**4. Summary and outlook**

zone of W11 is narrow, unfavorable to the performance. Similarly, the main exciton genera-

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

can pass through the thin interlayer via the tunneling process and then meet holes, which can generate excitons to guarantee the blue emission. For W13, by way of the bipolar interlayer and the suitable energy levels of 26DCzPPy, both holes and electrons can be easily passed through 26DCzPPy, as shown in **Figure 5c**. As a result, excitons can be formed at both the

which ensure the high performance of W23. Besides, since the Dexter energy transfer from

Another effective approach to develop WOLEDs with TADF and phosphorescence emitters is the mixture of green TADF and other-color phosphorescence emitters. In this case, the TADF emitters are adopted as the emitters for WOLEDs because they may be compatible with phosphorescence emitters and not quench triplet excitons of the phosphorescence emitters, other-

Kim et al. reported this approach by combining a green TADF with red/blue phosphorescence materials to organize high-efficiency hybrid-type WOLEDs [45]. In their WOLED, energy transfer between a blue phosphorescent material and a green TADF emitter was efficient and could be managed by controlling the doping concentration of emitters. A maximum EQE of 20.2% was achieved by optimizing the device structure of the hybridtype WOLEDs. The device structure is ITO (50 nm)/poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS, 60 nm)/TAPC (20 nm)/mCP (10 nm)/mCP: iridium(III)

nylphosphine oxide-4-(triphenylsilyl)phenyl (TSPO1, 35 nm)/LiF (1 nm)/Al (200 nm), where

cence emitter, respectively, as shown in **Figure 6**. To explore the possibility of this type of WOLEDs, hybrid OLEDs with blue-emitting FIrpic and green-emitting 4CzIPN were first fabricated. By optimizing the concentration of FIrpic and 4CzIPN, a maximum EQE of the hybrid OLEDs was 19.2% at 5% FIrpic and 0.5% 4CzIPN. Given that the EQE of mCP: FIrpic OLED is <20%, such superior EQE of hybrid OLED suggests that 4CzIPN would not quench

tribution to the 4CzIPN TADF emission. For this hybrid OLED, there are three main energy transfer processes, that is, energy transfer processes from mCP to FIrpic, mCP to 4CzIPN, and FIrpic to 4CzIPN dominate the blue and green emissions of the hybrid OLEDs. After the successful exploration of hybrid OLED, Kim et al. combined this system and red phosphorescence emitting layers, attaining high-efficiency WOLEDs. The factors for the highperformance of WOLEDs can be summarized as follows: (1) the hybrid OLEDs doped with FIrpic and 4CzIPN showed a high quantum efficiency, which ensure the high efficiency of blue-green-emitting layer. In this emitting layer, energy transfer from FIrpic to 4CzIPN is

(dpp) is close to the main exciton generation zone. However, a part of electrons

interface, as shown in **Figure 5b**. As

interfaces, leading to a broad exciton generation zone,

]picolinate (FIrpic): (4 s,6 s)-2,4,5,6-tetra(9H-

of FIrpic can be transferred to 4CzIPN and then make a con-

acac (12.5 nm, 3%)/diphe-

acac are blue and red phosphores-

(dpp) is also prevented due to the 3 nm 26DCzPPy, the yellow emis-

(dpp) instead of DDCzTrz since

tion zone of W12 is located at the mCP interlayer/Bepp<sup>2</sup>

sion mainly results from excitons on the yellow EML.

mCP/26DCzPPy and 26DCzPPy/Bepp<sup>2</sup>

wise triplet excitons will be wasted.

bis[(4,6-difluorophenyl)-pyridinato-N,C<sup>2</sup>

FIrpic triplet emission. In fact, T1

carbazol-9-yl)isophthalonitrile (4CzIPN) (12.5 nm)/TPBI: Ir(pq)<sup>2</sup>

4CzIPN is the green TADF emitter, and FIrpic and Ir(pq)<sup>2</sup>

DDCzTrz to Ir(dmppy)<sup>2</sup>

Ir(dmppy)<sup>2</sup>

a result, excitons are more easily harvested by Ir(dmppy)<sup>2</sup>

As a novel kind of 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 is now comparable to state-of-the-art phosphorescence WOLEDs and fluorescence/phosphorescence hybrid WOLEDs. In this chapter, the focus is the development of WOLEDs by manipulating TADF emitters. Specifically, we highlight the recent development of WOLEDs based on all TADF emitters, WOLEDs based on TADF and conventional fluorescence emitters, and WOLEDs based on TADF and phosphorescence emitters. Particularly, the device structures, design strategies, working mechanisms, and electroluminescent processes of the representative high-performance WOLEDs with TADF emitters are reviewed.

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 devices should be further enhanced.

**Conflict of interest**

**Author details**

Dongxiang Luo1

Guangzhou, China

China

**References**

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Science. 2011;**332**:944-947

of Materials Chemistry C. 2013;**1**:1699-1707

Chemical Society Reviews. 2015;**44**:8484-8575

The authors declare no competing financial interest.

, Zhiyuan He<sup>2</sup>

\*Address all correspondence to: bqliu1012@gmail.com

, Peng Xiao3

, Qunxing Liu<sup>2</sup>

White Organic Light-Emitting Diodes with Thermally Activated Delayed Fluorescence Emitters

1 School of Materials and Energy, Guangdong University of Technology, Guangzhou, China

2 China Electronic Produce Reliability and Environmental Testing Research Institute,

3 School of Physics and Optoelectronic Engineering, Foshan University, Foshan, China 4 Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou,

5 LUMINOUS! Center of Excellence for Semiconductor Lighting and Displays, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore

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[2] Helander MG, Wang ZB, Qiu J, Greiner MT, Puzzo DP, Liu ZW, Lu ZH. Chlorinated indium tin oxide electrodes with high work function for organic device compatibility.

[3] Sasabe H, Kido J. Development of high performance OLEDs for general lighting. Journal

[4] Yang X, Zhou G, Wong WY. Functionalization of phosphorescent emitters and their host materials by main-group elements for phosphorescent organic light-emitting devices.

[5] Liu B, Gao D, Wang J, Wang X, Wang L, Zou J, Ning H, Peng J. Progress of white organic

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light-emitting diodes. Acta Physico-Chimica Sinica. 2015;**31**:1823-1852

and Baiquan Liu4,5\*

http://dx.doi.org/10.5772/intechopen.75564

49

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 150,000 h at an initial luminance of 1000 cd/m<sup>2</sup> [64], while hybrid WOLEDs based on conventional blue fluorescence emitters can possess a long lifetime of >30,000 h at 1000 cd/ m2 [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 at 1000 cd/m<sup>2</sup> ) [66]. However, it is noted that the lifetime still cannot meet the requirement 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 to our human society.
