**5. Conclusions and outlook**

**Figure 7.** Chemical structures of green-yellow TADF emitters.

HOMO and LUMO. The fabricated device ITO/α-NPD/mCBP/6 wt% *TADF emitter*:DPEPO/ TPBI/LiF/Al) shows an excellent EQE of 21.4% with ELmax at ≈540 nm. In another study, Xiang et al. [58] reported a triazine based acceptor along with phenoxazine as donor and synthesized a yellow TADF emitter **TPXZ-as-TAZ** (**Figure 7**) with very low ΔEST of 0.03 eV. The molecule showed excellent EQE of 13%. The calculated PLQY in 1.5 wt% thin film in host CBP

Tang et al. revealed the strategy to synthesized solution processed green TADF emitters [59]. They synthesized emitter **4CzCNPy** (**Figure 7**) which has small ΔEST of 0.07 eV. The λmax for emitter was 560 nm. The calculated PLQY was quite low in toluene with a value of 55% and τd calculated of 8.4 μs. The fabricated device ITO/PEDOT:PSS/8 wt% *TADF emitter*:mCP/ TmPyPB/LiF/Al showed EQE of 11.3% and CIE coordinate was (0.34, 0.59). They suggested that without cyano group the TADF emission could not be observed in the molecule along

was 53% while the calculated τd was to be 1.10 μs. The λmax for emitter was 555 nm.

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

Past few years have witnessed tremendous development in the field of organic electronics and especially in synthesis of organic light emitting materials which helped to boost the cost reduction of OLEDs and performance enhancement. The potential inexpensive synthesis methods, OLEDs fabrication process, flexibility and lightweight make them one of the promising materials for energy devices. A large number of phosphorescent and fluorescent organic materials have been already synthesized for their use in OLEDs. Recently, TADF materials have been introduced in OLEDs fabrication to achieve 100% IQE and high EQE, which can be achieved near 50%.

**Author details**

Portugal

**References**

201300544

10.1063/1.2213017

Manish Kumar1,2, Miguel Ribeiro1

\*Address all correspondence to: luiz@ua.pt

Nanofabrication, University of Aveiro, Portugal

1987;**51**(12):913-915. DOI: 0.1063/1.98799

& Co. KGraA: Wiley-VCH; 2008

1st ed. Boca Raton: CRC Press; 2010

Letters. 1999;**75**(1):4-6. DOI: 10.1063/1.124258

2001;**90**(10):5048-5051. DOI: 10.1063/1.1409582

2011;**255**(21):2401-2425. DOI: 10.1016/j.ccr.2011.01.049

and Luiz Pereira<sup>2</sup>

1 CeNTI – Centre for Nanotechnologies and Smart Materials, R. Fernando Mesquita,

2 Department of Physics and i3N – Institute for Nanostructures, Nanomodulation and

[1] Tang CW, Van Slyke SA. Organic electroluminescent diodes. Applied Physics Letters.

[2] Yersin H. Highly Efficient OLEDs with Phosphorescent Materials. 1st ed. Verlag GmbH

[3] Franky S, editor. Organic Electronics Materials, Processing, Devices and Applications.

[4] Gaspar DJ, Polikarpov E, editors. OLED Fundamentals: Materials, Devices, and Processing of Organic Light-Emitting Diodes. 1st ed. Boca Raton: CRC Press; 2015 [5] Sasabe H, Kido J. Recent progress in phosphorescent organic light-emitting devices. European Journal of Organic Chemistry. 2013;**2013**(14):7653-7663. DOI: 10.1002/ejoc.

[6] Baldo M, Lamansky S, Burrows P, Thompson M, Forrest S. Very high-efficiency green organic light-emitting devices based on electrophosphorescence. Applied Physics

[7] Adachi C, Baldo MA, Thompson ME, Forrest SR. Nearly 100% internal phosphorescence efficiency in an organic light-emitting device. Journal of Applied Physics.

[8] Tsuzuki T, Nakayama Y, Nakamura J, Iwata T, Tokito S. Efficient organic light-emitting devices using an iridium complex as a phosphorescent host and a platinum complex as a red phosphorescent guest. Applied Physics Letters. 2006;**88**(24):243511. DOI:

[9] Kwong RC, Sibley S, Dubovoy T, Baldo M, Forrest SR, Thompson ME. Efficient, saturated red organic light emitting devices based on phosphorescent platinum(II) porphy-

[10] Kalinowski J, Fattori V, Cocchi M, Williams JG. Light-emitting devices based on organometallic platinum complexes as emitters. Coordination Chemistry Reviews.

rins. Chemistry of Materials. 1999;**11**(12):3709-3713. DOI: 10.1021/cm9906248

\*

New Generation of High Efficient OLED Using Thermally Activated Delayed Fluorescent Materials

http://dx.doi.org/10.5772/intechopen.76048

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The actual development of organic TADF emitters achieves a status that surpasses a simple curiosity. The new OLED framework based on highly efficient TADF materials opens a new field of applications for display and lightening. These materials, based on separated donor-acceptor moieties in one molecule, leads to a unique luminescent process of a triplet harvesting in the excited state with a further huge increase of internal efficiency up to the 100% limit. This simple way to tailored pure organic emitters is really one of the most recent advances in chemistry and OLEDs filed. Achieving very high external efficiencies without using an expensive and rare transition metals, puts the organic emitters towards new possibilities. Particularly important is the application in lighting from large area emitters, a field that is until now, a technological problem. The possibility to have a full range color emitters with high efficiency (as shown, and particularly in blue) is also an outstanding achievement towards a new device design and application, particularly in white emission. Nevertheless, improving the efficiency towards the theoretical efficiency limit, can only be achieved with a deep understand of the TADF process in these organic molecules. This is due to the fact that the interactions between energy levels of TADF emitter and the host can condition the emission due to the TADF process itself as referred before. Thus, the next steps must be focused on the physical models for this *rISC* pathway and the relationship with the molecular structure. In the OLEDs point of view, the design of reliable host materials that fulfills the requirements needs to allow an efficient luminesce emission form the TADFs. A new era is opened.

TADFs have been studied and used for OLEDs so far, and this field is relatively young but it has developed significantly during the past 5 years. By the incorporation of TADFs 40–50% EQE can be achieved. However, at the present stage despite of numerous characterization techniques to understand the TADF behavior, more rigorous efforts are required for their understanding and use for commercially production. The fabrication cost of the OLEDs based on TADF emitter should be less to make them major candidate in both display and lightening industry and the cost is critical. To reduce the cost, it is necessary to develop solution processed OLED fabrication methods and synthesis of polymeric or dendrimeric TADF emitters. These TADFs have already been found a significant role in next-generation displays and lightning materials and their use can only be realized as their synthesis characterization and device fabrication progresses.
