# Emergence of White Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence
## 4 Summary and Outlook  
As a novel kind OLED emitter, TADF materials show many unique characteristics, which have been demonstrated to develop high-performance WOLEDs. Thanks to the hard endeavors of researchers, the performance of WOLEDs based on TADF is now comparable to state-of-the-art phosphorescence WOLEDs and hybrid WOLEDs. In this review, the focus is the development of WOLEDs based on TADF. Specifically, we highlight the recent development of WOLEDs based on all TADF emitters, WOLEDs based on TADF and conventional fluorescence emitters, WOLEDs based on TADF and phosphorescence emitters, and WOLEDs based on TADF exciplex host and phosphorescence dopants. The detailed performance of representative WOLEDs based on TADF are shown in Table 1. \begin{table}
\begin{tabular}{l c c c c c c} \hline
**Devices \({}^{\ast}\)** & **V\({}_{\infty}\)/V\({}_{1000}\)\({}^{\text{b}}\) (v)** & **EQE\({}_{max}\)/EQE\({}_{1000}\)\({}^{\text{c}}\) (\%)** & **CE\({}_{max}\)/CE\({}_{1000}\)\({}^{\text{d}}\) (cd A\({}^{-1}\))** & **PE\({}_{max}\)/PE\({}_{1000}\)\({}^{\text{e}}\) (dm W\({}^{-1}\))** & **CE\({}_{1000}\)\({}^{\text{f}}\)** & **CR1 8** \\ \hline Ref. [36] & 2.5/3.6 & 28.1/2.15 & 83.6/61.0 & 105/59.5 & (0.40, 0.48) & - \\ Ref. [55] & 3.6/7.8 & 17/8.5 & 45.6/20.6 & 34.1/8.3 & (0.33, 0.41) & - \\ Ref. [56] & 3.4/5.4 & 19.2/14.2 & 51.4/37.3 & 47.5/21.8 & (0.348, 0.457) & 69 \\ Ref. [123] & 2.5/- & 19.2/- & 36.7/- & 46.2/- & (0.33, 0.38) & 82 \\ Ref. [126] & -/- & 12.1/- & -/- & 22.0/- & (0.25, 0.31) & 74 \\ Ref. [136] & 2.48/- & 7.48/7.31 & 20.2/19.8 & 15.9/14.7 & (0.36, 0.44) & - \\ Ref. [137] & -/- & 15.5/13.3 & 38.4/31.9 & 39.3/23.4 & (0.28, 0.35) & 58.6 \\ Ref. [138] & -/- & 15.2/11.8 & 35.1/28.4 & 36.2/18.9 & (0.31, 0.37) & - \\ Ref. [142] & -/- & 22.5/15.4 & -/- & 47.6/- & (0.45, 0.48) & - \\ Ref. [154] & 3.2/4.2 & 23.0/17.5 & 51.0/39.5 & 51.7/39.5 & (0.438, 0.438) & 89 \\ Ref. [155] & 2.9/4.65 & 23.5/15.1 & -/- & 70.92/30.09 & (0.30, 0.49) & 50 \\ Ref. [156] & 3.0/4.3 & 22.4/18.3 & 57.6/45.6 & 60.4/33.6 & (0.30, 0.37) & - \\ Ref. [162] & 2.5/- & 25.5/14.8 & 67.0/37.0 & 84.1/24.2 & (0.40, 0.43) & - \\ Ref. [171] & 2.5/- & 28.3/25.8 & 88.7/80.9 & 102.9/63.5 & (0.46, 0.43) & 86 \\ \hline \end{tabular} \({}^{\ast}\) The structures are mentioned before. \({}^{\text{b}}\) The turn-on voltage (cd/m\({}^{2}\)) and the voltage at 1000 cd/m\({}^{2}\). \({}^{\text{c}}\) Maximum EQE at the EQE at 1000 cd/m\({}^{2}\).\({}^{\text{d}}\) Maximum current efficiency (CE) and the CE at 1000 cd/m\({}^{2}\). \({}^{\text{c}}\) Maximum PE and the PE at 1000 cd/m\({}^{2}\). \({}^{\text{f}}\) CCE coordinates at about 1000 cd/m\({}^{2}\). \({}^{\text{f}}\) Maximum CRI.  
\end{table} 
Table 1: Summary of the performances of WOLEDs based on TADF. Although the performance of WOLEDs based on TADF has been enhanced over the past few years, there are still many challenges before they can be large-scale commercialized production, such as the efficiency, lifetime, color-stability and cost. However, it is deserved to point out that these issues are also hindrances for other kind of WOLEDs. For the issue of efficiency, there is still much room for the efficiency of WOLEDs to the theoretical limit of 248 lm W\({}^{-1}\) (standard light source (D65) from 400 to 700 nm wavelength) [172]. Generally, the EQE is decided by the photoluminescence quantum efficiency of the emissive materials (TADF or other type emitters), the fraction of excitons that can potentially radiatively decay, the charge balance and outcoupling factor, while the PE is also affected by the driving voltage. Therefore, the selection of excellent materials [173; 174; 175; 176], the careful manipulation of charges and excitons distribution [177; 178; 179; 180] and the introduction of outcoupling technique [181; 182; 183; 184] are crucial helpful to the efficiency. In addition, the introduction of tandem architecture is beneficial to the efficiency [185; 186; 187; 188; 189].