# Recent Advances of Exciplex-Based White Organic Light-Emitting Diodes
## 3 Approaches for Exciplex-Based WOLEDs
### WOLEDs Based on Exciplex Hosts
#### 3.3.2 WOLEDs Employing Exciplex as the Host of Phosphorescent Emitters  
Since the T\({}_{1}\) of blue phosphorescent emitters is usually very high (e.g., 2.62 eV for iridium(III) bis[(4,6-difluorophenyl)-pyridinato-N,C2\({}^{\prime}\)]-picolinate (FIrpic)) [178; 179; 180], the exciplex host should possess high T\({}_{1}\) to confine the excitons. Hence, for WOLEDs employing exciplex as the host of phosphorescent emitters, how to select a high-T\({}_{1}\) exciplex is critical. [PERSON] et al. realize this kind of WOLED by adopting the exciplex showing the high T\({}_{1}\) (2.97 eV) formed by mCP: bis-4,6-(3,5-di-3-pyridyl-phenyl)-2-methylpyrimidine (B3 PYMPM) as the host for FIrpic, and inserting an ultrathin nondoped PO-01 as the orange EML [181]. As a result, the WOLED exhibits maximum forward-viewing efficiencies of 20.0% and 75.3 lm W\({}^{-1}\). Even at 1000 cd m\({}^{-2}\), the efficiencies could be as high as 19.5% and 63.1 lm W\({}^{-1}\). As displayed in Figure 8, the device architecture is ITO/MoO\({}_{3}\) (10 nm)/TAPC: MoO\({}_{3}\) (10%, 50 nm)/TAPC (20 nm)/PO-01 (0.06 nm)/mCP: B3 PYMPM: FIrpic (1:1:0.4, 10 nm)/B3 PYMPM (15 nm)/B3 PYMPM: Li\({}_{2}\)CO\({}_{3}\) (3%, 40 nm)/Li\({}_{2}\)CO\({}_{3}\) (1 nm)/Al. Since TAPC possesses better hole mobility (1.0 \(\times\) 10\({}^{-2}\) cm\({}^{2}\) v\({}^{-1}\) s\({}^{-1}\)) and higher T\({}_{1}\) (2.98 eV) compared with mCP, it is key to the performance of the WOLED. The factors of the high performance could be summarized: (i) a good exciton confining EML structure has been formed via the large band gap hole transport and electron transport materials with high T\({}_{1}\), (ii) the exciplex host could harness all excitons and transfer energy from both S\({}_{1}\) and T\({}_{1}\) to the dopants, (iii) the hole transport along HOMO of mCP and the electron transport along LUMO of B3 PYMPM lower the electrical excitation energy, (iv) ultrathin PO-01 had a negligible effect on carrier transport, reducing the voltages, (v) p-doping HTL and n-doping ETL could further ensure the low voltage and high efficiency. For the emission mechanism, the FIrpic emission and PO-01 emission have been mainly resulted from energy transfer procedures. Specifically, holesand electrons have been first formed singlets as well as triplets on the exciplex host, then the energy of generated excitons has been transferred to FIrpic molecules, resulting in blue light. In addition, the energy of generated excitons on the exciplex host and FIrpic molecules could be transferred to PO-01 molecules, leading to orange light.  
Furthermore, [PERSON] et al. enhanced the performance of this kind of WOLED, achieving the EQE/PE as high as 28.1%/105.0 lm W\({}^{-1}\)[12]. For their exciplex host, mCP and 4,6-bis[3,5-(dipyridyl-4-yl)phenyl]-2-methylpyrimidine (B4 PyMPM) have been used as the donor and acceptor, respectively. Hence, by doping 15 wt.% FIrpic blue phosphor and low concentration of yellow phosphor PO-01 (0.2 wt.%) into this exciplex system, highly efficient WOLEDs have been obtained. Particularly, their TADF exciplex host is significant, since (i) the heterogeneity is reduced via energy-matching HTL/ETL; (ii) the voltage is lowered due to the excellent charge mobilities (e.g., 5 \(\times\) 10\({}^{-4}\) and 1 \(\times\) 10\({}^{-4}\) cm\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\) for mCP and B4 PyMPM, respectively); (iii) high-T\({}_{1}\) donor/acceptor eliminate exciton leakage. As a consequence, total singlets and triplets have been only harvested by FIrpic or PO-01 by virtue of the Dexter or Forster energy transfer, leading to 100% exciton use.