**3.1. Basic aspects of WOLEDs with TADF emitters**

Similar to phosphorescence emitters, the use of TADF emitters is very promising to achieve WOLEDs. This is because TADF emitters can (1) harness triplet excitons, (2) exhibit excellent efficiency, and (3) show usually broad emission spectra with rather large full width at half maximum of about 100 nm, which is wider than that of conventional fluorescent materials because of their charge-transfer nature [31–47].

To attain the high performance, the device structures, design strategies, working mechanisms, and electroluminescent processes of the WOLEDs with TADF emitters should be well manipulated. For example, unlike conventional fluorescence emitters, the T1 of TADF emitters is necessary to be considered when designing a WOLED architecture, since hosts or nearby layers with low T1 would quench the triplet excitons, which leads to the low efficiency. Besides, the location of TADF emitters is needed to be investigated, since the energy transfer would occur between the contacted different emitters (e.g., energy can transfer from high-energy TADF emitters to low-energy emitters). To date, various approaches have been reported to develop WOLEDs with TADF emitters, such as the exploitation of all TADF emitters, the combination of TADF and conventional fluorescence emitters, and the mixture of TADF and phosphorescence emitters.

(4CzPN): 3,3-Di(9H-carbazol-9-yl)biphenyl (mCBP, green EML) (x nm)/6 wt% 4CzPN: 2 wt% 41,4-dicyano-2,3,5,6-tetrakis (3,6-diphenylcarbazol-9-yl)benzene (CzTPN-Ph): mCBP (red EML) (y nm)/10 wt% 9-(3-(9H-carbazol-9-yl)-9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9H-carbazol-6-yl)-9H-carbazole (3CzTRZ): 2,8-bis(diphenylphosphoryl) dibenzo-[b,d] thiophene (PPT, blue EML) (z nm)/PPT (50 nm)/LiF/Al. In this device, the T1 level of mCBP host is 2.9 eV, which is much higher than that of blue, green, and red TADF emitters, ensuring the high efficiency.

optimizing the charge generation zone via the adjustment of different EML thickness (the total thickness is set to be x + y + z = 15 nm), the WOLED achieved a maximum EQE of over 17%, a

Considering that triplets could be harnessed by TADF emitters, it is promising to realize the unity IQE by combining traditional fluorescent materials with TADF emitters [61]. For such an approach, the TADF molecules would act as the triplet harvester, which is used to harness the triplets for the conventional other-color fluorescence materials. As a result, highly efficient

In 2016, Li et al. reported high-efficiency and high CRI WOLEDs with the chromaticityadjustable yellow TADF emitter 2-(4-phenoxazinephenyl)thianthrene-9,9′,10,10′-tetraoxide

of 3.1 eV, suggesting a good confinement of the triplet excitons. By

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41

Besides, PPT has a high T1

white light can be produced [42–44].

peak PE of 34.1 lm/W with CIE coordinates of (0.30, 0.38).

**3.3. WOLEDs with TADF and conventional fluorescence emitters**

**Figure 2.** The WOLED structures and energy level diagram. Reproduced from Ref. [46].

## **3.2. WOLEDs with all TADF emitters**

The most directed approach to develop WOLEDs with TADF emitters is the exploitation of all TADF emitters. That is to say, all blue, green, and red emitters are TADF materials. By selecting high-T1 hosts and charge transport materials, high-performance WOLEDs can be attained. More specifically, the conventional fluorescent host should possess high T1 , which should be particularly higher than the blue TADF materials. Otherwise, the triplet excitons of emitters would be quenched by the host, resulting in low performance. Moreover, high-T1 charge transport materials should be selected, which is used to confine the triplet excitons in the EML, leading to the excitons being well consumed.

Adachi and his coworkers for the first time adopted such an approach to develop highly efficient WOLEDs with all TADF emitters [46]. **Figure 2** depicts the device structure: ITO/1,4,5,8,9,11 hexaazatriphenylene hexacarbonitrile (HAT-CN, 10 nm)/9,9′,9″-triphenyl-9H,9'H,9"H-3,3′:6′,3″ tercarbazole (Tris-PCz, 35 nm)/10 wt% 1,2,3,4-tetrakis(carbazol-9-yl)-5,6-dicyanobenzene

White Organic Light-Emitting Diodes with Thermally Activated Delayed Fluorescence Emitters http://dx.doi.org/10.5772/intechopen.75564 41

**Figure 2.** The WOLED structures and energy level diagram. Reproduced from Ref. [46].

levels. However, to enhance thermal upconversion (i.e., T1 → S<sup>1</sup>

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

**3. Approaches to realize WOLEDs with TADF emitters**

ulated. For example, unlike conventional fluorescence emitters, the T1

in molecular conformation between its ground state (S0

**3.1. Basic aspects of WOLEDs with TADF emitters**

because of their charge-transfer nature [31–47].

can be 100%.

ers with low T1

selecting high-T1

phosphorescence emitters.

**3.2. WOLEDs with all TADF emitters**

the EML, leading to the excitons being well consumed.

of TADF materials requires small △EST, typically less than 0.2 eV, to overcome competitive non-radiative decay pathways, leading to highly luminescent TADF materials [57]. In addition, to enhance the photoluminescence efficiency of TADF materials, the geometrical change

suppress non-radiative decay. As a result, the maximum theoretical IQE of TADF emitters

Similar to phosphorescence emitters, the use of TADF emitters is very promising to achieve WOLEDs. This is because TADF emitters can (1) harness triplet excitons, (2) exhibit excellent efficiency, and (3) show usually broad emission spectra with rather large full width at half maximum of about 100 nm, which is wider than that of conventional fluorescent materials

To attain the high performance, the device structures, design strategies, working mechanisms, and electroluminescent processes of the WOLEDs with TADF emitters should be well manip-

necessary to be considered when designing a WOLED architecture, since hosts or nearby lay-

the location of TADF emitters is needed to be investigated, since the energy transfer would occur between the contacted different emitters (e.g., energy can transfer from high-energy TADF emitters to low-energy emitters). To date, various approaches have been reported to develop WOLEDs with TADF emitters, such as the exploitation of all TADF emitters, the combination of TADF and conventional fluorescence emitters, and the mixture of TADF and

The most directed approach to develop WOLEDs with TADF emitters is the exploitation of all TADF emitters. That is to say, all blue, green, and red emitters are TADF materials. By

should be particularly higher than the blue TADF materials. Otherwise, the triplet excitons of emitters would be quenched by the host, resulting in low performance. Moreover, high-T1 charge transport materials should be selected, which is used to confine the triplet excitons in

Adachi and his coworkers for the first time adopted such an approach to develop highly efficient WOLEDs with all TADF emitters [46]. **Figure 2** depicts the device structure: ITO/1,4,5,8,9,11 hexaazatriphenylene hexacarbonitrile (HAT-CN, 10 nm)/9,9′,9″-triphenyl-9H,9'H,9"H-3,3′:6′,3″ tercarbazole (Tris-PCz, 35 nm)/10 wt% 1,2,3,4-tetrakis(carbazol-9-yl)-5,6-dicyanobenzene

attained. More specifically, the conventional fluorescent host should possess high T1

would quench the triplet excitons, which leads to the low efficiency. Besides,

hosts and charge transport materials, high-performance WOLEDs can be

) and S1

RISC), the molecular design

states should be restrained to

of TADF emitters is

, which

(4CzPN): 3,3-Di(9H-carbazol-9-yl)biphenyl (mCBP, green EML) (x nm)/6 wt% 4CzPN: 2 wt% 41,4-dicyano-2,3,5,6-tetrakis (3,6-diphenylcarbazol-9-yl)benzene (CzTPN-Ph): mCBP (red EML) (y nm)/10 wt% 9-(3-(9H-carbazol-9-yl)-9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9H-carbazol-6-yl)-9H-carbazole (3CzTRZ): 2,8-bis(diphenylphosphoryl) dibenzo-[b,d] thiophene (PPT, blue EML) (z nm)/PPT (50 nm)/LiF/Al. In this device, the T1 level of mCBP host is 2.9 eV, which is much higher than that of blue, green, and red TADF emitters, ensuring the high efficiency. Besides, PPT has a high T1 of 3.1 eV, suggesting a good confinement of the triplet excitons. By optimizing the charge generation zone via the adjustment of different EML thickness (the total thickness is set to be x + y + z = 15 nm), the WOLED achieved a maximum EQE of over 17%, a peak PE of 34.1 lm/W with CIE coordinates of (0.30, 0.38).

#### **3.3. WOLEDs with TADF and conventional fluorescence emitters**

Considering that triplets could be harnessed by TADF emitters, it is promising to realize the unity IQE by combining traditional fluorescent materials with TADF emitters [61]. For such an approach, the TADF molecules would act as the triplet harvester, which is used to harness the triplets for the conventional other-color fluorescence materials. As a result, highly efficient white light can be produced [42–44].

In 2016, Li et al. reported high-efficiency and high CRI WOLEDs with the chromaticityadjustable yellow TADF emitter 2-(4-phenoxazinephenyl)thianthrene-9,9′,10,10′-tetraoxide (PXZDSO2) [47]. By combining the conventional deep-blue fluorescence emitter NI-1- PhTPA and PXZDSO2, the two-color WOLED showed a maximum EQE of 15.8% (device W3). Then, since the chromaticity of the EML containing PXZDSO2 could be tuned to yellowish green, they introduced a deep-red fluorescence emitter DBP (dibenzo{[f,f′]-4, 4′,7,7′-tetraphenyl}diindeno[1,2,3-cd:1′,2′,3′-lm]perylene) subtly to fabricate three-color WOLED, achieving the most efficient ever EQE of 19.2% with a CRI of 68 (device W4) and the highest ever CRI of 95 with an EQE of 15.6% (device W6). The configurations are ITO/HATCN/TAPC/EMLs/TmPyPB/LiF/Al, in which device W3 has the EML of CBP: 8 wt% NI-1-PhTPA (10 nm)/CBP (3 nm)/CBP: 6 wt% PXZDSO2 (15 nm)/CBP (3 nm)/CBP: 8 wt% NI-1-PhTPA (10 nm), device W4 has the EML of CBP: 7 wt% NI-1-PhTPA (10 nm)/ CBP (3 nm)/CBP: 3 wt% PXZDSO2 (5 nm)/CBP: 5 wt% PXZDSO2: 0.3 wt% DBP (5 nm)/ CBP:3 wt% PXZDSO2 (5 nm)/CBP (3 nm)/CBP: 7 wt% NI-1-PhTPA (10 nm), device W6 has the EML of CBP: 10 wt% NI-1-PhTPA (10 nm)/CBP (3 nm)/CBP: 5 wt%PXZDSO2: 0.35 wt% DBP (15 nm)/CBP (3 nm)/CBP: 10 wt% NI-1-PhTPA (10 nm). The device working mechanisms can be described as follows. For device W3, (1) since NI-1-PhTPA is a deep-blue fluorescence emitter and CBP: 6 wt% PXZDSO2 emits a yellow light with broad spectrum, high-performance two-color WOLEDs were realized; (2) given the almost equal T1 level of NI-1-PhTPA and PXZDSO2, the efficiency roll-off occurs if they are directly in contact due to the triplet exciton quenching by NI-1-PhTPA; (3) the efficiency roll-off can be further induced as the formed triplet excitons of NI-1-PhTPA cannot be utilized by PXZDSO2; (4) to stabilize the recombination zone which occurs in whole EMLs since NI-1- PhTPA/CBP are bipolar and avoid triplet exciton quenching by NI-1-PhTPA, two 3-nm CBPs were inserted between the blue and yellow EMLs, restraining the inevitable Förster

energy transfer from NI-1-PhTPA to PXZDSO2; (5) to reduce the triplet exciton energy loss via nonradiative transition process, blue-fluorescence emitter was dispersed in CBP for blue emission, leading to most excitons being generated at CBP; and (6) triplet energy transferred from CBP gives most of triplet excitons of PXZDSO2 since triplet excitons typically have long diffusion lengths (≈100 nm), as shown in **Figure 3a**. Hence, an EQE of 15.8% was achieved for device W3. For device W4, (1) a deep-red fluorescence emitter DBP was conceived to be used; (2) PXZDSO2 was an assistant host for DBP to realize a redlight emission due to an efficient energy transfer from the S1 of PXZDSO2; (3) the doping concentration of PXZDSO2 was decreased to reduce intermolecular aggregation and thus blue-shifted emission (20 nm), achieving green emission, complementary to emissions of NI-1-PhTPA and DBP; (4) a red EML of CBP: 5 wt% PXZDSO2: 0.3 wt% DBP was inserted between two green EMLs of CBP: 3 wt% PXZDSO2 to receive singlet exciton energy transferred from the PXZDSO2 molecules in both sides to give both green and red emissions; (5) the two doped blue EMLs and CBP interlayers located at both sides of the green EMLs to give a blue emission and to confine the PXZDSO2 triplet excitons, respectively, as shown in **Figure 3b**. Thus, an EQE of 19.2% was achieved for device W4. Furthermore, an EML consisting of improved DBP doping concentration was utilized instead of the green and

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red EMLs for candle-style warm WOLEDs (device W6), achieving a high CRI of 95.

Since both TADF and phosphorescence emitters can harvest singlet and triplet excitons, the mixture of TADF and phosphorescence emitters is a significant approach to construct WOLEDs. By virtue of their respective advantages, high efficiency and long lifetime can be realized simultaneously [44, 58]. In particular, there is tremendous interest in mixing blue TADF emitters with green/red or complementary color phosphorescence emitters. This is because blue TADF materials (1) are naturally advantageous to achieve high triplet energies due to their reduced singlet-triplet splits, (2) can possess high efficiency, and (3) can harvest the triplets [59–61]. For this kind of WOLED (i.e., mixing blue TADF and other-color phospho-

In 2014, Zhang and coworkers demonstrated hybrid WOLEDs via the use of blue TADF emitter, obtaining the peak efficiency as high as 47.6 lm/W [59]. The device structure is ITO/ HATCN (5 nm)/NPB (40 nm)/TCTA (10 nm)/mCP: 4,5-bis(carbazol-9-yl)-1,2-dicyanobenzene (2CzPN, 11 nm, blue EML)/TAZ: 4 wt% (acetylacetonato)bis[2-(thieno[3,2-c]pyridin-4-yl)phenyl]iridium(III) (PO-01, 4 nm, orange EML)/TAZ (40 nm)/LiF (0.5 nm)/Al (150 nm), as shown in **Figure 4**. The factors for the high performance are as follows: (1) mCP is chosen to be the

material plays an important role in determining the efficiency; (2) 2CzPN is placed nearest to the main recombination zone, ensuring that excitons can diffuse throughout the emissive region to produce a desired color-balanced output; (3) triplets formed on 2CzPN can be harvested by either energy transfer to the low-lying triplet states of the phosphor PO-01 (2.2 eV) or thermal upconversion to the emissive singlet states, eliminating the energy loss; and (4) the recombination zone is fixed as the voltage increases by 2CzPN due to its charge-trapping

of 3.0 eV, since the host of the TADF

**3.4. WOLEDs with TADF and phosphorescence emitters**

rescence emitters), it is generally called hybrid WOLEDs [21].

host for 2CzPN due to the wide energy gap and high T1

ability, achieving a stable white emission.

**Figure 3.** Function mechanisms of the use of singlets/triplets. (a) Device W3 and (b) devices W4 and W6. H, B, Y, and R represent CBP, NI-1-PhTPA, PXZDSO2, and DBP, respectively. Reproduced from Ref. [47].

energy transfer from NI-1-PhTPA to PXZDSO2; (5) to reduce the triplet exciton energy loss via nonradiative transition process, blue-fluorescence emitter was dispersed in CBP for blue emission, leading to most excitons being generated at CBP; and (6) triplet energy transferred from CBP gives most of triplet excitons of PXZDSO2 since triplet excitons typically have long diffusion lengths (≈100 nm), as shown in **Figure 3a**. Hence, an EQE of 15.8% was achieved for device W3. For device W4, (1) a deep-red fluorescence emitter DBP was conceived to be used; (2) PXZDSO2 was an assistant host for DBP to realize a redlight emission due to an efficient energy transfer from the S1 of PXZDSO2; (3) the doping concentration of PXZDSO2 was decreased to reduce intermolecular aggregation and thus blue-shifted emission (20 nm), achieving green emission, complementary to emissions of NI-1-PhTPA and DBP; (4) a red EML of CBP: 5 wt% PXZDSO2: 0.3 wt% DBP was inserted between two green EMLs of CBP: 3 wt% PXZDSO2 to receive singlet exciton energy transferred from the PXZDSO2 molecules in both sides to give both green and red emissions; (5) the two doped blue EMLs and CBP interlayers located at both sides of the green EMLs to give a blue emission and to confine the PXZDSO2 triplet excitons, respectively, as shown in **Figure 3b**. Thus, an EQE of 19.2% was achieved for device W4. Furthermore, an EML consisting of improved DBP doping concentration was utilized instead of the green and red EMLs for candle-style warm WOLEDs (device W6), achieving a high CRI of 95.

#### **3.4. WOLEDs with TADF and phosphorescence emitters**

(PXZDSO2) [47]. By combining the conventional deep-blue fluorescence emitter NI-1- PhTPA and PXZDSO2, the two-color WOLED showed a maximum EQE of 15.8% (device W3). Then, since the chromaticity of the EML containing PXZDSO2 could be tuned to yellowish green, they introduced a deep-red fluorescence emitter DBP (dibenzo{[f,f′]-4, 4′,7,7′-tetraphenyl}diindeno[1,2,3-cd:1′,2′,3′-lm]perylene) subtly to fabricate three-color WOLED, achieving the most efficient ever EQE of 19.2% with a CRI of 68 (device W4) and the highest ever CRI of 95 with an EQE of 15.6% (device W6). The configurations are ITO/HATCN/TAPC/EMLs/TmPyPB/LiF/Al, in which device W3 has the EML of CBP: 8 wt% NI-1-PhTPA (10 nm)/CBP (3 nm)/CBP: 6 wt% PXZDSO2 (15 nm)/CBP (3 nm)/CBP: 8 wt% NI-1-PhTPA (10 nm), device W4 has the EML of CBP: 7 wt% NI-1-PhTPA (10 nm)/ CBP (3 nm)/CBP: 3 wt% PXZDSO2 (5 nm)/CBP: 5 wt% PXZDSO2: 0.3 wt% DBP (5 nm)/ CBP:3 wt% PXZDSO2 (5 nm)/CBP (3 nm)/CBP: 7 wt% NI-1-PhTPA (10 nm), device W6 has the EML of CBP: 10 wt% NI-1-PhTPA (10 nm)/CBP (3 nm)/CBP: 5 wt%PXZDSO2: 0.35 wt% DBP (15 nm)/CBP (3 nm)/CBP: 10 wt% NI-1-PhTPA (10 nm). The device working mechanisms can be described as follows. For device W3, (1) since NI-1-PhTPA is a deep-blue fluorescence emitter and CBP: 6 wt% PXZDSO2 emits a yellow light with broad spectrum, high-performance two-color WOLEDs were realized; (2) given the almost equal

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

 level of NI-1-PhTPA and PXZDSO2, the efficiency roll-off occurs if they are directly in contact due to the triplet exciton quenching by NI-1-PhTPA; (3) the efficiency roll-off can be further induced as the formed triplet excitons of NI-1-PhTPA cannot be utilized by PXZDSO2; (4) to stabilize the recombination zone which occurs in whole EMLs since NI-1- PhTPA/CBP are bipolar and avoid triplet exciton quenching by NI-1-PhTPA, two 3-nm CBPs were inserted between the blue and yellow EMLs, restraining the inevitable Förster

**Figure 3.** Function mechanisms of the use of singlets/triplets. (a) Device W3 and (b) devices W4 and W6. H, B, Y, and R

represent CBP, NI-1-PhTPA, PXZDSO2, and DBP, respectively. Reproduced from Ref. [47].

T1

Since both TADF and phosphorescence emitters can harvest singlet and triplet excitons, the mixture of TADF and phosphorescence emitters is a significant approach to construct WOLEDs. By virtue of their respective advantages, high efficiency and long lifetime can be realized simultaneously [44, 58]. In particular, there is tremendous interest in mixing blue TADF emitters with green/red or complementary color phosphorescence emitters. This is because blue TADF materials (1) are naturally advantageous to achieve high triplet energies due to their reduced singlet-triplet splits, (2) can possess high efficiency, and (3) can harvest the triplets [59–61]. For this kind of WOLED (i.e., mixing blue TADF and other-color phosphorescence emitters), it is generally called hybrid WOLEDs [21].

In 2014, Zhang and coworkers demonstrated hybrid WOLEDs via the use of blue TADF emitter, obtaining the peak efficiency as high as 47.6 lm/W [59]. The device structure is ITO/ HATCN (5 nm)/NPB (40 nm)/TCTA (10 nm)/mCP: 4,5-bis(carbazol-9-yl)-1,2-dicyanobenzene (2CzPN, 11 nm, blue EML)/TAZ: 4 wt% (acetylacetonato)bis[2-(thieno[3,2-c]pyridin-4-yl)phenyl]iridium(III) (PO-01, 4 nm, orange EML)/TAZ (40 nm)/LiF (0.5 nm)/Al (150 nm), as shown in **Figure 4**. The factors for the high performance are as follows: (1) mCP is chosen to be the host for 2CzPN due to the wide energy gap and high T1 of 3.0 eV, since the host of the TADF material plays an important role in determining the efficiency; (2) 2CzPN is placed nearest to the main recombination zone, ensuring that excitons can diffuse throughout the emissive region to produce a desired color-balanced output; (3) triplets formed on 2CzPN can be harvested by either energy transfer to the low-lying triplet states of the phosphor PO-01 (2.2 eV) or thermal upconversion to the emissive singlet states, eliminating the energy loss; and (4) the recombination zone is fixed as the voltage increases by 2CzPN due to its charge-trapping ability, achieving a stable white emission.

LiF (1 nm)/Al (160 nm), where interlayers are none, mCP and 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy) for devices W11, W12, and W13, respectively. The configuration of the two-color WOLED is ITO/HAT-CN (100 nm)/TAPC (20 nm)/mCP: DDCzTrz (10 nm,

: Ir(dmppy)<sup>2</sup>

(35 nm)/LiF (1 nm)/Al (160 nm). Unlike previous TADF-based hybrid WOLEDs, the bipolar interlayer is demonstrated to enhance the performance. Particularly, it is demonstrated that the use of interlayer can enhance the lifetime (2.3 times). The working mechanism of the twocolor WOLED can be described as follows, which is beneficial to comprehend the reason why the bipolar interlayer can enhance the performance. For W11, since mCP and Bepp<sup>2</sup>

p-type and n-type materials, respectively, holes and electrons are easily accumulated at the

lets on blue EML can (1) convert into singlets via the RISC procedure and then generate the

the Dexter process and then generate part of yellow emission (the other part of yellow emission is originated from excitons on the yellow EML). However, the main exciton generation

**Figure 5 A schematic illustration of the working mechanism of (a) W11, (b) W12, and (c) W13.** The gray-filled rectangles are the main exciton generation zones. The Dexter energy transfer can occur in W11, while it is prohibited in both W12

blue emission, and (2) transfer to the low energy of yellow phosphor Ir(dmppy)<sup>2</sup>

interface, forming singlet and triplet excitons, as shown in **Figure 5a**. The trip-

(dpp): Ir(piq)3

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(15 nm, 1: 2%: 1.3%)/Bepp<sup>2</sup>

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are

45

(dpp) via

20%)/26DCzPPy (interlayer, 3 nm)/Bepp<sup>2</sup>

mCP/Bepp<sup>2</sup>

and W13. Reproduced from Ref. [62].

**Figure 4 Schematic diagrams of the working mechanisms.** The gray-filled rectangle represents the main exciton generation zone. PF is the prompt fluorescence while DF is the delayed fluorescence. RISC indicates the reverse ISC and ET denotes the energy transfer. Reproduced from Ref. [59] with permission from the Royal Society of Chemistry.

Although high-performance hybrid WOLEDs based on TADF materials have been demonstrated, there are still some problems, even for these state-of-the-art devices [59–61]. For example, (1) the driving voltages are somewhat high (e.g., 3.2 V at 1 cd m−2 [61]); (2) the luminances are very low (e.g., only ~10,000 cd m−2 [61]); (3) the efficiency at high luminance is not high (e.g., <6 lm W−1 at 10,000 cd m−2 [61]); (4) the CRI is not high enough; and (5) negligible attention has been paid to the lifetime of TADF-based hybrid WOLED.

To solve the issues, Luo et al. recently reported high-performance two-color and three-color hybrid WOLEDs [62]. The two-color WOLED exhibits (1) low voltage (i.e., 2.9 V at 1 cd m−2); (2) high luminance (103,756 cd m−2); (3) maximum EQE and PE of 23.5% and 70.92 lm W−1, respectively; and (4) 21.59 lm W−1 at 10,000 cd m−2. The three-color WOLED exhibits (1) low voltage and high luminance (51,514 cd m−2); (2) superior CRI of 94; and (3) EQE and PE of 17.3% and 46.09 lm W−1, respectively. The configuration of the two-color WOLEDs is ITO/HAT-CN (100 nm)/TAPC (20 nm)/mCP: 9,9′,9″,9′′′-((6-phenyl-1,3,5-triazine-2,4-diyl) bis(benzene-5,3,1-triyl))tetrakis(9*H*-carbazole) (DDCzTrz, 10 nm, 20%)/interlayers (3 nm)/ bis[2-(2-hydroxyphenyl)-pyridine] beryllium (Bepp<sup>2</sup> ): bis(2-phenyl-4,5-dimethylpyridinato) [2-(biphenyl-3-yl)pyridinato] iridium(III) (Ir(dmppy)<sup>2</sup> (dpp), 15 nm, 1:2%)/Bepp<sup>2</sup> (35 nm)/

LiF (1 nm)/Al (160 nm), where interlayers are none, mCP and 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy) for devices W11, W12, and W13, respectively. The configuration of the two-color WOLED is ITO/HAT-CN (100 nm)/TAPC (20 nm)/mCP: DDCzTrz (10 nm, 20%)/26DCzPPy (interlayer, 3 nm)/Bepp<sup>2</sup> : Ir(dmppy)<sup>2</sup> (dpp): Ir(piq)3 (15 nm, 1: 2%: 1.3%)/Bepp<sup>2</sup> (35 nm)/LiF (1 nm)/Al (160 nm). Unlike previous TADF-based hybrid WOLEDs, the bipolar interlayer is demonstrated to enhance the performance. Particularly, it is demonstrated that the use of interlayer can enhance the lifetime (2.3 times). The working mechanism of the twocolor WOLED can be described as follows, which is beneficial to comprehend the reason why the bipolar interlayer can enhance the performance. For W11, since mCP and Bepp<sup>2</sup> are p-type and n-type materials, respectively, holes and electrons are easily accumulated at the mCP/Bepp<sup>2</sup> interface, forming singlet and triplet excitons, as shown in **Figure 5a**. The triplets on blue EML can (1) convert into singlets via the RISC procedure and then generate the blue emission, and (2) transfer to the low energy of yellow phosphor Ir(dmppy)<sup>2</sup> (dpp) via the Dexter process and then generate part of yellow emission (the other part of yellow emission is originated from excitons on the yellow EML). However, the main exciton generation

Although high-performance hybrid WOLEDs based on TADF materials have been demonstrated, there are still some problems, even for these state-of-the-art devices [59–61]. For example, (1) the driving voltages are somewhat high (e.g., 3.2 V at 1 cd m−2 [61]); (2) the luminances are very low (e.g., only ~10,000 cd m−2 [61]); (3) the efficiency at high luminance is not high (e.g., <6 lm W−1 at 10,000 cd m−2 [61]); (4) the CRI is not high enough; and (5) negligible

**Figure 4 Schematic diagrams of the working mechanisms.** The gray-filled rectangle represents the main exciton generation zone. PF is the prompt fluorescence while DF is the delayed fluorescence. RISC indicates the reverse ISC and ET denotes the energy transfer. Reproduced from Ref. [59] with permission from the Royal Society of Chemistry.

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

To solve the issues, Luo et al. recently reported high-performance two-color and three-color hybrid WOLEDs [62]. The two-color WOLED exhibits (1) low voltage (i.e., 2.9 V at 1 cd m−2); (2) high luminance (103,756 cd m−2); (3) maximum EQE and PE of 23.5% and 70.92 lm W−1, respectively; and (4) 21.59 lm W−1 at 10,000 cd m−2. The three-color WOLED exhibits (1) low voltage and high luminance (51,514 cd m−2); (2) superior CRI of 94; and (3) EQE and PE of 17.3% and 46.09 lm W−1, respectively. The configuration of the two-color WOLEDs is ITO/HAT-CN (100 nm)/TAPC (20 nm)/mCP: 9,9′,9″,9′′′-((6-phenyl-1,3,5-triazine-2,4-diyl) bis(benzene-5,3,1-triyl))tetrakis(9*H*-carbazole) (DDCzTrz, 10 nm, 20%)/interlayers (3 nm)/

): bis(2-phenyl-4,5-dimethylpyridinato)

(35 nm)/

(dpp), 15 nm, 1:2%)/Bepp<sup>2</sup>

attention has been paid to the lifetime of TADF-based hybrid WOLED.

bis[2-(2-hydroxyphenyl)-pyridine] beryllium (Bepp<sup>2</sup>

[2-(biphenyl-3-yl)pyridinato] iridium(III) (Ir(dmppy)<sup>2</sup>

**Figure 5 A schematic illustration of the working mechanism of (a) W11, (b) W12, and (c) W13.** The gray-filled rectangles are the main exciton generation zones. The Dexter energy transfer can occur in W11, while it is prohibited in both W12 and W13. Reproduced from Ref. [62].

zone of W11 is narrow, unfavorable to the performance. Similarly, the main exciton generation zone of W12 is located at the mCP interlayer/Bepp<sup>2</sup> interface, as shown in **Figure 5b**. As a result, excitons are more easily harvested by Ir(dmppy)<sup>2</sup> (dpp) instead of DDCzTrz since Ir(dmppy)<sup>2</sup> (dpp) is close to the main exciton generation zone. However, a part of electrons can pass through the thin interlayer via the tunneling process and then meet holes, which can generate excitons to guarantee the blue emission. For W13, by way of the bipolar interlayer and the suitable energy levels of 26DCzPPy, both holes and electrons can be easily passed through 26DCzPPy, as shown in **Figure 5c**. As a result, excitons can be formed at both the mCP/26DCzPPy and 26DCzPPy/Bepp<sup>2</sup> interfaces, leading to a broad exciton generation zone, which ensure the high performance of W23. Besides, since the Dexter energy transfer from DDCzTrz to Ir(dmppy)<sup>2</sup> (dpp) is also prevented due to the 3 nm 26DCzPPy, the yellow emission mainly results from excitons on the yellow EML.

Another effective approach to develop WOLEDs with TADF and phosphorescence emitters is the mixture of green TADF and other-color phosphorescence emitters. In this case, the TADF emitters are adopted as the emitters for WOLEDs because they may be compatible with phosphorescence emitters and not quench triplet excitons of the phosphorescence emitters, otherwise triplet excitons will be wasted.

> efficient, leading to no non-radiative triplet exciton quenching of FIrpic by 4CzIPN. (2) The energy transfer from mCP to dopant materials in the FIrpic and 4CzIPN co-doped emitting layer is very efficient. (3) FIrpic activates the delayed emission of 4CzIPN through an efficient energy transfer, which resulted in the high quantum efficiency of the hybrid-emitting layer. (4) The balanced charge density in the emitting layer contributed to the high quantum effi-

**Figure 6.** The energy level diagram and device architecture of the WOLED. Reproduced from Ref. [45].

port properties of TPBI, which improved charge balance in the emitting layer in combination

As a novel kind of OLED emitter, TADF materials show many unique characteristics, which have been demonstrated to develop high-performance WOLEDs. Thanks to the hard endeavors of researchers, the performance of WOLEDs is now comparable to state-of-the-art phosphorescence WOLEDs and fluorescence/phosphorescence hybrid WOLEDs. In this chapter, the focus is the development of WOLEDs by manipulating TADF emitters. Specifically, we highlight the recent development of WOLEDs based on all TADF emitters, WOLEDs based on TADF and conventional fluorescence emitters, and WOLEDs based on TADF and phosphorescence emitters. Particularly, the device structures, design strategies, working mechanisms, and electroluminescent processes of the representative high-performance WOLEDs

with hole transport-type mCP: FIrpic: 4CzIPN-emitting layer.

acac-emitting layer efficiently injects electrons due to electron trans-

White Organic Light-Emitting Diodes with Thermally Activated Delayed Fluorescence Emitters

http://dx.doi.org/10.5772/intechopen.75564

47

ciency. The TPBI: Ir(pq)<sup>2</sup>

**4. Summary and outlook**

with TADF emitters are reviewed.

Kim et al. reported this approach by combining a green TADF with red/blue phosphorescence materials to organize high-efficiency hybrid-type WOLEDs [45]. In their WOLED, energy transfer between a blue phosphorescent material and a green TADF emitter was efficient and could be managed by controlling the doping concentration of emitters. A maximum EQE of 20.2% was achieved by optimizing the device structure of the hybridtype WOLEDs. The device structure is ITO (50 nm)/poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS, 60 nm)/TAPC (20 nm)/mCP (10 nm)/mCP: iridium(III) bis[(4,6-difluorophenyl)-pyridinato-N,C<sup>2</sup> ]picolinate (FIrpic): (4 s,6 s)-2,4,5,6-tetra(9Hcarbazol-9-yl)isophthalonitrile (4CzIPN) (12.5 nm)/TPBI: Ir(pq)<sup>2</sup> acac (12.5 nm, 3%)/diphenylphosphine oxide-4-(triphenylsilyl)phenyl (TSPO1, 35 nm)/LiF (1 nm)/Al (200 nm), where 4CzIPN is the green TADF emitter, and FIrpic and Ir(pq)<sup>2</sup> acac are blue and red phosphorescence emitter, respectively, as shown in **Figure 6**. To explore the possibility of this type of WOLEDs, hybrid OLEDs with blue-emitting FIrpic and green-emitting 4CzIPN were first fabricated. By optimizing the concentration of FIrpic and 4CzIPN, a maximum EQE of the hybrid OLEDs was 19.2% at 5% FIrpic and 0.5% 4CzIPN. Given that the EQE of mCP: FIrpic OLED is <20%, such superior EQE of hybrid OLED suggests that 4CzIPN would not quench FIrpic triplet emission. In fact, T1 of FIrpic can be transferred to 4CzIPN and then make a contribution to the 4CzIPN TADF emission. For this hybrid OLED, there are three main energy transfer processes, that is, energy transfer processes from mCP to FIrpic, mCP to 4CzIPN, and FIrpic to 4CzIPN dominate the blue and green emissions of the hybrid OLEDs. After the successful exploration of hybrid OLED, Kim et al. combined this system and red phosphorescence emitting layers, attaining high-efficiency WOLEDs. The factors for the highperformance of WOLEDs can be summarized as follows: (1) the hybrid OLEDs doped with FIrpic and 4CzIPN showed a high quantum efficiency, which ensure the high efficiency of blue-green-emitting layer. In this emitting layer, energy transfer from FIrpic to 4CzIPN is

White Organic Light-Emitting Diodes with Thermally Activated Delayed Fluorescence Emitters http://dx.doi.org/10.5772/intechopen.75564 47

**Figure 6.** The energy level diagram and device architecture of the WOLED. Reproduced from Ref. [45].

efficient, leading to no non-radiative triplet exciton quenching of FIrpic by 4CzIPN. (2) The energy transfer from mCP to dopant materials in the FIrpic and 4CzIPN co-doped emitting layer is very efficient. (3) FIrpic activates the delayed emission of 4CzIPN through an efficient energy transfer, which resulted in the high quantum efficiency of the hybrid-emitting layer. (4) The balanced charge density in the emitting layer contributed to the high quantum efficiency. The TPBI: Ir(pq)<sup>2</sup> acac-emitting layer efficiently injects electrons due to electron transport properties of TPBI, which improved charge balance in the emitting layer in combination with hole transport-type mCP: FIrpic: 4CzIPN-emitting layer.
