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

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 sev-

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 experimen-

Particularly important in this model, is the ability to modulate the energy of the excited 1

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

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*|*ψ*3*LE*〉|

*SOC*|*ψ*3*LE*〉〈*ψ*3*LE*|*H*̂

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ Δ(*E*3*LE* <sup>−</sup> *<sup>E</sup>*3*CT*) <sup>|</sup>

**Figure 2.** A simple representation of a TADF emitter in excited state, following the model proposed in [30]. In (a) the

CT and 3

2

*VIB*|*ψ*3*CT*〉

CT state and the lowest 3

). Following this two steps model, the probabilities of *krIC*

LE states whereas in (b) the 1

× Δ(*E*3*CT* − *E*1*CT*)

× Δ(*E*3*LE* − *E*3*CT*)

2

CT and 3

LE state

CT

(4)

CT state can be

CT energy

eral of such molecules, the energy gap between the lowest 1

tal results.

in **Figure 2**.

SOC operator is proportional to *<sup>s</sup>*̂<sup>∙</sup> *<sup>l</sup>*̂

(with π → π\* transition) are much higher than those found between 1

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

⁄ℏ2

and *krISC* can be written in terms of both physical process involved [31]:

<sup>ℏ</sup>*<sup>Z</sup>* <sup>|</sup>〈*ψ*3*CT*|*H*̂

*krIC* <sup>=</sup> \_\_\_ <sup>2</sup>*<sup>π</sup>*

<sup>ℏ</sup>*<sup>Z</sup>* <sup>|</sup>〈*ψ*1*CT*|*H*̂

*krISC* <sup>=</sup> \_\_\_ <sup>2</sup>*<sup>π</sup>*

SOC-ISC transition is enhanced by vibronic coupling between the 3

modulated by the environment polarity.

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 important figures of merit regarding efficiency can be easily obtained.

From **Figure 1**, we can formally consider two different kinds of radiative emissions arising in S1 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 transition probability *kF* , we have a prompt fluorescence (PF) whereas in the second situation, 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 is given by ϕ*rICS* <sup>=</sup> *<sup>k</sup>* \_\_\_\_\_\_\_\_\_\_ *rISC k rISC* + *k nRP* + *k rP* is approximately equal to 1. This appears when (and usually found in TADF materials) *k rISC* ≫ *k nRP* + *k rP*, meaning that all relaxation process from triplet excited state 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:

$$
\phi\_{\rm LADF} = \phi\_{\rm pF} + \phi\_{\rm DF} = \phi\_{\rm PF} \frac{1}{1 - \phi\_{\rm MC}} \tag{5}
$$

According [32], if the ratio <sup>ϕ</sup>*DF*⁄ <sup>ϕ</sup>*PF* is near (or above) four, the yield of the *rISC* process will be near 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, and is given by:

$$\phi\_{\rm{ISC}} = \frac{\phi\_{\rm{ro}}}{\overline{\phi\_{\rm{ro}}} \overline{\cdot} \overline{\phi\_{\rm{per}}}} \tag{6}$$

This relationship can be useful to determine the ratio of <sup>ϕ</sup> *DF*⁄ <sup>ϕ</sup> *PF* that is a fundamental key for the material 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 will give only (with great certainty) the PF (normal environment) and PF + DF (degassed environment). The direct ratio of the integrated spectra (intensity) further gives a very precise value for ϕ *DF*⁄ <sup>ϕ</sup> *PF*. Naturally this simple calculation is possible (quantum yield ratio from integrated intensity) because the intensity is proportional to the quantum yield and, in the case of TADF materials, the values of the proportionality constants for both emissions (DF and PF) are the same due to the fact that both arise from the same excited energy level [33]. The exact calculation is the performed considering the simple relationship is given by *<sup>I</sup>* \_\_\_\_ *DF*+*PF I PF* <sup>=</sup> <sup>ϕ</sup>*DF* <sup>+</sup> <sup>ϕ</sup> \_\_\_\_\_\_\_*PF* ϕ*PF* = 1 + ϕ \_\_\_*DF* ϕ*PF* . **Figure 3** shows a simple example of this behavior.

By another hand, the higher transition probability associated with PF compared to the DF probability (that in a crude way depends on the *rISC* process probability) allows an experimental emission separation under the time-resolved photoluminescence (TRP). In a typical TADF material, the PF lifetime is in the order of dozens of *ns* whereas the DF lifetime falls into *μs*. Therefore, measuring both lifetimes, an estimative of the <sup>ϕ</sup>*DF*⁄ <sup>ϕ</sup>*PF* can be done because the transition probabilities are related to the inverse of the lifetime. If we consider (and is a very good approach) that the emission follows a single exponential decay for both PF and DF, therefore the measured photoluminescence intensity under time can be simply given by the sum of the two single exponential decay expressions as follow:

$$I(t) \ = I\_0^{\mathrm{pr}} \exp\left(-\frac{t}{\mathsf{T}\_{\mathrm{pr}}}\right) + I\_0^{\mathrm{DF}} \exp\left(-\frac{t}{\mathsf{T}\_{\mathrm{D}}}\right) \tag{7}$$

being *τPF*, *<sup>I</sup>*

0

(and considering ϕ*rISC* ≈ 1)

2PX-TAZ emitter.

result for a TADF material.

*PF*, *τDF* and *<sup>I</sup>*

can be obtained and is given simply by:

<sup>ϕ</sup>

*krISC* <sup>=</sup> \_\_\_<sup>1</sup>

0

*DF* the lifetime and pre-exponential intensity factor for the PF and

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

<sup>ϕ</sup>*PF* ) (9)

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

ϕ*PF*

111

(8)

DF. Once again, calculating the intensity (integral) for both time-resolved emissions, the <sup>ϕ</sup>*DF*⁄

Finally, and by TRP is possible to estimate the transition probability of the *rISC* process. Knowing the values for quantum yields and lifetime, an estimation for *krISC* can be given by

*τDF*(

Due to several different kinds of triplet harvesting in an organic molecule, sometimes is not simple to attribute an enhanced luminescence to a TADF process. For instance, triplet-triplet annihilation (TTA) is also a wide investigated process for emission efficiency improvement. Distinguish both process is important. Due to the competition between the triplet quenching and decay of triplet states, usually the DF from TTA is non-linear on excitation energy; on the contrary, and because TADF process is purely intramolecular, its DF must follow a linear relationship with excitation energy. **Figure 4** shows an example of the

**Figure 4.** Emission intensity of 2PX-TAZ emitter as a function of excitation energy. The perfect linear fit is the expected

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

\_\_\_*DF* ϕ*PF* = *I* 0 *DF* <sup>×</sup> *<sup>τ</sup>* \_\_\_\_\_\_*DF I* 0 *PF* × *τPF*

**Figure 3.** Steady state photoluminescence spectra of 2PX-TAZ under normal (air) and degassed (vacuum) environments. The ratio of integrated spectrum in both situations allows a simple calculation of the DF/PF quantum yield. In this case, the ratio ϕ*DF*/ϕ*PF* is near 9.

being *τPF*, *<sup>I</sup>* 0 *PF*, *τDF* and *<sup>I</sup>* 0 *DF* the lifetime and pre-exponential intensity factor for the PF and DF. Once again, calculating the intensity (integral) for both time-resolved emissions, the <sup>ϕ</sup>*DF*⁄ ϕ*PF* can be obtained and is given simply by:

will give only (with great certainty) the PF (normal environment) and PF + DF (degassed environment). The direct ratio of the integrated spectra (intensity) further gives a very precise value for

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

\_\_\_\_ *DF*+*PF I PF*

By another hand, the higher transition probability associated with PF compared to the DF probability (that in a crude way depends on the *rISC* process probability) allows an experimental emission separation under the time-resolved photoluminescence (TRP). In a typical TADF material, the PF lifetime is in the order of dozens of *ns* whereas the DF lifetime falls into

sition probabilities are related to the inverse of the lifetime. If we consider (and is a very good approach) that the emission follows a single exponential decay for both PF and DF, therefore the measured photoluminescence intensity under time can be simply given by the sum of the

> *<sup>τ</sup>PF*) + *I* 0

**Figure 3.** Steady state photoluminescence spectra of 2PX-TAZ under normal (air) and degassed (vacuum) environments. The ratio of integrated spectrum in both situations allows a simple calculation of the DF/PF quantum yield. In this case,

*DF exp*(−\_\_\_*<sup>t</sup>*

<sup>ϕ</sup> *PF*. Naturally this simple calculation is possible (quantum yield ratio from integrated intensity) because the intensity is proportional to the quantum yield and, in the case of TADF materials, the values of the proportionality constants for both emissions (DF and PF) are the same due to the fact that both arise from the same excited energy level [33]. The exact calculation is the performed con-

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

= 1 + ϕ \_\_\_*DF* ϕ*PF*

. **Figure 3** shows a simple example

<sup>ϕ</sup>*PF* can be done because the tran-

*<sup>τ</sup>DF*) (7)

ϕ *DF*⁄

of this behavior.

sidering the simple relationship is given by *<sup>I</sup>*

*μs*. Therefore, measuring both lifetimes, an estimative of the <sup>ϕ</sup>*DF*⁄

0

*PF exp*(−\_\_\_*<sup>t</sup>*

two single exponential decay expressions as follow:

*I*(*t*) = *I*

the ratio ϕ*DF*/ϕ*PF* is near 9.

$$\frac{\Phi\_{\rm{DV}}}{\Phi\_{\rm{PV}}} = \frac{I\_0^{\rm{DC}} \times \tau\_{\rm{DV}}}{I\_0^{\rm{PV}} \times \tau\_{\rm{PV}}} \tag{8}$$

Finally, and by TRP is possible to estimate the transition probability of the *rISC* process. Knowing the values for quantum yields and lifetime, an estimation for *krISC* can be given by (and considering ϕ*rISC* ≈ 1)

$$k\_{\rm rSC} = \frac{1}{\overline{\tau}\_{\rm DV}} \left( \frac{\phi\_{\rm PV} \star \phi\_{\rm DT}}{\Phi\_{\rm PV}} \right) \tag{9}$$

Due to several different kinds of triplet harvesting in an organic molecule, sometimes is not simple to attribute an enhanced luminescence to a TADF process. For instance, triplet-triplet annihilation (TTA) is also a wide investigated process for emission efficiency improvement. Distinguish both process is important. Due to the competition between the triplet quenching and decay of triplet states, usually the DF from TTA is non-linear on excitation energy; on the contrary, and because TADF process is purely intramolecular, its DF must follow a linear relationship with excitation energy. **Figure 4** shows an example of the 2PX-TAZ emitter.

**Figure 4.** Emission intensity of 2PX-TAZ emitter as a function of excitation energy. The perfect linear fit is the expected result for a TADF material.

Besides the excitation energy dependence, the TADF emission is also strongly dependent on temperature. As the DF is thermally activated, we expect that its intensity decreases strongly with temperature and eventually vanishes at very low temperature. On the contrary, PF must be unaffected by thermal variations. This means that under TRP we must observe a decrease of the high lifetime emission as temperature decreases until remaining only the fast component.

for the development of TADF emitters such as these emitters are very limited and only few can be used for highly EQE and stable OLEDs, another is the lower maximum brightness and roll-offs at high brightness related triplet annihilation [20, 36]. These can be solved by understanding the structure-property relationship in emitters. In this chapter, we focused more on the photo physical relation of TADF emitter with device performance. In this section, we will describe different TADF emitters and their photo physics-device performance characteristics.

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113

The application of TADF emitters is generally focused on OLEDs applications. As we have discussed in photo physics behavior of TADFs, it requires a solid host to disperse TADF emitters and this host material has a strong influence on the photo physical properties of these emitters [37]. To encounter this, the design and optimization of TADF emitter is a key factor for the fabrication of OLEDs, and this requires the photo physical characterization of TADF in the host molecule which used in the device. Some of the most used hosts are DPEPO, CBP, mCP, mCBP, TPBi, TCTA and TAPC. The OLEDs are usually fabricated by thermally vacuum deposition, but several reports have been focused on fabrication via spin coating solution

Many groups reported various green TADF based on different donor and acceptor molecules, due to less space it is difficult to discuss all of them. Herein, we will discuss some of the TADF red, green and blue emitters based on their donor and acceptor groups, photo-physical char-

Herein, we present red-orange TADF emitters which exhibit an electroluminescence peak at wavelength (ELmax) > 580 nm. The first reported red TADF emitter, **4CzTPN-Ph** (**Figure 5**) with green emission by Adachi et al. [38] Figure which exhibits calculated PLQY of 26% in toluene and, τd 1.1 μs. The device showed remarkable EQE of 11.2%, the fabricated device structure was ITO/NPD/5 wt% **4CzTPN-Ph**:CBP/TPBi/LiF/Al. In another report [39], they demonstrated the effect of a higher transition dipole moment which is induced by increasing the distance between D-A units. They compared orange-red anthraquinone based TADFs based on D-A-D (**a1-a4**) and D-Ph-A-Ph-D (**b1-b4**) molecular scaffold showing higher PLQY. The fabricated device was ITO/HAT-CN/Tris-PCz/10wt%*TADF emitter*:CBP/T2T/Bpy-TP2/LiF/Al) (Tris-PCz = 9,9′-diphenyl-6-(9-phenyl-9H-carbazol-3-yl)-9H,9′H-3,39′H-bicarbazole; T2T =2, 4,6-tris(biphenyl-3-yl)-1,3,5-triazine; Bpy- TP2 = 2,7-di(2,2′-bipyridin-5-yl)triphenylene) using **b1** emitter. The compound showed 80% PLQY and τd 416 μs in a host CBP matrix. The calculated ΔEST from experimental value was 0.24 eV. The device exhibit 12.5% EQE and the CIE

In 2013, Li et al. [40] synthesized orange-red emitter, **HAP-3TPA** (**Figure 5**), based on heptaazaphenalene acceptor with a small ΔEST of 0.17 e ΔEST of 0.17 eV. The molecules show absorbance at 610 nm. The calculated PLQY of 6 wt% TADF in a host matrix 26mCPy was 91%, and the τd of 100 μs. The molecule showed very weak TADF behavior, and the ϕ*DF/*ϕ*PF* was 0.07 compared to ϕ*DF/*ϕ*PF* of 1.58 in the fabricated device i.e. ITO/NPD/6 wt% **HAP-3TPA**:26mCPy/

Bphen/Mg:Ag/Ag with a high EQE value of 17.5% and the CIE was (0.60, 0.40).

processed methods which is more suitable for large area OLEDs.

acteristics, and device performance.

**4.1. Red-orange TADFs**

coordinates were (0.61, 0.39).

The full understanding of the photo physics properties of the TADF emitter is naturally of extreme importance for further OLED development.
