# Recent Advances of Exciplex-Based White Organic Light-Emitting Diodes
## 3 Approaches for Exciplex-Based WOLEDs
### WOLEDs Based on Exciplex Hosts  
#### 3.3.1 The Understanding of WOLEDs Based on Exciplex Hosts  
In general, the \(\Delta_{\text{EST}}\) of exciplexes is lower compared with the intramolecular excited state, because distance between HOMO and LUMO in exciplexes is larger [177]. Hence, exciplexes can obtain the unity IQE by virtue of the RISC procedure due to the small \(\Delta_{\text{EST}}\). To take advantage of exciplex, another strategy is using exciplex as the host of emitters. To accomplish such strategy, the guest emitters can be phosphorescent, TADF or fluorescent materials. Particularly, since fluorescent emitters  
Figure 7: **(a)** Molecular structures of mCP, mCBP, CDBP as well as PO-T2T. **(b)** The PL spectra for CDBP film (solid line), PO-T2T film (line with square), PO-T2T solution (line with triangle), and CDBP: PO-T2T co-deposited film (line with circle). **(c)** The exciplex light generated from the donor CDBP and acceptor PO-T2T. **(d)** Physical processes in the exciplex host-dopant systems. ET, PF and DF are energy transfer, prompt fluorescence, and delayed fluorescence, respectively [172].  
are unable to harvest triplets, TADF exciplex hosts are necessary to guarantee the high efficiency of WOLEDs comprising exciplex as the host of fluorescent emitters. For this way, single-EML structures are usually utilized, since it is easy to lower the number of fluorescent molecules in single-EML WOLEDs. Generally, by using guests with low concentrations, the exciplex hosts also functioned as the blue emitters via the incomplete energy transfer, as introduced in the Section 3.2.4.  
Besides, since holes and electrons are transported from the HOMO of donors and the LUMO of acceptors, respectively, low voltages can be obtained. Furthermore, by choosing suitable donors and acceptors, energy levels of the exciplex can be easily adjusted to meet guests owing to the approximately equal S\({}_{1}\) and T\({}_{1}\) of exciplex. Therefore, by using exciplex as the host, it can be effective to realize high-performance devices. By combining high-T\({}_{1}\) donor and acceptor, the exciplex is possible to possess high T\({}_{1}\). If the blue phosphorescent or TADF guests show a lower T\({}_{1}\) than exciplexes, the exciplexes could be used as hosts to achieve high-performance WOLEDs. Generally, it is promising for the TADF exciplex to satisfy the demand of high T\({}_{1}\)[112]. Hence, when TADF exciplex hosts have been adopted, (i) the issue of carrier injection barrier can be eliminated, (ii) the RISC process can be efficient, (iii) the exciton leakage can be prevented by reducing the guest-host T\({}_{1}\) back energy transfer. As a consequence, by using TADF exciplex hosts in WOLEDs, it is possible to simultaneously realize high efficiencies, reduced voltages and efficient exciton confining. For this approach, it is required that T\({}_{1}\) of the exciplex should be lower compared with the donor as well as acceptor, which is helpful to avert the triplet exciton quenching.  
By using exciplexes as the host of phosphorescent emitters and TADF emitters, the concentration of the dopants is relatively high, which is beneficial for dopants to harvest the excitons on the exciplex host since the distance between host and dopants is decreased. Besides, if high concentration were used, excitons can be directly formed on the dopants, which also can be fully harvested by phosphorescent or TADF emitters. Hence, high efficiency can be guaranteed. However, by exploiting TADF exciplexes as the host of phosphorescent or TADF emitters, there is no need to use dopants with high concentrations. This is because the triplets on the host can be up-converted to singlets via the RISC process, and then the energy can be transferred to guests via the long-distance Forster procedure, leading to the high efficiency.