Chapter 11: Thermally Activated Delayed Fluorescence Materials Based on Donor–Acceptor Molecular Systems – Highly Efficient OLEDs

11
Thermally Activated Delayed Fluorescence Materials Based on Donor–Acceptor Molecular Systems

Ye Tao1,2, Runfeng Chen1, Huanhuan Li1,2, Chao Zheng1, and Wei Huang1,2

1Nanjing University of Posts and Telecommunications, Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials, Jiangsu National Synergistic Innovation Center for Advanced Materials, 9 Wenyuan Road, Nanjing, 210023, PR China

2Nanjing Tech University, Key Laboratory of Flexible Electronics & Institute of Advanced Materials, National Synergistic Innovation Center for Advanced Materials, 30 South Puzhu Road, Nanjing, 211816, PR China

Thermally activated delayed fluorescence (TADF) materials with efficient transition and interconversion between the lowest singlet (S1) and triplet (T1) excited states offer unique optical and electronic properties for organic light‐emitting diode (OLED) applications. In this chapter, we present an overview of the quick development in the molecular structure engineering of TADF materials in donor–acceptor (D–A) molecular architecture. Fundamental design principles and the common relations between the molecular structures and optoelectronic properties for the diversified device applications as emitters, sensitizers, or hosts in OLEDs have been discussed. Especially, a survey of recent progress in the studies of the D–A type TADF materials, with a particular emphasis on the different molecular building blocks for TADF phenomenon, is highlighted.

11.1 Introduction

TADF materials, developed to harvest triplet exciton for luminescence and to avoid the use of expensive and resource‐limited noble metals, can realize an internal quantum efficiency of up to 100% in OLEDs through the efficient utilization of both singlet and triplet excitons assisted by efficient reverse intersystem crossing (rISC) process for triplet‐to‐singlet transformation (Figure 11.1) [15]. Importantly, TADF can be facilely designed in purely organic donor–acceptor (D–A) molecular architecture; based on the abundant donors and acceptors already developed, a large number of D–A type TADF materials with varied molecular structures and optoelectronic properties have been reported in the recent years, leading to significant progress in TADF‐related OLED applications.

Figure 11.1 The electroluminescence from the singlet (S1) and triplet (T1) excitons of three generations of organic materials excited electrically for corresponding fluorescence (F) (a), phosphorescence (P) (b), and thermally activated fluorescence (TADF) (c) containing various processes of intersystem crossing (ISC), nonradiative relaxation (NR), and reverse intersystem crossing (rISC). Source: Ref. [1]. Reproduced with permission of Elsevier.

Important milestones in the development of TADF materials are illustrated in Figure 11.2. Firstly discovered in 1961 by Parker and Hatchard in eosin dye, TADF was previously named as E‐type delayed fluorescence (DF) [6]. Then, efficient TADF in metal‐containing materials (Cu(I)‐complex) was found in 1980 [7], followed by the observation of TADF in fullerenes in 1996, when Berberan‐Santos and Garcia firstly applied the triplet‐state‐related TADF in the detection of the oxygen and temperature [8]. The modern research of TADF in OLEDs began in 2009, although the devices required a rather high onset voltage of around 10 V [9]. Researches on TADF OLEDs culminated after the work of Adachi and coworkers in 2012 by designing TADF molecules in D–A molecular structure [11]. Thanks to the large varieties of organic donors and acceptors that have been already developed in organic electronics, the D–A type TADF materials with varied solubility, stability, emitting color, charge transport property, etc. can be rationally obtained with relatively straightforward molecular design strategy and facile synthetic route; the red, green, blue, and white TADF OLEDs with corresponding external quantum efficiency (EQE) of up to ∼13% [14], 31.2% [15], 36.7% [16], and 25.5% [17] have been reported, which undoubtedly break the efficiency limitations of fluorescent OLEDs and become comparable with PhOLEDs based on rare metal complexes. The distinguished developments of the noble metal‐free TADF materials have truly revolutionized our understandings of organic semiconductors and optoelectronics, and the replacement of source‐limited rare metal complexes, at least in OLEDs, is highly expected in the near future.

Figure 11.2 Milestones in the development of TADF materials. The TADF phenomenon in eosin was firstly discovered by Parker and Hatchard [6]. Blasse and McMillin reported the first TADF Cu‐complex in 1980 [7]. The TADF emission in fullerenes was found by Berberan‐Santos and Garcia [8]. Endo et al. demonstrated a TADF OLED using a Sn(IV)‐complex in 2009 [9]. In 2012, OLED using the TADF exciplex was reported by Goushi et al. [10]. The remarkable progress of TADF OLEDs was achieved by Adachi and coworkers with EQE of up to 19.3% [11]. In 2014, Adachi and coworkers reported a blue and white TADF OLED with EQE of 19.5% and 17%, respectively [12, 13].

In this chapter, we will summarize the recent progress on the molecular designs and properties of TADF materials based on D–A molecular systems as well as their recent advances in OLED applications. We begin by describing the TADF OLEDs, focusing on the device structures and operation mechanism. Next, we will describe the basic considerations in molecular design of D–A type TADF materials involving fundamental design principle, the control of the singlet–triplet energy splitting and the modulation of luminescent quantum yield. Subsequently, the emphasis will be placed on different types of TADF molecular systems with varied donor and acceptor building blocks, accompanied by thorough coverage of the recent efforts and latest research progresses on high efficient OLEDs to take advantage of TADF effects. A major goal of this chapter is to provide illustrative accounts on recent progress and to systematize our knowledge of the subject, extracting fundamental principles on design strategies of D–A type TADF materials and the common relationship between the molecular structures of TADF materials and their optoelectronic properties for OLEDs applications.

11.2 TADF OLEDs

11.2.1 Device Structures and Operation Mechanisms of TADF OLED

The device structure of TADF OLEDs has no difference from the conventional sandwich configuration of fluorescence (nondoped) and phosphorescence (doped) OLEDs, containing a series of functional layers including electrodes of metal cathode and indium tin oxide (ITO) anode, hole and electron injection and transport layers, an emissive layer (EML), and sometime exciton‐blocking layers. The device operation mechanisms of TADF OLEDs are also very similar to the traditional OLEDs, which require efficient charge injection and balanced charge transport for high device performance. However, significant difference begins when singlet and triplet excitons are electronically excited. For fluorescent OLEDs, only 25% singlet excitons can be harvested for luminescence, and 75% triplet excitons are wasted through nonradiative decays. For phosphorescence OLEDs, both singlet and triplet excitons are emissive with 100% internal electroluminescence quantum efficiency due to singlet–triplet mixing via the strong spin–orbit coupling of the heavy metal atoms. In the case of TADF OLEDs, although only singlet excitons are emissive, triplet excitons can be easily transformed to singlet excitons via facile rISC processes, leading to 100% exciton harvesting for electroluminescence.

Generally, there are four important processes for TADF emission in TADF OLEDs (Figure 11.3):

  1. The singlet and triplet excitons are formed after electron and hole recombination in a singlet‐to‐triplet ratio of 1 : 3 according to spin statistics.
  2. The higher‐lying exciton states are relaxed to the lowest singlet (S1) and/or triplet (T1) states via fast vibrational relaxation (VR), internal conversion (IC), and intersystem crossing (ISC).
  3. The formed triplet excitons at T1 are efficiently back transferred to S1 through rISC process with the aid of thermal activation.
  4. The singlet excitons at S1 formed either initially or back transferred from T1 are radiatively deactivated to S0 for prompt fluorescence (PF) and long‐lived DF, respectively.

Figure 11.3 Electroluminescence processes of TADF molecules.

In these processes, the efficient rISC process from T1 to S1 is very critical to utilize triplet excitons to improve the efficiency of the device. In order to facilitate the rISC process, small singlet–triplet splitting (ΔEST) is the key to enhance the T1 → S1 transition. Of course, when maintaining such an efficient rISC, the ISC is even more efficient than rISC due to the higher energy level of S1 than T1; the majority of the excitons are located at T1 with lower energy level in TADF materials. Therefore, T1 should be stable enough with slow nonradiative deactivation rate (knrT) to support the rISC process for DF emission.

11.2.2 TADF Molecules as Emitters for OLEDs

With apparent advantages in harvesting both singlet and triplet excitons for electroluminescence and theoretically 100% internal quantum efficiency, TADF molecules were widely used as emitters in OLEDs [2]. Upon electrical excitation, the theoretical maximum EQE of the TADF OLEDs can be estimated by Eq. (11.1) [2]:

11.1

where ηint is the internal quantum efficiency of the device, ηout is the out‐coupling constant, γ is the ratio of the charge combination to the electron and hole transportation, ηr is the excitation–production ratio, ηPL is the photoluminescence efficiency, ΦPF is the photoluminescence quantum yield (PLQY) of the PF, ΦDF is the PLQY of the DF, k is the number of ISC and rISC transitions, and ΦISC and ΦrISC are the ISC and rISC efficiencies, respectively. If the triplet exciton is very stable with very low nonradiative decay rate (knrT → 0), the ΦrISC will approach 1 and EQE = γ × ηPL× ηout, suggesting that 100% of the excitons can be used for electroluminescence. When ηPL = 1, the internal quantum efficiency will be 100%. Therefore, in order to obtain the high efficiency of TADF OLEDs, TADF emitters should have high PLQY (ηPL = 1), small ΔEST, and stable triplet state for total transformation of triplet excitons to singlet ones through efficient reverse ISC (ΦrISC = 1) and properly aligned the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels for efficient charge injection as well as good bipolar mobility for balanced and efficient charge transportation to increase the charge recombination ratio (γ → 1). In addition, the TADF emitters should also possess the other characteristics, such as good film‐forming properties, sufficiently high glass transition temperature to avoid crystallization within the desired operation lifetime of the device, and appropriate solubility and stability for fabrication procedures, for example, spin coating, inkjet printing, and vacuum deposition [16, 1820].

11.2.3 TADF Molecules as Host Materials and Sensitizers for OLEDs

A large number of TADF molecules, especially in D–A molecular architecture, are found to have low PLQY due to their too much separated HOMO and LUMO to efficiently support radiative decay for luminescence, eliminating their applications as efficient OLED emitters according to Eq. (11.1). Fortunately, these D–A type TADF molecules have balanced and bipolar charge transporting/injection properties, which are very attractive to function as high‐performance host materials and singlet exciton sensitizers in OLED applications [2, 21].

Host materials are important in OLEDs to disperse emitters, especially for the long‐lived triplet ones, to alleviate the quenching effects, such as concentration quenching, triplet–triplet annihilation (TTA), triplet exciton‐polaron quenching (TPQ), and electric field‐induced exciton dissociation. An efficient host material should possess the following features [2224]:

  1. Sufficiently higher triplet energy (ET) than the doped emitters (guests) to prevent reverse energy transfer from the guest to the host.
  2. Properly aligned HOMO and LUMO levels to match with those of the neighboring layers to ensure effective charge injection from the adjacent layers.
  3. Larger HOMO–LUMO energy gap (Eg) than that of the guest to facilitate direct charge trapping on the doped emitter.
  4. Balanced charge transport properties for hole and electron recombination process and confinement of the excitons formation zone in the EML.
  5. Good morphological stability, film‐forming ability, and structure compatibility to make the dispersion of dopant in host uniform during the device fabrication and operation. Moreover, the orientation of the host matrix may also have great impact on the device performance [2527].

Singlet exciton sensitizers can harvest triplet excitons generated in OLEDs for fluorescent emitters via energy transfer [28]. To guarantee efficient triplet exciton harvesting using a TADF sensitizer, two critical requirements should be satisfied [29]:

  1. The excitons should be formed in TADF materials, and the direct exciton trapping by fluorescent dopant should be avoided.
  2. The energy transfer from TADF sensitizer to fluorescent materials should only be mainly the singlet–singlet Förster energy transfer, and triplet–triplet Dexter energy transfer should be suppressed.

Therefore, the HOMO and LUMO energy levels of the TADF host/sensitizer should be close to that of fluorescent dopant to prevent the direct trapping of the guest/emitter; the doping concentration of fluorescent emitter should be very low to minimize the Dexter energy transfer to alleviate the triplet exciton formation in fluorescent materials.

11.2.4 Host‐free TADF OLEDs

The TADF emission is closely related to the long‐lived triplet excited states via rISC process. Therefore, TADF emitters are generally needed to be dispersed in a host and protected for high OLED performance. However, the host–guest systems are susceptible to crystallize and aggregate for phase separation due to the different structural compatibility of host and guest molecules, leading to a poor device performance especially in long‐term device operation. Moreover, the doping of emitters into the host requires precise control of the concentrations of both guest and host, which is a complicated process, causing a high cost of fabrication. And the host molecules may influence the photoluminescent (PL) and electroluminescent (EL) properties of the guests, inducing large variations in the emitting color, PLQY, and lifetime of the EML due to strong host and guest interactions [30]. Therefore, there is a trend, recently, to eliminate the use of host materials in both metal complex‐based PhOLEDs and TADF OLEDs. Thanks to the large structural diversity of purely organic molecules, TADF materials can be designed relatively easier to be not as sensitive as metal complexes toward host materials and doping levels; the doping concentrations of the metal‐free TADF emitters can be varied in a wide range from 1  to 40  wt% and even 100% (host free) [31, 32]. Of course, with the aforementioned advantages without using host materials, TADF molecules for the nondoped devices should have the following additional characteristics:

  1. The host‐free film of TADF materials should show a high PLQY.
  2. Special attentions should be paid during molecular design of TADF molecules to reduce quenching effects of the long‐lived triplet excitons excited electrically in OLEDs.
  3. The general requirements of OLED EMLs, including matched frontier orbital energy levels toward adjacent charge transport layers for efficient and balanced charge injection and high quality nanoscale thin film with smooth and stable morphology, should be satisfied.

11.3 Basic Considerations in Molecular Design of TADF Molecules

11.3.1 Design Principles of Donor–Acceptor Molecular Systems for TADF Emission

The key process in TADF emission is the rISC (T1 → S1) that upconverts excitons from the low‐lying T1 to S1 at a higher energy level, when the ΔEST between S1 and T1 is small enough (<0.37 eV) that can be overcome by the thermal vibration with the aid of surrounding temperature [33, 34]. The small ΔEST can be achieved through separated HOMO and LUMO distributions or, more specifically, through separated frontier orbital distributions [33]. Another important factor for TADF emission is the high luminescent efficiency, which requires overlapped HOMO and LUMO for high efficient luminescence with small transition dipole moment [4, 35]. Therefore, these two contradictory sides in simultaneously realizing small ΔEST for efficient rISC (separated HOMO and LUMO) and high luminescent efficiency for strong emission (overlapped HOMO and LUMO) should be taken into consideration to an optimized extent in the molecular design of the metal‐free organic materials for efficient TADF emissions.

The majority of the reported TADF materials based on D–A molecular system (Figure 11.4) were designed via intramolecular charge transfer (ICT) state with the following characteristics:

  1. A small ΔEST in donor–acceptor structure is realized by separating HOMO and LUMO on donor and acceptor groups, respectively [36].
  2. The steric hindrance structures, such as twisty, bulky, or spiro junctions, were introduced to connect donor and acceptor units to effectively separate the spatial overlaps between HOMO and LUMO [37, 38].
  3. The π‐conjugation length and redox potentials of donor and acceptor moieties along with the interruption extent of the conjugation between them should also be taken into consideration entirely to achieve an emission with desired color and high PLQY [39].
  4. Densely combined donors and acceptors are used to aggrandize the overlap between HOMO and LUMO wave functions and enhance molecular structural rigidity, hence to increase radiative luminescent efficiency for high PLQY [11].

Figure 11.4 Molecular design of the intramolecular TADF materials from the widely available donor and acceptor building blocks.

In addition to the intramolecular D–A type TADF emitters, the ICT between two electron‐donating and electron‐accepting molecules can also produce TADF emissions from the in situ formed exciplexes, showing broad and redshifted emissions compared with individual donor and acceptor molecules [40]. The electron transition from the LUMO of an acceptor to the HOMO of a donor in a large electron‐hole separation distance for exciplex emission should have a small ΔEST for TADF emission via efficient rISC from the triplet charge transfer (CT) state to the singlet CT state (Figure 11.5). However, this CT emission of exciplex is generally very low in efficiency. To enhance the TADF emission in exciplexes, appropriate donor and acceptor molecules should be carefully selected with the following considerations:

  1. Electron‐donating and accepting molecules should have high triplet energy levels to confine the triplet exciplex state by preventing the quenching of the triplet state via the triplet energy back transfer to the donor or acceptor [10].
  2. Both the shallow HOMO levels in the electron‐donating molecules and deep LUMO levels in the electron‐accepting molecules, as well as high PL efficiency, are important for exciplex‐based OLEDs (Figure 11.5).
  3. Electron‐donating and electron‐accepting molecules should have a planar structure for a flat‐on orientation in solid states to avoid the formation of excimer in the EML [41].

Figure 11.5 The intermolecular D–A system design for TADF emission in OLEDs.

11.3.2 Control of Singlet–Triplet Energy Splitting (ΔEST)

A small ΔEST is of fundamental importance for TADF materials featured by a fast rISC process [2]. Theoretically, the molecular energy of the lowest singlet (ES) and triplet (ET) excited state is determined by orbital energy (E), electron repulsion energy (K), and exchange energy (J) of the two unpaired electrons at the excited states, as illustrated in Eqs. (11.2) and (11.3). By definition, ΔEST is the difference between ES and ET and is then equal to the twice of J (Eq. (11.4)) by subtracting Eq. (11.3) from Eq. (11.2), suggesting J is the most decisive factor for the ΔEST [42]:

11.2
11.3
11.4

At S1 and T1, the unpaired two electrons are mainly distributed on the frontier orbitals of the HOMO and the LUMO, respectively, leading to the same J values regardless of the different spin states. Therefore, the exchange energy (J) of these two electrons at HOMO and LUMO can be determined by Eq. (11.5):

11.5

where ϕH and ϕL are the HOMO and LUMO wave functions, respectively, and e represents the electron charge. From Eq. (11.5), J is determined by spatial separation (r1 − r2) and overlap integral of ϕH and ϕL, i.e. spatial wave function separation of frontier orbitals. Thus, a small ΔEST can be expected when there is a small overlap and/or a large separation between HOMO and LUMO, and, in contrast, a large overlap and/or a small separation will lead to a large ΔEST (Eq. (11.6)):

11.6

The calculation of ΔEST in Eq. (11.6) can be simplified by using the orbital overlap integral (IH/L) and the mean separation distance (<rH/L>) of HOMO and LUMO. IH/L represents the extent of overlap between HOMO (H) and LUMO (L), which can be calculated using the overlap integral function of Multiwfn (Eq. (11.7)) [33, 43]:

11.7

<rH/L > is obtained from the barycenter (rtot) of the absolute value of the corresponding molecular orbitals to summarize the effects of the spatial separation (r1 − r2) (Eqs. (11.8)–(11.11)). Further adopting the function outputting statistic data of the points in specific spatial and value range, the rtot of the absolute value of the molecular orbital can be computed as in Eq. (11.8):

11.8

where r denotes coordinate vector, f represents the data value, and k runs over all grid points including positive and negative points, respectively. Thus, the barycenter of HOMO and LUMO (rH and rL, respectively) are

11.9
11.10

Thus, the mean distance between HOMO and LUMO is

11.11

Assuming that the variation of the separation distance between HOMO and LUMO (r1 − r2) is in a small range around <rH/L>, ΔEST could be related to IH/L and <rH/L > in Eq. (11.12):

11.12

where ΔEST is in eV and <rH/L > is in Å.

However, using the simple HOMO→LUMO transition to picture the transition nature of S1 or T1 states is not accurate. To give a whole picture of the excited states, natural transition orbital (NTO) analysis was performed to offer a compact orbital representation for the electronic transition density matrix to consider all the possible transitions [44, 45]. Similarly, the overlap extent (IS and IT in Eqs. (11.13))–(11.14))) and mean separation distance (<rS > and <rT > in Eqs. (11.15))–(11.20))) of the highest occupied natural transition orbitals (HONTOs) (ϕH′) and the lowest unoccupied natural transition orbitals (LUNTOs) (ϕL′) at both S1 and T1 states were calculated to give a full‐picture analysis of ΔEST using Multiwfn [33, 43]:

11.13
11.14
11.15
11.16
11.17
11.18
11.19
11.20

With the aid of NTO analysis, the description of ΔEST (Eq. (11.12)) can be updated to Eq. ((11.21) using above‐defined parameters of IS, IT, <rS>, and <rT > to consider the whole picture of the electron interactions of the corresponding excited states:

11.21

where CS and CT are the combination constants of S1 and T1 states, respectively. Consequently, the ultralow ΔEST can be obtained with both separated HONTO and LUNTO at S1 and T1 states (low IS and IT, long <rS > and <rT>); small ΔEST can be resulted with separated HONTO and LUNTO at S1 state but small overlapped at T1 state (low IS but high IT, long <rS > but short <rT>); and theoretically, ideal TADF molecules can be designed with small ΔEST from separated HONTO and LUNTO at T1 state (low IT, long <rT>) and high luminescent efficiency from overlapped HONTO and LUNTO at S1 state (high IS, short <rS>) [33].

11.3.3 Modulation of Luminescent Efficiency of TADF Emission

According to Eq. (11.1), the theoretical maximum EQE of the TADF OLED is proportion to the PLQY (ηPL) of TADF emitters [46]. In order to improve the device performance of TADF OLEDs to compete with that of PhOLEDs, the PLQY of TADF molecules should be very high. Compared with the relatively sophisticated control of ΔEST to obtain TADF by separating HOMO and LUMO of a molecule, the concurrently achieved small ΔEST and high PLQY are still a challenge. Considering important processes for both PF and DF in OLEDs, internal electroluminescence quantum efficiency (ηint) of TADF compounds can be expressed as follows (Eq. (11.22)):

11.22

Here, ФPF is typically expressed as

11.23

where kf is the PF decay rate and τPF is the PF lifetime that can be obtained by fitting the decay curve of the time‐resolved PL spectrum. The relationship between kf and the absorption coefficient in fluorescent molecules can be expressed as [47, 48]

11.24
11.25

where νf and f(νf) represent the fluorescence wave number and the fluorescence spectrum, νa and ε(νa) are the absorption wave number and the molar absorption coefficient at νa, and n is the refractive index. From Eq. (11.24), kf is closely related to ε(νa), which can be experimentally measured by absorption spectrum. To obtain high ε(νa) for high kf, the molecules can be designed according to the relationship between ε(νa) and oscillator strength (F) and transition dipole moment (Q) for absorption as [49]

11.26

where me, c, h, and e are the electron mass, light speed, Planck's constant, and elementary charge, respectively, and <vf > represents the average wave number of fluorescence. The values of F and Q can be either experimentally determined or theoretically predicted.

High ηint needs high ФPF, which can be achieved through large kf, based on Eqs. ((11.22))–((11.23)); large kf in turn requires large F and Q values, according to Eqs. (11.24)–(11.26). Therefore, for the development of TADF materials with high ηint, those key characteristics involving high kf, strong oscillator strength for absorption (F), and large transition dipole moment (Q) should be satisfied. From the molecular design point of view, one can extend the molecular orbitals to suppress a decrease in F and kf meanwhile limit the overlap between HOMO and LUMO for a low ΔEST. Following this strategy, Adachi and coworkers obtained nearly 100% of ηint in TADF molecules by inducing a large oscillator strength (for high PL efficiency) at even a small overlap between the two wave functions of the frontier orbitals (for small ΔEST) [48].

11.4 Typical Donor–Acceptor Molecular Systems with High TADF Performance

Generally, it is convenient to construct TADF molecules using donors and acceptors in D–A molecular systems in either intramolecular or intermolecular architectures for CT state with separated HOMO (distributed on donor) and LUMO (distributed on acceptor) and small ΔEST. However, a suitable bridge is needed to connect donor and acceptor units together properly to build an integral molecular system for TADF emissions. Therefore, it is important to smartly choose not only suitable donor and acceptor building blocks but also the connecting bridge to realize simultaneously small ΔEST, stable T1, and highly luminescent S1 with desired emission color, efficiency, and charge injection and transport properties for OLED applications. A large variety of organic donors and acceptors are available for the selection, but the major choice of donor units is concentrated on the N‐containing aromatics of carbazole, diphenyl amine, phenoxazine, and their derivatives, due to their strong electron‐donating ability, facile synthesis, stable and high triplet states, etc. For intermolecular D–A type TADF systems, the bridge between donors and acceptors are not needed, but interactions between them should be strong enough to form a large number of stable exciplexes with strong CT feature for small ΔEST and TADF emission. As a result, the suitable donor and acceptor pairs for efficient TADF emission in intermolecular system is relatively difficult to be satisfied, and there remains plenty of challenges and opportunities in developing intermolecular TADF materials for full‐color and high efficient TADF OLEDs. Besides these intra‐ and intermolecular TADF small molecules, TADF polymers with apparent advantages in solution‐processable devices are also developed recently. Such a wide range of available rare metal‐free D–A type TADF materials will certainly promote the investigations of organic electronics and boost the practical applications in the near future.

11.4.1 Cyano‐based TADF Molecules

Due to the strong electron‐withdrawing ability of cyano group, cyano‐substituted aromatophors exhibit promising electron affinities to act as electron‐accepting building blocks in the molecular scaffold for constructing high‐performance D–A type TADF molecules [50, 51]. Adachi and coworkers prepared a TADF emitter (1, Scheme 11.1) with two cyano electron‐accepting moieties and two di‐p‐tolylamino electron‐donating moieties, which were orthogonally connected through a spiro bridge to provide large steric hindrance and bulkiness between the donor and acceptor moieties for the effective separation of HOMO and LUMO with increased thermal and morphological stabilities and reduced aggregation tendency in the solid state [52]. Compound 1 shows yellow emission with low PL quantum efficiency of 27% and a small ΔEST of 0.057 eV. The TADF OLEDs doped with 1 as the emitter (ITO/α‐NPD/6 wt% 1: mCP/Bphen/Mg Ag/Ag) exhibited an EQE of 4.4%, maximum current efficiency (CE) of 13.5 cd A−1, and power efficiencies (PE) of 13.0 lm W−1, with the aid of N,N′‐4,4′‐dicarbazole‐3,5‐benzene (mCP) as the host materials and 4,4‐bis[N‐(1‐naphthyl)‐N‐phenylamino] biphenyl (a‐NPD) and 4,7‐diphenyl‐1,10‐phenanthroline (Bphen) as hole‐transport and electron‐transport layers (ETL), respectively. To improve the TADF OLED device performance, a modified spiroacridine derivative (2, ΔEST = 0.028 eV, Scheme 11.1) was also synthesized by the same group [53]. The efficient TADF emission with a high PLQY of 67.3% endowed the OLED devices of 2 (ITO/TAPC/mCP/6 wt% 2: TPSiF/TmPyPB/LiF/Al) with a maximum EQE of 10.1%, which undoubtedly breaks through the theoretical EQE limitation of fluorescent OLEDs (5%).

Scheme 11.1 Cyano‐based TADF molecules of 1–32.

To understand the influence of the twisting angle between donor and acceptor units on the TADF performance, Adachi and coworkers investigated two bicarbazolyl dicyanobenzene derivatives in A–D–A structure with different steric hindrance (3–4, Scheme 11.1) [54]. The two TADF molecules exhibit small ΔEST of 0.14 and 0.06 eV, bright blue photoluminescence at 470 and 488 nm, high PL quantum yields of 50% and 72%, and efficient devices performance with maximum EQE of 9.2% and 9.6%, respectively. The larger twisting angle induced by ortho‐substitution of CN because of steric hindrance in 4 results in more efficient separation of HOMO and LUMO for smaller ΔEST, shorter fluorescence lifetime, and reduced efficiency roll‐off at high luminance.

Further improvement of the TADF device efficiency with EQE of up to 19.3% was realized in 2012 by employing a series of highly efficient TADF materials based on carbazolyl dicyanobenzene, where the carbazoles and dicyanobenzenes are used as electron‐donating and electron‐accepting groups, respectively (Scheme 11.1) [11]. The emission color was easily tuned from sky blue to orange by changing the number of carbazolyl groups or by introducing different substituents on carbazole (Figure 11.6). The cyano group efficiently suppresses both nonradiative deactivation and geometries change at the excited states of those materials, and therefore their PL efficiencies are largely improved. The steric hindrance induced by the densely substituted donors results in the distorted carbazolyl units from the dicyanobenzene plane, leading to the breakage of π‐conjugation with localized HOMO and LUMO on the donor and acceptor, respectively, for small ΔEST (0.083–0.15 eV). The blue, green, and orange TADF OLEDs reached high maximum EQEs of 8.0 ± 1.0%, 19.3 ± 1.5%, and 11.2 ± 1.0%, respectively, which are comparable with that achieved in PhOLEDs based on noble metal–organic complexes. These TADF OLEDs suffer from high‐efficiency roll‐offs due to serious exciton quenching effects including singlet–triplet annihilation (STA) [55, 56] and TTA. In order to improve the device stability, a more stable device structure for 8 was fabricated through controlling the recombination zone position by carefully selecting the exciton‐blocking layers, tuning the interfaces of the EML, and optimizing the concentration of the emitter [57]. These 8‐doped TADF OLEDs showed excellent device stability with operation lifetime more than 2500 h at an initial luminance of 1000 cd m−2 and over 10 000 h at 500 cd m−2, demonstrating that the TADF material is intrinsically stable under electrical excitation. Furthermore, the performance of TADF OLEDs can be also influenced by the molecular orientation and the deposition temperature of the host materials [26]; when 8 is dispersed in a randomly orientated host of CBP, the efficiency roll‐off of the device was suppressed by 30% at a current density of 100 mA cm−2. Further studies show that the host materials are very important in realizing high‐performance TADF OLEDs based on 8; the EQE can be improved to 21.2% [58], 24.2% [59], 26.7% [60], 28.6% [61], 29.6% [62], and 31.2% [15] with various hosts. Using a carrier‐/exciton‐confining structure and energy transfer from an exciplex, the 8‐based TADF OLEDs showed a low turn‐on voltage of 2.33 V and maximum PE of 107 lm W−1 with a record of 79.4 lm W−1 at 1000 cd m−2, which is 1.6 times higher than that of state‐of‐the‐art TADF OLEDs [63].

Figure 11.6 Normalized photoluminescence spectra measured in toluene solution (a) and EQE as a function of current density for TADF OLEDs. Inset, normalized electroluminescence spectra of the OLEDs. The compounds of 5–12 are shown in Scheme 11.1.

To realize the ultimate potential of organic electronics in large‐area, low‐cost, and high‐efficiency display or lighting products, intense and growing interests were paid to solution‐based processes, such as spin‐coating or inkjet printing [6466]. Roll‐to‐roll solution processing techniques were also applied to the low‐cost fabrication of TADF OLEDs based on 8; high EQEs (19.1%) comparable with those achieved by conventional vacuum deposition were demonstrated [67]. Moreover, TADF molecules (9–10, Scheme 11.1) with improved solubility for the fabrication of high‐performance solution‐processed TADF OLEDs were further designed by modifying 8 with highly soluble methyl or tert‐butyl groups by Lee's group [18]. The resulting molecules show good solubility in common solvent and green TADF emissions with high PLQY of 67% for 9 and 78% for 10, and maximum EQEs of 8.2% and 18.3% were achieved in their solution‐processed TADF OLEDs, respectively.

Efficient TADF can be also observed when reducing the number of carbazole substituents on the dicyanobenzene core. The two carbazole‐substituted 12 shows blue TADF for OLEDs with EQE of up to 13.6% [68]. By changing the substitution site on the dicyanobenzene core, analogues of 13 (PLQY = 85%, ΔEST = 0.08 eV) and 14 (PLQY = 35%, ΔEST = 0.05 eV) are also efficient in blue TADF emission, exhibiting turn‐on voltages of 4.2 V and 3.5 V and EQEs of 15% [24] and 16.4% [69], respectively, in TADF OLEDs. Furthermore, 14 is an excellent host material for yellow PhOLEDs. The PhOLED of iridium(III) bis(4‐phenylthieno[3,2‐c]pyridinato‐N,C2′)acetylacetonate (PO‐01) hosted by 14 achieved a turn‐on voltage of 2.0 V and a maximum EQE of 24.9%, which are among the best results of yellow PhOLEDs reported so far. Still, 14 is excellent as a host for white OLEDs with a maximum EQE of up to 22.9% and a warm white color coordinate of (0.39, 0.43).

Besides carbazole, other electron donors are also effective in constructing TADF molecules when they were connected on the dicyanobenzenes. Lee et al. successfully developed two TADF molecules (15–16, Scheme 11.1) based on benzofurocarbazole and benzothienocarbazole as the donor [70]. These two TADF emitters exhibit high thermal property with Tg of 204 °C and 219 °C, Td of 397 °C and 434 °C, and small ΔEST of 0.13 eV and 0.17 eV; blue photoluminescence with high PLQY of 94.6% and 94.0% in solution; and high TADF OLED performance with maximum EQE of 12.1% and 11.8%, respectively. Yasuda and coworkers prepared a series of wedge‐shaped TADF molecules using various donor units (1‐methylcarbazole, 9,9‐dimethylacridan, and phenoxazine). These molecules (17–21) show small ΔEST of 0.04–0.36 eV, efficient TADF emissions that cover the entire visible range from blue to red and high‐performance TADF OLEDs with high EQE of up to 18.9% [71].

Besides the dicyanobenzene acceptors, tricyanobenzene (22) and mono‐/di‐cyanopyridines (23, 24) are also applicable in designing cyano‐based TADF molecules. The tricyanobenzene‐based TADF molecule (22) has six substitutions with two different types of alternating substituents of three cyano groups and three triphenylamines, resulting in reduced vibrational deactivation processes because of the high steric hindrance [72]. The molecule shows green emission with a peak at 533 nm, small ΔEST of 0.103 eV, and PLQY of 100% in doped film, demonstrating the almost complete suppression of nonradiative decay for a maximum EQE of 21.4% in its TADF OLEDs. Based on cyanopyridine, Tao and coworkers also synthesized a soluble TADF molecule (23, Scheme 11.1) with carbazole as the donor [19]. The compound shows green emission peaked at 536 nm in CHCl3 and yellow emission peaked at 560 nm in doped film with PLQY of 55%, ΔEST of 0.07 eV, and thermal property of 408 °C for Td and 159 °C for Tg. Its solution‐processed TADF OLED devices with the configuration of ITO/PEDOT: PSS/8 wt% 23: mCP)/TmPyPB/LiF/Al achieved a maximum EQE of 11.3%, CE of 38.9 cd A−1, and PE of 14.8 lm W−1. Zhang and coworkers prepared a blue TADF emitter with dicyanopyridine (pyridine‐3,5‐dicarbonitrile) as the acceptor and carbazole as the donor (24, Scheme 11.1). Separated HOMO and LUMO distribution for extremely low ΔEST (0.04 eV), high thermal property with Td of 350 °C, blue emission with high PLQY (∼50%) in doped film, and a maximum EQE of 21.2%, CE of 47.7 cd A−1, and PE of 42.8 lm W−1 in the TADF OLEDs were achieved [73]. Duan and coworkers developed a new series of blue TADF molecules based on cyanobenzene (25–28, Scheme 11.1). The four or five carbazole‐substituted cyanobenzenes show small ΔEST of 0.17–0.30 eV, high PLQY of 0.49–0.86, and high thermal properties with Td of 436–454 °C and Tg of 316–325 °C. The blue TADF OLEDs achieved high efficiencies with a maximum EQE of up to 21.2% and excellent stability with a record long T50 of 770 h at an initial luminance of 500 cd m−2, showing improved efficiency and stability by protecting luminance core with the steric shielding effects of the tert‐butyl substituents [74].

For the further development of TADF molecules, Lee and coworkers proposed a new design concept via a dual TADF‐emitting core containing four cyano substituents [75]. The molecule of 29 was designed and synthesized through a simple coupling of two TADF cores of 14. The resulted dual‐core TADF emitter shows small ΔEST of 0.13 eV, blue emission with PLQY of 91%, and a high TADF device performance with a low turn‐on voltage of 3.5 V, maximum EQE of 18.9%, and CIE coordinate of (0.22, 0.46). Following this design strategy, they further prepared three other twin TADF emitters (30–32, Scheme 11.1), which exhibit blue and green colors with high PLQY of 61–87%, small ΔEST of 0.11–0.21 eV, and high efficient OLEDs with EQE of up to ∼20% [76].

11.4.2 Nitrogen Heterocycle‐based TADF Molecules

Due to the electron‐deficient nature of aromatic systems containing electronegative nitrogen (N) atoms, N‐incorporated arenes are typical electron‐accepting moieties with promising electron affinities for the molecular design of D–A type TADF molecules.

Triazine derivatives. The highly electron‐deficient triazine containing three N atoms with three easily modifiable sites is highly attractive for the construction of TADF molecules by connecting electron‐donating substituents to the electron‐accepting triazine core either symmetrically or asymmetrically [77]. For example, Adachi and coworkers prepared an asymmetric triazine‐based TADF emitter (33, Scheme 11.2) by introducing two bulky indolocarbazoles and one biphenyl onto the triazine core; the strong donor of indolocarbazole with large steric hindrance results in a separated HOMO and LUMO with small ΔEST (0.11 eV) for blue‐green TADF emission [78]. The TADF device with the configuration of ITO/α‐NPD/mCP/6 wt% 33: mCP/BP4mPy/LiF/Al, where BP4mPy (3,3′,5,5′‐tetra[(M‐pyridyl)‐phen‐3‐yl]biphenyl) acts as ETL, achieved an maximum EQE of 5.3%. This TADF molecule of 33 is also a high‐performance host material for PhOLED due to its balanced charge transport and good injection properties; the green PhOLEDs (using phosphorescent guest of tris(2‐phenylpyridinato) iridium (III), Ir(ppy)3) hosted by 33 exhibited a very low onset voltage of 2.19 V, maximum CE of 68 cd A−1, and PE of 60 lm W−1 [21]. TADF‐sensitized fluorescent OLEDs using 33 as a sensitizer for a yellow fluorescent dopant was also reported to show a maximum EQE of 4.5% and a PE of 12.3 lm W−1 at 10 cd m−2. Using another TADF sensitizer of 34 with one bulky indolocarbazole and two biphenyl substituents on triazine, the EQE is further improved to 11.7%, which apparently exceeds the theoretical EQE upper limit (5%) of fluorescent OLEDs [79]. Similar to 33, 34 is also an excellent host for green PhOLED [80], showing a maximum EQE of 23.9% and a PE of 77.0 lm W−1 of the devices. After a minor modification of the number and structure of donors in 33 and 34, new TADF molecules of 35, 36, and 37 can be conveniently obtained due to the existence of strong ICT state when directly connecting carbazoles to the triazine framework through N atoms. Compared with 33, the TADF molecule of 35 has a smaller ΔEST of 0.02 eV, higher PLQY (45 ± 1%), and much better device performance of the TADF OLEDs with significantly improved EQE of 14% [81]. When the donors of indolocarbazoles were replaced with bicarbazoles, the ΔEST of the resulted molecules of 36 [82] and 37 [83] are 0.06 and 0.09 eV, respectively, leading to EQEs of up to 11% and 6% in their corresponding TADF OLEDs. Additionally, the yellowish‐green PhOLEDs employing 37 as the host materials reached a maximum EQE of 20.1% [84].

Scheme 11.2 Triazine‐based TADF molecules of 33–52.

Besides the direct connecting of donor substituents and acceptor core for triazine‐based TADF molecules, aromatic bridges are also useful in controlling the interactions between the donor and acceptor for TADF emission. When a phenyl bridge is adopted as in 38–39 (Scheme 11.2) [85], relatively larger ΔEST of ∼0.25 eV due to slightly overlapped HOMO and LUMO on the bridge were observed. These blue TADF materials show high thermal stability with Td of ∼500 °C and Tg of ∼220 °C, and their TADF OLEDs achieved high maximum EQE of 17.5% for 38 and 18.9% for 39, respectively. More importantly, the 39‐based TADF OLED showed a lifetime of 52 h of up to 80% of initial luminance at 500 cd m−2, which is almost three times as long as that of blue phosphorescent OLED under the same device structure using tris[1‐(2,4‐diisopropyldibenzo[b,d]furan‐3‐yl)‐2‐phenylimidazole]iridium (Ir(dbi)3) as the emitter. A subsequent study by Lee and coworkers reported the synthesis of triazine‐based TADF emitters (40–42, Scheme 11.2) following a similar design rule using dense substitution of the phenyl bridge [16]. These materials exhibit ΔEST in a range of 0.07–0.23 eV but have high PLQY of up to 100% and tunable emission color from blue to green. High device performance with maximum efficiency of 25% was achieved in those TADF emitters doped OLEDs. Dendritic diphenylamine and carbazole with large molecular size and electron‐donating ability were also connected to the phenyl bridge to prepare new TADF molecules with high PLQY by controlling the spatial overlap between the frontier orbitals to suppress nonradiative decay. The resulting blue (43) [48] and green (44) [86] TADF molecules show small ΔEST (<0.1 eV) and high PLQY (80% for blue and 100% for green) for high‐performance TADF OLEDs with maximum EQE of 20.6% and 13.8% for blue and green, respectively. Especially, using a carbazole and diphenylamino hybrid dendritic donor, the obtained green TADF emitter (45) has a small ΔEST of 0.026 eV and 100% PLQY and 100% reverse ISC efficiency; the TADF OLED with 45 as emitter showed a maximum EQE of 29.6% [87]. A white‐light TADF OLED based on red (7), green (8), and blue (46) triazine‐based TADF materials as the emissive dopants was also achieved, exhibiting a high EQE over 17% with CIE coordinate of (0.30, 0.38) [13].

Benefited from the phenyl bridge, which separates the donor substituents and triazine acceptor core and modulates their interactions effectively, various donors are applicable to prepare high‐performance triazine‐based TADF molecules with partially overlapped frontier orbitals on the bridge for small ΔEST and high PLQY simultaneously. Tanaka et al. introduced the electron donor of phenoxazine into the triphenyltriazine to construct TADF material (47, Scheme 11.2) [46]. Due to the high steric repulsion with large dihedral angle (74.9°) between the phenoxazine moiety and the phenyl ring, the twisted D–A structure facilitates a spatially separation of HOMO and LUMO, leading to a small ΔEST (0.07 eV) for efficient TADF emission with high PLQY of 65.7% in doped film. The TADF device with the structure of ITO/α‐NPD/6 wt% 47: CBP/TPBi/LiF/Al achieved an outstanding EL performance with a maximum EQE of 12.5% and high luminance of 10000 cd m−2. Further investigations on orientation order of 47 regulated by tuning the temperature during fabrication of the thin films revealed that the film deposited at 200 K showed a horizontal orientation and the EQE of the device can be enhanced by 24% compared with corresponding OLEDs with the vertically orientated film deposited at 300 K [27]. By increasing the numbers of the phenoxazine unit connected to the triphenyltriazine core, emission colors of the resulted TADF emitters (48, 49, Scheme 11.2) can be tuned from 545 to 568 nm, and the HOMO and LUMO are much more separated in 48 and 49 than that in 47 due to the increased steric hindrance, leading to a smaller ΔEST of 0.054 eV for 48 and 0.065 eV for 49 [88]. High PLQYs of 64% and 58% were observed in 48 and 49 doped mCBP (3,3′‐bis(N‐carbazolyl)‐1,1′‐biphenyl) films. The TADF OLEDs based on the two emitters with the configuration of ITO/α‐NPD/6 wt% 48 or 49: mCBP/TPBi/LiF/Al exhibited maximum EQE of 9.1 ± 0.5% and 13.3 ± 0.5%, respectively. In addition, the TADF‐sensitized fluorescent OLEDs with 47 and 49 as sensitizers were also studied by Adachi and coworkers. The resulting OLEDs showed high EQEs of 18% and 17.5% for yellow and red fluorescent dopants and a relatively long operational lifetime of up to 194 h [28]. They continued to modify the structure of 47 using phenothiazine to replace phenoxazine [89]. The resulted molecule (50, Scheme 11.2) demonstrates a dual ICT fluorescence with TADF characteristics peaked at 409 and 562 nm, which is composed of the direct 1CT1A → S0A transition with a large ΔEST between the 1CT1A and 3CT1A states and an indirect 1CT1E → S0E transition allowed through the successive 3CT1E → 1CT1E upconversion followed via rISC with a small ΔEST between the 1CT1E and 3CT1E states, due to the presence of two different quasi‐axial (CT1A) and quasi‐equatorial (CT1E) conformers at excited states, where the superscript E and A represent equatorial and axial, respectively. A maximum EQE of 10.8 ± 0.5% was observed in the 50‐doped TADF OLEDs with dual ICT fluorescence.

Kim and coworkers firstly introduced a highly rigid, bulky, and electron‐donating azasiline to the phenyl bridge of triazine to prepare TADF molecules [90]. The resulting 51 shows deep blue TADF emission with high PLQY of 74 ± 2% in doped film and small ΔEST of 0.14 eV. The TADF device achieved a maximum EQE of 22.3% with a CIE coordinate of (0.149, 0.197). Tsai et al. reported a TADF emitter (52, Scheme 11.2) by connecting 9,9‐dimethyl‐9,10‐dihydroacridine as the electron‐donating unit to the phenyl bridge of 1,3,5‐triazine [91]. The resulted molecule exhibits a small ΔEST of ∼0.05 eV, high glass transition of 91 °C, and high PLQY of 90% in a doped film and 83% in a nondoped (neat) films. Both the nondoped and doped TADF OLEDs achieved outstanding device performance with maximum EQE of 20% and 26.5%, respectively.

In light of the great potential of the nondoped solution‐processable TADF molecules for large‐scale and easy fabrication, Yamamoto and coworkers prepared a series of TADF dendrimers based on dendritic carbazole (D) and 2.4.6‐triphenyl‐1,3,5‐triazine (A) [92]. These dendritic TADF materials (53–56, Scheme 11.3) show similar emission profiles with high PLQY of ∼100% in the solution and small ΔEST of 0.03–0.06 eV. The solution‐processed undoped TADF OLEDs in a device structure of ITO/PEDOT: PSS/EML/TPBI/Ca/Al achieved EQEs of 2.4% (54), 3.4% (55), and 1.5% (56).

Scheme 11.3 Triazine‐based TADF dendrimers of 53–56.

Pyrimidine derivatives. Pyrimidine is a highly π‐deficient acceptor that can be easily modified on 2,4,6‐positions. Kido and coworkers synthesized a series of TADF molecules (57–59, Scheme 11.4) using pyrimidine as the acceptor and phenylacridine as the donor [93]. These molecules doped in bis‐(2(diphenylphosphino)phenyl)ether oxide) (DPEPO) films exhibit light blue emission with high PLQY of ∼80%, small ΔEST of <0.20 eV, and high device performance with low turn‐on voltage of 2.8 V, high EQE of 24.5%, and high PE of 61.6 lm W−1.

Scheme 11.4 Pyrimidine‐based TADF molecules of 57–59.

Triazole, oxadiazole, and thiadiazole derivatives. The N‐containing five‐membered aromatic rings of oxadiazole, triazole, and thiadiazole are widely used in building the electron‐transporting materials for OLEDs due to their good electron transportation and injection abilities [94, 95]. Adachi and coworkers developed a series of TADF emitters (60–63, Scheme 11.5) based on oxadiazole and triazole derivatives in D–A or D–A–D architecture to realize separated HOMO and LUMO distribution for small ΔEST [96]. The films with 61 and 63 as the dopants hosted by DPEPO show high PLQY of 52 ± 3% and 87 ± 3%, respectively. Employing these doped films as the EMLs, maximum EQE of 6.4% (sky blue) and 14.9% (green) with corresponding CIE color coordinates of (0.16, 0.15) and (0.25, 0.45) at 10 mA cm−2 were observed in the TADF OLEDs with the device configuration of ITO/α‐NPD/mCP/6 wt% 61 or 63: DPEPO/DPEPO/TPBi/LiF/Al. Adachi and coworkers also used 1,3,4‐thiadiazole to prepare TADF molecules (64, Scheme 11.5) [97]. They found that the vacant 3d orbitals of divalent sulfur in the thiadiazole heteroring can cause electron‐pair‐accepting conjugative effect for narrowed bandgap, enhanced S1 → T1 ISC, increased contribution of the DF component, and reduced ΔEST for enhancing reverse ISC. The maximum EQE of its TADF OLEDs reached 10.0%.

Scheme 11.5 N‐containing five‐membered heteroring‐based TADF molecules of 60–64, where triazole, oxadiazole, and thiadiazole are acceptors and phenoxazine is donor.

Heptazine derivatives. Heptazine, which possesses a rigid and planar heterocyclic system of six CN bonds surrounding a central sp2‐hybridized N‐atom, was also applied as an acceptor building block in constructing the TADF materials in intramolecular D–A molecular structure [98]. The orange‐red TADF emitter (65, Figure 11.7) containing a heptazine core and three electron‐donating tert‐butyl substituents of triphenylamine shows high decomposition temperature at 519.9 °C, relatively small ΔEST of 0.17 eV due to slight overlap of HOMO and LUMO, and high PLQY of 91 ± 0.9% in the doped film using 26mCPy as the host matrix. Although the component of DF of the 65 doped film is only 6% in PL, the OLED with the structure of ITO/α‐NPD/6 wt% 65: 26mCPy/Bphen/MgAg/Ag exhibited a high device performance with a turn‐on voltage of 4.4 V, a maximum luminance of 17000 cd m−2, and a maximum EQE of 17.5%, CE of 25.9 ± 1.6 cd A−1, and PE of 22.1 lm W−1, suggesting that rISC process is highly efficient under electrical excitation for high OLED efficiencies, even though 65 demonstrates quite weak TADF in the PL process.

Figure 11.7 EQE‐current density curve of 65. (inset, the molecular structure of 65).

1,4‐Diazatriphenylene derivatives. The 1,4‐diazatriphenylenes (ATP), which has a diaza‐heterocyclic and three fused benzene rings, are well known as a strong electron‐withdrawing unit for wide applications in OLEDs due to their large π‐conjugated framework and high triplet energy. A series of interesting TADF emitters (66–70, Scheme 11.6) based on ATP were developed by combining phenoxazine, 9,9‐dimethylacridane, and 3‐(diphenylamino)carbazole as the donor units, respectively, in a D–A–D molecular architecture [99]. These molecules show typical TADF characteristics with small ΔEST (0.04–0.26 eV), high PLQY of up to 81%, and high EQE of the doped TADF OLED of up to 12%. A near‐infrared (NIR)TADF molecule (71, Scheme 11.6) based on cyano‐substitution diazatriphenylenes and triphenylamine in V‐shaped D–A–D configuration was reported to have a relatively small ΔEST of 0.13 eV, a large kF value of 9.0 × 107 s−1, and strong aggregation‐induced emission (AIE) effect with high PLQY of 14% in solid film. The nondoped and doped TADF devices using 71 as the emitter achieved the maximum EQE of 2.1% and 9.8% with electroluminescence peaks at 668 nm and 708 nm, respectively. Those data are among the best results of the most efficient deep red/NIR OLEDs, including the PhOLEDs, reported so far [100].

Scheme 11.6 1,4‐Diazatriphenylenes‐based TADF molecules of 66–71.

Benzothiazole and benzoxazole derivatives. Benzothiazole and benzoxazole derivatives are efficient electron acceptors for charge transport materials in organic electronics due to their strong electron‐deficient nature. Therefore, they are promising building blocks for constructing TADF materials [101]. Adachi and coworkers reported a series of TADF emitters (72–76, Scheme 11.7) containing benzothiazole and benzoxazole as acceptor in D–A or D–A–D architecture to realize separated HOMO and LUMO distribution for small ΔEST (0.033–0.071 eV). These D–A–D type TADF materials exhibited higher PLQY of 98% (75), 81% (76), and 80% (74) compared with the D–A type molecules of 72 (72%) and 73 (75%). Maximum EQE of 16.6%, 14.4%, and 14.0% for the D–A–D type emitters and 9.1% and 12.1% for the D–A type emitters were observed in the TADF OLEDs employing the doped films as the EMLs. Due to the introduction of an exciton‐blocking layer between the EML and ETL, the maximum EQE of 75‐based OLED can be increased to 17.6%.

Scheme 11.7 Benzothiazole‐, benzoxazole‐, and quinoxaline‐based TADF derivatives of 72–79.

Quinoxaline derivatives. In light of the high electron‐withdrawing ability and good thermal stability of quinoxaline, Adachi and coworkers developed two TADF molecules (77–78, Scheme 11.7) using quinoxaline as the acceptor unit and different arylamines as the donor [102]. They found that the TADF properties can be controlled through tuning the twisting angle between the electron‐donating and accepting units. The 77 with small twisting angle of 50° exhibits higher PLQY of 74% and electroluminescence EQE of 12.8% compared with PLQY of 66% and EQE of 10.4% of 78 with large twisting angle of 77.9°. Swager and coworkers developed a TADF molecule (79, Scheme 11.7) using cyano‐substitution quinoxaline as the acceptor, triptycene as the scaffold, and diphenylamine as the donor [103]. The prepared molecule shows small ΔEST of 0.11 eV, high thermal stability (Td > 380 °C), and high PLQY of 44% in oxygen‐free cyclohexane solutions. The OLEDs using 79 as TADF emitters achieved an emission peak at 573 nm and a maximum EQE of 9.4%.

Diazafluorene derivatives. With orthogonally connected π‐conjugated system and strong electron‐accepting ability, diazafluorene would be a promising building block for TADF molecules. Adachi and coworkers reported a TADF molecule (80, Scheme 11.8) using diazafluorene as an electron‐accepting unit and bis(diphenylamino)acridane as an electron‐donating unit [104]. The prepared compound 80 exhibits a small ΔEST of 0.021 eV, blue‐greenish TADF emission with a PLQY of 70% in a mCP doped film at room temperature, and high EQE of the doped TADF OLEDs of up to 9.6%.

Scheme 11.8 Diazafluorene‐based TADF molecule of 80.

11.4.3 Diphenyl Sulfoxide‐based TADF Molecules

Diphenyl sulfoxide, containing a powerful electron‐withdrawing group with a twisted angle in the center, is one of the most famous and typical electron‐acceptor components for TADF molecule constructions [105]. A series of diphenyl sulfoxide‐based TADF molecules (81–83, Scheme 11.9) using diphenylsulfonyl and arylamines as acceptor and donor, respectively, have been developed [39]. These molecules show broad and structureless emission bands from the ICT singlet state (1CT) and well resolved and characteristic phosphorescence from 3π–π* state localized at the donor. Therefore, the mechanism of the observed TADF emission involves a reverse IC from T1 (3π–π*) to 3CT and a successive rISC from 3CT to 1CT (S1). In order to enhance the energy interchange between T1 and S1, the 3π–π* state should be close to or even higher than the 3CT state. Due to the multiple energy transfer from T1 to S1, those TADF materials of 81–83 show relatively large ΔEST of 0.54, 0.45, and 0.32 eV, respectively. By controlling the redox potential and π‐conjugation length of donor and acceptor moieties, bright pure blue TADF emission peaked at 421, 430, and 423 nm with high PLQY of 60, 66, and 80% in the doped film hosted by DPEPO, and efficient TADF devices with maximum EQEs of 2.9, 5.6, and 9.9% are achieved in 81–83, respectively. The best device performance was observed in the 83‐based TADF OLEDs, showing a standard blue emission with EQE of 9.9% and CIE of (0.15, 0.07) due to the lowest ΔEST induced by slightly raised 1CT but greatly raised 3π–π* after the replacement of diphenylamine with carbazole. However, serious EQE roll‐off was generally observed in these devices, because of the longer triplet exciton lifetime induced by the relatively larger ΔEST. In order to reduce the ΔEST for low‐efficiency roll‐off, the more electron‐rich methoxy groups were introduced to replace the tert‐butyl substituents on the carbazole units [106]. The resulting TADF emitter of 84 (Scheme 11.9) shows deep blue emission with small ΔEST = 0.21 eV, a relatively short triplet exciton lifetime, a high PLQY of 80% in the doped DPEPO film, and a maximum EQE of 14.5% with significantly reduced efficiency roll‐off in its TADF OLEDs. Even at a bright emission of 100 cd m−2, the EQE of the 84‐doped OLEDs maintains above 9%. The effects of the number and the linking position of the electron‐donating units on the ΔEST of the diphenylsulfoxide‐based TADF molecules (83, 85–88, Scheme 11.9) were also investigated [107]; the results show that ΔEST can be tuned from 0.39 to 0.22 eV. To further reduce the ΔEST of the diphenylsulfoxide‐based TADF emitters, Kido and coworkers enhanced the electron‐accepting diphenylsulfoxide by inserting one more phenyl sulfone into the diphenylsulfoxide core; two TADF molecules based on bis(phenylsulfonyl)benzene (89–90, Scheme 11.9) [108] show smaller ΔEST of 0.05 and 0.24 eV, blue emission peak at 446 and 422 nm, high PLQY of 66.6% and 71.0% in doped film, and maximum EQE of 5.5% and 10% in TADF OLEDs were observed, respectively.

Scheme 11.9 Diphenylsulfoxide‐based TADF molecules of 81–97.

Besides diphenylamines and carbazoles, other electron‐donating structures have been also employed as donors to interact with diphenylsulfone acceptor to build efficient TADF molecules. Zhang et al. reported a series of TADF molecules using different donors of 5‐phenyl‐5,10‐dihydrophenazine (91), phenoxazine (92), and 9,9‐dimethyl‐9,10‐dihydroacridine (93) [12]. These molecules with ΔEST around 0.08 eV and high kISC at the order of ∼107 s−1 show PL peaks at 577, 507, and 460 nm with high PLQY of 3, 80, and 80%, respectively. Blue TADF OLED employing 93 as the dopant achieved a maximum EQE of 19.5%, low turn‐on voltage of 3.7 V, and blue emission with CIE coordinate of (0.16, 0.20). Importantly, the EQEs of these TADF OLEDs still maintained 16.0% at a brightness of 1000 cd m−2, showing very low‐efficiency roll‐off and great potential in replacing the noble metal‐based phosphorescent complexes. Furthermore, a nondoped blue TADF OLED based on 93 without the help of host molecules still exhibits a high device performance with maximum EQE of 19.5% [32], mainly attributing to the special molecular structure composed of large twist structure of diphenyl sulfoxide and steric hindrance of methyl groups on the 9,9‐dimethyl‐9,10‐dihydroacridine that greatly suppress the aggregation‐caused quenching effect in film state. Furthermore, a hybrid white OLED using the blue emission TADF molecules of 93 as the triplet harvester and green and red fluorescent emitters as singlet harvesters realized a high EQE over 12% and CIE coordinates of (0.25, 0.31) [109].

Chi and coworkers designed two AIE TADF molecules (94–95, Scheme 11.9) with strong AIE property through regulation of molecular interactions by introducing phenothiazine to diphenylsulfoxide moieties [110]. The mono‐ (95) and bi‐phenothiazine (94) substituted TADF molecules show green emission peak with high PLQY of 93.3% and 52.8% and small ΔEST of 0.2 and 0.03 eV, respectively. Interestingly, the compound 95 also exhibits a significant mechanoluminescent induced by asymmetric molecular structure and formation of a noncentrosymmetric arrangement in crystals. When applying an asymmetric molecular structure design of the diphenylsulfone with two different donor substituents (96), dual‐emissive TADF compounds that fully inherit the photophysical properties of the parent molecules can be produced [111], and the asymmetric TADF emitters (97) are more efficient than the symmetric ones with enhanced EQE of the host‐free OLEDs of up to 17% [112].

11.4.4 X‐bridged Diphenyl Sulfoxide‐based TADF Molecules

For the further development of TADF molecules, X‐bridges are introduced to connect the diphenyl of the high‐performance TADF building block of diphenyl sulfoxide to construct a more rigid molecular structure with improved optical and electronic properties. With C‐bridge and acridine as donors, Lee and coworkers developed a deep blue TADF emitter (98, Scheme 11.10), which shows a zero ΔEST, high PLQY of 100% in doped film, and high device performance with a maximum EQE of 19.8% and deep blue CIE coordinate of (0.15, 0.13) [113]. Using C‐bridge of diphenyl sulfoxide and phenoxazine as donor, 99 (Scheme 11.10) was prepared and found to have an environment‐dependent TADF with a tunable ΔEST and a maximum EQE of 13.5% in OLEDs [114]. An additional sulfoxide bridge was also introduced into the diphenyl sulfoxide, providing a stronger acceptor of doubly OSO bridged diphenyl for the design of D–A type TADF molecules. Combining with donors of 9,9‐dimethyl‐9,10‐dihydroacridin and phenoxazine, Su and coworkers prepared two highly efficiently TADF molecules (100–101, Scheme 11.10), which exhibit small ΔEST of 0.058 and 0.048 eV and high PLQY of 71% and 62%, respectively. Their TADF OLEDs showed low turn‐on voltage of 3.5 and 3.7 V, maximum EQE of 19.2% and 16.7% for evaporation device and turn‐on voltage of 3.7 and 4.1 V, and maximum EQE of 17.5% and 15.2% for solution‐process device, respectively [115]. Besides X‐bridge, the direct linking of the diphenyl to produce the sulfurafluorene dioxide was also reported to be applicable to produce TADF molecules [116, 117].

Scheme 11.10 X‐bridged diphenyl sulfoxide‐based TADF molecules of 98–101.

11.4.5 Diphenyl Ketone‐based TADF Molecules

Containing a highly electron‐withdrawing CO group with a twist angle in the center, diphenyl ketone is known as a special purely organic phosphor with efficient ISC (kISC = 1011 s−1) that is highly attractive for building TADF materials [118120]. Recently, Adachi and coworkers developed a series of butterfly‐shaped D–A–D type TADF emitters (102–106, Scheme 11.11) using diphenyl ketone as the acceptor moiety and carbazoles or phenoxazines as the donor moiety [121]. These molecules show small ΔEST (0.03–0.21 eV) and highly efficient TADF emissions in full color with emission peaks at 444, 475, 538, 541, and 555 nm and PLQY of 55, 73, 70, 71, and 36%, respectively. High‐performance monochromatic TADF OLEDs of 102–106 with maximum EQEs of 8.1, 14.3 10.7, 6.9, and 4.2% (Figure 11.8); turn‐on voltages at 4.3, 4.4, 3.2, 3.6, and 2.8 V; and maximum luminance of 510, 3900, 86 100, 57 120, and 50 820 cd m−2 were observed, respectively. Notably, a white TADF OLED using blue (103) and red (106) dopants was also successfully realized, showing a maximum EQE of 6.7% with CIE coordinate of (0.32, 0.39). To obtain a white‐light‐emitting TADF molecule, Chi and coworkers designed an asymmetric TADF molecule (107, Scheme 11.11) based on 102 by asymmetrically connecting carbazolyl and phenothiazinyl substituents to the diphenyl ketone core [122]. A white TADF emission from two complementary colors emission induced by carbazole (blue) and phenothiazine (yellow) with ΔEST of 0.04 eV and obvious AIE phenomenon was observed. Researches on subsequent structural modification of 103 by Lee to cross the donor and acceptor in an X‐shaped molecular structure resulted in two new TADF molecules (108–109, Scheme 11.11), showing separated HOMO and LUMO with small ΔEST of 0.02 and 0.05 eV, green and sky blue emission band at 510 and 496 nm in the toluene, high PLQY in doped films of 57% and 46% with high kf of ∼7.0 × 106 s−1, and EQEs of 11.3% and 10.0% in the green TADF OLEDs, respectively [123]. Zhang et al. introduced the large and twisty 9,9‐dimethyl‐9,10‐dihydroacridine to diphenyl ketone [32]. The resulting molecule (110, Scheme 11.11) shows a green TADF emission with high PLQY of 90%, small ΔEST of ∼0.07 eV, and a maximum EQE of 18.9% with small efficiency roll‐off in an undoped TADF OLED.

Scheme 11.11 Diphenyl ketone‐ and phenyl(pyridin‐4‐yl)methanone‐based TADF molecules of 102–113.

Figure 11.8 EQE‐current density curves and electroluminescence spectra (inset) of TADF OLEDs based on 102–106.

Besides diphenyl ketone acceptors, phenyl(pyridin‐4‐yl)methanone is also applicable in preparing D–A type TADF molecules. Cheng and coworkers prepared 111 by connecting two meta carbazolyl groups on the phenyl ring of phenyl(pyridin‐4‐yl)methanone [124]. The TADF molecule (111) shows a small ΔEST of 0.06 eV, reversibly switchable emission via external stimuli, and high OLED performance with a low turn‐on voltage of 3.6 V, high EQE of 18.4%, and CIE coordinate of (0.18, 0.25). By tuning the substitution position of the carbazolyl units, two new TADF molecules (112 and 113) were resulted; those materials show blue and green emission with high PLQY of 88% and 91.4% (doped film), small ΔEST of 0.03 eV and 0.04 eV, and high efficient OLEDs with maximum EQE of 24.0% and 27.2%, respectively [125].

11.4.6 X‐bridged Diphenyl Ketone TADF Molecules

To improve the TADF performance of diphenyl ketone‐based molecules, various atoms including oxygen, carbon, and sulfur have been introduced into diphenyl ketone as an X‐bridge to promote the optical and electronic properties by constructing rigid molecular structure. Through the C‐bridge of diphenyl ketone (spiro‐anthracenone), Adachi and coworkers developed a blue‐greenish TADF molecule (114, Scheme 11.12) that shows a small ΔEST of 0.04 eV, a high PLQY of 81%, and a maximum EQE of 16.5% in the TADF OLEDs [30]. Also, high‐performance TADF‐sensitized OLEDs employing 114 and its O‐bridged analogue (115, Scheme 11.12) as the sensitizers to transfer ∼100% electrically excited excitons to conventional fluorescence molecules were found to show high EQEs of 13.5% and 15.8%, respectively [28].

Scheme 11.12 X‐bridged diphenyl ketone‐based TADF molecules of 114–126.

Another ketone bridge was also introduced to diphenyl ketone to enhance its electron deficiency and fasten ISC process; the formed CO bridge diphenyl ketones (anthraquinone) is highly effective as an acceptor to cooperate with different donors in D–A–D configuration for a series of TADF molecules (116–123, Scheme 11.12 ) [14]. These compounds show large fluorescence rate (kF) of up to 6.1 × 107 s−1, small ΔEST of 0.07–0.29 eV, tunable emission color from yellow to red with PLQY of up to 80%, and high‐performance TADF OLEDs with low turn‐on voltages around 3 V; high maximum EQEs of 12.5, 9.0, 9.0, and 6.9%; and varied emission bands at 624, 637, 574, and 584 nm, respectively.

The S‐bridged diphenyl ketones (thioxanthones) can benefit from both ketone and sulfoxide bridges for high rate ISC (kISC) and high quantum yield of triplet formation, showing great potential in constructing high‐performance TADF molecules. Recently, two novel TADF emitters (124–125, Scheme 11.12) with small ΔEST (0.052 and 0.073 eV) and high kISC of 107 s−1 using thioxanthone as the acceptor and arylamine as the donor were reported by Wang et al. [126] 124 and 125 show PL peaks at 625 and 570 nm with pronounced AIE characteristic, high PLQY of 36 ± 2% and 96 ± 2% in film, and highly efficient TADF OLEDs with turn‐on voltages at 5.3 and 4.7 V and maximum EQEs of 18.5% and 21.5%, respectively. Wang et al. also introduced a sulfur bridge to diphenyl ketone to prepare TADF molecule (126, Scheme 11.12). The resulting 126 containing a bulky donor shows a small ΔEST of 0.19 eV for efficient rISC to harvest the triplet excitons and high thermal property with Td of 456 °C and Tg of 127.3 °C for device operation. When 126 was used as a host material for the orange and red PhOLEDs, high EQEs of 11.8% and 15.6% with low‐efficiency roll‐off was observed, due to significantly reduced triplet concentration via efficient rISC from triplet to singlet and thus subdued TTA and TPA processes on host [127].

11.5 Organoboron‐based TADF Molecules

Boron‐containing fragment, which contains a boron‐incorporated aromatic moiety with strong electron‐accepting property due to the high accepting ability of the boron atom, has emerged as a new acceptor unit for TADF molecule design [128, 129]. Four TADF molecules (127–130, Scheme 11.13) containing 10H‐phenoxaborin and acridan as acceptor and donor were prepared by Adachi and coworkers [130]. These compounds show blue TADF emissions with high PLQY (56–100%) and small ΔEST of 0.06–0.12 eV; their TADF OLEDs exhibited a high EQE of 21.7, 13.3, 19.0, and 20.1%, respectively. Kitamoto et al. also developed two 10H‐phenoxaborin‐based TADF molecules (131–132, Scheme 11.13) [131], which show blue and green emissions with high PLQYs of 98% and 99% in the doped films, small ΔEST of 0.013 eV and 0.028 eV, and excellent blue (peaked at 466 nm) and green (peaked at 503 nm) TADF OLEDs with maximum EQEs of 15.1% and 22.1%, respectively. Recently, Wu and coworkers used boron atom as a hub for the spiro linker of light‐emitting molecules, leading to spatially separated HOMO and LUMO with orthogonal orientation in boron complexes (133–135, Scheme 11.13). The electron‐deficient pyridyl pyrrolide and electron‐donating phenylcarbazolyl fragments or triphenylamine connected by boron leads to intense green TADF, and the OLEDs showed an EQE of up to 13.5% [132]. Hatakeyama et al. constructed two new TADF molecules (136–137, Scheme 11.13) based on a rigid polycyclic aromatic framework containing triphenylboron and two nitrogen atoms; the multiple resonance effect of boron and nitrogen atoms results in separated HOMO and LUMO for TADF emission with small ΔEST of 0.14 eV and 0.18 eV, high PLQY of 84% and 90%, and ultrapure blue emission with emission peak at 462 and 470 nm, respectively [133]. The blue TADF OLEDs using 136–137 as emitters achieved high performance with EL peaks at 459 nm and 467 nm, FWHM of 28 nm and 28 nm, CIE coordinates of (0.13, 0.09) and (0.12, 0.13), and high EQE of 13.5% and 20.2%, respectively.

Scheme 11.13 Organoboron‐based TADF molecules of 127–137.

11.6 TADF Polymers

Currently, the majority of the materials capable of TADF emission are limited to small molecules; it is a great challenge to prepare TADF polymers for solution‐processable polymeric OLEDs (P‐OLEDs) with apparent advantages in reducing the fabrication costs and increasing the device size. Recently, Nikolaenko et al. proposed a strategy, which they called “intermonomer TADF,” to provide a promising route for molecular design of TADF polymers [134]. This concept involves of spatial separation of the HOMO and LUMO on donor and acceptor units, respectively, for the formation of intermonomer CT states to realize small ΔEST and act as the charge transporting units, and an additional comonomer with high triplet energy level to act as high T1 spacer units to prevent aggregation of the TADF emissive centers (intermonomer D–A structure) and to fulfill solution processibility. The synthesized TADF polymer (Scheme 11.14) shows a small ΔEST of 0.22 eV, a green emission peaked at ∼535 nm with PLQY of ∼41%, and a high performance in solution‐processed TADF P‐OLEDs with EQE of up to 10%.

Scheme 11.14 The design of TADF polymers based on “intermonomer TADF” with donor (D), acceptor (A), and backbone (B) units.

In other efforts to prepare TADF polymers, Dias and coworkers connected efficient TADF small molecules to the side chain of a flexible polystyrene and incorporated into a conjugated backbone (Scheme 11.15); an acceptable EQE of 3.5% at 100 cd m−2 was observed [135]. Yang and coworkers presented a series of TADF polymers through grafting the TADF emitter onto the side chain of the polymer backbone following side‐chain engineering strategy. The OLEDs using these TADF grafted polymers as the emitter exhibited a maximum EQE of 4.3% [136].

Scheme 11.15 Backbone and side‐chain type TADF polymer.

11.7 Intermolecular D–A System for TADF Emission

Besides intramolecular D–A molecules, intermolecular D–A systems are also applicable to compose TADF emissions. The well‐known exciplex is an intermolecular D–A structure formed between a donor (D) and an acceptor (A) via CT under electrical excitation in OLED devices. Exciplex emission, which is low in efficiency and broad in emission spectrum, was generally not favored in previous OLEDs studies [40]. However, recent studies show that for the exciplex emission of the intermolecular excited states, the electron transition from the LUMO of an acceptor to the HOMO of a donor over a large electron‐hole separation distance can provide a smaller exchange energy compared with that of intramolecular D–A systems, resulting in a small ΔEST for TADF emission [10]. The exciplex‐based TADF OLEDs can be fabricated either in bilayer with two layers of donor and acceptor molecules or in single layer with their mixture. A slight redshift between the PF and DF components was observed in exciplex‐type TADF, which is different from the TADF emission from single D–A molecules. The reason for the spectral shift has not been clarified at this moment. Adachi and coworkers explained that DF is generated immediately after the rISC process, so that the nuclear configuration of S1 formed through rISC is affected by that of T1. The Franck–Condon factor (including polarization in the host medium) of DF would be different from that of PF, leading to slight different PL spectra of PF and DF [10]. The existence of a broad distribution of the energy level of the exciplex originating from different geometric arrangements between the two molecules in the exciplex has been regarded as another plausible reason [137].

Blended single‐layer exciplex. The significant performance breakthrough of exciplex‐based OLEDs to utilize TADF effects was recently demonstrated in m‐MTDATA (D): 3TPYMB (A) intermolecular D–A system [10] (Scheme 11.16). The device based on single‐layer m‐MTDATA: 3TPYMB (1 : 1) exciplex exhibited a high EQE of 5.4% that is higher than the limit of conventional fluorescence‐based OLEDs even at a rather low PLQY (26%) of the exciplex; the efficient rISC (86.5%) greatly contributes to increase EQE of the device. Yang and coworkers [138] changed the ETL thickness of the same intermolecular D–A system and achieved a lower turn‐on voltage of 2.1 V, a higher luminescence of up to 17100 cd m−2,a higher CE of 36.79 cd A−1, and a lower efficiency roll‐off. To further improve the performance of OLEDs based on TADF emissions of exciplexes, a new electron‐accepting molecule 2, 8‐bis(diphenylphosphoryl) dibenzo‐[b,d]thiophene (PPT), which has higher triplet energy to confine the triplet exciplex, was adopted [139]. After carefully optimizing the ratios of the donor (m‐MTDATA) and the acceptor (PPT) for high PLQY and suitable recombination zone, the green OLEDs showed a high PE of 47.0 lm W−1 and an EQE of up to 10%. Peng and coworkers selected m‐MTDATA and BPhen as donor and acceptor components, respectively; a maximum EQE of 7.79% was achieved in the TADF OLEDs containing m‐MTDATA: 70 mol% BPhen as the EML with an extremely low ΔEST close to zero [140].

Scheme 11.16 Representative donors (a) and acceptors (b) applicable for intermolecular D–A type TADF emission.

Kim and coworkers reported a new D–A material pair for exciplex‐type TADF emission using a donor of 4,4′,4″‐tris(N‐carbazolyl)‐triphenylamine (TCTA) and an acceptor of bis‐4,6‐(3,5‐di‐3‐pyridylphenyl)‐2‐methylpyrimidine (B3PYMPM). The PLQY of the exciplex emission is only 10% at 195 K but almost 100% at 35 K [137]. The EQE of the OLEDs increased from 3.1% to 10% when the temperature dropped from 300 to 195 K. In‐depth studies of this exciplex system indicated that kISC (1.1 × 107 s−1) is faster than krS (7 × 106 s−1) and krISC has a distribution in the range from 3 × 106 to 3 × 104 s−1 depending on the delay time and is almost independent of temperature. The temperature‐independent rISC rate indicates that the rISC takes place without thermal activation, which is very different from that in single molecular D–A type TADF materials. The broad range of the krISC was suggested to result from the broad energy level distributions of the singlet and the triplet exciplexes. Further, the singlet CT states may lie energetically below the triplet CT states, because the kinetic exchange dominates the interactions of the donor and acceptor in the exciplex, which favors the singlet state with short intermolecular distance and coulombic interaction at CT states [141, 142]. Therefore, the singlet and triplet exciplex have broad energy level distributions, and energy difference between them (ΔEST) is extremely small that can be either positive or negative. And the rISC from the triplet to singlet exciplex can occur even without thermal activation in this system. The especially low ΔEST (estimated to be 5 meV) was also observed by Graves et al. in a green‐emitting exciplex (525–550 nm) between m‐MTDATA and PBD in a 50 : 50 blended film [143]. Recently, a new electron‐transporting material of ((1,3,5‐triazine‐2,4,6‐triyl)tris(benzene‐3,1‐diyl))tris (diphenylphosphine oxide) (PO‐T2T) that has very low LUMO and HOMO (−2.83 and −6.83 eV, respectively) was synthesized by Wong and coworkers [144]. Based on PO‐T2T, a panchromatic range of exciplex emission from blue to red was realized by systematically tuning the HOMO of the hole‐transporting materials. And a tandem, all‐exciplex‐based white OLED was also fabricated, showing excellent EQE (11.6%), CE (27.7 cd A−1), PE (15.8 lm W−1), CIE (0.29, 0.35), and color‐rendering indices (CRI) of 70.6. Zhang and coworkers also reported a new exciplex system with PO‐T2T as the acceptor and a new TADF emitter of MAC (6‐(9,9‐dimethylacridin‐10(9H)‐yl)‐3‐methyl‐1H‐isochromen‐1‐one) as the donor. The new exciplex D–A system has two rISC routes on both the single‐molecule TADF emitter of MAC and the exciplex emitter; this TADF molecular design can greatly enhance the utilization of the triplet excitons, increase the device efficiency, and decrease the efficiency roll‐off, achieving a maximum EQE of 17.8% and low‐efficiency roll‐off (12.3% at 1000 cd m−2) [145].

The widely used commercial host material of mCP was also found to be effective in constructing intermolecular D–A system for TADF emission when combined with a new electron‐accepting molecule of 2,5,8‐tris(4‐fluoro‐3‐methylphenyl)‐1,3,4,6,7,9,9b‐heptaazaphenalene (HAP‐3MF) [146]. The OLEDs containing 8 wt% HAP‐3MF: mCP as the EML exhibited a high EQE of up to 11.3%, which is far above the EQE limit (5%) of fluorescent OLEDs but is still far below the theoretical limit (20%) of TADF OLEDs due to the low PLQY. More recently, Liu et al. reported an exciplex pair of TAPC: DPTPCz, which shows the highest PLQY (68%) and EQE (15.4%) in the exciplex OLEDs reported so far [147]. Similar to the intramolecular D–A type TADF emitters, exciplexes were also used as singlet exciton sensitizers to harvest triplet excitons in OLEDs for fluorescent emitters. Liu et al. fabricated a TADF‐sensitized OLED by using TAPC:DPTPCz as singlet exciton sensitizer; a low turn‐on voltage of 2.8 V and high CE, PE, and EQE of 44.0 cd A−1, 46.1 lm W−1, and 14.5% were observed, respectively [29].

Bilayer‐type exciplex : Alternatively, the exciplex‐based TADF OLEDs can also function in a bilayer structure using an appropriate combination of donor and acceptor layers. The TADF OLED efficiencies of a bilayer‐type exciplex can be significantly enhanced according to the following criteria:

  1. Both the donor and acceptor molecules should have a planar structure for the flat‐on orientation of the molecules in solids to avoid the appearance of excimer emission in the EML.
  2. High hole and electron mobilities should be satisfied to allow sufficient charge carriers to the interface for exciplex generation.
  3. The charge carriers can be accumulated at the interfacial region by the large differences between the frontier energy levels of donor and acceptor, giving a high propensity of electron‐hole recombination to generate S1 and T1 states.
  4. More importantly, the triplet energies of donor and acceptor need to be higher than that of the exciplex to confine the electro‐generated triplet states within the interfacial region.

Consequently, the nonemissive T1 state can predominantly shuttle back to emissive S1 state, leading to a high‐efficiency bilayer‐type exciplex OLED.

In 2013, Hung et al. reported a simple bilayer‐type exciplex TADF OLED based on a carbazole‐centered electron‐donating material (TCTA) and a triazine‐based electron‐accepting material (3P‐T2T) [148]. The yellow OLED exhibited a high CE of 22.5 cd A−1 and an EQE of up to 7.7%. Su et al. reported a highly efficient bilayer TADF OLED containing TAPC as the donor and TmPyTZ as the acceptor, which exhibited an extremely low turn‐on voltage of 2.14 V, high maximum CE of 37.8 cd A−1, EQE of up to 12.02%, and PE of 52.8 lm W−1 [149]. Recently, Cherpak et al. used a metal complex (FIrpic) as an acceptor to form interface exciplex with THCA [41]. This new bilayer OLED emits yellow light consisted of a combination of the blue phosphorescent emission from FIrpic and a broad efficient DF from the exciplex, exhibiting a high maximum CE of 15 cd A−1, brightness of 38 000 cd m−2, and EQE of ∼5%.

11.8 Summary and Outlook

The design and characterization of TADF materials for optoelectronic applications represents an active area of recent research in organic electronics. Widely considered as the third generation OLED materials, noble metal‐free TADF materials either in small molecules, dendrimers, polymers, or exciplex pairs are promising candidates as emitters, sensitizers, and hosts with small ΔEST for theoretically 100% excitons utilization in D–A molecular structure for balanced charge transport and injection properties, which are highly attractive for a wide variety of high‐performance optoelectronic devices. TADF OLEDs using TADF emitters have shown significant improvements in PLQYs, color varieties, and device EQEs, since their first report in 2009. Up till now, high PLQY of ∼100%, maximum EQEs over 30%, low‐efficiency roll‐off, blue‐to‐red and white TADF OLEDs, and long operation lifetimes have been evidenced in many TADF molecular systems. In addition, the low sensitivity of TADF molecules toward concentrations allow the fabrication of nondoped TADF OLEDs with EQEs of up to 20%; solution‐processable TADF small molecules, dendrimers, and polymers have also been successfully developed, demonstrating the ultimate potentials of organic materials in large‐area, low‐cost, and high‐efficiency display and lighting products. TADF‐sensitized fluorescent OLEDs using TADF sensitizers can also harvest 100% excitons through the efficient rISC of the TADF molecules, exhibiting excellent device performance with full‐color emission, promising operation stability, and EQEs of up to 15% that obviously exceeds 5% limitation of traditional fluorescent OLEDs. PhOLEDs using TADF molecules as host materials show EQEs over 20%, extremely low driving voltages of ∼2.0 V, and low‐efficiency roll‐off (<10%) due to the balanced and bipolar charge transport/injection properties induced by the D–A configuration and small ΔEST. The outstanding performance in certain aspects of these judiciously designed D–A type TADF materials has even transcended that of noble metal‐containing phosphorescent complexes, in addition to their distinct relevant competitive advantages of low‐cost, rich resources, and easy material preparation. The success in the breakthrough of the theoretical and technical challenges and the revolution of the understandings of organic optoelectronics that arise in developing high‐performance TADF materials may pave the way to shape the future of organoelectronics.

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