# Recent Advances of Exciplex-Based White Organic Light-Emitting Diodes
## 3 Approaches for Exciplex-Based WOLEDs
### WOLEDs Based on Exciplex Emitters
#### 3.2.2 WOLEDs Combining Exciplex and Phosphorescent Emitters  
For WOLEDs combining exciplex and phosphorescent emitters, blue exciplex emitters are generally used to combine with phosphors, while other-color exciplexes are negligibly adopted. One of the reasons for this phenomenon may be attributed to the fact that it is not easy to synthesize efficient and stable blue molecular emitters, particularly for deep-blue ones. In contrast, by selecting appropriate donor and acceptor materials, high-performance blue exciplexes are promising to be the elegant emitters to organize WOLEDs. As a result, by incorporating blue exciplex with complementary or green/red phosphorescent emitters, high-performance WOLEDs can be established. For such WOLEDs, since blue light is originated from the blue fluorescence or TADF, they are also called hybrid WOLEDs [166]. Simply, this kind of hybrid WOLEDs can be classified into two types according to the use of blue exciplexes: hybrid WOLEDs with blue TADF exciplexes and hybrid WOLEDs with blue fluorescent exciplexes. In general, it is easy to understand that hybrid WOLEDs with blue TADF exciplexes can obtain high efficiencies, since the blue TADF exciplexes can harvest the triplets. However, by employing effective device engineering, hybrid WOLEDs with blue fluorescent exciplexes can also exhibit high performance, since the triplets of blue exciplexes which cannot be harnessed by the exciplexes can be harvested by the adjacent phosphorescent emitters (e.g., from the energy transfer and triplets diffusion mechanism) [167; 168; 169]. To fulfill these two types of hybrid WOLEDs, blue exciplexes are required to possess higher T\({}_{1}\) compared with phosphorescent emitters, otherwise the triplets of phosphors would be quenched by the exciplex, leading to the poor performance. Particularly, it is deserved to note that the blue exciplexes are simultaneously functioned as the hosts in single-EML hybrid WOLEDs. In this case, the concentration of phosphorescent guest emitters is usually very low, which ensure the incomplete energy transfer from the blue exciplex host to the phosphorescent guests, otherwise only the guest emission can be produced. In 2015, [PERSON] et al. explored a series of efficient WOLEDs, in which blue exciplexes have been combined with orange phosphorescent emitters, orange/red fluorescent emitters or orange TADF emitters [170]. By virtue of the blue TADF exciplex mCBP (donor): PO-T2T (acceptor), the energies have been transferred from the exciplex to orange fluorophore and phosphor dopants. For the double-EML hybrid WOLEDs combining blue exciplex and phosphorescent emitters, Figure 4 depicts the device structure: ITO/MoO\({}_{3}\) (3 nm)/9,9'-biphenyl-3,3'-diylbis-9H-carbazole (mCBP, 20 nm)/mCBP: PO-T2T (20 nm)/mCBP: PO-T2T: 4.0 wt.% bis(2-phenyl-1,3-benzothiozolato-N,C2') iridium (acetylacetonate) Ir(bt)\({}_{2}\)(acac) (2 nm)/PO-T2T (40 nm)/LiF/Al. Since triplets of the exciplex were up-converted to S\({}_{1}\) in a relatively long time, only part of triplets was converted as delayed fluorescence. Thus, the RISC efficiency has been less than unity. For extra triplets of the exciplex, they have been converted to heat  
Figure 3: (**a**) The electronic level configurations of tandem WOLED. (**b**) EL spectra of the WOLED at different brightness, and two decomposed bands corresponding to the blue and yellow exciplex emissions [165]. and would harm the device via non-radiative decay. When a small concentration of orange phosphor has been doped or an adjacent orange phosphor doped layer has been presented, the extra triplets of the exciplex would incompletely transfer the energy to phosphors and finally contribute to the white light. Hence, the additional 2 nm orange EML renders an excellent warm WOLED with EQE of 22.21% and PE of 52.28 Im W\({}^{-1}\). To simplify the structure, a single-EML hybrid WOLED was also developed by doping a low concentration 0.5 wt.% of Ir(bt)\({}_{2}\)(acac) of in the full EML.