Exciplex Host and Phosphorescence Dopants  
By combining high-T\({}_{1}\) p-type materials and high-T\({}_{1}\) n-type materials, high-T\({}_{1}\) exciplexes can be formed. When the T\({}_{1}\) of exciplex is higher than that of blue phosphorescence dopant, high-performance  
Figure 14: Schematic diagrams showing exciton allocation in the emissive zone. Reproduced with permission from [171], John Wiley and Sons, 2017. WOLEDs can be realized by using such high-T\({}_{1}\) exciplexes as the host of phosphorescence dopants. In general, TADF exciplexes are promising to meet the requirement of high T\({}_{1}\)[36]. Therefore, the utilization of TADF exciplex host can not only nearly eliminate the charge injection barrier issue but also give an efficient TADF upconversion due to the intermolecular donor-acceptor construction and sufficiently small S\({}_{1}\)-T\({}_{1}\) splitting (<0.1 eV). Moreover, the relatively high T\({}_{1}\) of exciplex will reduce the energy back-transfer (i.e., from T\({}_{1,\,\text{guest}}\) to T\({}_{1,\,\text{host}}\)) rate constant, preventing the exciton leakage. Hence, TADF exciplex hosts can possibly make a tradeoff among the 100% IQE, low voltage and good exciton confinement in the EML. However, in such way, it would be better that both the T\({}_{1}\) of donor and acceptor materials are higher than that of exciplexes, otherwise the triplets in the EML could be quenched. [PERSON] et al. reported a single-EML WOLED by using donor mCP and acceptor 4,6-bis[3,5-(dipyridyl-4-yl)phenyl]-2-methylpyrimidine (B4 PyMPM) as the TADF exciplex host of phosphorescence dopants, obtaining a maximum forward-viewing EQE of 28.1% and PE of 105.0 lm W\({}^{-1}\)[36]. The configuration is ITO (110 nm)/TAPC (40 nm)/TCTA (10 nm)/mCP (10 nm)/mCP: 50 wt. % B4 PyMPM: 15 wt. % bis(3,5-diuoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl) iridium III (Flrpic): 0.2 wt. % PO-01 (20 nm)/B4 PyMPM (50 nm)/Liq (0.8 nm)/Al (120 nm). The high performance can be explained as follows. (i) The exciplex was exclusively as the host and at the same time codoped it with two-color phosphors, prohibiting the shortcomings of low efficiency and pronounced efficiency roll-off; (ii) By using energy level matching hole and electron transporting layer, the structural heterogeneity is reduced. Besides, high hole mobility of mCP (5 \(\times\) 10\({}^{-4}\) cm\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\)) and electron mobility of B4 PyMPM (1 \(\times\) 10\({}^{-4}\) cm\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\)) are used, realizing low voltages (e.g., 1 cd m\({}^{-2}\) at 2.5 V); (iii) The perfectly confined excitons fully eliminate the exciton leakage from the emission zone, enhancing the efficacy. As shown in Figure 15 left, if acceptor molecules with low T\({}_{1}\), part of excitons from the exciton formation zone will be trapped by the acceptor (transfer mode I). Hence, excitons transferred to the acceptor molecules would decay via a nonradiative path owing to their rather low photoluminescence quantum yields, thereby causing undesirable exciton leakage and accordingly low efficiency. As shown in Figure 15 right, if acceptor molecules with high T\({}_{1}\), the leakage phenomenon is suppressed (transfer mode II). Hence, all excitons are transferred only to the emitter molecules via either long-range dipole-dipole coupling (Forster energy transfer) or short-range exchange interaction (Dexter energy transfer). Thus, the emitter molecules can utilize all excitons and consequently deliver a 100% IQE. Figure 15: Two different energy-transfer modes. Donor and acceptor are the constituting molecules for exciplex. S\({}_{1}\) and T\({}_{1}\) are the singlet and triplet states, respectively. k\({}_{\text{FRET}}\), k\({}_{\text{DEX}}\), k\({}_{\text{ISC}}\), k\({}_{\text{RISC}}\), k\({}_{\text{F}}\) and k\({}_{\text{P}}\) are the rate constant of Forster energy transfer, Dexter energy transfer, ISC, RISC, fluorescence and phosphorescence processes, respectively. Left: Incomplete energy transfer. Right: Complete energy transfer.