Aggregation-Induced Emission Luminogens (AIEgens) for Non-Doped

Chapter 7

Downloaded by KANSAS STATE UNIV on September 27, 2016 | http://pubs.acs.org Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1227.ch007

Aggregation-Induced Emission Luminogens (AIEgens) for Non-Doped Organic Light-Emitting Diodes Han Nie, Jian Huang, Zujin Zhao,*,1 and Ben Zhong Tang*,1,2,3 1State

Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China 2Department of Chemistry, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China 3Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, Hong Kong, China *E-mails: [email protected] (Z.Z.); [email protected] (B.Z.T.)

Organic light-emitting diodes (OLEDs) have shown great potential in full-color flat panel displays and solid-state white lighting. For the fabrication of high-performance OLEDs, the light-emitting materials are of high importance. However, most conventional luminescent materials generally suffer from aggregation-caused quenching (ACQ) problem, which weakens the fluorescence of their solid films, and thus, undermines OLED performance. Recently, a new kind of luminogens with aggregation-induced emission characteristics (AIEgens) is found to be free of ACQ problem. They can fluoresce strongly in solid films and perform outstandingly in non-doped OLEDs. Herein, representative AIEgens with fluorescence color covering from blue to red and varied carrier transport ability for the application in efficient non-doped OLEDs are described.

1. Introduction Organic light-emitting diodes (OLEDs) are being highly expected as the next generation technology for flat panel displays and solid-state lighting because they have excellent self-emitting properties, simple manufacture procedures © 2016 American Chemical Society Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


and flexible characteristics (1–3). Owing to the enormous effort devoted by a great number of scientists, more and more efficient OLEDs with various emission color have been obtained (4). In order to realize a further breakthrough, development of efficient and stable organic light-emitting materials with excellent optoelectronic properties is of significant importance. Many conventional luminescent materials showing robust emissions in the molecularly dispersive state usually experience serious quenching problem, and thus present weak or nearly no emissions in condensed phase or solid state, which is referred to as notorious aggregation-caused quenching (ACQ) effect (5). When these traditional luminescent molecules aggregate, the strong intermolecular π−π stacking interactions will promote the formation of delocalized excitons, which leads to low luminescence quantum efficiencies, and thus ACQ phenomenon. In OLEDs, the light emitters are practically fabricated into solid thin films, so the ACQ problem has impeded the progress of OLEDs field to some extent. To alleviate ACQ effect, various chemical, physical and/or engineering approaches have been proposed. Commonly, the luminogens are introduced with bulky groups or are doped into host materials at low concentration to suppress the aggregate formation. These methods, however, often end up with limited success and even are accompanied by some side effects in many cases (5). For example, the bulky groups can twist the conformations and shorten the effective conjugation lengths of the luminogens, which will barricade the charge transport in OLED devices. Because aggregate formation is a natural process when the luminogens are located in close proximity, the host-guest blending system may intrinsically suffer from phase separation, resulting in the limitation of efficiency and stability for OLEDs. Tang et al. reported a unique photophysical phenomenon from 1-methyl1,2,3,4,5-pentaphenylsilole (MPPS, Figure 1, 1) in 2001, whose weak emission in dilute solution was turned on by the formation of aggregates (6). This phenomenon was termed as aggregation-induced emission (AIE) that is essentially opposite to ACQ effect. During the past decade, lots of studies have been done to draw a clear picture on this novel AIE process by systemically experimental measurements and theoretical calculations (7). These studies had proposed that the restriction of intramolecular motions (RIM), including rotations (RIR) and vibrations (RIV), is the main cause for AIE effect. In the isolated state, the active intramolecular motions can enhance the nonradiative decay rate of the excited state. In the aggregated state, however, these nonradiative decay channels are blocked due to restricted intramolecular motions by the spatial constraints and interactions from the surrounding molecules. Moreover, the AIE molecules usually have twisted conformations which can also prevent the π−π stacking interactions. Consequently, they exhibit stronger emissions in the aggregated state. Since the first report of AIE phenomenon, many investigators with different backgrounds have focused their attentions on AIE research because of its potential significance in fundamental understanding and technological application. So far, a wide range of luminogens with AIE characteristics (AIEgens) have been developed and utilized in many frontier fields based on the understanding of AIE mechanism (8). By merging AIE elements and conventional ACQ chromophores (ACQphores) through molecular engineering, many of the resulting luminogens 174 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


presented AIE or aggregation-enhanced emission (AEE) characteristics with high emission efficiencies in the aggregated state. Therefore, AIE effect provides researchers with an effective strategy to address ACQ problem and develop efficient solid-state emitters for the construction of excellent non-doped OLEDs. As the most common AIE-active building blocks, silole (9) and teraphenylethene (TPE) (10) have been widely used to construct new fluorescent AIEgens with high fluorescent quantum yield (ΦF) values in solid films, which have been extensively employed to fabricate stable, efficient and simplified non-doped fluorescent OLEDs. The emission color of the fabricated OLEDs covered the entire visible region, and some of the devices performed excellently with high efficiencies approaching or reaching the theoretical limit of fluorescent OLEDs. In order to provide guidance for the further development of efficient solid state luminescent materials, in this chapter, silole-based and TPE-based AIEgens employed mainly as emitting materials for OLEDs with various EL emissions will be discussed. Then we will summarize the recent advances of multifunctional AIEgens, which can function efficiently as light emitter and carrier transporter simultaneously in OLEDs.

2. Silole-Based AIEgens Since the AIE behavior of MPPS (1) was first reported, silole has become an effective unit for the building of fluorescent AIEgens with high ΦF values in the aggregated state. In silole derivatives, the unique σ*-π* conjugation between the σ* orbital of two exocyclic C-Si σ-bonds and the π* orbital of the butadiene portion lowers the LUMO (lowest unoccupied molecular orbital) energy levels, which enables siloles to have good electron affinity and fast electron mobility. Therefore, silole derivatives can be adopted to transport electrons in optoelectronic devices. According to reported data, silole derivatives also exhibit excellent thermal and morphological stabilities as well as good solubility in common solvents (9). Therefore, siloles would certainly be the promising candidates of light-emitting materials in high-performance OLEDs. For example, MPPS had showed outstanding electroluminescence (EL) properties in early report (11). By utilizing dibenzosilole and TPE as the building blocks, a series of new fluorescent AIEgens (2-4) were achieved, whose chemical structures are presented in Figure 1 (12). These AIEgens possessed high thermal stabilities and good EL properties (Table 1). It is noteworthy that the EL spectra of OLEDs based on compounds 2 and 3 were located on deep-blue (CIE 0.16, 0.12) and sky blue (CIE 0.20, 0.33) region, respectively. Among them, compound 3 had the best EL efficiencies, and its OLED device with a configuration of ITO/MoO3 (10 nm)/NPB (60 nm)/3 (15 nm)/TPBi (30 nm)/LiF (1 nm)/Al had a low turn-on voltage (Von) at 3.0 V, a maximum luminance (Lmax) of 27161 cd m-2, a high maximum current efficiency (ηC,max) of 8.04 cd A-1, a maximum power efficiency (ηP,max) of 6.17 lm W-1, and a good maximum external quantum efficiency (ηext,max) of 3.38%. The device constructed from 4 radiated bright green EL at 512 nm with a Lmax of 28718 cd m-2 and displayed relatively high efficiencies shown in Table 1. From this work, it can be found that the EL properties of AIEgens can be easily tuned 175 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


by fine structural modulations, such as altering linkage mode and sharing benzene ring between the main building blocks.

Figure 1. Chemical structures of representative silole-based AIEgens with good EL performances. Their EL maxima are given in parentheses. As mentioned in Introduction section, ACQ effect is a knotty problem for the application of common luminescent materials in OLEDs, which is difficult to thoroughly solve without any negative effects by traditional approaches. It would be nice and highly desirable if the ACQ problem is eliminated, while the valuable properties of these conventional dyes are maintained at the same time. During the past few years, this desirability had been realized through transforming ACQphores to AIEgens on the basis of numerous AIE studies, which could not only address ACQ problem but also expand the AIE systems. One of the approaches on ACQ-to-AIE transformation is the replacement of the moieties of AIEgens with ACQ units. In addition, the electronic structures 176 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


and photophysical properties of silole derivatives can be easily modulated by the substituents at the 2,5-positions. With these considerations, a series of silole derivatives 5-14 (Figure 1) were designed and successfully synthesized. When the ACQ-active triphenylamine (TPA) units were introduced into the 2,5-positions of silole ring, the resulting luminogen 5 exhibited typical AEE characteristics with a high film-state ΦF value of 74% (13). The multilayered OLED fabricated from 5 [ITO/NPB (60 nm)/5 (20 nm)/TPBi (10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (100 nm)] was turned on at a low bias of 3.1 V, and radiated a yellow emission of 544 nm with a Lmax of 13405 cd m-2. The device displayed good EL performance with ηC,max, ηP,max and ηext,max of 8.28 cd A-1, 7.88 lm W-1 and 2.42%, respectively. It is worth mentioning that the simplified OLED [ITO/5 (80 nm)/TPBi (10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (100 nm)] built by using 5 simultaneously as light-emitting layer (EML) and hole-transporting layer (HTL) showed comparable EL efficiencies (7.60 cd A-1, 6.94 lm W-1, and 2.26%), revealing the good hole-transporting capability of 5 due to the presence of TPA groups. Another hole-transporting group, carbazole, is always used to construct efficient organic semiconductors. To investigate the influence of bonding pattern between substituents and silole ring on the EL property, two isomers 6 and 7 comprising of same building blocks (silole ring and carbazole units) through different bonding patterns were obtained (14). The powders of these two materials presented strong fluorescence with much higher ΦF values of 56-65% than those in THF solutions (2.3-6.0%), indicating AEE characteristics. And their non-doped OLEDs emitted yellow EL spectra peaking at 548-552 nm and displayed moderate EL performance (Table 1). Fluorene-based substituents are generally beneficial to the generation of efficient light emitters for OLEDs because they are intensely emissive and thermally stable. We developed several fluorene-substituted siloles (8-10) shown in Figure 1, which gave good thermal stabilities and high ΦF values up to 88% in solid films (15, 16). Consequently, the non-doped OLEDs based on them showed excellent EL performance. As shown in Table 1, compound 10 presented the best EL properties and its OLED with a simple and non-optimized configuration already performed very well. The OLED built from 10 with a further optimized configuration [ITO/MoO3 (5 nm)/NPB (60 nm)/10 (20 nm)/TPBi (60 nm)/LiF (1 nm)/Al (100 nm)] displayed a low Von of 3.3 V and radiated a strong yellow emission of 540 nm (CIE 0.36, 0.57) with a high Lmax of 37800 cd m-2. Notably, the ηC,max, ηP,max and ηext,max of this device were measured to be extraordinarily high with the values of 18.3 cd A-1, 15.7 lm W-1 and 5.5%, respectively, which reached the theoretical limit of fluorescent OLEDs. This breakthrough demonstrated the great potential of AIE emitters in OLED applications. Planar fluorescent chromophores (PFCs) are conducive to transport carriers in optoelectronic devices but can lead to undesirable ACQ effect. The introduction of representative PFC groups, anthracene and pyrene, into the 2,5-positions of silole ring produced some new silole derivatives (11-14) (15). These siloles were AEE-active and the OLEDs fabricated from them had good performance with EL emissions ranging from yellow to orange (Table 1). For instance, when compound 13 was employed as light-emitting materials to construct EL device, the Lmax, ηC,max, ηP,max and ηext,max attained by the resulting OLED were 49000 cd m-2, 9.1 cd A-1, 7.1 lm W-1 and 3.0%, respectively. 177 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

178


Table 1. Electroluminescent performances for some representative silole-based AIEgens AIEgen

Device configuration

λEL (nm)

CIE

Von (V)

Lmax (cd m-2)

ηC,max (cd A-1)

ηP,max (lm W-1)

ηext,max (%)

Ref.

2

ITO/MoO3 (10 nm)/NPB (60 nm)/2 (15 nm)/TPBi (30 nm)/LiF (1 nm)/Al (100 nm)

432

0.16, 0.12

3.7

4411

1.39

1.18

1.21

(12)

3


488

0.20, 0.33

3.0

27161

8.04

6.17

3.38

(12)

4


512

0.21, 0.37

3.5

28718

7.40

5.57

2.92

(12)

5

ITO/NPB (60 nm)/5 (20 nm)/TPBi (10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (100 nm)

544

0.39, 0.53

3.1

13405

8.28

7.88

2.42

(13)

5

ITO/5 (80 nm)/TPBi (10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (100 nm)

548

0.40, 0.57

3.1

14038

7.60

6.94

2.26

(13)

6

ITO/NPB (60 nm)/6 (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm)

552

0.41,0.56

4.6

28240

4.50

1.91

1.44

(14)

7


548

0.39,0.57

4.6

17280

4.26

2.23

1.35

(14)

8


552

0.40,0.53

4.8

2790

6.9

4.4

2.2

(15)

9


556

0.42,0.51

5.6

1900

8.1

4.6

2.9

(15)

10


544

0.37, 0.57

3.2

31900

16.0

13.5

4.8

(16)

Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

179


AIEgen


λEL (nm)

CIE

Von (V)

Lmax (cd m-2)

ηC,max (cd A-1)

ηP,max (lm W-1)

ηext,max (%)

Ref.

10

ITO/MoO3 (5 nm)/NPB (60 nm)/10 (20 nm)/TPBi (60 nm) /LiF (1 nm)/Al (100 nm)

544

0.36, 0.57

3.3

37800

18.3

15.7

5.5

(16)

11


568

0.45,0.52

3.3

21100

5.6

4.6

2.0

(15)

12


582

0.48,0.50

4.4

7660

3.9

2.8

1.5

(15)

13


546

0.36,0.53

3.5

49000

9.1

7.1

3.0

(15)

14


544

0.35,0.51

4.6

5170

2.9

1.5

0.9

(15)

Abbreviations: λEL = electroluminescence maximum; Von = turn-on voltage at 1 cd m−2 ; Lmax = maximum luminance; ηC,max = maximum current efficiency; ηP,max = maximum power efficiency; ηext,max = maximum external quantum efficiency; CIE = Commission Internationale de I’Eclairage coordinates; NPB = N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine; TPBi = 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene; Alq3 = Tris-(8-hydroxyquinoline)aluminum. NPB functions as hole-transporting layer (HTL), TPBi serves as electron-transporting layer (ETL) and hole-blocking layer (HBL), Alq3 functions as ETL and MoO3 serves as hole-injection layer (HIL).



3. TPE-Based AIEgens Amongst various AIEgens, TPE (15) is another star molecule, and possesses a very simple molecular structure but exhibits outstanding AIE characteristics with a ΦF value of 49% in solid state. When the naked TPE was used as light emitter to fabricate OLED, the generated device radiated a deep blue EL emission of 445 nm but showed inferior performance with the Lmax, ηC,max and ηext,max of only 1800 cd m-2, 0.45 cd A-1 and 0.4%, respectively (17). In the past decade, through the facile combination of TPE units and other functional groups, plenty of TPE derivatives have been created and their PL and EL properties have been improved significantly compared to the TPE parent. The OLEDs made from these TPE-based AIEgens emitted efficiently and displayed colorful emissions covering the whole range of visible lights. Herein, we will categorize some representative TPE-based AIEgens by EL emissions and give their EL performance data. The development of stable and efficient luminescent materials emitting three primary colors (red, green and blue) is crucial for OLEDs to be applied in full-color displays and solid-state lighting sources. The performance of blue OLEDs is often inferior to that of red and green ones due to the inherent wide band gap of blue emitters. Therefore, it is very challenging to hunt stable and efficient blue or deep blue emitting materials and devices for the realization of commercial applications. Recently, a great deal of effort has been invested to search highly efficient pure organic blue fluorescent materials and OLEDs owing to the inferior stability and short longevity of the electrophosphorescent devices (18). Since the conventional fluorophores would undergo ACQ effect in solid films, the blue fluorescent AIEgens could be good candidates for the construction of efficient blue OLEDs. As previously discussed, TPE showed a deep blue EL emission but a bad EL property. Frequently, spirofluorene and carbazole groups are used to develop efficient blue emitters. Thus Li and co-workers directly attached spirofluorene or carbazole moieties to TPE unit through sharing a benzene ring or Carbon–Nitrogen bonds, and the generated AIEgens (16 and 17) exhibited a good balance between blue light emission and improved EL properties (19). The performances of OLEDs based on 16 and 17 are listed in Table 2. Besides spirofluorene and carbazole groups, phenanthro[9,10-d]imidazole (PI) is another valuable building block for the fabrication of efficient blue fluorophores. In addition, triphenylethene is also a useful AIE unit, which has a shorter conjugation length and radiates a bluer solid-state emission relatively to TPE. The merging of triphenylethene units and a phenanthro[9,10-d]imidazole (PI) group by molecular engineering produced an efficient deep blue AIEgen 18 with a high ΦF value of 94% in solid film state (20). Compound 18 showed the best EL property among the deep blue AIEgens (Table 2). Its non-doped OLED device with a configuration of ITO/NPB (40 nm)/18 (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm) emitted a bright deep blue light with a peak at 450 nm and CIE coordinates of (0.15, 0.12) as well as a Lmax of 16400 cd m-2. The ηC,max, ηP,max, and ηext,max of this device were as high as 4.9 cd A-1, 4.4 lm W-1 and 4.0%, respectively. In addition, this OLED presented good stability, as demonstrated by the high efficiencies of 3.7 cd A-1, 3.1 lm W-1 and 3.0% at the luminance of 1000 cd m-2. As mentioned above, ACQ-to-AIE transformation is a feasible proposal to obtain efficient emitters for the application in OLEDs. 180 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Another effective approach of ACQ-to-AIE transformation is the decoration of ACQphores with AIE archetypes. The fluorophores 19-22 shown in Figure 2 are nice examples for this approach. Attaching two TPE units to anthracene, a blue ACQphore, yielded a new blue AIEgen 19 (21). The optimized nondoped OLED based on 19 [ITO/HATCN (150 nm)/NPB (20 nm)/19 (10 nm)/Bepp2 (40 nm)/LiF (1 nm)/Al (200 nm)] was turned on at a low voltage of 2.8 V and displayed an intense deep blue emission with CIE coordinates of (0.17, 0.14) and a high Lmax of 17721 cd m-2. The device exhibited good EL performance with an appreciable ηP,max of 4.3 lm W-1 and a low efficiency roll-off (22). By decorating the anthracene ring of 19 with a tert-butyl group, a new luminogen 20 was developed by Shu et al. (23) For compound 20, the ΦF value of solid film (89%) was much higher than that of its dilute solution (6%), which indicated the typical AEE characteristics. The non-doped OLED employing 20 as light-emitting layer emitted a deep blue EL of 456 nm (CIE 0.14, 0.12) and performed excellently with high ηC,max and ηext,max of 5.3 cd A-1 and 5.3%, respectively, reaching the theoretical limitation of fluorescent OLEDs. Pyrene is one other traditional blue fluorophore, whose emission in solid film is always quenched severely due to ACQ effect. Attaching TPE unit(s) to the periphery of pyrene afforded two AIE/AEE-active fluorophores (21 (24) and 22 (25)) which showed strong emissions in the solid state with extremely high ΦF values up to unity. The OLEDs constructed from 21 or 22 both radiated sky blue emissions and exhibited pretty good performance. The EL efficiencies of the device based on 21 were recorded to be 7.3 cd A-1, 5.6 lm W-1 and 3.0%. Compound 22 presented better EL property than 21. And its optimized OLED with a configuration of ITO/NPB (60 nm)/22 (26 nm)/TPBi (20 nm)/LiF (1 nm)/Al (100 nm) displayed fairly high Lmax, ηC,max, ηP,max, and ηext,max of 36300 cd m-2, 12.3 cd A-1, 7.0 lm W-1, and 4.95%, respectively. Connecting two TPE units by simple linkage modes generated AIEgens 23 (26) and 24 (27) (Figure 2), which showed efficient PLs with ΦF values as high as 100% in the solid state. As shown in Table 2, when 23 or 24 were utilized as emitting layers to fabricate multilayer OLEDs, the resulting EL devices displayed sky-blue ELs at 488 nm and comparable EL performance. Efficient green emitters can be easily achieved through rational fusing of TPE unit and pyrene groups at the molecular level. Luminogens 25 (28) and 26 (24) shown in Figure 3 were the good examples for this strategy. 26 was an AIE-active molecule possessing strong emission with a high ΦF of 100% in solid phase and thermally very stable. Its multilayer OLED radiated bright green light at 516 nm with a Lmax of 25500 cd m-2, showing good EL performance with a ηext,maxof 2.1%. The ΦF value of 25 in dilution solution (9.8%) was much higher than that of TPE (0.24%), which derived from the relatively stiffer molecular structure due to higher steric congestion. Its ΦF value was enhanced to unity in the amorphous film, indicating that 25 was AEE-active. The EL device based on 25 with a configuration of ITO/NPB (60 nm)/25 (20 nm)/TPBi (10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (100 nm) showed the EL peak at 516 nm and superior EL performance with Lmax, ηC,max and ηext,max of 49830 cd m-2, 10.2 cd A-1 and 3.3%, respectively. Linking two TPEs to the benzo-2,1,3-thiadiazole core created a yellowish green emitter 27 with excellent thermal and morphological stability as well as high solid-state ΦF value (89%) (29). The non-doped OLED fabricated by adopting 27 as emitter exhibited 181 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


a yellowish green emission peaking at 540 nm, a Lmax of 13540 cd m-2 and a ηC,max of 5.2 cd A-1.

Figure 2. Chemical structures of representative TPE-based blue AIEgens with good EL performances. Their EL maxima are given in parentheses.

Figure 3. Chemical structures of representative TPE-based green and yellow AIEgens with good EL performances. Their EL maxima are given in parentheses. In the field of OLEDs, it is equally important to develop efficient red emitters and devices for the full-color displays and lighting sources. However, many conventional red fluorophores containing planar polycyclic aromatic hydrocarbon (PAHs) units generally undergo strong ACQ problem in the solid state, whose film-state emission efficiencies are always unsatisfactory (30). Taking advantage of AIE effect is emerging as a promising strategy to address this issue. On the basis of the abundant experience on exploitation of efficient solid emitters with blue, green and yellow colors, Tang’s group had created a series of efficient solid red emitting materials, whose molecular structures are shown in Figure 4. The benzo-2,1,3-thiadiazole moiety, a heterocyclic ring and a famous strong electron-withdrawing group, is widely used to tune effectively the emission colors of the materials and thus develop red emitters. In addition, the introduction of thiophene ring can prompt the intramolecular charge transfer as well as elongate molecular conjugation, which are helpful for red-shifting the emissions of luminogens in many cases (29). Making use of AIE-active TPE 182 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


units, benzo-2,1,3-thiadiazole and thiophene rings as building blocks, several efficient solid red fluorophores (28-29) were prepared (29). 28 and 29 emitted orange-red and red emissions and presented relatively high ΦF values up to 55% in the solid state. In Table 2, the performance data of OLEDs based on them are listed in detail. The device of 28 radiated strong orange-red EL at 592 nm with a Lmax of 8330 cd m-2, a ηC,max of 6.4 cd A-1 and a high ηext,max of 3.1%. The device of 29 emitted at red region (668 nm) but the device performance was poor. When one more TPE unit was added into the conjugated backbone of 29 molecule to suppress the intermolecular interactions, a new luminogen 30 was generated (31). 30 presented better PL, EL properties and thermal stability than 29. The non-doped OLED of 30 [ITO/NPB (60 nm)/30 (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm)] displayed a red light at 650 nm (CIE 0.67, 0.32) and higher efficiencies (2.4 cd A-1 and 3.7%) than those measured from 29-based device. Because of good hole-transporting property and electron-donating capability, arylamines units are usually utilized to fabricate excellent solid red emitting materials (30). Thus another set of red AIEgens 31 and 32 were recently created (30). The film-state ΦF values of 31 and 32 were 48.8% and 63.0%, respectively. When they were adopted to build non-doped OLEDs, the resulting devices both had a red EL at 604 nm and showed outstanding EL performance. The device based on 32 displayed superior performance with excellent Lmax, ηC,max, ηP,max and ηext,max of 16396 cd m-2, 7.5 cd A-1, 7.3 lm W-1 and 3.9%, respectively, compared to those of the device based on 31 (15584 cd m-2, 6.4 cd A-1, 6.3 lm W-1 and 3.5%). At 1000 cd m-2, high efficiency values of up to 4.6 cd A-1, 2.6 lm W-1 and 2.4% were also achieved based on these two OLEDs. What’s more, their potential hole-transporting properties enabled the simplified OLEDs utilizing them as both EMLs and HTLs to perform well.

Figure 4. Chemical structures of representative TPE-based red AIEgens with good EL performances. Their EL maxima are given in parentheses. 183 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

184


Table 2. Electroluminescent performances for some representative TPE-based AIEgens AIEgen


λEL (nm)

CIE

Von (V)

Lmax (cd m-2)

ηC,max (cd A-1)

ηP,max (lm W-1)

ηext,max (%)

Ref.

16


466

0.18, 0.24

2.6

8196

3.33

2.10

-

(19)

17

ITO/NPB (40 nm)/17 (10 nm)/ TPBi (10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (100 nm)

462

0.17, 0.21

3.3

6179

2.80

2.51

-

(19)

18


463

0.15, 0.15

3.2

20300

5.9

5.3

4.4

(20)

18


450

0.15, 0.12

3.2

16400

4.9

4.4

4.0

(20)

19

ITO/HATCN (150 nm)/NPB (20 nm)/19 (10 nm)/Bepp2 (40 nm)/LiF (1 nm)/Al (200 nm)

449

0.17, 0.14

2.8

17721

-

4.3

-

(22)

20

ITO/PEDOT/TFTPA (30 nm)/20 (40 nm)/TPBi (40 nm)/Mg:Ag (100 nm)/Ag (100 nm)

456

0.14, 0.12

4.9

4165

5.3

2.8

5.3

(23)

21


484

-

3.6

13400

7.3

5.6

3.0

(24)

22

ITO/NPB (60 nm)/22(40 nm)/TPBI (20 nm)/LiF (1 nm)/Al (100 nm)

492

-

4.7

18000

10.6

5.0

4.04

(25)

22


488

-

3.6

36300

12.3

7.0

4.95

(25)

23


488

-

4.0

11180

7.26

-

3.17

(26)



185

AIEgen


λEL (nm)

CIE

Von (V)

Lmax (cd m-2)

ηC,max (cd A-1)

ηP,max (lm W-1)

ηext,max (%)

Ref.

24

ITO/NPB (60 nm)/24 (20 nm)/TPBi (40 nm) /LiF (1 nm)/Al (100 nm)

488

-

4.2

10800

5.8

3.5

2.7

(27)

25

ITO/NPB (60 nm)/25(20 nm)/TPBi (10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (100 nm)

516

-

3.2

49830

10.2

9.2

3.3

(28)

26


516

-

4.6

25500

6.0

2.7

2.1

(24)

27


540

-

3.9

13540

5.2

3.0

1.5

(29)

28


592

-

5.4

8330

6.4

2.9

3.1

(29)

29


668

-

4.4

1640

0.4

0.5

1.0

(29)

30


650

0.67, 0.32

4.2

3750

2.4

-

3.7

(31)

Continued on next page.


186


Table 2. (Continued). Electroluminescent performances for some representative TPE-based AIEgens AIEgen


λEL (nm)

CIE

Von (V)

Lmax (cd m-2)

ηC,max (cd A-1)

ηP,max (lm W-1)

ηext,max (%)

Ref.

31


604

-

3.2

15584

6.4

6.3

3.5

(30)

32


604

-

3.2

16396

7.5

7.3

3.9

(30)

Abbreviations: λEL = electroluminescence maximum; Von = turn-on voltage at 1 cd m−2 ; Lmax = maximum luminance; ηC,max = maximum current efficiency; ηP,max = maximum power efficiency; ηext,max = maximum external quantum efficiency; CIE = Commission Internationale de I’Eclairage coordinates; NPB = N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine; TPBi = 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene; Alq3 = Tris-(8-hydroxyquinoline)aluminum; TFTPA = tris[4-(9-phenylfluoren-9-yl)phenyl]amine; PEDOT = poly(styrenesulfonate)-doped poly(3,4-ethylenedioxythiophene); HATCN = 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile; Bepp2 = bis[2-(2-hydroxyphenyl)-pyridine] beryllium. NPB and TFTPA functions as hole-transporting layer (HTL), TPBi and Bepp2 serve as electron-transporting layer (ETL) and hole-blocking layer (HBL), Alq3 functions as ETL, HATCN, PEDOT and MoO3 serve as hole-injection layer (HIL).



4. Multifunctional AIEgens Besides high solid-state emission efficiencies, AIEgens can also be endowed with good carrier-transporting abilities by introducing hole- and/or electron-dominated units into their structures. The materials with multiple functionalities (p-type or n-type AIE emitters) can simultaneously act as EMLs and HTLs or electron-transporting layers (ETLs) in OLED devices, which are very appealing for the development of OLEDs as they can simplify the device configurations, shorten the fabrication process and cut down the manufacture cost (8). These attractive merits encourage material scientists to design multifunctional materials in a new way. Many AIEgens with excellent hole- or electron-transporting property have been developed and illustrated in Figure 5 as examples. The performance data of the OLEDs based on them are summarized in Table 3. Triphenylamine (TPA) segment is famous for its good hole-transporting capability and has been widely utilized to construct efficient p-type light emitters. As presented in Figure 5, fusing TPA group(s) and TPE unit(s) by different linkage modes generated a series of TPE-TPA adducts (33-40) that had not only high ΦF values but also excellent hole mobility in the film state. By directly attaching TPA unit(s) to TPE core, two new AIEgens (33 and 34) were obtained (32). They showed strong emissions in film phase with outstanding ΦF values up to unity. In addition, the amorphous film of 34 exhibited a reasonably high hole mobility of 5.2×10-4 cm2 V-1 s-1 attained by time-of-flight technique, manifesting that 34 was a good p-type emitter (10). Therefore, a simplified OLED fabricated using 34 simultaneously as EML and HTL [ITO/34 (60 nm)/TPBi (10 nm)/Alq3 (30 nm)/LiF/Al (200 nm)] emitted strong green EL with a Lmax of 33770 cd m-2 and presented appreciable efficiencies with the ηC,max, ηP,max and ηext,max of 13.0 cd A-1, 11.0 lm W-1 and 4.4%, respectively. These performance data were better than those measured from the device of 34 with NPB as the additional HTL. The simplified OLED based on 33 also displayed comparable performance than the one with “standard” configuration. Since NPB is a commercialized and useful hole-transporting material in the field of optoelectronic device, facilely inserting a TPE unit into the conjugated backbone of NPB produced a stable and efficient AIEgen (35) with a high film-state ΦF of 98% and good hole-transporting property (33). The non-doped multilayer OLED fabricated utilizing 35 as emitting material [ITO/NPB (40 nm)/35 (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm)] radiated green EL at 512 nm, and showed a high ηC,max of 11.9 cd A-1 and a pretty good ηext,max of 4.0%. Even better EL efficiencies (ηC,max = 13.1 cd A-1 and ηext,max = 4.2%) were obtained in its bilayer device without NPB [ITO/35 (60 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm)]. Decorating TPA and DTPA cores with three and four TPE units generated two compounds 36 and 37, respectively, whose optical and electrical properties were systematically investigated (34). 36 and 37 emitted nearly no light in their dilute THF solutions with the ΦF values of merely 0.42% and 0.55%, while in condensed phase the ΦF values were improved significantly to 91.6% and 100%, respectively, indicative of excellent AIE characteristics. As shown in Table 3, the OLED device using 36 as both EML and HTL presented higher EL efficiencies than its “standard” 187 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


device. 37 also showed good multifunctional properties as both efficient emitter and hole-transporter in the OLED. By modifying the TPA core of 36 with three methyl groups, a blue-shifted AIEgen (38) with a high ΦF value of 64% in the aggregate state and a hole-transporting potential was developed (35). The simplified OLED of 38 without NPB [ITO/MoO3 (10 nm)/38 (75 nm)/TPBi (35 nm)/LiF (1 nm)/Al (100 nm)] emitted sky blue EL at 469 nm (CIE 0.18, 0.25) and showed good performance with the Lmax, ηC,max, ηP,max and ηext,max up to 15089 cd m-2, 6.51 cd A-1, 6.88 lm W-1 and 3.39%, respectively, which were comparable to those (13639 cd m-2, 8.03 cd A-1, 7.04 lm W-1 and 3.99%) received from its multilayer device using NPB as HTL. By incorporating a hole-dominate N,N,N′,N′-tetraphenyl-p-phenylenediamine (PDA) or TPA core and TPE moieties, two starburst luminogens 39 and 40 were created by Adachi and co-workers (36). Thanks to the AIE effect, the ΦF values of solid films for 39 and 40 were as high as 56% and 73%, respectively. In addition, by means of the space-charge-limited current (SCLC) technique, the hole mobilities of their amorphous thin films were measured to be more than 10-2 cm2 V-1 s-1, being much higher than that of NPB, due to the presence of PDA or TPA segments and the spontaneous molecular orientations of the molecules. Hence, the bilayer EL devices of 39 and 40 [ITO/ 39 or 40 (65 nm)/BPhen (35 nm)/LiF (0.8 nm)/Al (70 nm)] performed extremely well as indicated by the data given in Table 3. When NPB was used as additional HTL to balance carriers, the resulting triple-layer devices exhibited remarkably high ηext,max values of up to 5.9%, which was contributed by the efficient solid-state luminescence and the enhancement of the hole mobility and the light out-coupling efficiency due to the horizontal orientation characteristics. Among the fabricated EL devices, the triple-layer device based on 40 showed the best performance with a current efficiency of 15.9 cd A-1 and a power efficiency of 16.2 lm W-1 at the luminance of 100 cd m-2. Surprisingly, its external quantum efficiency remained nearly 5% when the luminance was increased to ca. 10000 cd m-2. Compared with efficient p-type light emitters, the n-type organic semiconductors with fast electron mobility and excellent solid-state emission are rare and in urgent need. As mentioned earlier, silole derivatives have electron-transporting potential because of the unique electronic structures. Numerous good works have demonstrated that siloles are excellent solid emitters owing to their AIE attributes. So, decorating silole with functional groups by molecular engineering would create many efficient n-type solid emitters with AIE characteristics. On the basis of these considerations, we designed and prepared two silole-based AIEgens 41 and 42 that were comprised of a silole ring and dimesitylboryl groups (37). Thanks to the presence of the vacant pπ orbital on the boron center, the dimesitylboryl group is inherently electron-deficient and beneficial for reducing the LUMO energy level and thus elevating the electron-transporting capacity of the molecule. 41 and 42 possessed low-lying LUMO energy levels with the values of -3.06 and -3.10 eV, respectively, indicative of their good electron-transporting potentials. Moreover, the solid films of 41 and 42 presented high ΦF values about 60%. Based on these excellent integrated properties, their double-layer OLEDs employing 41 or 42 as both emitter and electron-transporter [ITO/NPB (60 nm)/41 or 42 (60 nm)/LiF (1 nm)/Al (100 188 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


nm)] were constructed, which presented outstanding performances (Table 3) with high ηC,max, ηP,max and ηext,max values up to 13.9 cd A-1, 11.6 lm W-1 and 4.35%, respectively. These EL performance data were much higher than those measured from their multilayered devices with an additional TPBi as ETL. Significantly, the simplified device based on 42 showed a small efficiency roll-off when luminance increased, and a high current efficiency of 7.0 cd A-1 was maintained at a luminance of 1000 cd m-2. These good results demonstrated that 41 and 42 were excellent n-type solid-state emitters. When the electron-withdrawing dimesitylboryl or diphenylphosphoryl functional groups were introduced into silole 10, two new silole derivatives 43 and 44 were prepared (38, 39). They emitted intense fluorescence in solid state with high ΦF values up to 88% and exhibited good thermal stabilities. They were also potential n-type solid emitters as indicated by their EL performance data listed in Table 3. Attaching a dimesitylboryl group to TPE unit could produce a new AIEgen molecule 45 (40). The presence of dimesitylboryl portion enabled 45 to act as both emitter and electron-transporter in OLED devices. As shown in Table 3, the triplelayer OLED of 45 radiated green EL at 496 nm and displayed good EL efficiencies (ηC,max = 5.78 cd A-1 and ηext,max = 2.3%). Superior EL performance was achieved from its simplified double-layer OLED [ITO/NPB (60 nm)/45 (60 nm)/LiF (1 nm)/Al (100 nm)] with the higher ηC,max and ηext,max of 7.13 cd A-1 and 2.7%, respectively. 2,5-Diaryl-1,3,4-oxadiazole (Oxa) is an electron-deficient aromatic group and another widely used moiety to construct electron-transporting materials. 46 containing Oxa group and TPE unit showed strong solid-state emission and high thermal stability (41). The 46-based OLED with a configuration of ITO/NPB (60 nm)/46 (60 nm)/LiF (1 nm)/Al (200 nm) presented better EL performance than that of the standard OLED device, which revealed the fine electron-transporting ability of 46 (Table 3). Apart from above p-type and n-type AIE emitters, bipolar AIE systems including both electron donors (D), acceptors (A) and AIE elements have been designed wisely and achieved. These bipolar AIE luminescent materials could not only function as excellent emitters but also balance carriers in OLEDs, which may improve the EL efficiencies and help to simplify the device configurations. Hence, by inserting TPE or two more phenyls attached TPE (TPEBPh) into a D-A framework containing diphenylamino as the electron donor and dimesitylboron as the electron acceptor, two nice bipolar AIE molecules (47 and 48) were developed (42). These two bipolar AIEgens showed highly satisfactory PL and EL properties (Table 3). For instance, 48 possessed a weak D-A interaction and fluoresced intensely in film state with an extremely high ΦF of 94%. The trilayer OLED fabricated from 48 [ITO/NPB (60 nm)/48 (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm)] was turned on at a low basis of 3.2 V and emitted bright green EL with a high Lmax of 49993 cd m-2. Its appreciable ηC,max, ηP,max, and ηext,max were measured to be 15.7 cd A-1, 12.9 lm W-1 and 5.12%, respectively. Its bilayer EL device without the NPB layer showed better performance (ηext,max up to 5.35%), demonstrating that 48 was an excellent p-type emitter. It is noteworthy that the OLEDs of 48 emitted efficiently and exhibited the small roll-off in EL efficiencies. When the luminance increased to 1000 cd m-2, the ηexts of its trilayer and bilayer devices were as high as 4.75% and 4.45% respectively. On the basis 189 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


of these good results, the authors thought that the hole mobility of 48 film should be a crucial factor for its outstanding EL property. So, the hole-transporting abilities of the thin films of 48 and NPB were evaluated by SCLC technique. As a result, the hole mobility values of 48 and NPB thin films were almost at the same level. Compounds 49-52 were new bipolar AIEgens based on TPE building block attached with N-ethyl-carbazole groups and dimesitylboron or (dimesitylboranyl)phenyl units by different linkage modes (43). The devices with the configuration of ITO/HATCN (20 nm)/NPB (40 nm)/49-52 (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (80 nm) radiated strong blue or green emissions with the Lmax up to 65150 cd m-2 and displayed good EL performances as shown Table 3. Another series of donor-AIE-accepter system (53-55) was reported in 2015, utilizing TPE as AIE-active core, TPA groups as electron donor and (dimesitylboryl)phenyl blocks as electron accepter (44). As illustrated in Figure 5, these novel bipolar TPE deviratives showed star-shaped conformation, which is good for the materials to be fabricate into good films by spin-coating technique. Moreover, they had very high film-state ΦF values up to 95%. So the solution-processed non-doped OLEDs employing these star-shaped AIEgens as emitters were constructed. The OLED based on 53 performed best and showed a Lmax of 11665 cd m-2 and a high ηC,max up to 8.3 cd A-1. Driven at 1000 cd m-2, this device presented slight efficiency roll-off and its current efficiency could also maintain at the value of 6.2 cd A-1.

190 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 5. Examples of representative multifunctional AIEgens. Their EL maxima are given in parentheses. 191 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

192


Table 3. Electroluminescent performances for some representative multifunctional AIEgens AIEgen


λEL (nm)

CIE

Von (V)

Lmax (cd m-2)

ηC,max (cd A-1)

ηP,max (lm W-1)

ηext,max (%)

Ref.

33


492

-

3.6

15480

8.6

5.3

3.4

(32)

33

ITO/33 (60 nm)/TPBi (10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (200 nm)

492

-

4.2

26090

8.3

4.9

3.3

(32)

34

ITO/NPB (40 nm)/34 (20 nm)/ TPBi (10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (200 nm)

514

-

3.4

32230

12.3

10.1

4.0

(32)

34

ITO/34 (60 nm)/TPBi (10 nm)/Alq3 (30 nm) /LiF (1 nm)/Al (200 nm)

512

-

3.2

33770

13.0

11.0

4.4

(32)

35


512

-

3.7

11981

11.9

8.9

4.0

(33)

35

ITO/35 (60 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm)

516

-

3.9

12607

13.1

7.8

4.2

(33)

36

ITO/NPB (60 nm)36 (20 nm)/TPBi (10 nm) /Alq3 (30 nm)/LiF (1 nm)/Al (100 nm)

493

-

5.4

1662

3.1

1.1

1.2

(34)

36

ITO/36 (80 nm)/TPBi (10 nm) /Alq3 (30 nm)/LiF (1 nm)/Al (100 nm)

499

-

4.5

6935

4.0

1.9

1.5

(34)

37

ITO/37 (30 nm)/TPBi (10 nm) /Alq3 (30 nm)/LiF (1 nm)/Al (100 nm)

488

-

4.1

10723

8.0

5.2

3.7

(34)

38

ITO/MoO3 (10 nm)/NPB (60 nm)/38 (15 nm)/TPBi (35 nm) /LiF (1 nm)/Al (100 nm)

480

0.17, 0.28

3.1

13639

8.03

7.04

3.99

(35)



193

AIEgen


λEL (nm)

CIE

Von (V)

Lmax (cd m-2)

ηC,max (cd A-1)

ηP,max (lm W-1)

ηext,max (%)

Ref.

38

ITO/MoO3 (10 nm)/38 (75 nm)/TPBi(35 nm)/LiF (1 nm)/Al (100 nm)

469

0.18, 0.25

2.9

15089

6.51

6.88

3.39

(35)

39

ITO/NPB (40 nm)/39 (25 nm)/ Bphen (35 nm)/LiF (0.8 nm)/Al (70 nm)

515

-

2.6

58300

-

-

4.5

(36)

39

ITO/39 (65 nm)/Bphen (35 nm) /LiF (0.8 nm)/Al (70 nm)

510

-

2.6

48300

-

-

3.6

(36)

40

ITO/NPB (40 nm)/40 (25 nm)/ Bphen (35 nm)/LiF (0.8 nm)/Al (70 nm)

523

-

2.4

53600

-

-

5.9

(36)

40

ITO/40 (65 nm)/Bphen (35 nm)/LiF (0.8 nm)/Al (70 nm)

523

-

2.4

54200

-

-

4.5

(36)

41

ITO/NPB (60 nm)/41 (20 nm)/ TPBi (40 nm)/LiF (1 nm)/Al (100 nm)

548

0.39, 0.55

5.4

15200

8.4

4.1

2.62

(37)

41

ITO/NPB (60 nm)/41 (60 nm)/LiF (1 nm)/Al (100 nm)

524

0.33, 0.56

4.3

12200

13.9

11.6

4.35

(37)

42

ITO/NPB (60 nm)/42 (20 nm)/ TPBi (40 nm)/LiF (1 nm)/Al (100 nm)

552

0.40, 0.54

7.5

9610

6.6

2.4

2.13

(37)

42


520

0.30, 0.56

3.9

13900

13.0

10.5

4.12

(37)

43


554

0.41, 0.56

3.8

48348

12.3

8.8

4.1

(38)

43


554

0.41, 0.56

4.6

34080

10.1

5.9

3.3

(38)

Continued on next page.


194


Table 3. (Continued). Electroluminescent performances for some representative multifunctional AIEgens AIEgen


λEL (nm)

CIE

Von (V)

Lmax (cd m-2)

ηC,max (cd A-1)

ηP,max (lm W-1)

ηext,max (%)

Ref.

44


542

0.38, 0.56

3.1

19019

9.2

9.0

3.1

(39)

44


544

0.38, 0.56

3.1

16656

8.7

8.6

2.9

(39)

45


496

-

6.3

5581

5.78

3.4

2.3

(40)

45


496

-

6.3

5170

7.13

3.2

2.7

(40)

46


466

-

4.4

2800

1.5

1.1

0.7

(41)

46


476

-

3.2

7000

2.4

2.2

1.0

(41)

47


544

0.35, 0.55

3.3

42924

10.5

9.40

3.24

(42)

47

ITO/47 (80 nm)/TPBi (40 nm) /LiF (1 nm)/Al (100 nm)

544

0.37, 0.54

3.3

7942

11.9

9.90

3.73

(42)

48


516

0.27, 0.51

3.2

49993

15.7

12.9

5.12

(42)

48

ITO/48 (80 nm)/TPBi (40 nm) /LiF (1 nm)/Al (100 nm)

516

0.25, 0.50

3.2

13678

16.2

14.4

5.35

(42)



195

AIEgen


λEL (nm)

CIE

Von (V)

Lmax (cd m-2)

ηC,max (cd A-1)

ηP,max (lm W-1)

ηext,max (%)

Ref.

49

ITO/HATCN(20 nm)/NPB (40 nm)/49 (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (80 nm)

513

0.23, 0.46

4.2

65150

8.60

5.07

3.28

(43)

50


500

0.20, 0.34

5.2

16410

4.49

2.57

2.16

(43)

51


489

0.19, 0.30

4.9

14980

2.53

0.99

1.26

(43)

52


524

0.25, 0.52

4.8

30210

9.96

5.43

2.73

(43)

53

ITO/PEDOT:PSS(40 nm)/53 (70 nm)/TPBi (30 nm)/Ba (4 nm)/Al (120 nm)

543

0.37, 0.54

3.4

11665

8.3

7.5

2.6

(44)

54


532

0.35, 0.53

3.4

7290

6.3

5.9

2.1

(44)

55


521

0.34, 0.50

8.1

838

1.8

0.6

0.6

(44)

Abbreviations: λEL = electroluminescence maximum; Von = turn-on voltage at 1 cd m−2 ; Lmax = maximum luminance; ηC,max = maximum current efficiency; ηP,max = maximum power efficiency; ηext,max = maximum external quantum efficiency; CIE = Commission Internationale de I’Eclairage coordinates; NPB = N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine; TPBi = 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene; Bphen = 4,7-diphenyl-1,10-phenanthroline; Alq3 = Tris-(8-hydroxyquinoline)aluminum; PEDOT:PSS = poly(3,4-ethylenedioxythiophene)–poly-(styrenesulfonic acid); HATCN = 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile. NPB functions as hole-transporting layer (HTL), TPBi and Bphen serve as electron-transporting layer (ETL) and hole-blocking layer (HBL), Alq3 functions as ETL, HATCN, PEDOT:PSS and MoO3 serves as hole-injection layer (HIL).



5. Conclusion and Outlook Since the first report of AIE phenomenon, numerous fluorescent AIEgens with high ΦF values in the solid state have been created for the construction of stable and efficient non-doped OLEDs. The emission colors of these devices have covered the whole range of visible lights. The efficiencies of some OLEDs have approached or reached the theoretical limit range of fluorescent OLEDs. This chapter has summarized the applications of siloe-based and TPE-based AIEgens for non-doped fluorescent OLEDs with various emission colors and the multifunctional AIEgens that played multiple roles in OLEDs. The successes of AIE effect in improving the performance of common fluorophors (the first-generation luminescent materials) for OLEDs demonstrate the great academic and practical significance of AIE research. However, there is still much room for further improvement of efficiencies of fluorescent OLEDs because 75% of the generated excitons (triplet excitons) remain unemployed. Hence, many current efforts have been devoting to fabricating efficient OLEDs based on pure organic thermally activated delayed fluorescence (TADF) materials (the third-generation luminescent materials) which can make full use of singlet and triplet excitons. The TADF materials, however, also have some disadvantages, such as severe efficiency roll-off in OLEDs, and doping technique is usually required for OLED fabrication based on TADF materials. Integrating of both AIE and TADF effects within a molecule could be a promising strategy to construct more robust luminescent materials for high-performance non-doped OLEDs.

References Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913–915. Grimsdale, A. C.; Chan, K. L.; Martin, R. E.; Jokisz, P. G.; Holmes, A. B. Chem. Rev. 2009, 109, 897–1091. 3. Yang, X.; Zhou, G.; Wong, W.-Y. Chem. Soc. Rev. 2015, 44, 8484–8575. 4. Jou, J.-H.; Kumar, S.; Agrawal, A.; Li, T.-H.; Sahoo, S. J. Mater. Chem. C 2015, 3, 2974–3002. 5. Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Chem. Soc. Rev. 2011, 40, 5361–5388. 6. Luo, J.; Xie, Z; Lam, J. W. Y.; Cheng, L.; Chen, H.; Qiu, C.; Kwok, H. S.; Zhan, X.; Liu, Y.; Zhu, D.; Tang, B. Z. Chem. Commun. 2001, 1740–1741. 7. Mei, J.; Hong, Y.; Lam, J. W. Y.; Qin, A.; Tang, Y.; Tang, B. Z. Adv. Mater. 2014, 26, 5429–5479. 8. Mei, J.; Leung, N. L. C.; Kwok, R. T. K.; Lam, J. W. Y.; Tang, B. Z. Chem. Rev. 2015, 115, 11718–11940. 9. Zhao, Z.; He, B.; Tang, B. Z. Chem. Sci. 2015, 6, 5347–5365. 10. Zhao, Z.; Lam, J. W. Y.; Tang, B. Z. J. Mater. Chem. 2012, 22, 23726–23740. 11. Chen, H. Y.; Lam, W. Y.; Luo, J. D.; Ho, Y. L.; Tang, B. Z.; Zhu, D. B.; Wong, M.; Kwok, H. S. Appl. Phys. Lett. 2002, 81, 574–576. 12. Yang, J.; Sun, N.; Huang, J.; Li, Q.; Peng, Q.; Tang, X.; Dong, Y.; Ma, D.; Li, Z. J. Mater. Chem. C 2015, 3, 2624–2631. 1. 2.



13. Mei, J.; Wang, J.; Sun, J. Z.; Zhao, H.; Yuan, W.; Deng, C.; Chen, S.; Sung, H. H. Y.; Lu, P.; Qin, A.; Kowk, H. S.; Ma, Y.; Williams, I. D.; Tang, B. Z. Chem. Sci. 2012, 3, 549–558. 14. Chen, L.; Nie, H.; Chen, B.; Lin, G.; Luo, W.; Hu, R.; Huang, F.; Qin, A.; Zhao, Z.; Tang, B. Z. J. Photon. Energy 2015, 5, 053598. 15. Chen, B.; Jiang, Y.; He, B.; Zhou, J.; Sung, H. H. Y.; Williams, I. D.; Lu, P.; Kwok, H. S.; Qiu, H.; Zhao, Z.; Tang, B. Z. Chem. Asian J. 2014, 9, 2937–2945. 16. Chen, B.; Jiang, Y.; Chen, L.; Nie, H.; He, B.; Lu, P.; Sung, H. H. Y.; Williams, I. D.; Kwok, H. S.; Qin, A.; Zhao, Z.; Tang, B. Z. Chem. Eur. J. 2014, 20, 1931–1939. 17. Dong, Y.; Lam, J. W. Y.; Qin, A.; Liu, J.; Li, Z.; Tang, B. Z.; Sun, J.; Kwok, H. S. Appl. Phys. Lett. 2007, 91, 011111. 18. Zhu, M.; Yang, C. Chem. Soc. Rev. 2013, 42, 4963–4976. 19. Huang, J.; Yang, X.; Wang, J.; Zhong, C.; Wang, L.; Qin, J.; Li, Z. J. Mater. Chem. 2012, 22, 2478–2484. 20. Qin, W.; Yang, Z.; Jiang, Y.; Jiang, Y.; Lam, J. W. Y.; Liang, G.; Kwok, H. S.; Tang, B. Z. Chem. Mater. 2015, 27, 3892–3901. 21. Kim, S.-K.; Park, Y.-II; Kang, I.-N.; Park, J.-W. J. Mater. Chem. 2007, 17, 4670–4678. 22. Liu, B.; Nie, H.; Zhou, X.; Hu, S.; Luo, D.; Gao, D.; Zou, J.; Xu, M.; Wang, L.; Zhao, Z.; Qin, A.; Peng, J.; Ning, H.; Cao, Y.; Tang, B. Z. Adv. Funct. Mater. 2016, 26, 776–783. 23. Shih, P. -I.; Chuang, C. -Y.; Chien, C. -H.; Diau, E. W. -G; Shu, C. -F. Adv. Funct. Mater. 2007, 17, 3141–3146. 24. Zhao, Z.; Chen, S.; Chan, C. Y. K.; Lam, J. W. Y.; Jim, C. K. W.; Lu, P.; Chang, Z.; Kwok, H. S.; Qiu, H.; Tang, B. Z. Chem. Asian J. 2012, 7, 484–488. 25. Zhao, Z.; Chen, S.; Lam, J. W. Y.; Lu, P.; Zhong, Y.; Wong, K. S.; Kowk, H. S.; Tang, B. Z. Chem. Commun. 2010, 46, 2221–2223. 26. Zhao, Z.; Chen, S.; Shen, X.; Mahtab, F.; Yu, Y.; Lu, P.; Lam, J. W. Y.; Kwok, H. S.; Tang, B. Z. Chem. Commun. 2010, 46, 686–688. 27. Chan, C. Y. K.; Zhao, Z.; Lam, J. W. Y.; Liu, J.; Chen, S.; Lu, P.; Mahtab, F.; Chen, X.; Sung, H. H. Y.; Kwok, H. S.; Ma, Y.; Williams, I. D.; Wong, K. S.; Tang, B. Z. Adv. Funct. Mater 2012, 22, 378–389. 28. Zhao, Z.; Chen, S.; Lam, J. W. Y.; Wang, Z.; Lu, P.; Mahtab, F.; Sung, H. H. Y.; Williams, I. D.; Ma, Y.; Kwok, H. S.; Tang, B. Z. J. Mater. Chem. 2011, 21, 7210–7216. 29. Zhao, Z.; Deng, C.; Chen, S.; Lam, J. W. Y.; Qin, W.; Lu, P.; Wang, Z.; Kwok, H. S.; Ma, Y.; Qiu, H.; Tang, B. Z. Chem. Commun. 2011, 47, 8847–8849. 30. Qin, W.; Lam, J. W. Y.; Yang, Z.; Chen, S.; Liang, G.; Zhao, W.; Kwok, H. S.; Tang, B. Z. Chem. Commun. 2015, 51, 7321–7324. 31. Zhao, Z.; Geng, J.; Chang, Z.; Chen, S.; Deng, C.; Jiang, T.; Qin, W.; Lam, J. W. Y.; Kwok, H. S.; Qiu, H.; Liu, B.; Tang, B. Z. J. Mater. Chem. 2012, 22, 11018–11021. 197 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


32. Liu, Y.; Chen, S.; Lam, J. W. Y.; Lu, P.; Kwok, R. T. K.; Mahtab, F.; Kwok, H. S.; Tang, B. Z. Chem. Mater. 2011, 23, 2536–2544. 33. Qin, W.; Liu, J.; Chen, S.; Lam, J. W. Y.; Arseneault, M.; Yang, Z.; Zhao, Q.; Kwok, H. S.; Tang, B. Z. J. Mater. Chem. C 2014, 2, 3756–3761. 34. Yuan, W. Z.; Lu, P.; Chen, S.; Lam, J. W. Y.; Wang, Z.; Liu, Y.; Kwok, H. S.; Ma, Y.; Tang, B. Z. Adv. Mater. 2010, 22, 2159–2163. 35. Huang, J.; Sun, N.; Yang, J.; Tang, R.; Li, Q.; Ma, D.; Li, Z. Adv. Funct. Mater. 2014, 24, 7645–7654. 36. Kim, J. Y.; Yasuda, T.; Yang, Y. S.; Adachi, C. Adv. Mater. 2013, 25, 2666–2671. 37. Chen, L.; Jiang, Y.; Nie, H.; Lu, P.; Sung, H. H. Y.; Williams, I. D.; Kwok, H. S.; Huang, F.; Qin, A.; Zhao, Z.; Tang, B. Z. Adv. Funct. Mater. 2014, 24, 3621–3630. 38. Quan, C.; Nie, H.; Hu, R.; Qin, A.; Zhao, Z.; Tang, B. Z. Chin. J. Chem. 2015, 33, 842–846. 39. Quan, C.; Nie, H.; Zhao, Z.; Tang, B. Z. Organic Light Emitting Materials and Devices XIX, Proc. SPIE 9566, 95660C, September 22, 2015. 40. Yuan, W. Z.; Chen, S.; Lam, J. W. Y.; Deng, C.; Lu, P.; Sung, H. H. Y.; Williams, I. D.; Kwok, H. S.; Zhang, Y.; Tang, B. Z. Chem. Commun. 2011, 47, 11216–11218. 41. Liu, Y.; Chen, S.; Lam, J. W. Y.; Mahtab, F.; Kwok, H. S.; Tang, B. Z. J. Mater. Chem. 2012, 22, 5184–5189. 42. Chen, L.; Jiang, Y.; Nie, H.; Hu, R.; Kwok, H. S.; Huang, F.; Qin, A.; Zhao, Z.; Tang, B. Z. ACS Appl. Mater. Interfaces 2014, 6, 17215–17225. 43. Shi, H.; Xin, D.; Gu, D.; Zhang, P.; Peng, H.; Chen, S.; Lin, G.; Zhao, Z.; Tang, B. Z. J. Mater. Chem. C 2016, 4, 1228–1237. 44. Chen, L.; Zhang, C.; Lin, G.; Nie, H.; Luo, W.; Zhuang, Z.; Ding, S.; Hu, R.; Su, S.-J.; Huang, F.; Qin, A.; Zhao, Z.; Tang, B. Z. J. Mater. Chem. C 2016, 4, 2775–2783.


Aggregation-Induced Emission Luminogens (AIEgens) for Non-Doped

Recommend Documents