Handbook of Aggregation-Induced Emission, Volume 3. Группа авторов

Based on these both advantages, the traditional fluorescent AIE emitters have covered wide range of spectrum from blue to green and red, and even the white OLEDs based on these emitters have been fabricated, although EQE limitation of 5% for traditional AIE emitters‐based OLEDs due to the theoretical out‐coupling efficiency of 20%. Among these monochromatic OLED, the blue and green AIE‐active emitters share the larger part, while the red counterparts take up the smaller portion. In this section, we will demonstrate the conventional AIE‐active emitters‐based OLEDs in detail, according to different colors [27].

      1.2.1 Blue Aggregation‐induced Emissive Emitters

      One major method to prepare AIE emitters is to integrate the AIE groups into some previous ACQ emitters, such as carbazole, fluorene, and anthracene to change these emitters’ aggregated behaviors. Among various AIE moieties, tetraphenylethene (TPE) moiety is the most popular building block to construct AIE emitters, due to its simple structure and facile modification. Due to the sky‐blue emitting behaviors of TPE moiety, the blue and green emitters took the majority of AIE‐active emitters.

Zhao et al. introduced TPE moiety at the blue‐emitting ACQ moieties of fluorene through 2,7‐position to obtain blue luminogen BTPEBCF (Figure 1.2) with AIE behavior, dim in solution but emitting strongly in the aggregate state with 100% of PLQY. Based on this emitter, two sky‐blue OLEDs were constructed with the structures of indium‐tin oxide (ITO)/NPB (60 nm)/BTPEBCF (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm) (device I) and ITO/NPB (60 nm)/BTPEBCF (20 nm)/TPBi (10 nm)/Alq₃ (30 nm)/LiF (1 nm)/Al (100 nm) (device II), reaching the maximum luminance of 13 760 cd/m², current efficiency (CE) and EQE of 7.9 cd/A and 2.9%, overperforming the ACQ counterparts [43]. Li et al. further linked TPE with blue‐emitting ACQ moieties of carbazole or spirofluorene to obtain two blue AIEgens STPE and TPE‐2Cz (Figure 1.2). And based on these both emitters, nondoped OLEDs were prepared with the construction of ITO/NPB (60 nm)/SFTPE (30 nm)/TPBi (20 nm)/LiF (1 nm)/Al (100 nm) and ITO/NPB (40 nm)/TPE‐2Cz (10 nm)/TPBi (10 nm)/Alq₃ (30 nm)/LiF (1 nm)/Al (100 nm) with blue emission peaking at 466 and 462 nm and CIE_x,y of (0.18, 0.24) and (0.17, 0.21). And the device based on SFTPE exhibited better EL performance with a maximum luminance and CE of 8196 cd/m² and 3.33 cd/A [44]. Zhu et al. further prepared subsequent numbers of TPE moieties decorated with carbazoles, i.e., Cz‐TPEs, based on which solution‐processed OLED could be prepared with structures of ITO/PEDOT (40 nm)/Cz‐TPEs (50 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm). All these OLEDs exhibited sky‐blue emission with maximum luminance of 2858 cd/m² and a maximum CE of 5.5 cd/A [45]. Pyrene is a good deep‐blue emitter with ACQ properties, which is often widely used for construct emitters [46]. Through strategic design, Li et al. linked four TPE moieties to a pyrene core to obtain two blue derivatives Py‐4MethylTPE and Py‐4mTPE (Figure 1.2). The OLED device based on Py‐4Mtpe exhibited a better performance, with maximum CE, PE, and EQE of 4.02 cd/A, 3.08 lm/W, and 2.5 %, respectively [47]. Similarly, Hu et al. used two TPE moieties to link TPE core to obtain two blue AIEgens Py(5,9)BTPE and Py(5,9)BTriPE of butterfly shape (Figure 1.2). The nondoped OLED based on Py(5,9)BTPE exhibited better performance with CIE coordinates of (0.19, 0.28), a maximum EQE of 3.35%, and CE of 6.51 cd/A [48]. By contract, another strategy was proposed with pyrene as a periphery and TPE acting as a core. Xie and Li et al. connected four pyrenes to the TPE cores to prepare the highly efficient blue emitter TPE‐4Pys (Figure 1.2). Its spin‐coated nondoped OLEDs can reach CE of up to 3.05 cd/A at 496 nm, while its doped OLED devices can achieve CE of up to 4.9 cd/A at 484 nm, among the best solution‐processable OLEDs based on blue AIE emitters [49].

Figure 1.2 Molecular structures of TPE‐based conventional blue AIE‐active emitters.

      In some other works, TPE served as single fluorescent moieties to construct more complicated AIE emitters. Li et al. merged two TPE units together through linking at different positions to obtain four BTPE (Figure 1.2) derivatives with deep‐blue emission. Nondoped sky‐blue OLED devices with a configuration of ITO/MoO₃ (10 nm)/NPB (60 nm)/EML (15 nm)/TPBi (35 nm)/LiF (1 nm)/Al (100 nm) were fabricated, with maximum luminance, CE, and PE of 3266 cd/m², 2.8 cd/A, and 2.0 lm/W, respectively [50]. They further decorated the benzene core with different number of TPE moiety peripheries to obtain three AIE emitters PhTPE, Ph2TPE, and Ph3TP (Figure 1.2), based on which nondoped blue OLEDs were fabricated with the structure of ITO/MoO₃ (10 nm)/NPB (80 nm)/EML (20 or 30 nm)/TPBi (30 nm)/LiF (1 nm)/Al, with emission ranging from 457 to 488 nm, and maximum CE, luminance, and PE of 5.0 cd/A, 3966 cd/m², and 3.87 lm/W, respectively [51].

      On the other hand, as an important factor, balanced factor of carriers’ transport (electrons and holes) can also decide the overall electroluminescent performance. However, owing to the electron‐rich nature of most organic materials, they exhibit high hole transport but relatively low electron mobility, and finally lead to unbalanced electron‐hole recombination. In this regard, bipolar materials with donor−acceptor (D−A) structures can favor both holes and electrons injection and transport to result in balanced factor of carriers’ transport (electrons and holes), which finally enhance the efficiency of the OLEDs. Additionally, the dipolar luminogens tend to align horizontally in solid state, which can also result in high out‐coupling efficiency over 20%. In previous research, most dipolar emitters were served as host or dopant, because the strong D−A interaction usually showed lower PL efficiency at solid state [52, 53]. For example, Xie et al. designed and prepared two novel AIE materials of TPEPBN and TPE‐2PBN (Figure 1.2) with the property of liquid crystal due to 4‐cynobiphenyl moiety serving as the mesogenic unit, and the PLQYs can be obtained 71 and 83% in solid state, respectively. Due to liquid crystals of electron‐accepting moiety of CN, these both compounds showed high hole mobility. Based on these two AIE emitters, both nondoped and doped OLED devices were fabricated and the better OLED device were obtained for nondoped ones based on the emitter of TPE‐PBN (Figure 1.2), with maximum EQE of 4.1%, which is one of the highest values reported for blue fluorescent OLEDs base on AIE emitters [54].

Silole derivatives, with both good electron affinity and electron mobility, due to its σ *– π * conjugated electronic structures, also exhibit the propeller‐like molecular structures and therefore the property of AIE, which also attracted the research interest in taking this moiety into the OLED emitters’ design. Li et al. covalently incorporated TPE moiety to a dibenzosilole core through three different link modes to obtain compounds Si‐pTPE, Si‐tPE, and Si‐mTPE with light emitting from green to deep blue. The nondoped OLED based on Si‐tPE can reach the maximum CE and EQE of 8.04 cd/A and 3.38%, respectively [55]. They also linked the TPE moiety to tetraphenylcyclopentadiene (TPCP) with substitution of Si of silole by C to prepare six novel AIEgens with blue emission, and the nondoped OLED device can reach the maximum luminance and CE of 8721 cd/m² and 3.40 cd/A, respectively. Recently, Tang et al. reported a new deep‐blue AIE emitter of tetraphenylbenzosilole (TPBS),