# 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
The structure is ITO/MoO\({}_{3}\) (3 nm)/mCBP (20 nm)/mCBP: PO-T2T: 0.5 wt.% Ir(bt)\({}_{2}\)(acac) (20 nm)/PO-T2T (40 nm)/LiF (0.8 nm)/Al. Due to the competition for the blue exciplex and phosphorescent guest, the single-EML WOLED showed a lower performance with a maximum PE of 39.89 Im W\({}^{-1}\) and an EQE of 17.06%. In 2015, [PERSON] et al. achieved an efficient blue exciplex host via (4-dimesitylboryl)phenyltriphenylamine (TPAPB) and 1,3,5-tri(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl (TPBi), exhibiting a PL quantum efficiency (PLQY) of 44.1%, where the up-conversion of triplets to singlets is mainly via triplet\(-\)triplet annihilation (TTA) [109]. By doping an orange phosphor into this exciplex host, a single-EML hybrid WOLED exhibited a maximum EQE and PE of 15.7 \(\pm\) 0.3% and 29.6 \(\pm\) 0.2 Im W\({}^{-1}\), respectively. Then, their group realized single-EML hybrid WOLEDs with blue TADF exciplexes by using a low phosphor guest concentration to ensure the incomplete energy transfer, in which the blue emission was resulted from the TADF exciplex host 4,4'-bis(9-carbazolyl)-2,2'-dimethylliphenyl (CDBP): PO-T2T [171]. Due to the efficient exciplex and well management of excitons, the WOLED shows the maximum EQE and PE of 25.5% and 84.1 Im W\({}^{-1}\), respectively. Furthermore, to improve performances of such kind of WOLEDs, [PERSON] and coworkers also used the blue exciplex CDBP: PO-T2T to manage exciton allocation [172]. By doping iridium(III) bis(4-phenylthieno[3,2-c]pyridinato-N,C2') acetylacetonate (PO-01) into the CDBP: PO-T2T host, their device having triple EMLs exhibited the peak EQE and PE as high as 28.3% and 102.9 Im W\({}^{-1}\), respectively. Even at 1000 cd m\({}^{-2}\), the efficiencies were very impressive (25.8% and 63.5 Im W\({}^{-1}\)), which are the highest among hybrid WOLEDs with blue exciplexes. For hybrid WOLEDs with blue fluorescent exciplexes, [PERSON] et al. reported this method by using 1,3,5-tri(m-pyridyl-3-yl-phenyl)benzene (TmPyPB) as an acceptor while TAPC as a donor [173]. Aside from the exciplex, electrolyte can also be produced from the TAPC/TmPyPB interface, rendering that the blue emission is very broad, which is beneficial to yield white light. By taking advantage of the doping-free technology, single-molecular-emitter based WOLEDs (WOLED-I) showed an EQE as high as 16.8%. Besides, two-molecular-emitter WOLEDs (WOLED-II) possessed high EQE (15.1%), high CRI  
Figure 4: Device architecture of the WOLEDs and the incomplete energy transfer from blue exciplex to orange phosphor/fluorophor. Left: I and II indicate the doped areas of orange dopant, corresponding to WOLED with two EMLs and one EML. Right: incomplete energy transfer from exciplex to orange phosphor/fluorophor. F, DF, ET, NR and P denote fluorescence, delayed fluorescence, the part of triplet/singlet exciplex transfer its energy to the orange dopant, non-radiative process and phosphorescence, respectively [170]. (92.1) and low CCT (2319 K). For their device architectures, the main difference between WOLED-I and WOLED-II is the used EMLs. For example, only 0.3 nm doping-free yellow phosphorescent EML bis(2-phenyl-4,5-dimethylpyridriatino)[2-(biphenyl-3-yl)pyridriatino] iridium(III) [Ir(dmppy)2(dpp)] has been adopted in WOLED-I, while additional 0.3 nm doping-free red phosphorescent EML tris(1-phenylisoquinolinolato-C2,N) iridium(III) [Ir(piq)3] and the 0.3 nm Ir(dmppy)2(dpp) have been placed both sides of the TAPC/TmPyPB system in WOLED-II. Besides, it is deserved to point out that the thin EMLs are conducive to avoid the concentration quenching, ensuring the high performance. Further working mechanism for WOLED-I and WOLED-II can be seen in Figure 5. In both WOLED-I and WOLED-II, the blue light has been generated by the TAPC/TmPyPB system. For the complementary light in WOLED-I, it could be originated from the TAPC/TmPyPB system. Besides, triplet excitons of the TAPC/TmPyPB system, the donor TAPC and the acceptor TmPyPB could be harvested by Ir(dmppy)2(dpp) by dint of diffusion mechanism.