# 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  
#### 3.3.1 WOLEDs Using Blue TADF and Complementary Fluorescence Materials  
Given that (i) TADF materials can harvest triplet excitons; (ii) hybrid WOLEDs composed of blue fluorescence and green-red phosphorescence emitters can realize high performance [60; 61; 62; 63; 64; 65; 66], WOLEDs based on a TADF blue exciton generation combined with green and red fluorescence emitters are expected to realize the IQE of 100%. This way is inspired by the TADF-assisted fluorescence monochromatic OLEDs, where direct charge recombination on the conventional fluorescent molecules should be eliminated to obtain high IQE by using a suitable combination of TADF and fluorescent  
Figure 3: The exciplex host exciton energy diagram and possible decay ways of singlet and triplet excitons. (Solid arrow represents Forster energy transfer, dash arrow represents Dexter energy transfer and blocking mark represents singlet excitons energy stopping transfer). Reproduced with permission from [123], Elsevier, 2017.  
molecules [124; 125]. In other words, a TADF molecule acts as a triplet harvester for other-color fluorescent emitters to achieve white emission.  
In 2015, [PERSON] et al. demonstrated this approach by using blue TADF emitter bis[4-(9,9-dimethyl-9,10-dihydroracidine)phenyl]sulfone (DMACDPS) with green fluorescent emitter 9,10-bis[\(N\),\(N\)-di-(p-tolyl)-amino]anthracene (TTPA) and red emitter tetraphenyldibenzoperifallenthene (DBP), achieving a maximum EQE of 12% [126]. The device structure is ITO/\(N\),\(N^{\prime}\)-di(naphthalen-1-yl)-\(N\),\(N^{\prime}\)-diphenyl-benzidine (\(\alpha\)-NPD, 30 nm)/1 wt. % DBP: 10 wt. % TTPA: 1,3-bis(9-carbazolyl) benzene (mCP,8 nm)/mCP (2 nm)/DMACDPS (7.5 nm)/bis[2-(diphenylphosphino)phenyl]Jether oxide (DPEPO, 10 nm)/2,2\({}^{\prime}\),2\({}^{\prime\prime}\)-(1,3,5-benzinetryl)-tris(1-phenyl-1-H-benzimidazole) (TPBi, 40 nm)/LiF (0.5 nm)/Al (100 nm). The high performance can be explained as follows. (i) There is a large spectral overlap between the ground state absorption of the exciton acceptor and fluorescent emission of the exciton donor, indicating that the energy of singlet excitons, i.e., Forster energy transfer, up-converted from a triplet state on a TADF molecule can be efficiently transferred to the fluorescent exciton acceptors; (ii) A large LUMO energy barrier is presented at the mCP/DMACDPS interface, leading to the carrier recombination mainly occurring on DMACDPS molecules for blue emission; (iii) 2 nm spacer mCP is used to eliminate the direct carrier recombination on fluorescent molecules and produce delayed emission from the fluorescent molecules. As a consequence, the exciton donor and acceptor molecules are separated into different layers, because Forster energy transfer involves long-range interactions; (iv) The cascade energy transfer occurs from S\({}_{1}\) of DMACDPS to S\({}_{1}\) of DBP through S\({}_{1}\) of TTPA, in which controlling the TTPA concentration can tune the spectrum by influencing the amount of energy absorbed from DMACDPS and the rate of Forster energy transfer to DBP, as shown in Figure 4.