Recent progress in thermally activated delayed fluorescence emitters for nondoped organic light-emitting diodes

Nondoped organic light-emitting diodes (OLEDs) have drawn immense attention due to their merits of process simplicity, reduced fabrication cost, etc. To realize high-performance nondoped OLEDs, all electrogenerated excitons should be fully utilized. The thermally activated delayed fluorescence (TADF) mechanism can theoretically realize 100% internal quantum efficiency (IQE) through an effective upconversion process from nonradiative triplet excitons to radiative singlet ones. Nevertheless, exciton quenching, especially related to triplet excitons, is generally very serious in TADF-based nondoped OLEDs, significantly hindering the pace of development. Enormous efforts have been devoted to alleviating the annoying exciton quenching process, and a number of TADF materials for highly efficient nondoped devices have been reported. In this review, we mainly discuss the mechanism, exciton leaking channels, and reported molecular design strategies of TADF emitters for nondoped devices. We further classify their molecular structures depending on the functional A groups and offer an outlook on their future prospects. It is anticipated that this review can entice researchers to recognize the importance of TADF-based nondoped OLEDs and provide a possible guide for their future development.


Introduction
Organic light-emitting diode (OLED) technology has been extensively applied in high-end solid-state lighting and atpanel displays owing to its superior advantages of autoluminescence, low driving voltage, exibility, fast response, and so on. 1 According to the spin statistics, electrogenerated singlet and triplet excitons have a ratio of 1 : 3. Traditional uorescent materials with high photoluminescence quantum yields (PLQYs) can only utilize singlet excitons, while the energies of triplet excitons are depleted via nonradiative transition processes. To realize full exciton utilization, phosphorescence and thermally activated delayed uorescence (TADF) emitters have been proposed. 2,3 Phosphorescence emitters can utilize triplet excitons due to the strong spin-orbit coupling (SOC) induced by heavy atoms (e.g., platinum and iridium), which can help to enhance the triplet radiation (i.e., phosphorescence) rate. Although phosphorescence-based OLEDs are now the main commercial OLED technologies, they have several obvious drawbacks, such as high costs and toxicity due to heavy atoms and the absence of stable blue phosphorescence emitters.
TADF-based OLEDs are an alternative approach with potential 100% internal quantum efficiencies (IQEs) and have attracted considerable attention within the last decade. [4][5][6] Fig. 1 Illustration of the exciton utilization channels of the TADF mechanism upon electrical excitation. GS and ES are ground and excited states, respectively; S and T are the electrogenerated singlet and triplet excitons; ISC and RISC are the intersystem crossing and reverse intersystem crossing processes; nr S and nr T are the nonradiative transition processes of the singlet and triplet excitons; PF and DF are the prompt and delayed fluorescence. Fig. 1 illustrates a schematic illustration of exciton energy transfer and utilization in a TADF emitter upon electrical excitation. Singlet excitons have three possible energy decay routes: uorescence radiation, nonradiative transition to the ground state (S 0 ) and intersystem crossing (ISC) from the lowest singlet excited state (S 1 ) to the lowest triplet excited state (T 1 ), while triplet excitons can either be upconverted into singlets via reverse intersystem crossing (RISC) or suffer nonradiative decay (assuming that the phosphorescence process can be ignored in conventional pure organic systems). RISC and uorescence radiation are the key steps for triplet and singlet exciton harvesting, respectively. To realize a competitive RISC rate (k RISC ) to that of the corresponding triplet nonradiative process (k T nr ), a small S 1 -T 1 energy splitting (DE ST ) is required. To date, several design strategies have been proposed, such as highly twisted D-A molecular structures, 7-10 exciplex systems, [11][12][13][14][15][16][17] and the recently developed multiple resonance (MR) effect-induced TADF. [18][19][20] A common characteristic of these molecular designs is that they can all restrain the overlaps between the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs), which is essential for small DE ST s. Singlet-triplet spin-orbit coupling (SOC) is another factor that signicantly affects the RISC process. It can be efficiently enhanced via either internal/external heavy atom effects [21][22][23][24] or excited states with a different nature. [25][26][27][28] Fluorescence radiation is the ultimate pathway for not only the initial singlet exciton harvest (prompt uorescence, PF) but also the triplet exciton harvest via delayed uorescence (DF). Thus, enhancing its rate (k r ) is also required to sufficiently surpass the singlet nonradiative decay rate (k S nr ). While unlike small DE ST s, which require the separation of frontier molecular orbitals (FMOs), a high k r requires a sufficient integral between the HOMO and LUMO wavefunctions. Thus, how to strike a balance between small DE ST s and fast k r becomes key to obtaining optimized performances. PLQY is a parameter that contains both a prompt component that is directly related to the uorescence process and a delayed component that is related to both the RISC and uorescence processes. Thus, the PLQY reects the overall exciton utilization and is typically positively correlated with the corresponding electroluminescent (EL) properties. A high PLQY is a prerequisite for a TADF emitter to realize high performances in OLEDs.
Aer the rapid development of the past dozen years, considerable progress has been made in developing highperformance TADF-OLEDs, and the reported maximum external quantum efficiencies (EQEs) have been beyond 30%. [29][30][31][32][33][34][35][36][37][38][39] In these OLEDs, emitting layers (EMLs) are all obtained by adopting optimized host-dopant congurations. This is mainly to suppress the annoying concentration quenching, especially arising from the long-lived triplet excitons. On the other hand, introducing host matrixes not only complicates device fabrication but also increases the cost of production. 40 For mass production, OLEDs with nondoped EMLs would be simpler and more attractive.
In this review, we rst summarize the recent progress in the eld of TADF emitters toward high-performance nondoped OLEDs, especially the important progress within the last two years. Then, we categorize the TADF emitters according to their different electron-withdrawing units and devote particular emphasis to molecular design strategies, which has not been done in previous reviews. In addition, their physical properties, as well as nondoped device performances, are also summarized. We further discuss the present pros and cons of TADF-based nondoped OLEDs and provide an outlook on their prospects.

Molecular design strategies of TADF emitters for nondoped OLEDs
To exploit novel TADF materials toward high-performance nondoped OLEDs, the abovementioned keys of general TADF molecular designs are still essential. In addition, energy loss channels in nondoped EMLs are more complicated than ideal host-guest EMLs and should be carefully blocked. In earlier studies, researchers have recognized that methods of rigidifying organic luminogens through viscous or solid environments or forming J-aggregates can give rise to emission enhancement. 41,42 In 2001, Tang et al. accidentally discovered enhanced emission upon aggregation for hexaphenylsilole (HPS) and named the phenomenon "aggregation-induced emission (AIE)". 43,44 Since then, AIE has been widely applied in the elds of optical devices, luminescent sensors, bioimaging, phototherapy, etc. Both theoretical simulation and experimental results suggest that in their aggregate states, molecular vibrations and rotations are evidently suppressed, resulting in decreased k S nr and thus enhanced emission. In particular, some emitters exhibit aggregation-induced delayed uorescence (AIDF) behaviors (as shown in Fig. 2a). 45 Emitters with such AIE/AIDF behaviors are promising candidates for high-efficiency nondoped OLEDs. Many AIE-TADF/AIDF emitters have been reported to show decent performances in nondoped OLEDs.
In nondoped OLEDs, intermolecular exciton quenching plays a vital role in causing exciton loss. Many studies have suggested that suppressing the annihilation process associated with long-lived triplet states is key to determining nondoped device performance. The rate constant of triplet-related quenching (k CQ ) can be expressed as follows: where R 0 is the critical intermolecular distance at which concentration quenching and RISC occurred with the same probability, and L ET is the effective electron tunneling distance, which quanties the intermolecular electron-exchange interactions. According to eqn (1), k CQ exhibits an exponential dependence on the average intermolecular distance (R) in the thin lms, which agrees well with the Dexter energy-transfer model, and thus, short-range electron-exchange interactions should be well blocked. Therefore, isolating triplet excitons in amorphous neat lms is key to suppressing their related quenching and realizing high-performance nondoped OLED performances. Introducing steric hindrance of TADF emitters is one of the most feasible methods to enlarge the intermolecular distance. Fig. 2b displays a typical intermolecular interaction of such molecules.
Theoretical calculations indicate that triplet spin-density distributions (SDDs) are centralized on the main body of the TADF emitters, while the phenyl moieties act as steric hindrances to realize loosely aligned molecules. 46 Distances between triplet excitons can be evidently enhanced in such molecular packings, and thus, it is expected that triplet exciton collision-induced concentration quenching can be well restrained.
In addition to introducing extra steric groups, integrating the host matrix and guest TADF emitters into new "self-host" materials is also a wise molecular design method for nondoped OLEDs. As displayed in Fig. 2c, introducing carbazoles as steric shields can not only keep the intrinsic TADF feature unchanged but also effectively suppress the exciton quenching caused by intermolecular interactions. Moreover, such "emission-core and host-encapsulation" molecular structures generally meet the requirements of the solution process and are thus usually applied in solution-processed nondoped OLEDs. 47,48 It was recently recognized that some organic emitters actually have dual stable conformations. TADF is associated with the highly twisted conformation, which is also known as the "quasiequatorial (QE)" conformation. On the other hand, the other mildly twisted form, referred to as the "quasi-axial (QA)" conformation, oen shows only conventional uorescence with higher energy instead of TADF. As displayed in Fig. 2d, by using the different characteristics of these conformers, our group proposed a new concept of "self-doping" for realizing highefficiency nondoped OLEDs. 49 Interestingly, this "compositionally" pure lm actually behaves as a lm with a dopant (QE form) in a matrix (QA form). The concentration-induced quenching that may seriously occur at high concentrations is thus expected to be effectively relieved.
Hydrogen bonds, especially intermolecular bonds, have been widely used to construct rigid intermolecular frameworks to relieve molecular vibrational and rotational motions. Enormous highly efficient chromophores have been exploited based on hydrogen bonds. On this basis, we further proposed that in neat lms of TADF emitters with suitable intermolecular hydrogen bonds, continuous and oriented interactions could be achieved. More importantly, in delicate supramolecular frameworks, not only can unimolecular nonradiative transitions be suppressed, but the annoying intermolecular quenching can also be relieved with distanced electron-rich segments (shown in Fig. 2e). 50 Intermolecular exciton quenching in OLEDs is dominated by electron-exchange interactions of triplet excitons and is generally serious between TADF emitters based on intramolecular charge-transfer (CT) transitions, while there is another type of TADF emitter based on intermolecular CT transitions, which is an exciplex. Intermolecular mutual collisions in the exciplex favor the recombination of excitons rather than exciton annihilation. In a neat lm containing a single material with molecular congurations such as D-spacer-A structures (shown in Fig. 2f), a space-enough and conjugationforbidden diphenyl ether linkage can effectively suppress the intramolecular charge-transfer (CT) transition while forming an exciplex-type emitter via the intermolecular CT transition. 51

Cyano-substituted aromatics as acceptors
Cyano (CN)-substituted aromatics have been widely used to construct TADF emitters due to their strong electronwithdrawing abilities and high chemical stabilities. The molecular structures of CN-based TADF emitters are shown in Fig. 3, and the corresponding key parameters of their nondoped devices are summarized in Table 1. Grazulevicius et al. 52 designed ve benzonitrile (BN)-based TADF emitters 1-5 with symmetrical donor-acceptor-donor (D-A-D) and asymmetrical donor-acceptor-donor* (D-A-D*) molecular structures to carefully study the inuence of different substitution patterns on device performance. Charge transport tests show that molecular structures signicantly affect the charge carrier mobilities, which can be ascribed to the different molecular packings in amorphous states. However, their nondoped devices all show poor results with maximum external quantum efficiencies (EQEs) below 5.0% because their electronwithdrawing core BN moiety is unprotected.
Yang et al. 53 reported a series of sky-blue TADF emitters 6-8 by linking di-tert-butylcarbazole (tBuCz) and diuorocyanobenzene (CNDF) groups on the phenyl bridge simultaneously with mutual ortho-positions. The large steric tBuCz groups not only effectively protect the electron-withdrawing core CNDF moieties but also favor the coexistence of throughspace CT and through-bond CT. Compared with the large DE ST values of 0.24 eV and 0.21 eV for 6 and 7, the highly twisted multi-(D/A) molecular structure of 8 can not only effectively lower its DE ST (0.03 eV) but also suppress the intermolecular close p-p packing. As a result, the 8 amorphous neat lm gives a high PLQY of 0.76. Therefore, the 8-based solution-processed nondoped device realizes sky-blue emission with a peak at 484 nm and a maximum EQE of 21.0%.
Wang et al. 54 also proposed a facile and efficient molecular design strategy for shielding the BN moiety with carbazole derivatives (Cz and tBuCz). In their newly designed compounds 9 and 10, 9,9-diphenylacridan (DPAc) and BN were the D and A groups, respectively. Introduction of Cz and tBuCz groups as the protective units can not only effectively isolate the triplet excited states but also enhance the rigidity of the molecular skeletons and decrease molecular vibrations/rotations. Thus, in nondoped OLEDs, 9 and 10 achieved high performance with maximum EQEs of 10.1% and 20.0% and CIE coordinates of (0. 16, 0.19) and (0.16, 0.26), respectively. In particular, the inferior performance of the 9-based device can be reasonably attributed to its poorer carrier mobilities. Based on the results, we can notice that a more delicate balance between the protection functions and carrier mobilities is needed. By further replacing the DPAc unit with a more electron-rich phenoxazine (PXZ) unit, two novel TADF emitters 11 and 12 were also synthesized. 55 Nevertheless, due to the unprotected rigid and planar PXZ unit, which is prone to close intermolecular packing, the 11and 12-based nondoped OLEDs show poor performance with maximum EQEs of 6.6% and 13.6%, respectively, which are much lower than those of DPAc-based emitters. These results further demonstrate that both D and A moieties should be well protected to shield potential intermolecular charge exchange.
Since the pioneering work of Adachi et al. 3 in 2012, carbazolyl dicyanobenzene (CDCB) derivatives have been carefully studied. As a member of the CDCB family, 2,3,5,6-tetra(9H-carbazol-9-yl) benzonitrile (4CzBN) and its derivatives have been widely used to construct high-performance blue TADF emitters. 56,57 By using the self-host molecular design strategy, Sun et al. 58 reported two 4CzBN-based TADF emitters 13 and 14 for fully solutionprocessed blue nondoped devices. Both emitters possess identical emissive cores but different bulky shielding groups. As displayed in Fig. 4, unlike the formation of the 14-based electromer state during electroexcitation, the rigid phenylcarbazole (PCz) unit in 13 can efficiently block such energy leakage. As a result, the 13-based fully solution-processed blue OLED realizes a maximum EQE of 22.6%, which is almost 20 times higher than that of the 14-based OLED. With the same emission core, Tang et al. 59 also reported two 4CzBN-based blue TADF emitters 15 and 16 by introducing steric groups. Both emitters exhibit blue emission at 464 nm with a CIE y < 0.2 and an EQE over 20% in doped devices. Meanwhile, compared with the steric tertbutyl (tBu) group in 15, the terminal phenyl group in 16 can effectively weaken the intermolecular p-p packing in the solidstate lm and signicantly enhance the PLQY. Therefore, the 16-based nondoped device achieves a maximum EQE of 21.6% with an extremely low turn-on voltage (V on ) of 2.7 V. Sun et al. 60 also synthesized two 4CzBN-based blue TADF emitters 17 and 18 with the host-s-guest molecular conguration for solutionprocessed nondoped OLEDs. The oxygen bridge-linked host can not only suppress triplet exciton quenching between guest units but also improve the carrier transport ability.
As another representative member of the CDCB family, 2,3,4,5,6-penta(9H-carbazol-9-yl)benzonitrile (5CzBN) is an excellent precursor to exploit TADF emitters for efficient skyblue nondoped devices. 61,62 By applying a self-host molecular design strategy, Sun et al. 63 conveniently constructed TADF emitter 19 for solution-processed nondoped OLEDs. The alkyl chain-linked Cz units can sufficiently encapsulate the 5CzBN emission core and effectively suppress intermolecular interaction-induced exciton quenching. Therefore, compared with the low PLQY of 0.21 for precursor 5CzBN, the 19 amorphous neat lm exhibits a higher PLQY of 0.52. By further applying 19 as the pure EML, the corresponding solutionprocessed nondoped device achieves a low V on of 3.1 V and a high maximum EQE of 17.1%. To enhance device performance, they further decorated precursor 5CzBN with different peripheral functional groups (20)(21)(22)(23). 64 Sufficient Cz decoration can effectively encapsulate the central chromophore and thus prevent these units from being redissolved by isopropanol. Therefore, they can be used in fully solution-processed devices with a device structure of ITO/PEDOT:PSS (40 nm)/EML (40 nm)/PO-T2T (30 nm)/Cs 2 CO 3 (2 nm)/Al (100 nm). Compared with their monosubstituted counterparts (20 and 22), the disubstituted compounds (21 and 23) show superior device performances, which can be ascribed to the well-protected TADF emission core. Tang et al. 65 also tried to decorate the 5CzBN core with different numbers of alkyl chain-linked spi-robiuorene (SPF) groups (24)(25)(26). All these emitters show obvious AIDF characteristics. Moreover, with increasing exible dendron numbers, intermolecular interactions can be well restricted. Thus, the 26 pristine lm shows better resistance to isopropyl alcohol than the others, beneting the solution fabrication process. The corresponding fully solution-processed nondoped device achieves a maximum EQE of 20.1%. All these results suggest that sufficiently peripheral decorations act as a feasible method to construct TADF emitters for fully solutionprocessed devices.
To further broaden the luminescence range of CN-based TADF materials, adjusting the numbers and positions of the CN groups is a feasible method. 4,5-di(9H-carbazol-9-yl) phthalonitrile (2CzPN) is a well-known blue TADF emitter; however, due to its poor solubility, it is rarely used in solutionprocessed devices. To exploit 2CzPN-based materials for solution-processed OLEDs, Choi et al. 66 reported a novel AIE and TADF-active triad 28 and a control compound, 4,5-bis(3-phenyl-9H-carbazol-9-yl)phthalonitrile (27). Compared with the parent molecule (27), introducing PCz and nonconjugated cyclohexane units as the host and bridge moieties in 28, respectively, can effectively enlarge the intermolecular distance without affecting the electroluminescence (EL) spectrum. In solution-processed nondoped OLEDs, they exhibit nearly identical green EL emissions with peaks at 524 nm, while the maximum EQE of 28 (13.4%) is signicantly better than that of 27 (7.5%).
Wang et al. 67 also applied the isophthalonitrile moiety, in which two cyano groups are at the meta-positions, to design D-p-A-type TADF materials 29-31. The nondoped device using Czdecorated 31 as the EML shows a superior device performance to the others, which can be reasonably ascribed to the following reasons: (1) relatively high hole mobility of the TPA group; (2) the rigid and twisted molecular conformation, which can effectively suppress the aggregation-caused quenching (ACQ) effect. Likewise, 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) is also decorated for nondoped devices due to its excellent device performance. Nevertheless, it shows a high concentration sensitivity and poor nondoped device efficiency due to the tendency of forming Cz-based dimers. 68 To achieve 4CzIPN-based highperformance nondoped devices, Jiang et al. 69 connected the emission core 4CzIPN with the host matrix via exible alkyl chains by applying a self-host molecular design strategy. In particular, the peripherally decorated 1,3-di(9H-carbazol-9-yl) benzene/tris(4-(9H-carbazol-9-yl)phenyl)amine (mCP/TCTA) moiety can form a high triplet level interfacial exciplex with the electron-transporting layer (ETL). Due to the extra intramolecular exciplex under electrical stress, the 33-based nondoped device exhibits a much lower device performance than 32.
Circularly polarized luminescence (CPL) has great applications in optical data storage, quantum computing, bioresponsive imaging, liquid crystal displays, and backlights in 3D displays. Pan et al.

Carbonyl-substituted aromatics as acceptors
Aromatic units containing strong electron-withdrawing carbonyl groups in the center, such as benzophenone (BP) and its derivatives, have been widely used as the A unit for designing TADF emitters. The molecular structures of BP-based TADF emitters are shown in Fig. 5, and the corresponding key parameters of the nondoped devices are summarized in Table 2. Adachi et al. 72 reported the rst D-A-D type BP-based TADF emitter 36 for high-performance nondoped OLEDs. A rigid and steric 9,9-dimethyl-9,10-dihydroacridine (DMAC) moiety can effectively restrain long-lived triplet exciton quenching, and its amorphous neat lm thus displays a high PLQY of 0.85 and a short delayed lifetime (s d ) of 2.7 ms, guaranteeing efficient nondoped devices. As summarized in Table 2, the 36-based nondoped OLED exhibits a superior performance with a maximum EQE of 18.9% with mild efficiency roll-off at high luminance. Yang et al. 73 further modied precursor 36 by encapsulating the DMAC moiety with tBuCz and 3,6-bis-(3,6-ditert-butylcarbazol-9-yl)-carbazole (tBuCz2) groups and developed two novel TADF dendrimers 37 and 38 for solutionprocessed nondoped devices. Both dendrimers exhibit good thermal and morphological stabilities, guaranteeing homogeneous and stable amorphous thin lms via solution-processed fabrication. Owing to the signicantly decreased carrier transport arising from the large steric hindrance of the tBuCz2 unit, the 37-based device exhibits a much lower driving voltage of 4.8 V at a luminance of 10 cd m À2 than that of 38 (7.7 V). Moreover, the nondoped device using 37 as the EML shows a maximum EQE of 12.0%, three times larger than that of the 38-based device. Cheng et al. 74 also reported a series of TADF polymers 39 by using conjugated Cz/acridine moieties as the backbone and BP as the pendant A moiety. The solutionprocessed nondoped device achieves a high device performance with a very mild efficiency roll-off.
By replacing the DMAC moiety with a soluble tBuCz2 moiety, Sun et al. 75 synthesized a novel dendronized TADF emitter (40) for solution-processed nondoped green OLEDs. Although the 40-based neat lm exhibits a high PLQY (0.65) and a small DE ST (0.08 eV), its solution-processed device shows a poor performance with a maximum EQE of 4.3%, which may be ascribed to the decreased bipolar carrier transport. 54 Fujita et al. 76 also designed a series of BP-based AIDF dendrimers (41 and 42) for solution-processed nondoped devices. By further modulating the terminal groups (43), 77 they found that the terminal groups can dramatically alter the photophysical properties of dendritic TADF emitters. The introduction of tBu and phenyl groups favors the AIE character, while the introduction of a methoxy group results in emission quenching. Thus, it is important to choose suitable terminal functional groups for designing TADF emitters for solution-processed nondoped devices.
Triazatruxene (TAT), a planar conjugated system with three fused carbazole fragments that share a benzene ring, acted as a powerful D group to construct TADF emitters. Wang et al. 78 rst characterized two TAT-based TADF emitters 44 and 45 for solution-processed devices. Compared with the long alkyldecorated TAT unit in 45, the four peripheral phenyl substituents in 44 can effectively enlarge the distance of adjacent triplet excitons. Therefore, although both emitters realize identical device performances with maximum EQEs of approximately 6.0%, the 45-based device displays a more serious efficiency rolloff at high luminance. To further improve the device performance, they also synthesized two asymmetrical D-A-type AIDF emitters 46 and 47 by using phenyl-substituted TAT and BP as the D and A groups, respectively. 79 The 47-based solutionprocessed nondoped device exhibits a higher device efficiency than 46, possibly due to the balanced carrier transport caused by the introduction of one more BP unit.
Compared with D-A-D-type symmetric molecular structures, BP-based TADF materials with unsymmetrical structures have   81 also developed a D-A-D 0 -type AIDF emitter 52 by using DMAC and dibenzothiophene (DBT) moieties as the D and D 0 , respectively. The DMAC-BP segment is anticipated to form a twisted molecular conformation to realize AIE and TADF properties, while planar DBT may help to increase the chargetransporting ability. Therefore, the 52 neat lm displays a high PLQY of 0.80 and a smaller DE ST of 0.08 eV, beneting the utilization of electrogenerated excitons. By further replacing the DMAC moiety with electron-rich PXZ and phenothiazine (PTZ) moieties, two analog AIDF emitters 53 and 54 were also synthesized. 82 However, since unprotected PXZ/PTZ groups tend to aggregate in solid-states and can induce severe triplet exciton quenching, their nondoped device performance is much worse than that of 52 (maximum EQEs of 14.2%, 9.2%, and 9.7% for 52, 53, and 54, respectively).
By introducing a diphenylphosphoryl (DPO) unit as the A 0 , Chi et al. designed an unsymmetrical D-A-A 0 -type TADF emitter 55. 83 The corresponding nondoped device demonstrates orange emission with a peak at 586 nm and a maximum EQE of 16.7%, which is among the best performances of TADF-based nondoped OLEDs in the orange-red region.
To enhance the rigidity of the BP skeleton, Lu et al. 84 prepared a novel triptycene-fused BP moiety, and a D-A-type TADF emitter 56 was then designed. For comparison, D-A-type emitter 57 was also synthesized by using DMAC and BP as the D and A moieties, respectively. Introducing a rigid and steric triptycene scaffold can not only strengthen the molecular rigidity but also effectively restrict intermolecular interactions in aggregated states. Therefore, the nondoped device using 56 as the EML achieves a maximum EQE of 15.6%, much higher than that of 57 (a maximum EQE of 4.8%).
Tang et al. 85 also used steric uorene derivatives to decorate the precursor BP-PXZ and developed three AIDF emitters 58-60 for nondoped OLEDs. The highly twisted conformation with large sterically hindered uorene groups can effectively suppress the short-range Dexter energy transfer (DET) process at high exciton concentrations, thus signicantly relieving the annihilation of triplet excitons. On this account, the corresponding nondoped OLEDs all exhibit good device performance with maximum EQEs over 12.3%.
Qi et al. 86 demonstrated a class of efficient AIDF materials 61-65 bearing pyridine-decorated BP as the A moiety and tBuCz/ DMAC/PXZ as the D moieties. The presence of intramolecular hydrogen bonding can effectively suppress nonradiative decay and improve the luminescence efficiency of the aggregated solid-state. Meanwhile, compared with the planar PXZ unit (64 and 65), the steric DMAC group in 61-63 can effectively restrict the intermolecular p-p packing.
By replacing tBuCz with a 9-phenyl-9H-carbazole (PCz) moiety, three unsymmetrical AIDF emitters 66-68 were synthesized by Tang et al. 44 As shown in Fig. 6, the PCz moiety can effectively impede close molecular packing and weaken intermolecular interactions. Therefore, concentration-induced emission quenching and exciton annihilation can be greatly suppressed. Meanwhile, the electron-rich PXZ group in 67 favors a smaller DE ST (0.024 eV) and shorter lifetime s d (2.1 ms) than those of 66 (0.07 eV and 2.7 ms) in neat lm states. Therefore, the 67-based nondoped device shows a superior performance with a maximum EQE of 18.4% compared to 66 (maximum EQE of 15.0%). To further explore the methods to alleviate the concentration quenching of PXZ-based TADF emitters, they proposed accelerating the RISC process by introducing heavy atoms (69)(70)(71). 22 Compared with the precursor AIDF emitter 67, chlorine-modied 69-71 exhibit higher PLQYs and shorter s d values, which can be attributed to the heavy atom effect that greatly increases the SOC values and signicantly accelerates the RISC process. As a result, their nondoped OLEDs exhibit outstanding device performances with excellent maximum EQEs over 20.4%. To further improve the device performance, they also proposed a self-host molecular design strategy by graing the AIDF 4-(phenoxazin-10-yl) benzoyl moiety to common host materials, including 1,4di(carbazol-9-yl)benzene (DCB), CBP, mCP, and 3,3 0 -di(carbazol-9-yl)biphenyl (mCBP), and developed materials 72-75. 87 The photophysical and electrochemical behaviors of 72-75 are very similar and seem to be mainly determined by the parent 4-(phenoxazin-10-yl)benzoyl fragment. The nondoped OLEDs using these generated luminogens exhibit outstanding performances with excellent maximum EQEs (22.1-22.6%) and high efficiency stability. All these results demonstrate that introducing a host matrix can effectively relieve triplet exciton annihilation without affecting the EL emission. They also decorated the PCz moiety with large steric carbazole-substituted uorene derivatives (76)(77)(78) to study the inuences of subtle modulations on the bulkiness and stiffness of uorene derivatives (9,9-dimethyl-9H-uorene, 9,9-diphenyl-9H-uorene, and 9,9 0 -spirobi[uorene]) on the PL and EL properties of AIDF materials. 88 The results reveal that uorene derivatives can effectively inhibit intermolecular interactions. On the other hand, the distorted and large steric conformation of the 9,9 0spirobi[uorene] segment in 78 signicantly decreases the electron mobility compared with the others (76 and 77). Eventually, the 78-based nondoped device exhibits a much lower maximum EQE of 3.3% than the others (maximum EQEs of 19.0% and 18.5% for the 76and 77-based devices, respectively).
To improve the solubility of AIDF emitters for solutionprocessed nondoped OLEDs, Qi et al. 89  Yasuda et al. 91 attempted to improve the IQE of BP-based AIDF emitters by increasing the rigidity of the BP unit. Compared with precursor 82, xing the BP unit with an oxygen bridge (xanthone (XT)) in 83 can effectively restrict the nonradiative process of the A moiety. Therefore, the 83 amorphous neat lm exhibited a higher PLQY value of 0.53 than the 0.31 for 82. They further claried the actual mechanism of concentration quenching of TADF emitters by replacing the PTZ unit in 83 with rigid and steric 1,3,6,8-tetramethylcarbazole (MCz), 9,9diphenylacridan (DPAc), spiro[acridan-9,9 0 -xanthene] (SXAc), and spiro[acridan-9,9 0 -uorene] (SFAc) groups (84)(85)(86)(87). 46 As shown in Fig. 7, they rst revealed that the concentration quenching of TADF molecules was dominated by electronexchange interactions for triplet excitons, and the inhibited intermolecular electron-exchange interactions in their condensed solid-states were the key to exploiting efficient TADF emitters for nondoped OLEDs.
By using XT as the A group, Tang et al. 92 also reported two D-A-D-type robust AIDF emitters 88 and 89. Owing to the AIE character and well-restricted intermolecular interactions, both emitters show small DE ST s of 0.025 and 0.024 eV, and excellent PLQYs of 0.96 and 0.94 in neat lms, guaranteeing highperformance nondoped devices. Therefore, the 88and 89based nondoped devices emit intense green and sky-blue light with maximum EQEs of approximately 21.0%. Moreover, the 88based device displays outstanding efficiency stability with no efficiency roll-off ($0%) when the luminance increases to 1000 cd m À2 , which can be ascribed to the high and balanced hole and electron mobilities. However, compared with the performance of optimized doped devices (maximum EQEs of 25.0% and 27.0% for 88 and 89, respectively), the efficiencies of nondoped devices are still less satisfactory, which can be reasonably ascribed to the unprotected electron-rich XT moiety. These results further indicate that both D and A moieties should be well shielded to suppress potential intermolecular packing-induced exciton quenching.
Wang et al. 93 reported a special ultrathin EML with a quantum well (QW) structure by using a previously reported TADF emitter 90. The QW structure can effectively conne the charge carriers and excitons, affording high-efficiency and lowefficiency roll-off. A maximum EQE of 22.6% was achieved for the 90-based nondoped device with seven quantum wells without any light outcoupling enhancement. However, the device shows severe efficiency roll-off at high luminance, maintaining only a small EQE of $4.0% at a luminance of 1000 cd m À2 .
Conformational status is a fundamental issue for organic molecules to determine their overall properties. Wang et al. 94 developed TADF emitter 91 with dual stable conformations at the ground state. Both theoretical and physical tests show that the QE conformer with TADF characteristics plays a dominant role at the ground state, guaranteeing effective utilization of the excitons. Therefore, the 91-based yellow nondoped device realizes a maximum EQE of 17.1%, current efficiency (CE) of 50.5 cd A À1 , and power efficiency (PE) of 60.0 lm W À1 .
To exploit efficient BP-based blue TADF emitters for nondoped OLEDs, Yasuda et al. TADF emitters with AIE features are hot candidates for nondoped devices, as they are highly emissive in the solid-state upon photoexcitation. Nevertheless, not every AIDF emitter in the past had guaranteed decent efficiencies in nondoped devices, indicating that the AIE character alone does not necessarily afford ideal nondoped TADF emitters. As shown in Fig. 8, our group designed three AIDF emitters 95-97 to investigate the critical molecular design rules. 96 The two new emitters (96 and 97) both retain the electron-donating and electronwithdrawing cores, as in reference compound 95. The BP and  DPAc moieties were locked via a rigid uorene group. Detailed physical measurements prove that these three compounds have similar energy levels as well as characteristics of AIE and TADF, while in neat lms, 96 and 97 exhibit signicantly higher exciton utilization in PL and EL processes, which can be ascribed to rigid steric uorenes hindering intermolecular triplet-triplet interactions. Therefore, the nondoped OLEDs based on 96 and 97 display excellent maximum EQEs of 22.8% and 21.3%, respectively, which are evidently higher than that of the 95-based OLED (12.9%). All these results demonstrate a feasible strategy of molecular modication to improve the nondoped EL performance of practical AIDF molecule candidates.
You et al. 97 designed a brand-new A moiety of heptagonal diimide (N-(4-(tert-butyl)phenyl)-1,1 0 -biphenyl-2,2 0 -dicarboximide (BPI)) and synthesized a D-A-D-type TADF emitter 98. The rigidity of BPI can restrict excessive intramolecular rotation, thus enhancing k r , while moderate rotatability can inhibit close intermolecular p-p packing, reducing exciton quenching in the aggregated state. Therefore, the corresponding nondoped device demonstrates a stable green emission with an emission peak at 511 nm and CIE coordinates of (0.25, 0.51). Moreover, the device achieves a maximum EQE of 24.7% with a small efficiency roll-off. Takano et al. 98 also applied phthalimidebased TADF emitter 99 to construct self-host cellulose derivatives for solution-processed nondoped OLEDs.

Sulfone-substituted aromatics as acceptors
Sulfone-substituted aromatics, such as diphenyl sulfoxide (DPS) and derivatives, have been widely used to construct TADF emitters due to their suitable electron-withdrawing abilities and limited molecular conjugations. The molecular structures of sulfone-based TADF emitters are shown in Fig. 9, and the corresponding key parameters of the nondoped devices are summarized in Table 3. By replacing the BP group in D-A-Dtype TADF emitter 36 with a DPS moiety, Adachi et al. 72 also reported a novel efficient blue TADF emitter 100. The methyl groups on the DMAC unit can effectively inhibit the intermolecular interactions; thus, the performances of the 100-based devices are nearly concentration-independent. Moreover, in the neat lm, 100 displays an even higher PLQY (0.88) than that of the doped lm (0.80). To further lower the DE ST and accelerate the RISC process, Guo et al. 99 designed a novel AIDF emitter 101 with a highly twisted zigzag molecular conguration. Compared with the para-positioned control compound 100, connecting the DPS and DMAC units with a meta-position in 101 can effectively decrease the DE ST and accelerate the RISC process. Therefore, the 101-based pristine lm exhibits a smaller DE ST value of 0.02 eV, and the OLED exhibits a higher maximum EQE of 14.0% than those of 100 (a DE ST of 0.18 eV and a maximum EQE of 11.2%).
Lee et al. 100 also designed DPS-based asymmetric and symmetric AIDF emitters 102 and 103 to investigate the relationship between molecular conguration and device performance. Since buttery shaped PTZ groups induce conformational isomerizations, 101 PXZ-based asymmetric AIDF emitter 102 thus exhibits superior nondoped device performance with a maximum EQE of 17.0%.
To exploit DPS-based TADF emitters for solution-processed nondoped OLEDs, Yang et al. 102 proposed a multicarbazole Fig. 9 Molecular structures of sulfone-based TADF emitters for nondoped OLEDs. encapsulation strategy. By introducing carbazole dendrons (tBuCz and tBuCz2) into TADF emissive core 100, two novel solution-processable DPS-based dendrimers 104 and 105 were synthesized. Multicarbazole encapsulation can not only improve the solubility but also minimize the concentration quenching effect, consequently reducing the quenching efficiency in OLEDs. Meanwhile, compared with tBuCz-decorated 104, applying large steric tBuCz2 as the encapsulation group can signicantly lower carrier transport. Thus, the 105-based nondoped device exhibits a higher V on of 5.2 V and a lower maximum EQE of 2.2% than those of 104 (a V on of 3.6 V and a maximum EQE of 12.2%).
To further reduce exciton annihilation, increasing the rigidity of the DPS group is a feasible method. By oxidizing the sulfur atom in the PTZ unit, Wang et al. 104 designed a novel A moiety named 10-phenyl-10H-phenothiazine-5,5-dioxide (2PTO). Further introducing rigid and planar PXZ units as the D moiety, green AIDF emitter 107 with a highly stereoscopic structure was thus synthesized. The 107-based nondoped device exhibits a high efficiency with a maximum EQE of 16.4%, which is comparable with that of the corresponding doped counterpart. They also tried to replace the planar PXZ unit (107) with the buttery shaped PTZ unit (108). 94 Owing to the stable QA conformer at the ground state, 108 shows conventional uorescence emission. Therefore, compared with the BP-based analog 91, 108 exhibits a lower PLQY and poorer device performance. These results indicate that molecular conformational distributions will signicantly inuence device performance and need more in-depth research.
Dias et al. 105 designed TADF emitter 109 by using buttery shaped PTZ and rigid dibenzothiophene-S,S-dioxide (DBTO2) as the D and A moieties, respectively. The presence of a suitable 3 LE (2.66 eV of the DBTO2 group) close to the CT manifold (2.61 eV of 109) is expected to accelerate the RISC process and facilitate the utilization of triplet excitons. Nevertheless, compared with the doped ones (a maximum EQE of up to 22.0%), its nondoped device only exhibits a very poor performance with a maximum EQE of only 3.1%. This is mainly attributed to the low charge mobilities. By utilizing the dendronization strategy on precursor molecule 2,8-bis(3,6-di-tertbutyl-9H-carbazol-9-yl)dibenzo[b,d]thiophene-5,5-dioxide (tBuCz-DBTO2), Yang et al. 106 designed a series of DBTO2-based dendrimers 110-111 for solution-processed nondoped OLEDs. Encapsulation of the parent tBuCz-DBTO2 TADF core with a soluble tBuCz moiety can not only enhance the morphological stabilities but also effectively lower the DE ST s by decreasing the overlap of the FMOs. As a result, their solution-processed nondoped devices exhibit gradually redshied EL emissions (456, 508 and 522 nm for tBuCz-DBTO2, 110 and 111, respectively) with increasing generation number of dendrimers. In addition, the 110-based device displays the best EL performance with a maximum EQE of 6.1%, which is nearly double that of the 111- based device (3.3%) and more than 20-fold higher than that of the tBuCz-DBTO2-based device (0.3%).
To regulate the molecular packing and bipolar carrier transport more conveniently, asymmetric DPS-based TADF emitters have been extensively investigated. Chi et al. 107 designed two asymmetric D-A-D 0 -type AIDF emitters 112 and 113 by using PTZ, DPS, and Cz as the D, A, and D 0 moieties, respectively. Further introducing an ortho-positioned Cz unit in 113 guarantees the coexistence of intramolecular electrostatic attractions and through-space CT, leading to reduced structural vibrations, suppressed nonradiative decay, and rapid radiative decay to avoid excited state energy loss. As a result, the 113based nondoped device exhibits a state-of-the-art performance with a maximum EQE of 28.7%. However, at 1000 cd m À2 , the EQE of this nondoped device dramatically declined to approximately 2.8%. Such a serious efficiency roll-off can be ascribed to the unbalanced carrier mobilities.
Yan et al. proposed that the poor device performance of DPSbased fully conjugated (104) or fully nonconjugated (106) structures can be attributed to the unwanted degenerate excited states generated by multiple identical dendrons. To suppress the formation of degenerate excited states, two new asymmetrical "half-dendronized" (114) and "half-dendronized-halfencapsulated" (115) emitters have been designed and synthesized. 108 Both emitters show distinct AIDF properties, which minimize the exciton quenching in aggregated states, and consequently reduce their efficiency roll-off in their nondoped devices. Moreover, the asymmetric molecular structures successfully limit the unwanted degenerate excited states, favoring the TADF characteristics. As a result, the solutionprocessed nondoped devices using 114 and 115 as the pure EMLs achieved maximum EQEs of 23.3 and 24.0%, respectively.
Wang et al. also reported two asymmetric AIDF emitters 116 and 117 by using DMAC/PXZ, DPS, and PCz as the D, A, and D 0 moieties, respectively. 109 Introducing the PCz moiety can effectively restrain the intermolecular interactions in the aggregated states. Therefore, both emitters exhibit typical AIDF characteristics. Moreover, due to the obviously enhanced electrondonating ability of the PXZ unit, the 117 amorphous neat lm exhibits a smaller DE ST of 0.03 eV and shorter s d of 1.0 ms than 116 (DE ST and s d of 0.07 eV and 2.4 ms, respectively). Therefore, the nondoped OLEDs obtained by using 116 and 117 as the pure EMLs achieve maximum EQEs of 17.9 and 9.1%, respectively.

Nitrogen-containing heterocycles as acceptors
Nitrogen-containing heterocycles, such as terpyridine, pyrimidine, quinoline, quinoxaline, triazine, and dibenzo[a,c]phenazine, have been widely used to exploit TADF emitters for nondoped OLEDs. Herein, we classify nitrogen-containing heterocycles based on their different electron-withdrawing abilities (exhibited in Fig. 10 and summarized in Table 4). To effectively restrict exciton quenching in the aggregated solidstate, we have proposed a novel strategy of exploiting highperformance nondoped electroluminescence by tuning intermolecular hydrogen bonding. 50 Three TADF isomers 118-120 were synthesized by using PXZ and o/m/p-terpyridine as the D and A moieties, respectively. As displayed in Fig. 11, suitable intermolecular hydrogen bonding (119) enables the formation of a 3D supramolecular framework, which can evidently restrict  the nonradiative process and suppress triplet exciton quenching caused by p-p packing of triplets. Moreover, such a 3D supramolecular structure also favors horizontal molecular orientations, which can evidently increase the outcoupling efficiency and benet realization of high device performance. 110 Therefore, the 119-based nondoped device exhibits state-of-theart performance with a maximum EQE of 23.6% with only 7.2% roll-off at 1000 cd m À2 . Moreover, it is the rst report that the performance of a TADF-based nondoped device can evidently surpass that of its corresponding optimized doped counterpart (maximum EQE ¼ 16.3%), indicating that the intermolecular hydrogen bonding mechanism can not only provide a new pathway for designing TADF emitters for high-performance nondoped devices, but also has great potential in other solidstate luminescence applications. By replacing the peripheral pyridine ring in 119 with a more electron-withdrawing pyrimidine unit, we further synthesized a novel TADF emitter 121. 111 Unlike the formation of a 3D framework in 119, the X-ray data show that a 1D chain can be observed in 121. Such a linear chain structure can effectively keep the electron-rich PXZ cores in neighboring molecules apart from each other such that triplet-related exciton quenching can be well suppressed. In addition, it also improves the balance of carrier mobilities and the optical outcoupling from the EML. With these merits, OLEDs using 121 as a dopant emitter, from 10 to 100 wt%, can maintain high EQEs of over 20%. More importantly, the nondoped OLED shows yellow emission and an excellent maximum EQE of 21.8% with a small efficiency roll-off, which is even comparable with that of state-of-the-art yellow OLEDs. These results provide new insight into the role of hydrogen bonding in molecular packing and its subsequent inuences on the performance of OLEDs. Conformational status is a fundamental issue for TADF emitters to determine their overall properties. Recently, TADF emitters with dual conformations have been widely studied. [112][113][114][115] However, unlike the detailed investigations between conformational isomerization and excited-state properties in the dilute solution state, their inuences in the amorphous solid-state are far from clear due to the lack of a suitable quantitative analysis method. By using temperaturedependent time-resolved photoluminescence spectroscopy, we quantitatively measured the conformational populations for the rst time. 49 The populations of the quasi-axial (QA) and quasiequatorial (QE) forms of newly designed TADF emitter 122 in the disordered solid-state were thus measured to be 86 and 14%, respectively. Interestingly, this "compositionally" pure lm actually behaves as a lm with a dopant (QE form) in a matrix (QA form). In terms of the conformational distribution aspect, we further propose a new concept of "self-doping" for realizing high-efficiency nondoped OLEDs (Fig. 12). The "selfdoping" OLED realizes superior device performance with a maximum EQE of 25.4% and neglectable efficiency roll-off. This is similar to those in state-of-the-art OLEDs with "dopant-in-host" systems. On the other hand, as the emitting lm employs only one compound, the issues of higher manufacturing cost and potential dopant segregation associated with "dopant-in-host" systems can be effectively avoided.
Chen et al. 116 designed and synthesized three AIDF emitters 124-126 by using the quinoline unit as a new A group. Owing to the highly twisted D-A molecular framework, all three quinoline-based emitters display small DE ST values ($0.04 eV), good photoluminescence, and AIDF properties. Meanwhile, the steric DMAC group in 124 can effectively retain the intermolecular mutual interactions and favor the utilization of the excitons. Therefore, the 124-based nondoped device displays a superior performance with a maximum EQE of 17.4%, much higher than that of the others (maximum EQEs of 14.9% and 13.1% for 125 and 126, respectively). To further exploit quinoline-based emitters for high-performance red/orange nondoped OLEDs, Yang et al. 117 designed a series of AIDF emitters 127-130 by simultaneously modulating the D units and regulating the degree of ICT characteristics. They also decorated the quinoline unit with the uorine atom (131 and 132). 118 Introducing the uorine atom can not only slightly enhance the electron-withdrawing ability but also enrich the intermolecular electron coupling capability. The nondoped OLEDs obtained by using 131 and 132 as the EMLs achieve orange emission with EL peaks of 584 and 588 nm and maximum EQEs of 10.1% and 9.8%, respectively.
Tao et al. 119 modied the efficient sky-blue precursor 5CzBN by replacing the CN unit with a 1,3,4-oxadiazole (OXD) moiety (133). It exhibits bright light-blue emission, with a maximum EQE of 3.6% of the nondoped device. Moreover, it functions as a highly efficient host for green and yellow TADF OLEDs. Kippelen et al. 120 further modied the molecule by introducing two tBu-substituted OXD moieties (134). Due to effectively restricted exciton quenching owing to the steric tBu group, the 134 pristine neat lm exhibits a high PLQY of 0.71. The corresponding nondoped OLED yields a maximum EQE of 20.9% and a high luminance of 120 000 cd m À2 .
By applying TPA and 2,3-dicyanopyrazino phenanthrene (DCPP) as the D and A moieties, respectively, Wang et al. 121 synthesized the rst V-shaped TADF emitter 135 for nearinfrared (NIR) nondoped OLEDs. A large and rigid p-conjugated DCPP moiety can effectively restrain the molecular vibration and rotational processes, while an electron-donating TPA moiety can not only offer excellent hole-transporting capability but also diminish exciton quenching in the aggregated state. Therefore, a small DE ST and high k r value can be simultaneously obtained, beneting the utilization of electrogenerated excitons. The corresponding nondoped device realizes NIR emission with a peak at 710 nm and CIE coordinates of (0.70, 0.29). Xu et al. 37 also developed a novel NIR TADF emitter by using the CN-decorated quinoline unit as the A moiety (136). The 136-based nondoped device realizes a maximum EQE of 4.6% with NIR emission at a peak of 700 nm.
By modulating the substitution position of the TPA unit from the o-to the p-position, Xu et al. 122 developed a novel "T-shaped" deep-red TADF emitter 137 by using the planar dipyridophenazine (DPPZ) moiety as the A. In comparison to the osubstituted precursor, 123 the rational spatial arrangement of D and A groups in 137 can not only remarkably improve the carrier transport but also dramatically accelerate the rate of k r by 90fold without worsening the nonradiative transition process (k S nr ) and amplify the thermodynamic advantage of RISC with a nearly unitary efficiency, giving rise to a PLQY as high as 0.87. Therefore, the corresponding nondoped device realizes a record high EQE of 12.3% for the maximum and 10.4% at 1000 cd m À2 . All these results demonstrate that it is important to obtain deep insight into the inuence of the spatial arrangement of functional groups on the optoelectronic properties.
Based on the DCPP moiety, our group also designed two TADF emitters 138 and 139 to demonstrate the relationship between molecular rigidity and intermolecular packing. 124 Although the fused dibenzo[a,c]dipyrido[3,2-h:2 0 ,3 0 -j]phenazine (BPPZ) moiety can effectively relieve the nonradiative process, a large planar molecular structure induces unavoidable intermolecular p-p interactions, which are detrimental to the performance of nondoped devices. The rotational pyridine unit in 2,3-di(pyridin-3-yl)dibenzo[f,h]quinoxaline (mDPBPZ) can provide suitable steric hindrance against intermolecular p-p packing. The 138-based doped device realizes a remarkably high maximum EQE of 25.2%, while the 139-based nondoped device shows a relatively high EQE of 5.2% with deep red/NIR emission at a peak of 680 nm.
Wang et al. 125 designed two AIDF emitters 140 and 141 by using rigid TAT and dibenzo[a,c]phenazine (DBPZ)/uorinedecorated DBPZ as the D and A moieties, respectively. The large steric hindrance between planar TAT and DBPZ segments can not only restrain the overlap of the FMOs but also suppress aggregation-caused quenching. Therefore, both compounds possess TADF characteristics with small DE ST s and AIE properties. The 140and 141-based solution-processable nondoped OLEDs exhibit red and deep-red emissions (CIE coordinates of (0.61, 0.38) and (0.64, 0.35)) with maximum EQEs of 5.6 and 2.9%, respectively.
With three potential modication sites, the triazine (TRZ) group with highly electron-decient properties is a suitable building block for constructing TADF emitters. By using DMAC/ PXZ and TRZ as the D and A groups, respectively, Wu et al. designed two TADF emitters, PXZ-TRZ and 142. 126 Compared with PXZ-TRZ, the DMAC unit in 142 can effectively restrict exciton quenching in the solid-state and guarantee that the 142 neat lm realizes a high PLQY of 0.83. As a result, a highly efficient nondoped device with a maximum EQE of 20.0% is also realized. Kaji et al. 127 further applied 142 for solution-processed nondoped OLEDs. The peripheral phenyl units and large steric methyl units in the TRZ and DMAC moieties, respectively, can effectively suppress the intermolecular interactions. Therefore, the spin-coated 142 neat lm displays a high PLQY of 0.84, which is almost identical to that of a vacuum vapour deposited neat lm. Hong et al. 128 also connected the DMAC and TRZ units via a meta-position and synthesized two TADF emitters 143 and 144 with molecular structures of D-A and D-p-A, respectively. Compared with D-A-type molecule 143, introducing the p group in 144 more efficiently benets the formation of intermolecular exciplexes. Therefore, the 144-based nondoped device exhibits a superior device performance with a maximum EQE of 18.7%, much higher than that of 143 (5.3%).
To relieve triplet exciton quenching in the aggregated solidstate, our group proposed a novel molecular model to develop TADF emitters radiating via the intermolecular CT transition for nondoped OLEDs. 51 As displayed in Fig. 2f, the DMAC and TRZ moieties were connected with a space-enough and conjugationforbidden linkage (diphenyl ether linkage), which was called the D-spacer-A structure. Such a molecular structure can effectively suppress the intramolecular CT transition while forming an exciplex-type emitter via the intermolecular CT transition. Therefore, 145 exhibits local excited properties in a singlemolecule state, as the D-spacer-A molecular backbone strongly suppresses the intramolecular CT transition. With increasing doping concentration, the intermolecular CT transition becomes signicant, opening another effective radiation channel and resulting in TADF characteristics. The corresponding devices thus exhibit similar performance under various high doping ratios (40-100 wt%), and the nondoped device retains a high maximum EQE of 14.7%. These results prove that the intermolecular CT transition not only endows emitters with natural TADF but also helps to inhibit the undesired concentration quenching effect. Therefore, the D-spacer-A structure molecule could act as a novel candidate to develop TADF emitters with intermolecular CT transitions for efficient nondoped OLEDs.
At present, the majority of TADF emitters are based on largely twisted aromatic amine-based compounds. To broaden the choice of the D moieties, Su et al. 129 designed two TADF emitters 146 and 147 by using thianthrene (TE) and TRZ, respectively, as the D and A groups. Unlike only the existing pconjugated CT channel in linear 147, ortho-substituted 146 exhibits a twisted D-A backbone with coexistence through the bond and through spatial CT. As a result, the 146 amorphous neat lm has a high PLQY of 0.79, which is much higher than that of 147 (0.44). Therefore, the 146-based nondoped device produces a green emission with a maximum EQE of 20.6%, which is comparable to those of the reported aromatic aminebased counterparts. They also designed two novel tri-spiral units, 10H-dispiro[acridine-9,9 0 -anthracene-10 0 ,9 00 -thioxanthene] (TspiroS) and 10H-di acridine-9,9 0 -anthracene-10 0 ,9 00 -uorene] (TspiroF), with long bar shapes for high-performance nondoped OLEDs. 130 By further integrating a strong TRZ acceptor moiety, two linear TADF emitters 148 and 149 were designed and synthesized. Compared with the nonspiral and dual-spiral acridine units, the elongation of the molecular backbone with a large nonconjugated fragment can unambiguously increase the distance between the adjacent luminophores and weaken the intermolecular interactions. Moreover, the long and linear molecular congurations favor the horizontal dipole ratio, which can signicantly enhance the outcoupling efficiency of the device. 110 As a result, sky-blue nondoped OLEDs realize maximum EQEs of 20.0% and 16.5% for 148 and 149, respectively.
Hexaarylbenzene (HAB) derivatives, in which six aromatic rings are arranged around the central phenyl ring with a propeller-shaped conguration, can be considered the desired model to carefully investigate the through-space CT process. Wang et al. 132 designed a novel AIDF emitter 152 with circularly arrayed DMAC and TRZ units in the periphery of the HAB core. Such a propeller-shaped molecular structure not only favors a reduced DE ST but also facilitates k r . Nevertheless, the corresponding solution-processed nondoped device can only realize poor device performance with a maximum EQE of 3.5%. Tang et al. 133 also decorated the HAB with DMAC/PXZ and TRZ units positioned in close proximity (153 and 154) to ensure that electron clouds of D and A exchange through spatial interactions. Due to the enhanced electron-donating ability of the PXZ unit, the 154 amorphous neat lm shows a smaller DE ST value of 0.02 eV than DMAC-based 153 (0.09 eV). Moreover, the AIEactive HAB moiety can effectively enlarge the intermolecular distance and relieve the ACQ effect. Therefore, the 154-based nondoped device realizes a superior performance with a maximum EQE of 12.7% and a neglectable efficiency roll-off at high luminance.
Chang et al. 134 modied the TRZ unit with a long alkyl chain to inhibit unwanted intermolecular D-D/A-A-type p-p interactions and accordingly designed two D-A-type TADF emitters 155 and 156. Owing to the nature of the electron-donating ability of the alkyl chain, the LUMO of the modied TRZ unit can be slightly weakened, generating blueshied emission. Meanwhile, the crowded molecular architecture favors well-separated FMOs and benets TADF properties. As a result, blue-emitting nondoped devices with 155 and 156 as the EMLs reveal satisfactory efficiencies of 12.8% and 15.8%, respectively.
Yamamoto et al. 135 rst designed Cz-decorated TRZ-based TADF dendrimers 157 and 158 by decorating the precursor 2,4,6-tris(4-(9H-carbazol-9-yl)phenyl)-1,3,5-triazine (TCz-TRZ) for solution-processed nondoped OLEDs. Intermolecular interactions between the adjacent emissive cores decrease when the generation number of the Cz unit increases. The secondgeneration 158-based solution-processable nondoped device exhibits a superior performance compared to 157. Although device performance should be improved by careful optimization, more importantly, the recorded EQE value indicated that the dendrimer emitting layer can harvest triplet excitons. These TRZ-based dendrimers are the rst high-molecular-weight TADF materials for solution-processable nondoped OLEDs. To realize fully solution-processed devices, they also tried to decorate precursor 157 by introducing methyl, tBu, and phenyl groups (159-161), respectively. 136 All the modied dendrimers exhibit excellent solubility in common organic solvents except alcohol solution, fullling the requirements of fully solutionprocessable devices. As a result, fully solution-processed nondoped OLEDs with a device conguration of ITO/PEDOT-PSS (70 nm)/PVK (poly-vinylcarbazole, 20 nm)/EML (20-25 nm)/TPBi (40 nm)/Ca (10 nm)/Al (80 nm) were fabricated by spin-coating all organic materials. The maximum EQEs are 9.4% (159), 9.5% (160), and 8.2% (161), which are comparable to or even higher than those of the TPBi vacuum evaporated devices. Sun et al. 137 also designed self-host dendrimers 162 and 163 by connecting the emission core TCz-TRZ with the peripherally Cz-based host matrix through nonconjugated aliphatic chains. The prevalent concentration quenching effect of TADF materials can thus be effectively restrained by the encapsulation of the emissive core. Moreover, the device performances are found to be greatly dependent on the peripheral dendrons, suggesting the importance of the role in molecular modulation for performance improvement. Using the same self-host strategy, they also decorated the emitting core 4,4 0 ,4 00 -(1,3,5-triazine-2,4,6-triyl) tris(N,N-diphenylaniline) (TTPA-TRZ) with Cz-based dendrons for high-performance solution-processed nondoped OLEDs (164 and 165). 138 Choi et al. 139 also designed two dendrimers 166 and 167 by choosing the efficient nondoped green TADF material 142 as the emitting core. Using a methylene group as the linkage can effectively disrupt the conjugation between the peripheral Czbased dendrons and the emitter core. Thus, the performances of the emitter core are independent of the dendrons. Their nondoped solution-processable OLEDs demonstrate acceptable performance with maximum EQEs of 9.5% for 166 and 8.1% for 167.
In addition to the abovementioned TRZ-based dendrimers, the TRZ unit has also been applied to design novel polymers for solution-processed devices. Cheng et al. 140 reported a series of conjugated TADF polymers 168 by using conjugated acridine/ carbazole and tBu-substituted TRZ moieties as the backbones and pendant, respectively. The long alkyl branched chains signicantly enhance the solubility of the target polymers in common solvents, which is very benecial for the solution process. Meanwhile, the TADF core can be sparsely incorporated into the poly-3,6-carbazole backbone to realize molecular dispersion, and the distance between the adjacent TADF chromophores can exibly be regulated through a subtly lengthened carbazole oligomer. Therefore, a high PLQY of 0.90 was realized with the 5% molar content of the TADF emission unit. They also modied 168 by replacing the tBu group with phenyl units on the triazine parent (169). 141 Although introducing the Cz group is expected to isolate the TADF emission core and relieve the concentration quenching effect, all these constructed polymers still exhibit high concentration sensitivities, and this needs further in-depth study.
The conjugated acridine/carbazole backbones would lower the triplet excited state, which is detrimental to realizing the TADF characteristic. To realize high-performance TADF polymers, Wang et al. 142 proposed exploiting through-space CT polymers 170-173 containing a nonconjugated polystyrene backbone and spatially separated D and A units for solutionprocessed OLEDs. Such a molecular design strategy can not only avoid the large bathochromic shi of the TADF emission but also limit the electron cloud overlap of the FMOs, which is benecial for reducing DE ST and achieving TADF characteristics. By further tuning the CT strengths, an emission color spanning from the deep blue (455 nm) to red (616 nm) region can be achieved. Meanwhile, by incorporating two kinds of D/A pairs into one polymer to create double through-space CT channels, blue and yellow emission can be simultaneously obtained, indicating that white electroluminescence can be realized from a single polymer. They also designed two throughspace CT polymers 174 and 175 based on a nonconjugated poly(cis,exo-2,3-diarylnorbornene) backbone with D and A units xed in edge-to-face/face-to-face alignment patterns. 143 Compared to its edge-to-face counterpart 174, the 175 neat lm with face-to-face aligned D/A units exhibits a much larger oscillator strength (f) and higher PLQY up to 0.81 owing to the enhanced spatial electron cloud overlaps. Therefore, the 175based solution-processed nondoped device realizes a maximum EQE of 18.8%, which is the best efficiency for blue TADF polymers thus far.

Boron-based units as acceptors
Due to the vacant p-orbital of boron atoms, triarylboron-based acceptors have been utilized to exploit emitters for optoelectronics. The molecular structures of boron-based TADF emitters are shown in Fig. 13, and the corresponding key parameters of the nondoped devices are summarized in Table 5. Lu et al. 144 reported two efficient blue TADF emitters 176 and 177 bearing triarylboron as the A group and ortho-positioned Cz and tCz, respectively, as the D group. Such ortho-positioned D-A molecular structures favor the coexistence of through-bond and through-space CT processes, guaranteeing a high transition dipole moment and small DE ST s. Moreover, both highly twisted D/A moieties and steric alkyl substituents can efficiently inhibit aggregation-induced quenching. Meanwhile, compared with Cz-based emitter 176, additional tBu groups in 177 can not only improve the solubility, beneting solution processing but also restrict the close intermolecular p-p packing. Therefore, solution-processed nondoped OLEDs using 176 and 177 as the EMLs achieve blue EL emission with peaks at 463 nm and 474 nm and maximum EQEs of 8.0% and 19.1%, respectively. By replacing the tBuCz unit in 177 with a buttery shaped PTZ unit, Lu et al. 145 synthesized a novel boron-based TADF emitter 178. Owing to the well-restricted intermolecular interactions, its amorphous neat lm achieves a high PLQY of 0.65. The 178based nondoped device also obtains a high maximum EQE of 19.7%. It should be noted that there is a shoulder emission below 450 nm when 178 is doped in mCP with 5 wt%. The authors believe that this shoulder emission is due to the host material, but the EL emission of mCP cannot reach this position. We suspect that this emission arrives from the QA form of 178 caused by conformational isomerism of the PTZ group.
Yasuda et al. 146 developed a series of TADF materials with different bridging heteroatoms (sulfur for 179 and oxygen for 180). Highly twisted D-A molecular structures with steric alkyl substituents benet not only small DE ST s but also wellrestricted concentration quenching effects. Meanwhile, as shown in Fig. 14a, compared with oxygen-linked emitter 180, incorporating a phenothiaborin (PTB) unit into 179 can significantly enhance the SOC effect, thereby accelerating the RISC and shortening the TADF emission lifetime to $1 ms, which is among the shortest lifetimes ever reported for efficient TADF emitters and comparable to those of organometallic phosphorescent emitters. The 179and 180-based nondoped devices attain considerably high EQEs of up to 22.8% and 21.3%, respectively. More importantly, owing to the short s d , the 179based device can still retain a high EQE of 19.0% at a high luminance of 1000 cd m À2 , much higher than that of 180 (an EQE of 8.0%) (shown in Fig. 14c). To further enhance the SOC effect, they also exploited a series of acridan analogues, phenazasiline and phenazagermine, with heavy elements in group 14 (silicon and germanium atoms). By using the same PTB unit as the weak A group, novel blue TADF emitters 181-183 were synthesized. 147 Incorporating larger silicon and germanium atoms can effectively enhance the structural exibility of the D units, leading to conformational heterogeneity as well as dual uorescence capability. Meanwhile, terminal bulky substituents can effectively suppress the ACQ, and thereby, high PLQY values of 0.74 and 0.83 are obtained for 182 and 183 neat lms, respectively. Hence, 182and 183-based nondoped devices demonstrate excellent EL performances, with maximum EQEs as high as 20.9% and 17.4%, respectively, and suppressed efficiency roll-off at practically high luminance. Although TADF-based blue nondoped devices with maximum EQEs exceeding 20% have been achieved, it should be noted that their emissions are mostly in the sky-blue range with peaks at above 470 nm. In contrast, high-performance deep-blue TADF materials (<460 nm) for nondoped devices have been rarely reported. This is because developing them should not only consider the suppression of concentration quenching like with other color ones, but more importantly, it requires suitable D and A pairs that provide wide enough bandgaps. To achieve high-performance deep-blue nondoped OLEDs, Wang et al. 148 designed a deep-blue D-p-A-type TADF emitter 184 by combining SFAc and weak electron-withdrawing siprodipenozoborinine (sDBB) as the D and A moieties, respectively. Both the uorene groups appended on D/A moieties and the bulky isopropyl group on the bridge unit can effectively suppress the intermolecular interactions of neighboring molecules. Therefore, the annoying concentration quenching can be well relieved. Moreover, bulky isopropyl groups can effectively restrict the molecular free rotations between the bridge and the A groups, beneting a high PLQY and a narrow emission bandwidth. As a result, the corresponding nondoped device realizes a deep-blue EL emission with a maximum EQE of 22.5% and CIE y < 0.1.
Boron-containing polycyclic aromatic hydrocarbons have emerged as very promising candidates for optoelectronic applications by virtue of their unique photophysical properties, good thermal stability, rigid planar structure, and easy structural modication. One typical framework is 5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene (DOBA) (1,3-diphenoxybenzene fused with a boron atom). Wang et al. 149 designed and synthesized three isomeric derivatives 185-187 by incorporating two DMAC units at different locations of two of the phenyl rings (wing phenyl groups) in the DOBA unit. The location and orientation of the DMAC unit signicantly inuence the physical properties of the three isomers. The para-substituted isomer 186 realizes a short s d of 1.8 ms and a higher PLQY value of 0.94 in the neat lm state accumulation and quenching of long-lived triplet excitons particularly at high voltages in the EML. Therefore, exploiting TADF emitters with stable chemical structures and short s d s to dilute exciton concentrations should be the main pursuits to improve the operational lifetimes of TADF-based nondoped OLEDs. In addition, MR-TADF emitters have recently attracted tremendous attention due to their excellent color purities and device efficiencies. However, due to their typical rigid and planar molecular structures, precise modulation of doping weight is still necessary to achieve these superior device performances. How to effectively suppress its concentration sensitivity without affecting the light-emission wavelengths and color purity is still unknown and needs further in-depth study. Despite the current challenges, exploiting novel TADF emitters for high-performance nondoped OLEDs remains an important research direction at the current stage.

Conflicts of interest
The authors declare no conicts of interest.