# Emergence of White Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence
## 3 Approaches for WOLEDs Based on TADF
### WOLEDs Based on TADF and Conventional Fluorescence
#### 3.3.3 Single-EML WOLEDs Utilizing Blue TADF Host and Complementary Fluorescence Dopant  
According to the number of EMLs, the device architectures can be simply classified into two kinds, namely multi-EML and single-EML WOLEDs [127, 128, 129, 130, 131, 132]. Compared with multi-EML WOLEDs, the structures, fabrication procedures and device engineering of single-EML WOLEDs are relatively simple. For single-EML WOLEDs, by managing the incomplete energy transfer from hosts to guests in EMLs, the host materials can also be functioned as the blue emitters to produce blue emissions, in which the concentration of guests is usually low (e.g., \(<\)1%) to ensure the white emissions [133, 134, 135]. Generally, this simple single-EML structures can be used to develop any type of WOLEDs. Therefore, by carefully manipulating the charges and excitons distribution, the development of single-EML WOLEDs based on TADF is possible. [PERSON] et al. demonstrated a single-EML WOLED by using blue host TADF material of DMAC-DPS and traditional orange fluorescent dopant of (5,6,11,12)-tetraphenyl-naphthacene (rubrene), achieving a highly efficient and color stable single-EML WOLEDs with the EQE of 7.48% and PE of 15.9 lm W\({}^{-1}\)[136]. The device architecture is ITO/MoO\({}_{3}\) (3 nm)/4,4',4''-tri(N-carbazolyl)triphenylamine (TCTA, 20 nm)/DMAC-DPS: 0.6 wt. % rubrene (15 nm)/4,7-diphenyl-1,10-phenanthroline (Bphen, 40 nm)/LiF (1 nm)/Al. In this single-EML WOLED, DMAC-DPS is a highly efficient blue TADF material with small \(\Delta\)E\({}_{\text{ST}}\) of 0.08 eV. Hence, the 75% triplets produced on this TADF host could up-convert to its S\({}_{1}\), then the total singlets of \(>\)25% would transfer to S\({}_{1}\) of dopant by Forster energy transfer process, as shown in Figure 6. Besides, the Dexter energy transfer between T\({}_{1}\) of host and dopant could be suppressed effectively by decreasing the doped concentration (e.g., 0.6%). Therefore, the white emission is derived from both the emissions of host and dopant through an incomplete energy transfer from the blue TADF host to orange fluorescent dopant. When a traditional fluorescent 4,4'-bis(9-ethyl-3-carbazo-vinylene)-1,1'-biphenyl (BCzVBi) is used as the host, only a maximum EQE of 3.72% can be obtained, indicating the advantage of the TADF materials in this type of WOLEDs. Figure 5: Device function mechanisms of the conceptual utilization of singlet and triplet excitons generated in EMLs. (**a**) device W3; and (**b**) devices W4 and W6. H, B, Y and R are CBP, NI-1-PhTPA, PXZDSO2 and DBP, respectively. Reproduced with permission from [56], John Wiley and Sons, 2016. Figure 6: The energy transfer and electroluminescent emission processes in the single-EML WOLEDs based on TADF host materials. Reproduced with permission from [136], Elsevier, 2015. 3.4. Single-EML WOLEDs Exploiting Fluorescent Host, Blue TADF and Complementary Fluorescence Dopants  
Another way to develop single-EML WOLEDs based on TADF is the exploitation of fluorescent host, blue TADF and complementary fluorescence dopants. In such way, an efficient blue TADF materials is required. Besides, the concentration of complementary fluorescence dopants should be very low, otherwise the triplet excitons could be quenched by the conventional fluorescence dopants. In 2015, [PERSON] et al. reported this way by codoping a blue TADF emitter and a yellow fluorescence emitter into a fluorescent host, obtaining a maximum EQE of 15.5% and PE of 39.3 Im W\({}^{-1}\)[137]. The device configuration is ITO (120 nm)/poly(3,4-ethylenedioxythiophene):poly(styreneesulfonate) (PEDOT:PSS, 60 nm)/TAPC (10 nm)/TCTA (10 nm)/mCP (10 nm)/DPEPO:DMAC-DPS: 2,8-ditert-butyl-5,11-bis(4-tert-butylphenyl)-6,12-diphenylTetracene (TBRb, 25 nm)/diphenylphosphine oxide-4-(triphenylsilyl)phenyl (TSPO1, 5 nm)/TPBI (30 nm)/LiF (1 nm)/Al (200 nm). The high performance can be explained as follows. (i) High-efficiency blue emitter is used, because the dominant emission mechanism is energy transfer from a blue emitter to a yellow emitter.