**1. Introduction**

Since the first invention of the organic light emitting diodes (LEDs) in 1987 by Tang and Van Slyke [1] that represented an advancement in display and lightening technologies, Organic light-emitting diodes (OLEDs) have emerged as an extensive active field in the both scientific as well technological aspects. Organic light-emitting diodes (OLEDs) are cheap, flexible, and cheap like a movie projector screen. They have attracted considerable attention due to their promising applications in cheap, energy-saving, eco-friendly and solid-state lighting [2–4].

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The current research in OLEDs is emerging technology which is also a growing market and expected to cross 20 billion by 2030 [5]. OLEDs are used in several flat and roll displays, also in white EDs for the lightning. OLEDs give freedom of taking advantage of emission in different colors, color modulation (color coordinates, temperature and color rendering white lighting), diffused light—(light from flat panels (large area) and high viewing angle), Freedom design (thin, lightweight, flexible transparent—easy incorporation into 3D surfaces), etc. Along with these characteristics, there is some difficulty in getting a large homogeneous emitter area, where the organic materials rapidly degrade in the presence of oxygen and/or humidity. Although solved with rigid OLEDs, flexible ones have low lifetime (no efficient encapsulation system has been developed). The main principle behind OLED technology is electroluminescence and such devices offer brighter, thinner, high contrast, and flexibility.

covalent bond is made using two *sp.* orbitals, from each carbon atom, giving rise to a usually called π bond; the third makes a covalent bond around the inter-nuclear axis and is usually called as σ bond. This simplified framework can explain the major electro-optical behavior of organic compounds. Effectively, whereas the π bonds located above and below (respectively, π\* - anti-bonding and π bond), originating an overlapping of the *sp.* orbitals in each side, the σ bond is a pure bond between two adjacent atoms. Besides the orbitals geometry, the electrical carriers are allowed to hopping among the ππ\* cloud, in contrary to σ carriers that are confined. The ππ\* cloud is the basic formation of the occupied and unoccupied energy levels and the further definition of HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) levels. The σ energy region is typically a forbidden gap. From this simple configuration, organic molecule energy levels are typically singlet (S) and triplet

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

levels. Under excitation (electrical and/or optical), the electrons are promoted to these excited

fundamental one, and *r* the electrical dipole quantum operator. By spin multiplicity rules,

nonradiative process, remaining only 25% of excited carriers available for the radiative (lumi-

of a pure organic luminescent material is only 25%. Overcoming this constraint is an absolute priority for achieving highly efficient organic electroluminescent devices. Besides the wellknown transition metals organic–inorganic complexes [22] that promote a strong spin-orbit coupling (SOC) with further enhanced phosphorescence, several other paths have been con-

The analysis of this process can be based on the exciton formation (electron-hole pair) in a conjugated organic material. An electronic charge can be transferred between both entities in a two molecules system (or also in different parts of the same molecule). This process is called of charge-transfer (donor-acceptor complex) leading origin to the CT energy levels. The primary effect of these levels is to provide an electrostatic attraction, stabilizing the molecule. But, interestingly, this CT state is spin selective and is supposed to be able to change the triplet / singlet balance, allowing a conversion of triplet excitons to singlet ones. Although being still an unclear mechanism, was the fundamental starting point to the TADF materials. The **Figure 1** shows, in a simple scheme, the fundamental process involved in the excitation / de-

An efficient TADF emission needs to enhance the transition probability *krISC* (T1 → S1

states, *ΔEST* according to the following simple equation [23]:

*rISC* transition probability, that, in turns, depends on the energy difference between the S1

<sup>Δ</sup> *<sup>E</sup>* \_\_\_\_*ST*

transition is strictly forbidden and 75% of excited electrons can only relax by the

and Tn energy states, although in a simple model we may consider the first S1

sidered. The most promising, and subject of this chapter, is the TADF materials.

, *Ψ<sup>0</sup>*

[22]. The excited levels comprise, therefore, several Sn

. This is the main drawback of pure organic compounds: the

an r are the wave-functions of the excited level and of the

. Moreover, the TADF efficiency is directly related with the

*kB <sup>T</sup>* ) (1)

ground state can be evaluated by the transition probability that

(spin allowed). This means that the maximum internal efficiency

and T1

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

excited

105

) rela-

(T) with the ground state a singlet S0

and three in T1

<sup>|</sup>*r*|*<sup>ψ</sup>* <sup>0</sup>〉 where *Ψ<sup>1</sup>*

levels, one in S1

is given by 〈*<sup>ψ</sup>* <sup>1</sup>

the T1 → S0

de-excitation towards the S0

nescent) transition S1 → S0

excitation of an organic molecule.

tively to the *KnrP* transition T1 → S0

*krISC* = *Aexp*(−

and T1

In the conventional OLEDs, the materials used are π-electron-rich molecules, which helps in the fast charge transfer at the interface. But in these OLEDs, the internal quantum efficiency (IQE) is lower which results to the lower external quantum efficiency (EQE) of 5% and limits the OLEDs development because of the nonradiative triplet exciton non-harnessing. Usually, materials used for OLEDs are phosphorescent emitters such as iridium [6–8] or platinum complexes [9, 10] that are used to achieve the electroluminescence efficiency. In such systems, both 25% singlet excitons and 75% triplet excitons can be used for harnessing the electroluminescence. In phosphorescent OLEDs, the internal quantum efficiency was reported close to 100% [11–16], but the disadvantage in such phosphorescent material is their high cost and poor stability. Along with phosphorescent material harnessing phosphorescence [7, 17], triplet-triplet annihilation [18] were also used. Therefore, to achieve 100% low-cost IQE, the development of an alternative to harvest the 75% triplet exciton is important for the future of OLEDs. In this context, response to this need, the development of the thermally activated delayed fluorescence (TADF) materials with the most promising exciton harvesting mechanism used in OLED devices, which was firstly reported by Adachi et al. in [19] received tremendous attention, and in recent years, considerable efforts have been devoted towards the fabrication of OLEDs based on TADF materials where the IQE can be easily achieved up to 100% [20, 21].

In this chapter, we summarize the fundamentals of thermally activated delayed fluorescence process, their optoelectronic behavior linking with the device performance and recent experimental studies of the introduction of TADF emitters used as the doping/guest material for OLED fabrication. Along with, a summary of the best TADF emitters used for fabrication of orangered, blue and green-yellow OLEDs is provided. In addition, a correlation is provided between the structure and doping percentage of TADF emitters and their optoelectronic properties.
