**4. OLEDs based on TADF emitters**

In the contest of finding best organic emitters for the lightening industry, in 2011 Adachi et al. [34] reported the very first purely organic TADF emitter **PIC-TRZ** (**Figure 5**) which showed promising calculated PLQY in a host matrix of mCP and was 39% and the device showed 5.3% EQE. Since then, in the past 5 years, over 200 new compounds have been reported. Among various emitters, many of them showed EQE of more than 20% composed in a device and this can be reached up to 30% using stipulated device structure with an optimized concentration of TADF emitters and fabrication process [35]. There have been several reviews published focusing on photo physics, device characteristics of TADF and chemistry of TADF emitters [20, 23, 35]. Apart from the use of TADF molecules for OLEDs, there are some challenges must be taken

**Figure 5.** Chemical structures of red-orange TADF emitters.

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.

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 processed methods which is more suitable for large area OLEDs.

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 characteristics, and device performance.

## **4.1. Red-orange TADFs**

**Figure 5.** Chemical structures of red-orange TADF emitters.

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. The full understanding of the photo physics properties of the TADF emitter is naturally of

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

In the contest of finding best organic emitters for the lightening industry, in 2011 Adachi et al. [34] reported the very first purely organic TADF emitter **PIC-TRZ** (**Figure 5**) which showed promising calculated PLQY in a host matrix of mCP and was 39% and the device showed 5.3% EQE. Since then, in the past 5 years, over 200 new compounds have been reported. Among various emitters, many of them showed EQE of more than 20% composed in a device and this can be reached up to 30% using stipulated device structure with an optimized concentration of TADF emitters and fabrication process [35]. There have been several reviews published focusing on photo physics, device characteristics of TADF and chemistry of TADF emitters [20, 23, 35]. Apart from the use of TADF molecules for OLEDs, there are some challenges must be taken

extreme importance for further OLED development.

**4. OLEDs based on TADF emitters**

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 coordinates were (0.61, 0.39).

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).

In another study, Wang et al. [41] demonstrated the effect of the twist angle during the designing strategy of TADF emitters. This twist angle can be reduced by increasing the D-A distance which gives an orbital overlap to increase kt . They synthesized the first near-infrared (NIR) TADF emitter **TPA-DCPP** (**Figure 5**) based on dicyanodiazatriphenylene acceptor moiety. The experimental calculated ΔEST was 0.13 eV. The calculated PLQY of 10 wt% **TPA-DCPP** in TPBi host matrix was 50%, and the τd of 86.2 μs. The fabricated device ITO/NPB/TCTA/20 wt% **TPA-DCPP**:TPBi/TPBi/LiF/Al exhibit EQE value of 9.8% with the CIE at (0.68, 0.32). Similarly, Chen et al. [42] also reported a novel solution processed red TADF using **red-1b** molecule which can undergo both TADF and TTA process, depends on current density. The calculated ΔEST was 0.40 eV. The photo-physical characteristics of the molecule are λmax of 622 nm and the solid state calculated PLQY of 28%, while the EQE was very low 1.75% in fabricated device. Very recently in another report, Data et al. [43] reported dibenzeno-phenanzine acceptors based TADF emitters which showed a formation of exciplex when doped in m-MTDATA host and a strong NIR emission at 741 nm was observed with EQE of 5%, but no concluded evidence was provided in the work to support the TADF mechanism of this exciplex system.

#### **4.2. Blue TADF emitters**

In 2012, Adachi et al. reported the very first class of blue TADF emitter based on diphenylsulfone (**DPS**) as an acceptor [27] (see **Figure 6**).

To synthesize efficient blue TADF emitter and their use in the device, it is important to take account of the π-conjugation length and the redox potential of the donor and acceptor moieties and in DPS derivatives the advantage is that the oxygens of the sulfonyl group have significant electronegativity, which gives the sulfonyl group electron-withdrawing properties and sulfonyl group exhibit tetrahedral geometry which limit the conjugation [35]. The device fabricated with 10 wt% emitter showed good results, the device ITO/α-NPD/TCTA/ CzSi/10 wt% *TADF emitter*:DPEPO/ DPEPO/TPBI/LiF/Al and the EQE was 9.9%, the emitter incorporated in the device was **DTS-DPS** [44]. The photo-physical characteristics of the molecules are λmax of 423 nm. The calculated PLQY in DPEPO host was 80% and τd was to be 540 μs. The ΔEST was 0.32 eV. They suggested that for small ΔEST, the energy gap between 3 LE and 3 CT must be small, and the *rIC* occurs from 3 LE to 3 CT which was followed by *rISC* to 1 CT and similar results were seen in other derivatives. In a similar approach, Dias et al. proposed a mechanism to understand the mechanism of *rISC* in such type of molecules [45]. The results suggested that *rISC* mechanism is still possible if the ΔEST is greater than 0.3 eV. They studied a molecule DTC-DBT (**Figure 6**), where the ΔEST is 0.35 eV, but in the molecule, it is possible to harvest 100% triplet excitons. According to the authors, the presence of heteroatom loan pairs form "hidden" 3n-π\* state sandwiched between higher 3 CT and lower 3 LE states and the up-conversion follow the pattern: 3 LE → -3n-π\* → <sup>3</sup> CT → <sup>1</sup> CT. In similar study, Chen et al. [46] demonstrated the importance of" non-adiabatic effects in butterfly donor-acceptor-donor molecules" **DTC-DBT**, and suggested that upon the rotation of D-A groups, a conical intersection (C1 ) between long-lying excited states and at C1 "the non-adiabatic coupling matrix element between two excited stated becomes infinite, which is proportional to the RISC rate." This was further supported by Etherigton et al. [47].

Adachi et al. [48] reported deep blue emitters **DMOC-DPS** (**Figure 6**) with the ΔEST of 0.21 eV. The calculated PLQY in DPEPO host at 10 wt% was 80% along with measured τd of 114 μs and the device ITO/a-NPD (30 nm)/TCTA (20 nm)/CzSi (10 nm)/ **DMOC-DPS:**DPEPO (20 nm)/ DPEPO (10 nm)/TPBI (30 nm)/LiF (0.5 nm)/Al showed an EQE of 14.5%. They further reported another blue TADF **DMAC-DPS** with the absorbance of 464 nm. The PLQY in mCP host was 80%, and τd was 3.1 μs. For this **DMAC-DPS,** the experimentally calculated ΔES

very low and was 0.08 eV. The fabricated device performed at an excellent EQE of 19.5% [44],

dimethylacridan donor. Another molecule **DMAC-BP** which ΔEST is 0.07 eV with calculated

LE state than 3

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where the reduced ΔEST is a result of higher 3

**Figure 6.** Chemical structures of blue TADF emitters.

PLQY of 85%, and τd of 2.7 *μs* showed EQE of 18.9%.

was

CT state, caused by inducement of

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**Figure 6.** Chemical structures of blue TADF emitters.

In another study, Wang et al. [41] demonstrated the effect of the twist angle during the designing strategy of TADF emitters. This twist angle can be reduced by increasing the D-A distance

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

TADF emitter **TPA-DCPP** (**Figure 5**) based on dicyanodiazatriphenylene acceptor moiety. The experimental calculated ΔEST was 0.13 eV. The calculated PLQY of 10 wt% **TPA-DCPP** in TPBi host matrix was 50%, and the τd of 86.2 μs. The fabricated device ITO/NPB/TCTA/20 wt% **TPA-DCPP**:TPBi/TPBi/LiF/Al exhibit EQE value of 9.8% with the CIE at (0.68, 0.32). Similarly, Chen et al. [42] also reported a novel solution processed red TADF using **red-1b** molecule which can undergo both TADF and TTA process, depends on current density. The calculated ΔEST was 0.40 eV. The photo-physical characteristics of the molecule are λmax of 622 nm and the solid state calculated PLQY of 28%, while the EQE was very low 1.75% in fabricated device. Very recently in another report, Data et al. [43] reported dibenzeno-phenanzine acceptors based TADF emitters which showed a formation of exciplex when doped in m-MTDATA host and a strong NIR emission at 741 nm was observed with EQE of 5%, but no concluded evidence was provided in the work to support the TADF mechanism of this exciplex system.

In 2012, Adachi et al. reported the very first class of blue TADF emitter based on diphenylsul-

To synthesize efficient blue TADF emitter and their use in the device, it is important to take account of the π-conjugation length and the redox potential of the donor and acceptor moieties and in DPS derivatives the advantage is that the oxygens of the sulfonyl group have significant electronegativity, which gives the sulfonyl group electron-withdrawing properties and sulfonyl group exhibit tetrahedral geometry which limit the conjugation [35]. The device fabricated with 10 wt% emitter showed good results, the device ITO/α-NPD/TCTA/ CzSi/10 wt% *TADF emitter*:DPEPO/ DPEPO/TPBI/LiF/Al and the EQE was 9.9%, the emitter incorporated in the device was **DTS-DPS** [44]. The photo-physical characteristics of the molecules are λmax of 423 nm. The calculated PLQY in DPEPO host was 80% and τd was to be 540 μs. The ΔEST was 0.32 eV. They suggested that for small ΔEST, the energy gap between 3

LE to 3

CT → <sup>1</sup>

and similar results were seen in other derivatives. In a similar approach, Dias et al. proposed a mechanism to understand the mechanism of *rISC* in such type of molecules [45]. The results suggested that *rISC* mechanism is still possible if the ΔEST is greater than 0.3 eV. They studied a molecule DTC-DBT (**Figure 6**), where the ΔEST is 0.35 eV, but in the molecule, it is possible to harvest 100% triplet excitons. According to the authors, the presence of heteroatom loan

LE → -3n-π\* → <sup>3</sup>

[46] demonstrated the importance of" non-adiabatic effects in butterfly donor-acceptor-donor molecules" **DTC-DBT**, and suggested that upon the rotation of D-A groups, a conical inter-

element between two excited stated becomes infinite, which is proportional to the RISC rate."

. They synthesized the first near-infrared (NIR)

LE

CT

LE states and

CT which was followed by *rISC* to 1

CT and lower 3

CT. In similar study, Chen et al.

"the non-adiabatic coupling matrix

which gives an orbital overlap to increase kt

fone (**DPS**) as an acceptor [27] (see **Figure 6**).

CT must be small, and the *rIC* occurs from 3

This was further supported by Etherigton et al. [47].

the up-conversion follow the pattern: 3

pairs form "hidden" 3n-π\* state sandwiched between higher 3

) between long-lying excited states and at C1

**4.2. Blue TADF emitters**

and 3

section (C1

Adachi et al. [48] reported deep blue emitters **DMOC-DPS** (**Figure 6**) with the ΔEST of 0.21 eV. The calculated PLQY in DPEPO host at 10 wt% was 80% along with measured τd of 114 μs and the device ITO/a-NPD (30 nm)/TCTA (20 nm)/CzSi (10 nm)/ **DMOC-DPS:**DPEPO (20 nm)/ DPEPO (10 nm)/TPBI (30 nm)/LiF (0.5 nm)/Al showed an EQE of 14.5%. They further reported another blue TADF **DMAC-DPS** with the absorbance of 464 nm. The PLQY in mCP host was 80%, and τd was 3.1 μs. For this **DMAC-DPS,** the experimentally calculated ΔES was very low and was 0.08 eV. The fabricated device performed at an excellent EQE of 19.5% [44], where the reduced ΔEST is a result of higher 3 LE state than 3 CT state, caused by inducement of dimethylacridan donor. Another molecule **DMAC-BP** which ΔEST is 0.07 eV with calculated PLQY of 85%, and τd of 2.7 *μs* showed EQE of 18.9%.

It is very important to design the geometry of TADF molecule to induce TADF process, to counter this, Rajmalli et al. [49] reported novel blue TADF emitters based on benzoylpyridine acceptor **DCBPy** and **DTCBPy**, in **DTCBPy** (**Figure 6**) a tert-butyl group is present. Upon photo-physical characterization, the second molecule **DTCBPy** showed a small redshift in emission by 4 nm. The ΔEST was smaller in both emitters, and was 0.07 and 0.08 eV in **DCBPy** and **DTCBPy**, respectively. The calculated PLQY in host matrix was 88 for DCBPy and 91% for DTCBPy. The PLQY in the solid state was 91% compared to lower 14–36% in solution. The device exhibit sky-blue and green emission. The EQEs was 24.0% CIE (0.17, 0.36) and 27.2% CIE (0.30, 0.64) for **DCBPy** and **DTCBPy**, respectively. The efficiency roll-off was low at practical brightness level.

acceptors are used as most usual building blocks for the synthesis of deep-blue TADFs. The first Cyano based blue TADF **2CzPN** (**Figure 6**) was firstly reported by Adachi et al. [36] PLQY of 47% in solution state and the EQE was 13.6% in device. Sun et al. [54] reported a blue TADF emitter showing an excellent EQE of 21.8% in composed OLED structure of ITO/4 wt% ReO3:mCP/5 wt% **TADF emitter**:mCP:PO15/4 wt% Rb2CO3:PO-15/Al) (PO-15 = poly[N,N′-bis(4-butylphenyl)-N,N′-bisphenylbenzidine]) by using a mixed co-host sys-

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Solution-processed TADF materials was reported by Cho et al. [55], they synthesized two blue TADF emitters named **3CzFCN** and **4CzFCN** (**Figure 6**), showing photo-physical characteristics of calculated PLQY in 10 wt% diphenyldi(4-(9-carbazolyl)phenyl)silane (SiCz) host at 10 wt%, and was 74% for **3CzFCN** and 100% for **4CzFCN**, while the experimentally calculated small ΔEST of 0.06 eV for both emitters. The λmax was 440 and 460 nm, respectively. The fabricated device with **4CzFCN** emitter showed an excellent EQE of 20% with CIE coordinates of (0.16, 0.26), the device structure was ITO/PEDOT:PSS/PVK/15 wt%**4CzFCN**:SiCz/TPBI/LiF/Al.

In another study, very recently, Hatakeyama et al. [56] demonstrated synthesis of boronbased acceptor TADFs. They synthesized TADF emitters **DABNA-1** and **DABNA-2** (**Figure 6**) showing ΔEST of 0.18 and 0.14 eV, respectively. The λmax was 460 and 469 nm, while calculated PLQY in mCBP host at 1 wt% was 88 and 90%. The device fabricated with **DABNA-2** emitter showed an excellent EQE of 20.2%. In such boron based TADFs, the ΔEST is very low, this is due to the presence of strong LUMO localization called as "multiple resonance effect", induced by boron atom. The PLQY in solid state is 87–100%, which makes boron based TADF

Among various TADF emitters, plenty of them are the green-yellow emitter and most those green to yellow emitters are based on cyano-based acceptors. The molecular design of these cyano-based emitters is based "on the presence of a twisted conformation of donor carbazoles with respect to phthalonitrile plane" to confer the HOMO-LUMO separation and result to lower ΔEST. These cyano-based green TADF emitters are classified in three categories: (a) monomeric series with orthosteric hindrance, (b) homoconjugation series, and (3) dimeric emitters. In monomeric emitters, ΔEST is very small and high PLQY yield. In homoconjugation series, the HOMO and LUMO separation is easily achievable but lower PLQY yield and in dimeric series the ΔEST is very higher i.e. 0.11–0.21 eV. The first green emitter was reported by Adachi et al. [38] in 2012, **4CzIPN** (**Figure 7**), exhibiting ΔEST of 0.08 eV. The calculated PLQY

in CBP host was 82% and τd of 3370 μs and the device showed excellent 19.3% EQE.

In the similar contest, Taneda and co-workers [57] synthesized a highly efficient green TADF emitter **3DPA3CN** (**Figure 7**) and it showed PLQY of 100% in solid state and 100% triplet harvesting via *rISC* mechanism. The molecule showed photo-physical characteristics of absorbance of 533 nm. The calculated PLQY of 6 wt% thin film in DPEPO host matrix was 100% while the τd was 550. The ΔEST was to be small 0.10 eV and is due to the strongly localized molecular geometry the HOMO and LUMO, which gives the small separation between

tem (mCP:PO15 = 1:1).

emitter a potential candidate for blue TADFs.

**4.3. Green-yellow TADFs**

In 2015, Kim et al. [50] synthesized two new blue TADF emitters **DCzTrz** and **DDCzTrz** (**Figure 6**), where "two additional carbazole moieties attached to phenyl ring in a meta fashion" in **DDCzTrz** and as "meta linkage" both emitters have similar emission as well as ΔEST. The photo-physical characteristics of both molecules are absorbance maximum of 420 and 430 nm. The calculated PLQY for both emitters was 43 and 66%, respectively. The smaller ΔEST 0.25 eV for **DCzTrz** and 0.27 eV for **DDCzTrz.** The OLED using **DDCzTrz** showed an excellent EQE of 18.9% but in the device, the LT80 (time required to drop 80% luminescence) was 52 h, three-time longer than conventional blue phosphorescent Iridium complex. It is due to first stabilization through carbazole moiety, secondly, stable positive carbazole and negative triazine excited state pair, and thirdly excellent glass transition temperature for both 160 for **DCzTrz** and 218°C for **DDCzTrz** give the stability to the device. But the device composed of **DCzTrz** was stable only for 5 h and this caused due to the high emission of the molecule. Upon modification of **DCTrz** through the addition of more carbazole donors, they synthesized another three molecules **TCzTrz, TmCzTrz**, and **DCzmCzTrz** (**Figure 6**). Where calculated PLQY was 100% in DPEPO host, and the ΔEST was 0.16, 0.07, and 0.20 eV, respectively [51]. It was found that in **TCzTrz** the ΔEST is lower by 0.09 eV compare to **DCzTrz**. The device composed of **TCzTrz** showed an excellent EQE of 25.5%, while **TmCzTrz** and **DCzmCzTrz** showed EQE of 25.3 and 21.3%, respectively.

In another study, Lin et al. demonstrated [52] a novel triazine-based blue TADF emitter named **spiroAC-TRZ** (**Figure 6**) which showed photo-physical characteristics of calculated PLQY of 100% in 12 wt% mCPCN host, τd of 2.1 μs. The experimentally calculated ΔEST was very small and to be 0.07 eV. The device ITO/ MoO3/TAPC/mCP/12 wt% **spiroAC-TRZ**:mCPCN/3TPYMB/ LiF/Al an EQE of 37%, which is highest among the reported blue TADF.In a similar approach, Komatsu etal. [53] reported three novel deep blue TADF (**Figure 6**). **Ac-RPMs**, on the modification of triazine acceptor to pyrimidine. All the compounds exhibited similar photo-physical properties i.e. PLQY of 80%, τd of 26.2 μs in a host matrix at 10 wt% in DPEPO and the calculated ΔEST was 0.19 eV for **Ac-MPM**, and the OLEDs showed an EQE of 24.5% and CIE coordinates of (0.19, 0.37), interestingly the turn-on voltage vas very low and was 2.80 V, such device can be used for highly efficient light devices.

Among various D-A and D-A-D structured TADF molecules, the main chemical moiety plays an important role for the exhibition of the TADF behavior, and as we have discussed various acceptors have been used for the synthesis of TADF materials, among them Cyano-based acceptors are used as most usual building blocks for the synthesis of deep-blue TADFs. The first Cyano based blue TADF **2CzPN** (**Figure 6**) was firstly reported by Adachi et al. [36] PLQY of 47% in solution state and the EQE was 13.6% in device. Sun et al. [54] reported a blue TADF emitter showing an excellent EQE of 21.8% in composed OLED structure of ITO/4 wt% ReO3:mCP/5 wt% **TADF emitter**:mCP:PO15/4 wt% Rb2CO3:PO-15/Al) (PO-15 = poly[N,N′-bis(4-butylphenyl)-N,N′-bisphenylbenzidine]) by using a mixed co-host system (mCP:PO15 = 1:1).

Solution-processed TADF materials was reported by Cho et al. [55], they synthesized two blue TADF emitters named **3CzFCN** and **4CzFCN** (**Figure 6**), showing photo-physical characteristics of calculated PLQY in 10 wt% diphenyldi(4-(9-carbazolyl)phenyl)silane (SiCz) host at 10 wt%, and was 74% for **3CzFCN** and 100% for **4CzFCN**, while the experimentally calculated small ΔEST of 0.06 eV for both emitters. The λmax was 440 and 460 nm, respectively. The fabricated device with **4CzFCN** emitter showed an excellent EQE of 20% with CIE coordinates of (0.16, 0.26), the device structure was ITO/PEDOT:PSS/PVK/15 wt%**4CzFCN**:SiCz/TPBI/LiF/Al.

In another study, very recently, Hatakeyama et al. [56] demonstrated synthesis of boronbased acceptor TADFs. They synthesized TADF emitters **DABNA-1** and **DABNA-2** (**Figure 6**) showing ΔEST of 0.18 and 0.14 eV, respectively. The λmax was 460 and 469 nm, while calculated PLQY in mCBP host at 1 wt% was 88 and 90%. The device fabricated with **DABNA-2** emitter showed an excellent EQE of 20.2%. In such boron based TADFs, the ΔEST is very low, this is due to the presence of strong LUMO localization called as "multiple resonance effect", induced by boron atom. The PLQY in solid state is 87–100%, which makes boron based TADF emitter a potential candidate for blue TADFs.

### **4.3. Green-yellow TADFs**

It is very important to design the geometry of TADF molecule to induce TADF process, to counter this, Rajmalli et al. [49] reported novel blue TADF emitters based on benzoylpyridine acceptor **DCBPy** and **DTCBPy**, in **DTCBPy** (**Figure 6**) a tert-butyl group is present. Upon photo-physical characterization, the second molecule **DTCBPy** showed a small redshift in emission by 4 nm. The ΔEST was smaller in both emitters, and was 0.07 and 0.08 eV in **DCBPy** and **DTCBPy**, respectively. The calculated PLQY in host matrix was 88 for DCBPy and 91% for DTCBPy. The PLQY in the solid state was 91% compared to lower 14–36% in solution. The device exhibit sky-blue and green emission. The EQEs was 24.0% CIE (0.17, 0.36) and 27.2% CIE (0.30, 0.64) for **DCBPy** and **DTCBPy**, respectively. The efficiency roll-off was low at prac-

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

In 2015, Kim et al. [50] synthesized two new blue TADF emitters **DCzTrz** and **DDCzTrz** (**Figure 6**), where "two additional carbazole moieties attached to phenyl ring in a meta fashion" in **DDCzTrz** and as "meta linkage" both emitters have similar emission as well as ΔEST. The photo-physical characteristics of both molecules are absorbance maximum of 420 and 430 nm. The calculated PLQY for both emitters was 43 and 66%, respectively. The smaller ΔEST 0.25 eV for **DCzTrz** and 0.27 eV for **DDCzTrz.** The OLED using **DDCzTrz** showed an excellent EQE of 18.9% but in the device, the LT80 (time required to drop 80% luminescence) was 52 h, three-time longer than conventional blue phosphorescent Iridium complex. It is due to first stabilization through carbazole moiety, secondly, stable positive carbazole and negative triazine excited state pair, and thirdly excellent glass transition temperature for both 160 for **DCzTrz** and 218°C for **DDCzTrz** give the stability to the device. But the device composed of **DCzTrz** was stable only for 5 h and this caused due to the high emission of the molecule. Upon modification of **DCTrz** through the addition of more carbazole donors, they synthesized another three molecules **TCzTrz, TmCzTrz**, and **DCzmCzTrz** (**Figure 6**). Where calculated PLQY was 100% in DPEPO host, and the ΔEST was 0.16, 0.07, and 0.20 eV, respectively [51]. It was found that in **TCzTrz** the ΔEST is lower by 0.09 eV compare to **DCzTrz**. The device composed of **TCzTrz** showed an excellent EQE of 25.5%, while **TmCzTrz** and **DCzmCzTrz**

In another study, Lin et al. demonstrated [52] a novel triazine-based blue TADF emitter named **spiroAC-TRZ** (**Figure 6**) which showed photo-physical characteristics of calculated PLQY of 100% in 12 wt% mCPCN host, τd of 2.1 μs. The experimentally calculated ΔEST was very small and to be 0.07 eV. The device ITO/ MoO3/TAPC/mCP/12 wt% **spiroAC-TRZ**:mCPCN/3TPYMB/ LiF/Al an EQE of 37%, which is highest among the reported blue TADF.In a similar approach, Komatsu etal. [53] reported three novel deep blue TADF (**Figure 6**). **Ac-RPMs**, on the modification of triazine acceptor to pyrimidine. All the compounds exhibited similar photo-physical properties i.e. PLQY of 80%, τd of 26.2 μs in a host matrix at 10 wt% in DPEPO and the calculated ΔEST was 0.19 eV for **Ac-MPM**, and the OLEDs showed an EQE of 24.5% and CIE coordinates of (0.19, 0.37), interestingly the turn-on voltage vas very low and

Among various D-A and D-A-D structured TADF molecules, the main chemical moiety plays an important role for the exhibition of the TADF behavior, and as we have discussed various acceptors have been used for the synthesis of TADF materials, among them Cyano-based

was 2.80 V, such device can be used for highly efficient light devices.

tical brightness level.

showed EQE of 25.3 and 21.3%, respectively.

Among various TADF emitters, plenty of them are the green-yellow emitter and most those green to yellow emitters are based on cyano-based acceptors. The molecular design of these cyano-based emitters is based "on the presence of a twisted conformation of donor carbazoles with respect to phthalonitrile plane" to confer the HOMO-LUMO separation and result to lower ΔEST. These cyano-based green TADF emitters are classified in three categories: (a) monomeric series with orthosteric hindrance, (b) homoconjugation series, and (3) dimeric emitters. In monomeric emitters, ΔEST is very small and high PLQY yield. In homoconjugation series, the HOMO and LUMO separation is easily achievable but lower PLQY yield and in dimeric series the ΔEST is very higher i.e. 0.11–0.21 eV. The first green emitter was reported by Adachi et al. [38] in 2012, **4CzIPN** (**Figure 7**), exhibiting ΔEST of 0.08 eV. The calculated PLQY in CBP host was 82% and τd of 3370 μs and the device showed excellent 19.3% EQE.

In the similar contest, Taneda and co-workers [57] synthesized a highly efficient green TADF emitter **3DPA3CN** (**Figure 7**) and it showed PLQY of 100% in solid state and 100% triplet harvesting via *rISC* mechanism. The molecule showed photo-physical characteristics of absorbance of 533 nm. The calculated PLQY of 6 wt% thin film in DPEPO host matrix was 100% while the τd was 550. The ΔEST was to be small 0.10 eV and is due to the strongly localized molecular geometry the HOMO and LUMO, which gives the small separation between

with enhancement to the PLQY. In this molecule, the emission is dominated by 1

*TADF emitter*:CBP/BmPyPhB/Liq/Al, the EQE was 18.6% along with ELmax at 520 nm.

In 2014, Wang et al. reported [62] sulfone-based acceptor green TADF emitters. They synthesized two green TADF emitter named **TXO-PhCz** and **TXO-TPA** (**Figure 7**). The photo-physical characteristic of these molecules is λmax of 520 nm, calculated PLQY of thin film was 90% in 5 wt% mCP, and a small ΔEST value of 0.07 eV for TXO-PhCz and λmax of 580 nm, the PLQY of thin film was 83% and even smaller ΔEST of 0.05 eV were demonstrated. The fabricated device structure was ITO/PEDOT (30 nm)/ TAPC (20 nm)/*TADF emitter* 5 wt%: mCP (35 nm)/TmPyPB (55 nm)/ LiF(0.9 nm)/Al and the device showed an EQEs of 21.5% for **TXO-TPA** emitter and 18.5% for **TXP-PhCz** emitter, respectively with the turn-on voltage of 4.7–5.2 V which is low for such devices.

1,3,4-Oxadiazole was used to synthesized green TADF emitter and these emitters are most commonly used for the applications. Three new green TADF emitters were synthesized by Lee et al. [63]. The three emitters were **PXZ-OXD, PXZ-TAZ, 2PXZ-OXD,** and **2PXZ-TAZ** (**Figure 7**). In emitters, the D-A moieties, where the donor was phenoxazine group while the acceptor was 1,3,4-oxadiazole and 1,2,4-triazole. In molecules, the PLQY was calculated in a host material and the PLQY was high in donor-acceptor-donor **2PXZ-OXD** and **2PXZ-TAZ** with high *rISC* compare to donor-acceptor **PXZ-OXD** and **PXZ-TAZ** molecules. The best device showed an EQE of 14.9% with emitter as **2PXZ-OXD**. The emitter exhibit photo-physical characteristics of λmax of 517 nm, PLQY yield was 87%, τd of 520 μs in 6 wt% DPEPO and the ΔEST of 0.15 eV and the fabricated device structure was ITO/NPD (30 nm)/mCP (10 nm)/6 wt% *TADF* 

Many groups reported various green TADF based on different donor and acceptor molecules.

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

*emitter*: DPEPO (15 nm)/DPEPO (10 nm)/TPBi (40 nm)/ LiF (0.8 nm)/Al (90 nm).

The more significant are focused here.

**5. Conclusions and outlook**

Apart from Cyano-based green TADF emitters, many researchers reported TADF emitters based on 1,2,5-triazine acceptor (**TRZ**). Tanaka and co-workers [60] reported in 2012 a TADF emitter **PXZ-TRZ** (**Figure 7**) with a small ΔEST exhibiting λmax: near 540 nm. The calculated PLQY of the thin film in 6 wt% CBP was 66%, while τd was 0.68 μs. The twisted structure of the phenoxazine can easily be achieved and induces the charge transfer, results to a small ΔEST of 0.07 eV. In such molecule, the dihedral angle between donor and acceptor is 74.9, which helps to localize the HOMO and LUMO for efficient TADF exhibition. The device was fabricated using the emitter ITO/α-NPD/6 wt% *TADF emitter*:CBP/TPBi/LiF/Al showed EQE of 12.5%. Adachi et al. [61] also reported anther green emitter based on TRZ acceptor, **3ACR-TRZ** (**Figure 7**), with a small ΔEST of 0.02 eV which can be solution processed. It exhibits the Photo-physical characteristics of λmax of 504 nm. The calculated PLQY in 16 wt% CBP was near a unit value of 98% and τd of 6.7 μs in Host matrix, while in device ITO/PEDOT:PSS/16 wt%

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

reduce the n-π\* quenching of the 4CzPy group.

CT state to

119

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

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

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 with enhancement to the PLQY. In this molecule, the emission is dominated by 1 CT state to reduce the n-π\* quenching of the 4CzPy group.

Apart from Cyano-based green TADF emitters, many researchers reported TADF emitters based on 1,2,5-triazine acceptor (**TRZ**). Tanaka and co-workers [60] reported in 2012 a TADF emitter **PXZ-TRZ** (**Figure 7**) with a small ΔEST exhibiting λmax: near 540 nm. The calculated PLQY of the thin film in 6 wt% CBP was 66%, while τd was 0.68 μs. The twisted structure of the phenoxazine can easily be achieved and induces the charge transfer, results to a small ΔEST of 0.07 eV. In such molecule, the dihedral angle between donor and acceptor is 74.9, which helps to localize the HOMO and LUMO for efficient TADF exhibition. The device was fabricated using the emitter ITO/α-NPD/6 wt% *TADF emitter*:CBP/TPBi/LiF/Al showed EQE of 12.5%. Adachi et al. [61] also reported anther green emitter based on TRZ acceptor, **3ACR-TRZ** (**Figure 7**), with a small ΔEST of 0.02 eV which can be solution processed. It exhibits the Photo-physical characteristics of λmax of 504 nm. The calculated PLQY in 16 wt% CBP was near a unit value of 98% and τd of 6.7 μs in Host matrix, while in device ITO/PEDOT:PSS/16 wt% *TADF emitter*:CBP/BmPyPhB/Liq/Al, the EQE was 18.6% along with ELmax at 520 nm.

In 2014, Wang et al. reported [62] sulfone-based acceptor green TADF emitters. They synthesized two green TADF emitter named **TXO-PhCz** and **TXO-TPA** (**Figure 7**). The photo-physical characteristic of these molecules is λmax of 520 nm, calculated PLQY of thin film was 90% in 5 wt% mCP, and a small ΔEST value of 0.07 eV for TXO-PhCz and λmax of 580 nm, the PLQY of thin film was 83% and even smaller ΔEST of 0.05 eV were demonstrated. The fabricated device structure was ITO/PEDOT (30 nm)/ TAPC (20 nm)/*TADF emitter* 5 wt%: mCP (35 nm)/TmPyPB (55 nm)/ LiF(0.9 nm)/Al and the device showed an EQEs of 21.5% for **TXO-TPA** emitter and 18.5% for **TXP-PhCz** emitter, respectively with the turn-on voltage of 4.7–5.2 V which is low for such devices.

1,3,4-Oxadiazole was used to synthesized green TADF emitter and these emitters are most commonly used for the applications. Three new green TADF emitters were synthesized by Lee et al. [63]. The three emitters were **PXZ-OXD, PXZ-TAZ, 2PXZ-OXD,** and **2PXZ-TAZ** (**Figure 7**). In emitters, the D-A moieties, where the donor was phenoxazine group while the acceptor was 1,3,4-oxadiazole and 1,2,4-triazole. In molecules, the PLQY was calculated in a host material and the PLQY was high in donor-acceptor-donor **2PXZ-OXD** and **2PXZ-TAZ** with high *rISC* compare to donor-acceptor **PXZ-OXD** and **PXZ-TAZ** molecules. The best device showed an EQE of 14.9% with emitter as **2PXZ-OXD**. The emitter exhibit photo-physical characteristics of λmax of 517 nm, PLQY yield was 87%, τd of 520 μs in 6 wt% DPEPO and the ΔEST of 0.15 eV and the fabricated device structure was ITO/NPD (30 nm)/mCP (10 nm)/6 wt% *TADF emitter*: DPEPO (15 nm)/DPEPO (10 nm)/TPBi (40 nm)/ LiF (0.8 nm)/Al (90 nm).

Many groups reported various green TADF based on different donor and acceptor molecules. The more significant are focused here.
