# Emergence of White Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence
## 3 Approaches for WOLEDs Based on TADF
### WOLEDs Based on All TADF Emitters
#### 3.2.2 WOLEDs Combining TADF Exciplex Hosts and all TADF Materials  
Although TADF-based WOLEDs can exhibit high efficiency, their color-stability is still unsatisfactory. In fact, the development of TADF-based WOLED simultaneously with high efficiency, stable electroluminescent spectra and high CRI is a big challenge. Therefore, to compete with their counterparts, further improvement on emission quality is required for TADF-based WOLEDs [119; 120; 121; 122]. To alleviate this difficulty the combination of TADF exciplex hosts and all TADF materials can be an alternative to develop WOLEDs. For this way, electron-donating and electron-accepting materials are carefully chosen to form the exciplex host, generating efficient TADF characteristic via RISC. Then, the singlet excitons on the exciplex are harvested by the TADF materials via Forster energy transfer. Besides, the two host materials should have higher T\({}_{1}\) than the exciplex, otherwise the triplet excitons decay non-radiatively. Moreover, the T\({}_{1}\) of exciplex should be higher than that of TADF emitters.  
Figure 2: The WOLED structures and energy level diagram. Reproduced with permission from [55], AIP Publishing LLC, 2014.  
Towards this end, [PERSON] et al. recently constructed an efficient, color-stable and high CRI WOLED employing all TADF system, yielding a maximum forward-viewing EQE of 19.2%, PE of 46.2 lm W\({}^{-1}\), CRI of 82 and a stable color with the variation of CIE coordinates of (0.00, 0.02) at 100-3000 cd m\({}^{-2}\)[123]. The optimized device configuration is ITO/4,4\({}^{\prime}\)-cyclohexylidenebis[\(N\),\(N\)-bis(4-methylphenyl)aniline] (TAPC, 40 nm)/4,4\({}^{\prime}\)-bis(9-carbazolyl)-2,2\({}^{\prime}\)-dimethylbiphenyl (CDBP, 10 nm)/CDBP: 1,3,5-triazine-2,4,6-triyl)tris(benzene-3,1-diyl)tris(diphenylphosphineoxide) (PO-T2T) (2:1) 1 wt. % AnbTPA (14 nm)/CDBP:PO-T2T (3:1, 6 nm)/CDBP:PO-T2T (1.5:1): 10 wt. % 2 CzPN (10 nm)/PO-T2T (45 nm)/LiF (1 nm)/Al (100 nm), where (2 CzPN (1,2-bis(carbazol-9-yl)-4,5-dicyanobenzene) and AnbTPA (2,6-bis[4-(diphenylamino)phenyl)-9,10-anthracenedione) are light-blue and red TADF emitters, respectively. The high performance can be explained as follows. (i) CDBP:PO-T2T exciplex system possessing efficient TADF characteristic has been used as the host, which is beneficial to the up-conversion of triplet excitons, improving the efficiency; (ii) CDBP:PO-T2T exciplex system having suitable energy levels. For example, the HOMO of dopants are lower than that of CDBP, which can not only help the formation of the \"barrier-free\" structure but also minimize the trapping effects on dopants; (iii) Beneficial from the good hole and electron transporting properties from CDBP and PO-T2T respectively, the transporting ability of each EML can be precisely adjusted and balanced by controlling the ratio between CDBP and PO-T2T; (iv) An interlayer is introduced between emitting layers (EMLs) to confine the exciton, further improving the color-stability, as shown in Figure 3. This is because the 6 nm interlayer CDBP:PO-T2T can effectively prevent the energy transfer between two emitters for the Forster radius is ~3 nm, confining excitons in each EML. With precisely controlled ratios of each EML, the hole and electron transport can be balanced and excitons can be generated in the whole EML and it keeps the same exciton distributions of two EMLs under different driving voltages. Therefore, a stable color is obtained.