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

of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, China

       3 Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education, Zhejiang Sci‐Tech University, Hangzhou, China

1.1 Introduction

      Organic light‐emitting diode (OLED) can convert electricity into light within organic emissive layers (EMLs) under external voltage. Since the pioneering work of Tang and VanSlyke [1], this technology has seen a rapid progress in the past three decades and has already realized commercial application in various electronic appliances, such as smartwatches, mobile phones, cameras, and so on [2–4], owing to its significant advantages, such as low cost, low‐driving voltage, light weightiness, and fast response [5–10]. Furthermore, this technology was regarded as the future major displaying and lighting technology [11–13].

Typical OLED devices consist of two electrodes, i.e., anode and cathode, and several organic layers sandwich between these two electrodes. Within OLED devices, electrons and holes are injected from cathode and anode, respectively, and pass through several organic layers, such as hole injection layer (HIL), electron injection layer (HIL), hole transport layer (HTL), and electron transport layer (ETL), and finally recombine in the EML with a small part of photons passing through transparent electrode to emit light (Figure 1.1) [12]. As for OLEDs, the organic EML plays a vital role in determining the devices’ performances, especially for color, luminance, and efficiency. OLED devices’ color was judged according to Commission Internationale de l’Eclairage (CIE) coordinates [15, 16]. Internal quantum efficiency (IQE) and external quantum efficiency (EQE), defined as the numerical ratio of photons generated within or out of the devices to electron‐hole pairs injected, are usually applied to evaluate the OLEDs’ capability of electron–photon conversion [17, 18]. Compared with IQE, the parameter EQE are more reliable, which is determined by the photoluminescence quantum efficiency (PLQY) of emitters in the solid films, balanced factor of electrons and holes, and out‐coupling efficiency. Because photons are not only emitted out of but also generated within the EMLs, the emitters’ structures, aggregated state, and properties can strongly determine OLED manufacturing process and performance [19].

Figure 1.1 The OLED devices and aggregated‐state lightening. (a) The construction of OLED emitter, (b) OLED’s working mechanism, (c) aggregation‐caused quenching, and (d) aggregation‐induced emissive phenomenon.

      Source: Reproduced with permission from John Wiley and Sons [3, 13, 14].

Under electrical excitation, emitter can produce singlet excitons and triplet excitons in the ratio of 1 : 3 according to spin statistics; there are several types of emitters with different photophysical mechanisms in terms of utilizing the electrons. Traditional fluorescent emitters can take advantage of singlet excitons to produce fluorescence, with maximum IQE of 25% and maximum EQE of merely 5% provided that out‐coupling efficiency is 20%. In order to improve OLEDs’ efficiency, triplet excitons were further taken into consideration and various mechanisms were put forward. According to photophysical mechanisms of utilizing triplet‐harvesting, OLEDs with high exciton utilizing efficiency (EUE) can be classified into different groups. Phosphorescent emitters can reach 100% IQE with phosphorescent emission through the mechanism of spin–orbit coupling [20]. Thermally activated delayed fluorescent (TADF) emitters can also reach the maximum IQE of 100% by upconverting the lowest triplet to the lowest singlet via reverse‐intersystem crossing (RISC) under external energy [21–27]. The emitters with hybridized local and charge transfer (HLCT) state also share 100% IQE with fluorescent emission, through upconverting triplet charge transferring (CT) state to singlet local excited (LE) state in higher excited states [28–31]. In addition, some other new mechanisms were reported in OLEDs with high EQE, such as triplet–triplet annihilation (TTA) [32], triplet exciton‐polaron annihilation [33], neutral π ‐radical [34], and singlet fission [35].

      Furthermore, as a kind of solid‐state lightening technologies [5], not only molecular structures but also aggregated state of emitters can significantly influence the light‐emitting behaviors. One of the frustrating problems associated with most conventional luminogens is aggregation‐caused quenching (ACQ) effects, which mean that the fluorescence of the emitters was dim or totally quenched in the solid state, in spite of their strong emission in solution, and it may significantly reduce the device’s efficiency. The ACQ phenomenon is usually caused by the emitter’s large conjugated plane, which tends to experience serious intermolecular interactions, such as π–π stacking. Various methods, including chemical modifications or physical doping processes were proposed to reduce the molecule aggregation and the ACQ phenomena, but only limited achievement has been made and also some other side effects were brought about. Fortunately, some luminogenic molecules feature a distinctive photophysical process of AIE [14]. Opposite to the ACQ phenomena, the luminogens with AIE properties, or AIEgens for short, radiate rather stronger emission in solid films than they do in dilute solutions, with the restriction of intramolecular motions (RIM) in solid state considered as its mechanisms. The AIEgens are favored in the OLEDs’ applications, because of their potentials in increasing the OLEDs’ efficiency. When applied in OLED devices, the ACQ emitters are usually applied in the complex host–guest structure through doping method to overcome the quenching problems in solid states, with the disadvantages of complexed fabricated process, phase separation, and short lifetimes, while AIE emitters prefer the aggregated state for higher photoluminescence (PL) efficiency, and they could avoid the complex host–guest structure of the traditional OLEDs, with higher stability device, lower cost fabrication, and longer lifetime (Figure 1.1). Till now, numerous AIEgens have been prepared and even applied as nondoped emitters, which not only increase the efficiency of OLED devices but also simplify the fabricating process and increase the device’s stability [27, 36–41].

      In this chapter, we demonstrate the progress of emitters designed by combining AIE property with traditional fluorescent TADF and HLCT materials. The different AIEgens with various photophysical mechanisms were described with the related emissive materials applied in OLED devices. As for the traditional fluorescent AIE emitters, it currently took up the majority of AIE emitters applied in OLED devices and will be introduced according to different wavelengths from blue to green, red, and even white. However, due to the limited value of 5% for traditional AIE emitters‐based OLEDs, AIE property combined with other EUE mechanisms to prepare highly efficient emitters for OLED devices were also reported, such as the aggregation‐induced phosphorescent emissive (AIPE) emitters, aggregation‐induced delayed fluorescence (AIDF), and HLCT‐AIE materials. Finally, we give the outlook about the development of novel AIE emitters. From the molecular basis, it can also be noticed that not only did the small molecule but also the polymeric emitters develop fast. In 1987, Tang and VanSlyke reported that the first practical OLED was with tris‐(8‐hydroxyquinoline)‐aluminum as EML to achieve EQE of 1% [1]. And three years later, the first polymeric OLED based on poly(p‐phenylene vinylene) was reported by Friend et al. in 1990 [42]. Till now both small‐molecule emitters and polymeric emitters have experienced fast progress and been applied in the OLEDs; however, in this chapter we emphasize on small‐molecule emitters, due to their vital and basic role of mechanism study and structural design that are also suitable for polymers [13]. Thus, we hope that through this chapter, not only will current status of AIE emitters with different photophysical mechanisms be carefully demonstrated but also more interest will be brought to the areas of AIE‐active emitters and move the OLED research forward.

1.2 Conventional Aggregation‐induced Emissive Emitters

      The conventional fluorescent AIE emitters took the majority of all the AIE‐active