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

with high CRI (>90), EQE_max > 25%, and PE_max = 99.9 lm/W [88].

      WOLEDs can also be obtained through the method of blue OLED with color down conversion layer (CCL), with the advantage of simple fabrication process and high color stability [89, 90]. Kowk et al. utilized orange‐red emissive emitter BTPETTD (Figure 1.5) as a CCL on top‐emitting blue OLEDs by thermal evaporation, with 74.5% of the blue emission converted to red emission at the efficiency of 40%. And resulting top‐emitting WOLEDs were realized by mixing the left blue and red emission, with CIE coordinate, CE, and PE of (0.34, 0.35), 17.7 cd/A, and 8.7 lm/W, respectively [91].

1.3 High Exciton Utilizing Efficient Aggregation‐induced Emissive Materials

      Although AIE‐active conventional fluorescence have took up the majority of all AIE emitters, the maximum EQE values of their OLEDs are too low, with only 5% in theory, due to their capabilities of using only singlet excitons, with the loss of 75% triplet excitons. To further improve the efficiency of OLED devices, the AIE emitters were designed with other photophysical mechanisms, which can take advantage of triplet excitons for emission in both highly efficient and cost‐effective OLEDs. At present, the AIE properties have been combined with various photophysical strategies of high EUE such as phosphorescence [20], TADF [92], and HLCT [28–31] to prepare highly efficient emitters for OLED devices. Despite different photophysical mechanisms, all of them have the potentials to realize 100% EQE in theory. In the following paragraph, we will introduce each of these emitters combined with AIE and high EUE mechanisms in details.

      1.3.1 Aggregation‐induced Phosphorescent Emissive Emitters

      The phosphorescent OLEDs, as the second generation technique, can fully utilize both singlet and triplet excitons for 100% phosphorescent emission with the process of intersystem crossing from the lowest singlet excitons S₁ to lowest triplet excitons T₁ and further to the ground state S₀, due to spin–orbital coupling effect usually caused by heavy metals. In 1998, Ma et al. [93] and Forrest et al. [94] first reported that heavy‐metal complex of osmium(II) or platinum(II) complexes can generate phosphorescence within OLED devices. In last few decades, the phosphorescent OLED developed quite rapidly with numerous phosphorescent emitters, including Ir(III) [95, 96], Au(III) [97], Pt(II) [98], Cu(I) [99, 100], and Os(II) [101] complexes merchandized, and even some pure organic compounds with room‐temperature phosphorescence [102, 103] and AIPE materials [104, 105] were reported.

This AIPE phenomena were discovered first by Lu et al. in the metal complexes of oxo‐bridged Re(I) AIPE 1–3 (Figure 1.6), with dim emission in pure acetonitrile solution, but a dramatic increase by adding water to this solvent [106]. Similar to AIE property, the AIPE phenomenon was also a result of the intramolecular motion restricted in mixed solvents. Till now, numerous AIPE compounds have been designed, but mainly applied in the areas of gas [107] or explosive detection [108], with limited applications in AIPE‐based OLEDs [109]. In 2011, Zhang, Zuo, and Zheng et al. designed the AIPE iridium complexes Ir(tfmppy)2(tpip) (Figure 1.6), and by doping it into the host of mCP, they prepared highly efficient OLED devices, exhibiting maximum luminance of 64 351 cd/m² and a low efficiency roll‐off [110]. Likewise, He et al. took TPBI (Figure 1.6) and mCP as the bihosts for the AIPE emitter Ir(tfmppy) ₂(tpip), with the ratio of TPBI : mCP : Ir(tfmppy) ₂(tpip) at 1 : 1 : 1. The inverted PhOLED based on 0.5 nm doped EML experience significant and efficient roll‐off, with EQE from 31.1% at 100 cd/m², to 24.2% at 1000 cd/m², and finally to 17.0% at 10 000 cd/m², as a result of TTA effect. By increasing the thickness of EML to 10.5 nm, the efficiency roll‐off of the PhOLED was sharply reduced, with EQE from 23.3% at 1000 cd/m² to 19.2% at 10 000 cd/m² [111]. AIPE emitters can also be applied in the nondoped OLED emitters. Su et al. utilized AIPE‐active iridium(III)‐based molecules to fabricate OLED through solution processing. The devices can reach the maximum CE of 25.7 cd/A, and EQE of 7.6% was achieved based on this AIPE‐active complex [109]. Recently, Wu et al. took advantage of Pt(II) metal complex DPA‐Pt (Figure 1.6) with simple structure as the emitter to prepare both nondoped and doped OLED devices. The doped ones with 6% weight content showed lower EQE of 9.2%, while the nondoped ones can reach EQE of 9.8% with deep‐red/NIR emission, showing facile fabrication and high performance of the nondoped device [112]. Therefore, it could be expected that increasingly more AIPE emitters could find applications in OLED, owing to their excellent solid‐state lightening performance.

Figure 1.6 Aggregation‐induced phosphorescent emitters.

      1.3.2 Aggregation‐induced Delayed Fluorescent Emitters

      TADF or E‐type delayed fluorescent emitters can also achieve 100% IQE, and these emitters tend to have a small energy between S₁ and T₁ states (ΔE_ST) [21] to enable upconverting triplet excitons into singlet ones via the process of RISC under external heat [113]. Early in 1961, Parker and Hatchard have already discovered this phenomenon in the eosin dye [114]; however, it is not until 2009 when Adachi group utilized the TADF emitters Sn(IV)‐complexes to fabricate the first TADF OLEDs [21]. In last decade, the TADF emitters have seen a rapid progress in terms of both category [115] and efficiency [116], and they are on their way for replacing phosphorescent emitters and conventional fluorescent materials, referred to as the third‐generation emitters for OLED devices [117]. Among these emitters, the emitters with both TADF and AIE propriety have already been introduced and have attracted tremendous attention, because they can prepare highly efficient OLEDs without doping process [92].

The first emitters with both AIE and TADF properties applied in OLED devices were reported by Yasuda et al. in 2016, as organic–inorganic compounds of o‐carborane derivatives PCZ‐CB‐TRZ, PCZ‐CB‐PCZ, and TPA‐CB‐TRZ (Figure 1.7), with emission peaking at wavelengths of 557, 624, and 571 nm, respectively, and also high solid‐state PLQYs of 97, 55, and 94% in neat films, respectively, compared with their PLQY below 3% in solution. Their nondoped OLED devices can reach the maximum EQEs of 11, 10.1, and 9.2%, respectively [118]. Zhao and Tang et al. synthesized and reported a series of AIE and TADF luminogens, as AIDF emitters [119]. For example, they reported three unsymmetrical D‐A‐D′ structured AIDF emitters of DBT‐BZ‐DMAC, DBT‐BZ‐PXZ, and DBT‐BZ‐PTZ (Figure 1.7), and their doped OLEDs exhibited highest EQE of 19.2% with larger efficiency roll‐off, while their nondoped counterparts showed the data of up to 14.2%, but with slight efficiency roll‐off [120, 121]. Chen et al. reported another AIE‐TADF materials AI‐Cz and AI‐TBCz (Figure 1.7), based on which OLEDs gave stable emission and low efficiency roll‐off, with maximum EQEs of 23.2 and 21.1%, respectively [122].

Figure 1.7