# Recent Advances of Exciplex-Based White Organic Light-Emitting Diodes
## 4 Summary and Outlook  
Since exciplexes can be (i) fluorescent or TADF emitters; (ii) the hosts of fluorescent, phosphorescent and TADF dopants, these excellent characteristics have rendered that exciplexes have been actively investigated in the WOLED. Currently, the efficiency for the exciplex-based WOLED can be comparable to that of other kinds of best WOLED. In this review, we have mainly focused on recent advances for the WOLED using exciplex emitters or hosts. Particularly, we have emphasized representative WOLEDs with all exciplex emitters, WOLEDs combining exciplex and phosphorescent emitters, WOLEDs using exciplex and TADF emitters, WOLEDs exploiting exciplex and fluorescent emitters, WOLEDs employing exciplex as the host of phosphorescent emitters and WOLEDs adopting exciplex as the host of TADF emitters. The detailed performances for exciplex-based WOLEDs have been described in Table 1.  
Figure 9: Exciton energies for exciplex hosts and decay channels for singlets as well as triplets. (Dash and solid arrows are Dexter and Förster energy transfer, respectively. The locking mark is singlets energy transfer stopping) [182]. Over recent years, the performance of exciplex-based WOLEDs has been step-by-step enhanced. In particular, their performance has been boosted after the recognition of exciplexes possessing TADF property. To date, there are still many challenges hindering the development of commercial productions, including the efficiency, efficiency roll-off, operational stability as well as color stability. For the issue of efficiency, since the theoretical efficiency limit of WOLEDs is 248 lm W\({}^{-1}\), there is still much room for exciplex-based WOLEDs [183]. In OLEDs, the EQE is determined by outcoupling factor, carrier balance, fraction of excitons potentially decayed radiatively, as well as PLQY of materials. On the other side, the driving voltage can greatly affect PE. Hence, careful introduction for the donors and acceptors plays a significant role in exciplex-based WOLEDs. When the exciplex is used as the emitter, high PLQY of the exciplex is essential to the high performance, apart from the high RISC efficiency. Besides, regardless of WOLEDs based exciplex emitters or hosts, the elaborative management of the carrier and exciton distribution [184; 185; 186; 187; 188] as well as adoption of outcoupling technology [189; 190; 191; 192] can improve efficiencies. Additionally, efficiency roll-off is somewhat serious, particularly for the WOLEDs using the TADF exciplex emitters. To overcome this issue, the charge balance, charge transport barriers and materials selection are required to be regulated, which are also beneficial to the color-stability [193; 194; 195; 196; 197; 198; 199; 190; 190; 191; 192; 193; 194; 195; 196; 197; 198; 199; 200]. Another possibility to affect fluorescence and phosphorescence of organic compounds is photonic structures, i.e., the photonic band gap (PhBG), which can manipulate the efficiency of OLEDs since PhBG is crucial for molding the propagation of light [201]. The periodic changes of refractive index within the colloidal crystal, i.e., crystal/voids, can cause a forbidden region of frequencies for photons, where the region is PhBG. Fluorescence can be influenced by the PhBG since fluorescence is the emission of an electromagnetic wave [202]. Particularly, in the Forster energy transfer process, PhBG can suppress the donor emission, which enhances the energy transfer, and in turn increases the acceptor emission. For example, the PhBG enhanced the Forster energy transfer efficiency from 37% to 61% [203]. On the other hand, phosphorescence emission can be obtained for forced intersystem crossing by engineering a PhBG (e.g., the phosphorescence was generated from bis(2-methyl-8-quinolinato)-4-phenylphenolate aluminum (BALq)) [204]. In terms of the lifetime, it is a key factor to determine whether LEDs can meet the demand of the commercialization [205; 206; 207; 208]. However, it is deserved to point out that exciplex-based WOLEDs have not yet achieved long lifetime.