**2. Theory and concepts of thermally activated delayed fluorescence**

The starting point to understand the TADF principle in organic luminescent materials is to consider the fundamental *sp.* orbitals hybridization. The carbon–carbon (C-C) conjugation, employing two *2 s*-orbital electrons and two *2p*-orbitals electrons, leads to the *s* and *p* orbitals mix, giving place to three *sp.* orbitals and one non-hybridized p orbital. The C-C 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 (T) with the ground state a singlet S0 [22]. The excited levels comprise, therefore, several Sn and Tn energy states, although in a simple model we may consider the first S1 and T1 excited levels. Under excitation (electrical and/or optical), the electrons are promoted to these excited levels, one in S1 and three in T1 . This is the main drawback of pure organic compounds: the de-excitation towards the S0 ground state can be evaluated by the transition probability that is given by 〈*<sup>ψ</sup>* <sup>1</sup> <sup>|</sup>*r*|*<sup>ψ</sup>* <sup>0</sup>〉 where *Ψ<sup>1</sup>* , *Ψ<sup>0</sup>* an r are the wave-functions of the excited level and of the fundamental one, and *r* the electrical dipole quantum operator. By spin multiplicity rules, the T1 → S0 transition is strictly forbidden and 75% of excited electrons can only relax by the nonradiative process, remaining only 25% of excited carriers available for the radiative (luminescent) transition S1 → S0 (spin allowed). This means that the maximum internal efficiency 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 considered. The most promising, and subject of this chapter, is the TADF materials.

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.

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

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.

**2. Theory and concepts of thermally activated delayed fluorescence**

The starting point to understand the TADF principle in organic luminescent materials is to consider the fundamental *sp.* orbitals hybridization. The carbon–carbon (C-C) conjugation, employing two *2 s*-orbital electrons and two *2p*-orbitals electrons, leads to the *s* and *p* orbitals mix, giving place to three *sp.* orbitals and one non-hybridized p orbital. The C-C 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 / deexcitation of an organic molecule.

An efficient TADF emission needs to enhance the transition probability *krISC* (T1 → S1 ) relatively to the *KnrP* transition T1 → S0 . Moreover, the TADF efficiency is directly related with the *rISC* transition probability, that, in turns, depends on the energy difference between the S1 and T1 states, *ΔEST* according to the following simple equation [23]:

$$k\_{\rm tSC} = A \exp\left(-\frac{\Delta E\_{\rm ST}}{k\_y T}\right) \tag{1}$$

where *ψH*, *ψ<sup>L</sup>*

3

, *r1* , *r2*

if a high value for *kISC* and *krISC* exist, T1

cess [28] (for instance with different characters like 1

LE and 3

LE → <sup>1</sup>

LE and 3

involving a mixture of 3

depends on the SOC (3

the first equilibrium between 3

sation); next, a coupling between the 3

→ <sup>1</sup>

, and *q* are the HOMO and LUMO wave functions, the coordinate posi-

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

CT and

107

CT

can be found in LE (local excited) and CT states

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

LE); or a more sophisticated model

LE state (very efficient) promotes the

tions and electronic charge, respectively. From this equation, is very easy to verify that a minimization of *J* requires a negligible wave functions overlapping and therefore, a very low (or absence) of spatial overlapping between HOMO and LUMO levels (a spatial separation between HOMO and LUMO frontier orbitals). In a single molecule, this basic rule can be obtained if the molecule has independent structural moieties, one containing electron-donor (D) and another with electron-acceptor (A) which promotes the D-A charge transfer in the excited state. Therefore, the basic rule for an efficient *rISC* process in a single molecule is to guarantee if such molecule has at least, two unities (D and A, with non-overlapped orbitals) spatially separated. This can be achieved by increasing the spatial distance between such unities using a π-conjugated link or forcing a large dihedral angle between the planes of the donor and the acceptor, i.e. roughly speaking, forces a twist between D and A unities around the common axis [25]. In any case, we expect a strong CT character in the excited states.

The physical model to explain the *rISC* process is so far little understood. The first successful application [26] precisely focuses on the twisted D-A unities. In the first explanations, the CT

CT). Following the known rules, the *ISC* (and therefore the *rISC*) process will be efficient under an occurrence of symmetry change of the excited states. This means in triplet/singlet systems,

respectively. However, we know that efficient TADF can occur even if such rule is not observed. Moreover, the excited states involved in TADF organic material, are typically a mixture of CT-LE states and not pure CT or LE states. Furthermore, extensive studies involving photoluminescence data reveal some discrepancies and new hypothesis involving also the LE states begin to be considered. In a general organic molecule (system), the excited states can be described in terms of their binding energy: the CT (low binding energies) and LE (strong binding energies). This LE has a very high radiative probability (with a strong emission) due to the high orbitals overlapping (the dipole electrical operator in the wave functions gives a high resulting probability). The *rISC* process model with only CT states starts to pose some problems with the discovery of SOC between such intramolecular states are zero [27]. Several hypotheses have been discussed and it was found that it is possible to tune independently two excited states involved in the *rISC* pro-

CT and 3

CT). However, and in spite of an allowed SOC between these states, the calculations of the *rISC* probabilities (relatively low) cannot explain the experimental data that gives much higher values. Recently [30] a more complex model involving two steps was proposed. In such model,

(also called as *rIC* – reserve intersystem crossing, helping the thermally assisted internal conver-

*rISC* finalization process. In this model, both SOC and vibronic coupling are involved. All these models are, in general, well supported by several experimental data but at this moment, and depending on the specific organic emitter studied, different pathways need to be considered,

CT states via 3

CT and 1

leading to several open questions. In any case, this topic remains heavily investigated.

CT states [29] giving rise to an hypothesis where the *rISC* process

CT states is promoted by vibronic coupling between them

CT) and also on a hyperfine coupling induced *ISC* process (3

states are used as a key to promoting the *rISC* process (singlet and triplet character, 1

and S1

**Figure 1.** A simple scheme of the electroluminescence process involving an organic light emitter material. The transition probabilities (the inverse gives the transition lifetime) are *krF*, *krP*, *knrF*, *knrP, kISC* and *krISC* (radiative fluorescence, radiative phosphorescence, nonradiative, inter-system crossing and reverse inter-system crossing). TADF emission is related to *krTADF* and strictly depends on *krISC*.

where *kB* is the Boltzmann constant, *T* the temperature and *A* the pre-exponential factor. The value of *ΔEST*, naturally depends on the typical energy arising from the electrostatic interactions among the molecular orbitals. Particularly important, we need to consider the oneelectron orbital energy in excited state (*E*) (supposing a fixed nuclear model), the typical electron repulsion energy (*K*) and the exchange energy (*J*) resulting from electron–electron repulsion based on the Pauli exclusion principle, affecting two excited unpaired electrons (one in LUMO and another one in HOMO levels) [24]. As the singlet and triplet excited states have a different spin ordering, the *J* value is usually higher in S1 state and lower (in the same amount) in T1 state [20]. With such consideration, the energy associated with the S1 and T1 states (respectively *ES1* and *ET1*) and therefore the *ΔEST*, can be easily established, by the following relationships:

$$\begin{aligned} E\_{\mathbf{S}\_i} &= E + \mathbf{K} - \mathbf{J} \\ E\_{T\_i} &= E + \mathbf{K} - \mathbf{J} \\ \Delta E\_{ST} &= E\_{\mathbf{S}\_i} - E\_{T\_i} \implies \Delta E\_{ST} = \mathbf{2J} \end{aligned} \tag{2}$$

The immediate conclusion is that the minimization of *ΔEST* implies a lowest possible exchange energy. Remembering that the two unpaired electrons should be considered as distributed in the frontier orbitals of the LUMO and HOMO levels (T1 or S1 , excited states) resulting in pure LUMO-HOMO transitions, *J* can be given by [20]:

$$J = \iint \psi\_{\mathbf{1}}(r\_1) \psi\_{\mathbf{1}\emptyset}(r\_2) \left(\frac{q^2}{r\_1 - r\_2}\right) \psi\_{\mathbf{1}\{r\_2\}} \psi\_{\mathbf{1}\{r\_1\}} dr\_1 dr\_1 dr\_2 \tag{3}$$

where *ψH*, *ψ<sup>L</sup>* , *r1* , *r2* , and *q* are the HOMO and LUMO wave functions, the coordinate positions and electronic charge, respectively. From this equation, is very easy to verify that a minimization of *J* requires a negligible wave functions overlapping and therefore, a very low (or absence) of spatial overlapping between HOMO and LUMO levels (a spatial separation between HOMO and LUMO frontier orbitals). In a single molecule, this basic rule can be obtained if the molecule has independent structural moieties, one containing electron-donor (D) and another with electron-acceptor (A) which promotes the D-A charge transfer in the excited state. Therefore, the basic rule for an efficient *rISC* process in a single molecule is to guarantee if such molecule has at least, two unities (D and A, with non-overlapped orbitals) spatially separated. This can be achieved by increasing the spatial distance between such unities using a π-conjugated link or forcing a large dihedral angle between the planes of the donor and the acceptor, i.e. roughly speaking, forces a twist between D and A unities around the common axis [25]. In any case, we expect a strong CT character in the excited states.

The physical model to explain the *rISC* process is so far little understood. The first successful application [26] precisely focuses on the twisted D-A unities. In the first explanations, the CT states are used as a key to promoting the *rISC* process (singlet and triplet character, 1 CT and 3 CT). Following the known rules, the *ISC* (and therefore the *rISC*) process will be efficient under an occurrence of symmetry change of the excited states. This means in triplet/singlet systems, if a high value for *kISC* and *krISC* exist, T1 and S1 can be found in LE (local excited) and CT states respectively. However, we know that efficient TADF can occur even if such rule is not observed. Moreover, the excited states involved in TADF organic material, are typically a mixture of CT-LE states and not pure CT or LE states. Furthermore, extensive studies involving photoluminescence data reveal some discrepancies and new hypothesis involving also the LE states begin to be considered. In a general organic molecule (system), the excited states can be described in terms of their binding energy: the CT (low binding energies) and LE (strong binding energies). This LE has a very high radiative probability (with a strong emission) due to the high orbitals overlapping (the dipole electrical operator in the wave functions gives a high resulting probability). The *rISC* process model with only CT states starts to pose some problems with the discovery of SOC between such intramolecular states are zero [27]. Several hypotheses have been discussed and it was found that it is possible to tune independently two excited states involved in the *rISC* process [28] (for instance with different characters like 1 CT and 3 LE); or a more sophisticated model involving a mixture of 3 LE and 3 CT states [29] giving rise to an hypothesis where the *rISC* process depends on the SOC (3 LE → <sup>1</sup> CT) and also on a hyperfine coupling induced *ISC* process (3 CT → <sup>1</sup> CT). However, and in spite of an allowed SOC between these states, the calculations of the *rISC* probabilities (relatively low) cannot explain the experimental data that gives much higher values. Recently [30] a more complex model involving two steps was proposed. In such model, the first equilibrium between 3 LE and 3 CT states is promoted by vibronic coupling between them (also called as *rIC* – reserve intersystem crossing, helping the thermally assisted internal conversation); next, a coupling between the 3 CT and 1 CT states via 3 LE state (very efficient) promotes the *rISC* finalization process. In this model, both SOC and vibronic coupling are involved. All these models are, in general, well supported by several experimental data but at this moment, and depending on the specific organic emitter studied, different pathways need to be considered, leading to several open questions. In any case, this topic remains heavily investigated.

where *kB* is the Boltzmann constant, *T* the temperature and *A* the pre-exponential factor. The value of *ΔEST*, naturally depends on the typical energy arising from the electrostatic interactions among the molecular orbitals. Particularly important, we need to consider the oneelectron orbital energy in excited state (*E*) (supposing a fixed nuclear model), the typical electron repulsion energy (*K*) and the exchange energy (*J*) resulting from electron–electron repulsion based on the Pauli exclusion principle, affecting two excited unpaired electrons (one in LUMO and another one in HOMO levels) [24]. As the singlet and triplet excited states

**Figure 1.** A simple scheme of the electroluminescence process involving an organic light emitter material. The transition probabilities (the inverse gives the transition lifetime) are *krF*, *krP*, *knrF*, *knrP, kISC* and *krISC* (radiative fluorescence, radiative phosphorescence, nonradiative, inter-system crossing and reverse inter-system crossing). TADF emission is related to

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

state [20]. With such consideration, the energy associated with the S1

states (respectively *ES1* and *ET1*) and therefore the *ΔEST*, can be easily established, by the fol-

= *E* + *K* − *J*

<sup>=</sup> *<sup>E</sup>* <sup>+</sup> *<sup>K</sup>* <sup>−</sup> *<sup>J</sup>*

⟹ Δ *EST* = 2*J*

or S1

*ES*1

*ET*1

− *E<sup>T</sup>*<sup>1</sup>

The immediate conclusion is that the minimization of *ΔEST* implies a lowest possible exchange energy. Remembering that the two unpaired electrons should be considered as distributed in

\_\_\_\_

Δ *EST* = *E<sup>S</sup>*<sup>1</sup>

the frontier orbitals of the LUMO and HOMO levels (T1

LUMO-HOMO transitions, *J* can be given by [20]:

*<sup>J</sup>* <sup>=</sup> <sup>∬</sup>*ψL*(*r*1) *<sup>ψ</sup>H*(*r*2)( *<sup>q</sup>* <sup>2</sup>

state and lower (in the same

, excited states) resulting in pure

*<sup>r</sup>*<sup>1</sup> <sup>−</sup> *<sup>r</sup>*2) *<sup>ψ</sup>L*(*r*2) *<sup>ψ</sup>H*(*r*1)*<sup>d</sup> <sup>r</sup>*<sup>1</sup> *<sup>d</sup> <sup>r</sup>*<sup>2</sup> (3)

and T1

(2)

have a different spin ordering, the *J* value is usually higher in S1

amount) in T1

lowing relationships:

*krTADF* and strictly depends on *krISC*.

Besides the usual D-A molecule separated structure, some new molecules based on D-A-D (so-called "butterfly-shaped" structure) also exhibits TADF emission. Surprisingly, in several of such molecules, the energy gap between the lowest 1 CT state and the lowest 3 LE state (with π → π\* transition) are much higher than those found between 1 CT and 3 CT energy states in the conventional D-A molecules. The explanation was the two-step model above referred. This model appears to be the most interesting and well supported by experimental results.

with *Z* being the canonical partition function for vibrational motion in the initial electronic state. These terms are therefore the starting point for a more detailed description of the TADF process. Knowing these probabilities, that we can estimate, several possibilities to further tuning the TADF process is possible. Despite several well-founded hypothesis, the fully under-

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

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

109

The starting point for developing an efficient OLED using TADF emitter is based on the luminescence properties of the emitter itself. As an earlier point, the physical process involved are not really straightforward, but leaving aside the pure photophysics process studies, the

From **Figure 1**, we can formally consider two different kinds of radiative emissions arising

depending on a lower *krISC* probability, we have a delayed fluorescence (DF). A strong TADF emission is usually observed in molecules where the yield of triplet levels formation (by intersystem crossing), as well the singlet level formation (by reverse intersystem crossing) are high (particularly the last one as expected). In this condition, we must assume that the *rISC* yield that

are much less probable that the *rISC* (as expected). The emission from a TADF material is naturally the sum of the observed from the PF and DF and therefore its quantum yield is given by:

100%. In practice, most TADF materials where the value of *ΔEST* is less than near 150 mV, such yield is obtained. In this situation, the triplet yield is relatively easy to obtain with precision,

rial characterization. In simple but practical ways, two different approaches can be used for such determination. Both are related to the fact that almost know TADF materials exhibit very poor or none DF in the presence of oxygen. Thus, measuring the luminescence emission parameters under a normal or degassed environment, we can achieve either PF or PF + DF. Under steady state photo physics, the direct measurement of the luminescence spectra in both environment conditions

 state: from its own electrons population (25% of excited ones) and from the population via *rISC* process (the remaining 75% of excited electrons). In the first case, with a very high

, we have a prompt fluorescence (PF) whereas in the second situation,

is approximately equal to 1. This appears when (and usually found

\_\_\_\_\_\_\_\_\_ 1 1 − ϕ*rISC* ϕ*ISC*

<sup>ϕ</sup>*PF* is near (or above) four, the yield of the *rISC* process will be near

<sup>ϕ</sup> *PF* that is a fundamental key for the mate-

*rP*, meaning that all relaxation process from triplet excited state

(5)

(6)

stand of TADF process in organic emitters, still remains under major studies.

**3. Fundamental photophysics of an organic TADF emitter**

important figures of merit regarding efficiency can be easily obtained.

in S1

transition probability *kF*

is given by ϕ*rICS* <sup>=</sup> *<sup>k</sup>* \_\_\_\_\_\_\_\_\_\_ *rISC k rISC* + *k nRP* + *k rP*

According [32], if the ratio <sup>ϕ</sup>*DF*⁄

and is given by:

*rISC* ≫ *k*

*nRP* + *k*

ϕ*TADF* = ϕ*PF* + ϕ*DF* = ϕ*PF*

<sup>ϕ</sup>*ISC* <sup>=</sup> <sup>ϕ</sup> \_\_\_\_*DF*\_\_\_ <sup>ϕ</sup>*DF* <sup>+</sup> <sup>ϕ</sup>*PF*

This relationship can be useful to determine the ratio of <sup>ϕ</sup> *DF*⁄

in TADF materials) *k*

Particularly important in this model, is the ability to modulate the energy of the excited 1 CT state via the environment polarity [31]. In solution, the photophysics analysis can help in revealing the main process involved, in turn, dependent on the solvent polarity. On film (solid state), this possibility opens a wide range of choice for the organic host material in order to significantly improve the efficiency of an OLED based on a TADF material. In a simple scheme, we can, therefore, represent the excited state of the TADF molecule as shown in **Figure 2**.

It must be noted that, according to this model, and following several experimental data (see [30] and references therein) the energy transfer SOC-ISC is more efficient in a D-A perpendicular geometry, in a transition *n*π\*-like. This is a consequence of the maximum change in the orbital angular momentum as the SOC depends on the spin magnetic quantum number of the electrons and simultaneously on its spatial angular momentum quantum number (the SOC operator is proportional to *<sup>s</sup>*̂<sup>∙</sup> *<sup>l</sup>*̂ ⁄ℏ2 ). Following this two steps model, the probabilities of *krIC* and *krISC* can be written in terms of both physical process involved [31]:

$$k\_{\rm rSC} = \frac{2\pi}{\hbar\bar{Z}} \left| \left\langle \psi\_{\rm 3CT} \mid \hat{H}\_{\rm sSC} \mid \psi\_{\rm 3LE} \right\rangle \right|^2 \times \Delta \left(E\_{\rm 3LE} - E\_{\rm 3CT} \right)$$

$$k\_{\rm rSC} = \frac{2\pi}{\hbar\bar{Z}} \left| \frac{\left\langle \psi\_{\rm 3CT} \mid \hat{H}\_{\rm sSC} \mid \psi\_{\rm 3LE} \right\rangle \left\langle \psi\_{\rm 3LE} \mid \hat{H}\_{\rm vBs} \mid \psi\_{\rm 3CT} \right\rangle}{\Delta \left(E\_{\rm 3LE} - E\_{\rm 3CT} \right)} \right|^2 \times \Delta \left(E\_{\rm 3CT} - E\_{\rm 1CT} \right)$$

**Figure 2.** A simple representation of a TADF emitter in excited state, following the model proposed in [30]. In (a) the SOC-ISC transition is enhanced by vibronic coupling between the 3 CT and 3 LE states whereas in (b) the 1 CT state can be modulated by the environment polarity.

with *Z* being the canonical partition function for vibrational motion in the initial electronic state. These terms are therefore the starting point for a more detailed description of the TADF process. Knowing these probabilities, that we can estimate, several possibilities to further tuning the TADF process is possible. Despite several well-founded hypothesis, the fully understand of TADF process in organic emitters, still remains under major studies.
