# Recent Advances of Exciplex-Based White Organic Light-Emitting Diodes
## 3 Approaches for Exciplex-Based WOLEDs
### WOLEDs Based on Exciplex Emitters  
#### 3.2.1 WOLEDs Utilizing All Exciplex Emitters  
For WOLEDs utilizing all exciplex emitters, the combination of blue exciplex emission with complementary exciplex emission or green/red exciplex emissions is the most directed approach [158]. In such approach, blue, green, yellow, orange as well as red emitter is formed by exciplexes. Additionally, to enhance efficiency, it is more desirable to use TADF mechanism, since TADF exciplexes can harvest the triplet excitons. Otherwise, the triplets of exciplexes would be decayed nonradiatively, resulting in low performance. In 2009, [PERSON] and coworkers adopted this way to develop WOLED via four exciplex emission bands, achieving very broad white-emission spectrum [159]. As shown in Figure 2, the device architecture is ITO/4_A_'_A_''-tris[methylphenyl(phenyl)amino]-triphenylamine (m-MTDATA, 40 nm)/m-MTDATA: tris(dibenzoyl methane)-aluminum (Al(DBM)\({}_{3}\)) (R, x)/N,N''-bis(3-methylphenyl)-N,N''-diphenylbenzidine (TPD): 4,7-diphenyl-1,10-phenanthroline (Bphen, 10 nm)/Bphen (40 nm)/LiF (1 nm)/Al (200 nm), where R denotes the molar ratio of m-MTDATA-to-Al(DBM)\({}_{3}\) and x denotes its layer thickness. To regulate the spectra, R was changed from 1:1 to 1:2 to 1:3, while x was changed among 2, 3, 4, and 5 nm. In this device, white light is emitted by inserting two blend layers of m-MTDATA: Al(DBM)\({}_{3}\) and TPD: Bphen between the m-MTDATA HTL and Bphen ETL. The white light spectrum consists of four broad bands that arise from blue-emitting TPD/Bphen (463 nm), green-emitting m-MTDATA/Bphen (528 nm), orange-emitting TPD/Al(DBM)\({}_{3}\) (572 nm), and red-emitting m-MTDATA/Al(DBM)\({}_{3}\) exciplexes (650 nm), respectively. These exciplex emissions are different from the PL spectra of m-MTDATA, TPD, Bphen, and Al(DBM)\({}_{3}\) neat films, which give a monomer (i.e., bulk exciton) emission with peaks at about 375, 401, 413, and 425 nm, respectively, as shown in Figure 2c. More specifically, the exciplex of m-MTDATA/Al(DBM)\({}_{3}\) is formed at the interface between m-MTDATA HTL and m-MTDATA: Al(DBM)\({}_{3}\) layers, exciplexes of TPD/Al(DBM)\({}_{3}\) and m-MTDATA/Bphen are at the interface between m-MTDATA: Al(DBM)\({}_{3}\) and TPD: Bphen layers, and the exciplex of TPD/Bphen is at the interface between TPD: Bphen and Bphen electron-transporting layers. The intralayer exciplexes of m-MTDATA/Al(DBM)\({}_{3}\) and TPD/Bphen are formed in m-MTDATA: Al(DBM)\({}_{3}\) and TPD: Bphen layers, respectively. As a result, an ultrahigh CRI of 94.1 can be achieved. Although the current efficiency (CE) is low (\(\sim\)0.1 cd A\({}^{-1}\)), such findings demonstrated a concept that high-quality white emissions can be generated without molecular emitters. Figure 2: _Cont._  
Currently, the utilization of tandem OLEDs, which refer to devices using charge generation layers (CGLs) or transparent conductive layers (e.g., ITO) to interconnect multiple EL units, has been considered to be an effective way to boost the performance [160; 161; 162; 163]. Compared with single-unit OLEDs, the CE, EQE and lifetime of tandem OLEDs can be N-fold (N, the number of EL units) enhanced, since N-fold luminance can be achieved with the current density similar to that of single-unit OLEDs. Therefore, tandem OLEDs have aroused both academic and industry interest. In terms of WOLEDs with all exciplex emitters, the introduction of exciplexes in each unit can be an effective scheme. Therefore, the molecular emitter is also unnecessary. For such approach, how to maximize the efficiency of the exciplex in each unit is crucial, apart from the design of efficient CGLs. Particularly, since it is relatively difficult to attain blue exciplexes, the development high-efficiency blue exciplexes is essential to the high performance [164]. In 2014, [PERSON] et al reported the first tandem, all-exciplex-based WOLED, where blue and yellow exciplexes have been generated at each unit [165].