**Conflict of interest**

Although the performance of WOLEDs with TADF emitters 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, and cost. However, it is deserved to point out that these issues are also hindrances for other kinds of WOLEDs. For example, there is still much room for the efficiency of WOLEDs to the theoretical limit of 248 lm/W (standard light source (D65) from 400 to 700 nm wavelength) [63]. Therefore, the photoluminescence quantum efficiency of the emissive materials (TADF or other-type emitters), the charge balance, and outcoupling efficiency of the

48 Light-Emitting Diode - An Outlook On the Empirical Features and Its Recent Technological Advancements

Besides, despite the efficiency of WOLEDs with TADF emitters that can be high enough, there are some other parameters that are needed to be enhanced. Particularly, the stability and efficiency roll-off of WOLEDs with TADF emitters still lags behind other kinds of WOLEDs. For example, fluorescence WOLEDs can show an extremely long lifetime of

ventional blue fluorescence emitters can possess a long lifetime of >30,000 h at 1000 cd/

ment of large-scale commercialized productions. Hence, to solve this issue, stable TADF emitters are urgently explored [57]. In addition, fluorescence WOLEDs, hybrid WOLEDs based on conventional blue fluorescence emitters or even phosphorescence WOLEDs with extreme color stability (△CIE = (0.00, 0.00)) in the whole luminance/driving voltage have been reported [28, 67–70]. However, the color stability of WOLEDs with TADF emitters is usually unstable, indicating that more efforts are required to manage this difficulty. Furthermore, the efficiency roll-off in WOLEDs with TADF emitters is not ideal, particularly for the PE roll-off [71]. As a consequence, only low efficiency can be attained at high luminances, which is not beneficial to the practical applications. To loosen this bottleneck, the charge balance, energy barriers between nearby layers, and materials selection should be well manipulated [72–76]. With the endeavor of academic and industrial researchers to enhance the materials design and device engineering, we believe that WOLEDs with TADF emitters can play a significant role in the marketplace in the near future, which is beneficial

The authors are grateful to the National Natural Science Foundation of China (Grant No. 61704034), the Key Platforms and Research Projects of Department of Education of Guangdong Province (Grant Nos. 2016KTSCX034 and 2016KTSCX031), the Guangdong Natural Science Foundation (Grant No. 2016A030310360), and the financial support from Scientific Research Starting Foundation of Foshan University (Gg040926) and Foshan Science and Technology

 [65]. However, it is still difficult for WOLEDs with TADF emitters to achieve long lifetime, which may be attributed to the instability of TADF emitters. For example, Wang et al. recently reported the first WOLED with TADF emitters realizing long lifetime (2025 h

) [66]. However, it is noted that the lifetime still cannot meet the require-

[64], while hybrid WOLEDs based on con-

devices should be further enhanced.

m2

at 1000 cd/m<sup>2</sup>

to our human society.

**Acknowledgements**

innovation special funds (2017EZ100111).

150,000 h at an initial luminance of 1000 cd/m<sup>2</sup>

The authors declare no competing financial interest.
