Aggregation-induced Emission Luminogens for Non-doped Organic Light-emitting Diodes

Introduction

Organic Light-emitting Diodes (OLEDs) are solid-state devices that transform electrical energy into light. OLEDs are considered the next generation technology for high-resolution flexible displays and solid state lighting, attracting intense scientific and industrial interest.¹ As the fluorescent OLEDs, which are the first generation of OLEDs and employ conventional fluorophores as emitters, have been significantly studied due to the excellent stability and relatively long operating lifetime of the resulting devices.²

The external quantum efficiency of an OLED (η_ext) can be determined by calculating the product of the internal quantum efficiency (η_int) and the light out-coupling factor (η_out):

η_ext = η_int × η_out

Typically, η_out is approximately 20–30% for most OLED devices without an optical out-coupling layer. The η_int value can be obtained from the equation, η_int = γ × β × Φ_PL, where γ is the carrier balance ratio of holes and electrons, β is the fraction of excitons that are capable of radiative decay, and Φ_PL is the intrinsic photoluminescence (PL) quantum yield of the emitting layer.³ According to spin statistics, 25% of the excitons generated by electronic excitation are singlet excitons that decay to ground state via fluorescence. Therefore, the β for fluorescent OLEDs is limited to 25% and the theoretical maximum η_ext of the device is about 5–7.5% even if the γ and Φ_PL of a fluorophore (Φ_F) are 100%.³ Thus development of efficient and stable fluorescent materials with high Φ_F values is of great importance. Many conventional fluorophores emit strongly as isolated molecules in solutions; however, these emissions experience partial or complete quenching in the aggregated state. This effect is known as aggregationcaused quenching (ACQ).⁴ The ACQ effect is believed to be controlled by the formation of delocalized excitons via strong intermolecular π−π stacking interactions, resulting in red-shifted emissions and low Φ_PL values.⁴ Because of the negative impact on Φ_PL, the ACQ effect has obstructed the application of conventional fluorophores in OLEDs for some time.

Aggregation-induced emission (AIE) is a unique chromophore aggregation-associated phenomenon that is essentially the opposite of ACQ.⁵ Luminogens with AIE characteristics (AIEgens) are weakly fluorescent or nonfluorescent when molecularly dispersed in dilute solutions, but they fluoresce strongly upon the formation of aggregates. Figure 1 presents the AIE phenomenon based on a typical AIEgen, 1,1,2,3,4,5-hexaphenylsilole (HPS, Prod. No. 797294),⁶ in which the emission of HPS activates upon aggregation.

During the past decade, the AIE phenomenon has been thoroughly characterized through systematic experimental measurements and theoretical calculations. These studies have established that the restriction of intramolecular motion, RIM (comprised of both restriction of intramolecular rotations, RIR, and restriction of intramolecular vibrations, RIV) is largely responsible for the AIE effect.⁶ In the solution state, active molecular motions act as a nonradiative channel for energetic decay from the excited state to the ground state. In the aggregated state, however, these motions are restricted greatly by the spatial constraints and interactions of the surrounding molecules, blocking the nonradiative decay channels and resulting in emission.

Attracted by the potential applications of the AIE phenomenon, many research groups have focused on the development of new AIEgens and how best to harness their potential. A thorough understanding of the AIE mechanism has enabled the development of a wide range of new AIEgens, providing researchers with an alternative strategy for addressing problems caused by fluorescence quenching and enabling the development of new highly efficient solid-state emitters.⁷ Fluorescent AIEgens with high Φ_F values in solid films, in particular silole⁸ and tetraphenylethene (TPE)⁹ derivatives, have been widely used to fabricate stable and simplified non-doped fluorescent OLEDs. Some of these OLEDs show excellent electroluminescence (EL) performance with high efficiencies approaching or reaching the theoretical limit. These achievements are summarized in this review.

Silole-based AIEgens

The first reported AIEgens,⁵ propeller-shaped siloles (Prod. Nos. 797294 and 797286) have generated the most attention from researchers in the field of organic electronics research. Due to their AIE characteristics, most siloles present high solid-state Φ_F values. Their unique σ*–π* conjugation results in low-lying LUMO (lowest unoccupied molecular orbital) levels, which arise from the interaction between the σ* orbital of two exocyclic single C–Si bonds and the π* orbital of the butadiene moiety. As a result, siloles exhibit good electron affinity and fast electron mobility, allowing them to be used to transport electrons in OLEDs.⁸ In addition, siloles display high thermal and morphological stability and good solubility in common solvents, facilitating film fabrication by vapor deposition or using solution processing techniques.⁸ The excellent integrated performance of siloles is a good indicator of their potential for use in the fabrication of non-doped OLEDs, and many efficient solid-state luminescent materials for OLEDs have been developed recently through efforts to engineer new types of siloles.

Fluorene-based substituents have been widely used in the construction of efficient light emitters for OLEDs because of their intense emissivity and good thermal stability. Siloles incorporating dimethylfluorene as substituents at the 2,5-positions of the silole ring show excellent PL and EL properties.¹⁰ For example, the MFMPS film (ITO/NPB (60 nm)/emitter (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm)) shown in Figure 2 exhibits a strong fluorescence peak at 534 nm with a much higher Φ_F value (88%) than that in THF solution (2.6%), indicating AIE characteristics. When MFMPS is adopted as a light-emitting layer to fabricate non-doped OLEDs, e.g., MFMPS [ITO/NPB (60 nm)/MFMPS (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm)], the resulting device can be turned on at a low voltage (3.2 V) and radiates yellow light at 544 nm with CIE coordinates (x = 0.37, y = 0.57), a maximum luminance (L_max) of 31,900 cd m^-2. The device gives good EL efficiencies with a maximum current efficiency (ηC,_max) of 16.0 cd A^-1, a maximum power efficiency (ηP,_max) of 13.5 lm W^-1, and a maximum external quantum efficiency (ηext,_max) of 4.8%. This approaches the theoretical limit of OLEDs based on traditional fluorescent materials. The EL performance of MFMPS is further demonstrated in a device configuration that includes ITO/MoO³ (5 nm)/NPB (60 nm)/MFMPS (20 nm)/TPBi (60 nm)/LiF (1 nm)/ Al (100 nm). In this configuration, the device shows a turn-on voltage (V_on) of 3.3 V, a yellow EL peak at 540 nm (CIE 0.36, 0.57), and outstanding maximum luminance, current, and power efficiencies of 37,800 cd m^-2, 18.3 cd A^-1 and 15.7 lm W^-1, respectively. Notably, a η_ext,max of 5.5% is obtained in this optimized device, exceptional for fluorescent OLEDs.

As previously discussed, both the outstanding AIE attributes and the unique electronic structures of siloles make them ideal materials for utilization as light-emitting materials and electron transporters in efficient non-doped OLEDs. By combining the two merits of siloles, efficient bifunctional n-type light emitters can also be produced, an approach that is favored for fabricating simplified OLEDs. (MesB)₂MPPS and (MesB)₂HPS are both good examples of this approach. Both contain the silole ring and dimesitylboryl, a functional group that contains inherently electrondeficient groups.¹¹ The tetraphenylsilole moiety endows the luminogens with AIE characteristics and efficient solid-state emissions, while the dimesitylboryl group effectively lowers the LUMO energy level and improves the electron-transporting ability of the molecule by virtue of the vacant low-lying pπ orbital on the boron center. (MesB)₂MPPS and (MesB)₂HPS possess low-lying LUMO energy levels of 3.06 and 3.10 eV, respectively, indicating significant potential as an electron transporter for OLEDs. Solid films of (MesB)₂MPPS and (MesB)₂HPS are highly emissive with peaks at 524 and 526 nm, and exhibit high Φ_F values of 58% and 62%, respectively. Based on these properties, double-layer OLEDs [ITO/ NPB (60 nm)/silole (60 nm)/LiF (1 nm)/Al (100 nm)] were fabricated by adopting (MesB)₂MPPS or (MesB)₂HPS simultaneously as both lightemitting (emissive layer, EML) and electron-transporting layers (ETL). These two simplified OLEDs display excellent performance with high ηC,_max (up to 4.35%), ηP,_max (up to 13.9 cd A^-1), and η_ext,max (up to 11.6 lm W^-1). All of these results are much higher than those attained using triple-layer devices with an additional TPBi (Prod. No. 806781) electron-transporting layer. The excellent performance of these OLEDs is attributed to the efficient electron transport and the LUMO level of siloles matching the cathode work function, resulting in a good carrier balance of holes and electrons (γ). This shows that dimesitylboryl functionalized siloles are efficient n-type solid-state light emitters and are promising bifunctional materials for constructing high-performance and simplified OLEDs. A similar silole derivative, (MesBF)₂MPPS, developed by integrating dimesitylboryl substituents into MFMPS,¹² also exhibits excellent solid-state emission (Φ_F = 88%) and great electron-transporting potential, consequently presenting a good EL property as well (Table 1).

TPE-based AIEgens

The TPE functionality is one of the most popular AIEgen moieties because of its simple molecular structure and remarkable AIE effect. A TPE unit can easily be introduced into ACQ fluorophores to create new fluorescent AIEgens with high emission efficiencies in the aggregated state that can be effectively used to fabricate efficient non-doped OLEDs. Some examples of TPE-based AIEgen structures are shown in Figure 3. With subtle structural alteration the emission colors of TPE-based fluorescent AIEgens can be tuned to cover the whole range of visible light. This has been used to construct a number of high performance and highly efficient blue, cyan, green, yellow, red, and even white OLEDs.^7,9

Abbreviations: λ_EL = electroluminescence maximum; V_on = turn-on voltage at 1 cd m⁻²; L_max = 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. NPB functions as a hole-transporting layer (HTL); TPBi and Bphen serve as an electron-transporting layer (ETL) and a hole-blocking layer (HBL), respectively; Alq₃ functions as ETL; and MoO₃ serves as a hole-injection layer (HIL).

The development of efficient blue OLEDs, especially deep blue OLEDs, is critical for realizing commercial applications of full-color display and solid-state lighting. However, because of the intrinsic large band gap, robust organic blue light-emitting materials and OLEDs are still rare. Considerable effort has been invested in the development of pure organic blue fluorophores because of the formidable challenges in designing efficient blue phosphors and the difficulty of improving the stability and longevity of the resulting phosphorescent OLEDs. Recently, Li’s group has developed a number of blue AIEgens by utilizing different linkage patterns and increasing the intramolecular twisting degrees using steric hindrance to tune the balance between the molecular rotation and conjugation.¹³ Among these AIEgens, MethylTPA-3pTPE, consisting of a methyl-substituted triphenylamine (TPA) core and three TPE units in peripheries, shows a high Φ_F value of 64% in the aggregate state with good EL efficiency. A multilayer OLED with a configuration of ITO/MoO₃ (10 nm)/NPB (60 nm)/MethylTPA-3pTPE (15 nm)/TPBi (35 nm)/LiF(1 nm)/ Al radiates blue light at 480 nm (CIE 0.17, 0.28) and presents excellent performance with L_max, η_C,max, η_P,max, and _ηext,max of 13,639 cd m^-2, 8.03 cd A^-1, 7.04 lm W^-1, and 3.99%, respectively. While the methyl-substituted TPA group creates good hole-transporting properties for MethylTPA-3pTPE, a simplified device without the HTL, [MoO₃ (10 nm)/MethylTPA-3pTPE (75 nm)/TPBi (35 nm)/LiF (1 nm)/Al], shows comparable EL efficiencies (6.51 cd A^-1, 6.88 lm W^-1, and 3.39%) and emits bluer light at 469 nm (CIE 0.18, 0.25).

Triphenylethene is another highly useful AIE unit with a very simple molecular structure. Compared to TPE, triphenylethene possesses a shorter conjugation length and exhibits a bluer solid-state emission, making it a promising building block for the fabrication of efficient solid-state blue fluorophores. The melding of triphenylethene with a phenanthro[9,10-d]imidazole (PI) group at the molecular level produces an efficient deep blue AIEgen, BTPE-PI.¹⁴ A non-doped multilayer EL device constructed using BTPE-PI as an emitting layer [ITO/NPB (40 nm)/BTPEPI (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm)] allows for the display of deep blue EL, peaking at 450 nm (CIE 0.15, 0.12) with an outstanding η_ext,max of 4.4% and a small efficiency roll-off.

Pyrene (Prod. Nos. 185515 and 571245) is a conventional fluorophore whose film emission is generally weakened as a result of the notorious ACQ effect. Attaching four TPE units to the periphery of pyrene generates a new fluorophore (TTPEPy), which features obvious AIE characteristics and efficient solid-state fluorescence (Φ_F = 70%).¹⁵ The non-doped OLEDs using TTPEPy as light-emitting layers radiate sky blue emissions with a peak at ~490 nm, while showing excellent performance (η_C,max up to 12.3 cd A^-1 and η_ext,max 4.95%).

In order to use organic semiconductors in an OLED device, emitters should have both efficient solid-state emissions and high carriertransporting capabilities. Such multifunction materials help simplify device configuration, shorten the fabrication process, and reduce the manufacturing cost because they simultaneously serve as light-emitting layers and hole- and/or electron-transporting layers.⁹ TPA is widely used in semiconductor fabrication due to its good hole-injection/transporting capability, but it suffers from the ACQ effect in the condensed phase. By integrating the TPA groups with the TPE unit, a new and highly versatile semiconductor, 2TPATPE, was prepared.¹⁶ 2TPATPE exhibits not only an extremely high Φ_F value (~100%) but also an excellent hole mobility value of 5.2×10^-4 cm² V^-1 S^-1 in solid amorphous film, as determined by timeof- flight technique, a widely used method to measure carrier mobility. A simplified OLED of 2TPATPE without the HTL [ITO/2TPATPE (60 nm)/ TPBi (10 nm)/Alq₃ (30 nm)/LiF/Al (200 nm)] was constructed that emits green light with an L_max value of 33,770 cd m^-2 and displays excellent EL efficiencies (4.4%, 13.0 cd A^-1, and 11.0 lm W^-1), an improvement over devices that include an HTL [ITO/NPB (40 nm)/2TPATPE (20 nm)/TPBi (10 nm)/Alq₃ (30 nm)/LiF/Al (200 nm)] that showed EL efficiencies of 4.0%, 12.3 cd A^-1, and 10.1 lm W^-1.

Adachi and co-workers developed two novel starburst AIEgens, PDA-TPE and TPA-TPE, by integrating a hole-transporting N,N,N΄,N΄-tetraphenylp- phenylenediamine (PDA) or TPA core with TPE units.¹⁷ The amorphous films of the two AIEgens emit strong fluorescence with high ΦF values of 56–73% and possess much higher hole mobility than the typical commercialized hole transporter, N,N΄-di(1-naphthyl)-N,N΄-diphenylbenzidine (NPB, Prod. No 556696). This is due to the presence of PDA or TPA groups as well as a spontaneous molecular orientation of the star-burst molecules. Hence, the resulting simplified OLEDs in which PDA-TPE or TPA-TPE function both as light-emitting and holetransporting bifunctional layers perform extremely well (Table 1). Because of the optimal charge balance and the enhanced η_out ascribed to the spontaneous molecular orientation, triple-layer devices such as [ITO/ NPB (40 nm)/PDA-TPE or TPA-TPE (25 nm)/BPhen (35 nm)/LiF (0.8 nm)/Al (70 nm)] display notably high η_ext,max values of up to 5.9% with green EL emissions at 510–530 nm.

Bipolar luminescent materials that contain both electron donors and acceptors (D-A) are preferred materials for balancing the injection and transport of carriers in OLEDs and also help simplify device structure. In an effort to create a more highly efficient solid-state bipolar luminogen, our group designed a new luminogen by connecting an electron donor (diphenylamino) and an electron acceptor (dimesitylboryl) with a TPE unit resulting in a D-A framework with AIE units.¹⁸ This new bipolar AIEgen (TPE-PNPB) exhibits a weak D-A interaction and fluoresces strongly in solid film with a Φ_F value of 94%. The resulting trilayer OLED [ITO/NPB (60 nm)/ TPE-PNPB (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm)] can be turned on at a low voltage of 3.2 V and radiates a bright EL emission at 516 nm (CIE 0.27, 0.51) with a high L_max of 49,993 cd m^-2. The resulting η_C,max, η_P,max, and η_C,max values attained from this device were excellent, measuring 15.7 cd A^-1, 12.9 lm W^-1, and 5.12%, respectively. A remarkably high η_C,max of 5.35% is recorded when TPE-PNPB functions both as an EML and HTL in a bilayer OLED, demonstrating that TPE-PNPB is an excellent p-type light emitter. In addition, the η_C,max values of 4.75% and 4.45% are maintained at 1,000 cd m^-2 for its trilayer and bilayer OLEDs, respectively, suggesting good stability of OLED based on TPE-PNPB.

For full-color display applications, it will be necessary to develop efficient blue, green, and red fluorescent materials and OLEDs. Like the blue OLEDs, the current performance of existing red fluorescent OLEDs is also unsatisfactory. In general, many traditional red fluorophores are constructed from planar polycyclic aromatic hydrocarbon (PAHs) units with extended π-conjugation. These materials also display a strong ACQ effect, exhibiting weak emission in the solid state.¹⁹ The use of AIE units to develop efficient red fluorophores is emerging as a promising approach to address these shortcomings. For example, a new orange-red fluorophore named BTPETTD, composed of two TPE units tethered to a conjugate of benzo-2,1,3-thiadiazole and thiophene, has been developed.²⁰ BTPETTD shows AIE characteristics and emits efficiently in solid films with a Φ_F value of 55%. OLEDs fabricated using BTPETTD as the light-emitting material exhibited an orange-red EL of 592 nm and high EL efficiencies of 6.1 cd A^-1 and 3.1%. Another set of red AIEgens, TPE-TPA-BTD and TPE-NPA-BTD, have also been recently developed.¹⁹ These new materials have high Φ_F values of 48.8% and 63.0%, respectively. The resulting non-doped OLEDs both emit at 604 nm with high η_ext,max values up to 3.9%. Moreover, these new red AIEgens possess good hole-transporting characteristics due to the presence of arylamino moieties, and double-layer EL devices using these materials as both EMLs and HTLs also perform well (Table 1). When more TPE units are introduced to the conjugated backbone, the resulting luminogen TTPEBTTD shows a highly twisted conformation, and the intermolecular interactions are greatly suppressed. The solid film of TTPEBTTD emits red PL peaking at 646 nm. High-performance red non-doped OLED is achieved using TTPEBTTD as the light-emitting layer, radiating EL at 650 nm (CIE 0.67, 0.32) and offers an L_max of 3,750 cd m^-2 and a high η_ext,max of 3.7%.²¹

Conclusion and Outlook

Since the AIE phenomenon was first reported, many fluorescent AIEgens with high Φ_F values in the solid state have been developed for the fabrication of stable and efficient non-doped OLEDs. The emission colors of the devices cover the whole range of visible light. Some of these OLEDs approach or reach the theoretical limit of η_ext,max (5–7.5%), several of which have been summarized in this review. A number of high-performance white OLEDs have also been successfully produced using these AIEgens.²² The ability to use the AIE effect to improve common fluorophores (the first-generation luminescent materials) for OLEDs demonstrates the great academic and practical significance of AIE research. However, since 75% of the generated excitons (triplet excitons) have not yet been employed using AIE, there is still much room to improve the efficiencies of fluorescent OLEDs. Much effort has recently been devoted to the creation of third-generation luminescent materials for fabricating efficient OLEDs using pure organic thermally activated delayed fluorescence (TADF) materials. These materials allow the devices to have a large β that can theoretically reach up to 100%, but the efficiency roll-off of these devices is usually severe. Integrating both AIE and TADF effects within a molecule is another promising strategy for constructing increasingly more robust luminescent materials for high-performance OLEDs.

Acknowledgments

We greatly acknowledge financial support from the National Natural Science Foundation of China (51273053), the National Basic Research Program of China (973 Program, 2015CB655004 and 2013CB834702), the Guangdong Natural Science Funds for Distinguished Young Scholars (2014A030306035), the ITC-CNERC14S01, the Guangdong Innovative Research Team Program (201101C0105067115), and the Fundamental Research Funds for the Central Universities (2015PT020 and 2015ZY013).