Developing red thermally activated delayed fluorescence (TADF) emitters for high‐performance OLEDs is still facing great challenge. Herein, three red TADF emitters, pDBBPZ‐DPXZ, pDTBPZ‐DPXZ, and ...oDTBPZ‐DPXZ, are designed and synthesized with same donor–acceptor (D‐A) backbone with different peripheral groups attaching on the A moieties. Their lowest triplet states change from locally excited to charge transfer character leading to significantly enhance reverse intersystem crossing process. In particular, oDTBPZ‐DPXZ exhibits efficient TADF feature and exciton utilization. It not only achieves an external quantum efficiency (EQE) of 20.1 % in red vacuum‐processed OLED, but also realize a high EQE of 18.5 % in a solution‐processed OLED, which is among the best results in solution‐processed red TADF OLEDs. This work provides an effective strategy for designing red TADF molecules by managing energy level alignments to facilitate the up‐conversion process and thus enhance exciton harvesting.
By introducing phenyl or o‐tolyl groups into different positions of the same acceptor backbone, the lowest triplet energy levels of red thermally activated delayed fluorescence emitters can be tuned from locally excited triplet (3LEA) to charge transfer triplet (3CT) states, resulting in enhancement of the rates of reverse intersystem crossing (RISC), and boosting efficiencies in both vacuum‐ and solution‐processed OLEDs.
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.
The mechanism, exciton leaking channels, and reported molecular design strategies of TADF emitters for high-performance nondoped OLEDs are summarized. Their molecular structures depending on the functional A groups are further classified.
The recent introduction of thermally activated delayed fluorescence (TADF) emitters is regarded as an important breakthrough for the development of high efficiency organic light‐emitting devices ...(OLEDs). The planar D and A groups are generally used to construct TADF emitters for their rigid structure and large steric hindrance. In this work, it is shown that many frequently used nonaromatic (noncontinuous conjugation or without satisfying Hückel's rule) planar segments, such as 9,9‐dimethyl‐9,10‐dihydroacridine, are actually pseudoplanar segments and have two possible conformations–a planar form and a crooked form. Molecules constructed from pseudoplanar segments can thus have two corresponding conformations. Their existence can have significant impact on the performance of many TADF emitters. Two design strategies are presented for addressing the problem by either (1) increasing the rigidity of these groups to suppress its crooked form or (2) increasing the steric hindrance of the linked group to minimize energy of the emitters with the highly twisted form. Following these strategies, two new emitters are synthesized accordingly and successfully applied in OLEDs demonstrating high external quantum efficiencies (20.2% and 18.3%).
A schematic energy level diagram of (2‐(9,9‐dimethylacridin‐10(9H)‐yl) thianthrene‐5,5,10,10‐tetraoxide) shows that molecules constructed from pseudoplanar segments can have two corresponding conformations, which have significant impact on the performance of many thermally activated delayed fluorescence emitters. By either increasing the rigidity of these groups, or by increasing the steric hindrance of the linked group, the problem can be addressed.
A novel molecular model of connecting electron‐donating (D) and electron‐withdrawing (A) moieties via a space‐enough and conjugation‐forbidden linkage (D‐Spacer‐A) is proposed to develop efficient ...non‐doped thermally activated delayed fluorescence (TADF) emitters. 10‐(4‐(4‐(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl) phenoxy) phenyl)‐9,9‐dimethyl‐9,10‐dihydroacridine (DMAC‐o‐TRZ) was designed and synthesized accordingly. As expected, it exhibits local excited properties in single‐molecule state as D‐Spacer‐A molecular backbone strongly suppress the intramolecular charge‐transfer (CT) transition. And intermolecular CT transition acted as the vital radiation channel for neat DMAC‐o‐TRZ film. As in return, the non‐doped device exhibits a remarkable maximum external quantum efficiency (EQE) of 14.7 %. These results prove the feasibility of D‐Spacer‐A molecules to develop intermolecular CT transition TADF emitters for efficient non‐doped OLEDs.
Non‐doped OLEDs: A novel molecular model of connecting electron‐donating (D) and electron‐withdrawing (A) moieties through a conjugation‐forbidden spacer is proposed to develop efficient thermally activated delayed fluorescence emitters for non‐doped organic light‐emitting diodes. The structure shows intermolecular charge‐transfer (CT) transitions as the major radiative channel and a suppressed effect of photoluminescence concentration quenching.
Exciplex emitters have been extensively studied owing to their natural thermally activated delayed fluorescence characteristic, and many efforts have been made to improve their performance in organic ...light emitting diodes (OLEDs). In this work, the authors propose a novel strategy by introducing intermolecular hydrogen bond (HB) between electron‐donating and electron‐accepting constituting molecules (D and A) to suppress non‐radiative transition of exciplex emitters and thus improve their efficiencies and stabilities in the OLEDs. Accordingly, three exciplex emitters are constructed by using 1,3‐di(10H‐phenoxazin‐10‐yl)benzene (13PXZB) as donor and 4,6‐bis(3,5‐di(pyridin‐4‐yl)phenyl)‐2‐methylpyrimidine (B4PyMPM), 4,6‐bis(3,5‐di(pyridin‐3‐yl)phenyl)‐2‐methylpyrimidine (B3PyMPM), and 4,6‐bis(3,5‐di(pyridin‐2‐yl)phenyl)‐2‐methylpyrimidine (B2PyMPM) as acceptors. With the most intermolecular HBs, 13PXZB:B4PyMPM shows the highest photoluminescence quantum yield (69.6%) and the lowest rate constant of non‐radiative process of triplet excitons (3.4 × 105 S−1). And the OLED using 13PXZB:B4PyMPM as the emitter successfully exhibits a much higher external quantum efficiency of 14.6% than other contrastive devices. Moreover, the half lifetime of 13PXZB:B4PyMPM is 4.8 and 8.2 times higher than those of 13PXZB:B3PyMPM and 13PXZB:B2PyMPM in the devices. This work not only demonstrates that intermolecular HBs between D and A molecules can improve the performance of exciplex emitters, but also paves a new route to develop efficient and stable exciplex emitters.
A novel strategy is proposed to develop exciplex emitters by introducing an intermolecular hydrogen bond (HB) between electron‐donating and electron‐accepting constituting molecules, and a series of exciplex emitters are constructed accordingly. With the most intermolecular HBs, 1,3‐di(10H‐phenoxazin‐10‐yl)benzene:4,6‐bis(3,5‐di(pyridin‐4‐yl)phenyl)‐2‐methylpyrimidine successfully realizes the best efficiencies and stabilities in the organic light‐emitting diodes compared to other contrastive exciplex emitters.
Highly twisted electron donor (D)–electron acceptor (A)-type thermally activated delayed fluorescence (TADF) emitters can achieve high efficiency while suffering from serious structural relaxations ...and broad emissions. Multiple resonance (MR)-type TADF emitters can realize narrow emission. However, until now, only a few efficient MR-emitting cores are reported and custom tunning of their emission color remains a major challenge in their wider applications. In this work, by combining the conventional TADF and MR-TADF designs, we demonstrate that color tuning and narrowing the spectral width of conventional TADF emission can be easily achieved simultaneously. We select a prototypical carbonyl (CO)/N-based MR core as a backbone and attach it with D segments of different electron-donating abilities and numbers to obtain three different TADF emitters with emissions from sky blue to green and orange-red while maintaining the narrow emission of the original MR core. The corresponding sky blue, green, and orange-red organic light-emitting diodes achieve maximum external quantum efficiencies of 20.3, 27.3, and 26.3%, respectively, and narrow full widths at half-maximum all below 0.28 eV. These results provide a new molecular design strategy for developing narrowband TADF emitters with easily tunable emissions covering the full visible range.
The conformational distribution and mutual interconversion of thermally activated delayed fluorescence (TADF) emitters significantly affect the exciton utilization. However, their influence on the ...photophysics in amorphous film states is still not known due to the lack of a suitable quantitative analysis method. Herein, we used temperature‐dependent time‐resolved photoluminescence spectroscopy to quantitatively measure the relative populations of the conformations of a TADF emitter for the first time. We further propose a new concept of “self‐doping” for realizing high‐efficiency nondoped OLEDs. Interestingly, this “compositionally” pure film actually behaves as a film with a dopant (quasi‐equatorial form) in a matrix (quasi‐axial form). The concentration‐induced quenching that may occur at high concentrations is thus expected to be effectively relieved. The “self‐doping” OLED prepared with the newly developed TADF emitter TP2P‐PXZ as a neat emitting layer realizes a high maximum external quantum efficiency of 25.4 % and neglectable efficiency roll‐off.
Until now, the influence of the conformational distribution on the photophysics and device performance of thermally activated delayed fluorescence (TADF) emitters has not been clear. Herein, conformational populations in disordered solid states are quantitatively measured for the first time. A high‐performance “self‐doping” OLED with a maximum external quantum efficiency of 25.4 % is achieved.
A novel thermally activated delayed fluorescence (TADF) emitter 12,15‐di(10H‐phenoxazin‐10‐yl)dibenzoa,cdipyrido3,2‐h:2′,3′‐jphenazine (DPXZ‐BPPZ) is developed for a highly efficient red organic ...light‐emitting diode (OLED). With rigid and planar constituent groups and evident steric hindrance between electron‐donor (D) and electron‐acceptor (A) segments, DPXZ‐BPPZ realizes extremely high rigidity to suppress the internal conversion process. Meanwhile, the highly twisted structure between D and A segments will also lead to an extremely small singlet–triplet energy split to DPXZ‐BPPZ. Therefore, DPXZ‐BPPZ successfully realizes an efficient fluorescent radiation transition and reverse intersystem crossing process, and possesses an extremely high photoluminescence quantum efficiency of 97.1 ± 1.1% under oxygen‐free conditions. The OLED based on DPXZ‐BPPZ shows red emission with a peak at 612 nm and a Commission Internationale de L'Eclairage (CIE) coordinate of (0.60, 0.40), and it achieves high maximum forward‐viewing efficiencies of 20.1 ± 0.2% (external quantum efficiency), 30.2 ± 0.6 cd A−1 (current efficiency), and 30.9 ± 1.3 lm W−1 (power efficiency). The prepared OLED has the best performance among the reported red TADF OLEDs. These results prove that DPXZ‐BPPZ is an ideal candidate for red TADF emitters, and the designing approach is valuable for highly efficient red TADF emitters.
A thermally activated delayed fluorescence emitter with rigid and planar constituent segments and highly twisted electron‐donor–electron‐acceptor framework for highly efficient red organic light‐emitting diode is developed. It results in an efficient fluorescent radiation transition and reverse intersystem crossing process simultaneously. The device based on this emitter exhibits red emission as well as a high maximum forward‐viewing external quantum efficiency of 20.1 ± 0.2%.
Realizing efficient red/near‐infrared (NIR) electroluminescence (EL) by precisely modulating molecular aggregations of thermally activated delayed fluorescence (TADF) emitters is an attractive ...pathway, yet the molecular designs are elusive. Here, a new approach is proposed to manage molecular aggregation via a mild‐twist acceptor‐donor‐acceptor (A‐D‐A)‐type molecular design. A proof‐of‐concept TADF molecule, QCN‐PhSAC‐QCN, is developed that furnishes a fast radiative rate and obvious aggregation‐induced emission feature. Its emission bands can be facilely shifted from intrinsic yellow to the red/NIR region via fine‐tuning doping levels and molecular aggregates while maintaining elegant photoluminescence quantum yields benefiting from suppressed exciton annihilation processes. As a result, a QCN‐PhSAC‐QCN‐based organic light‐emitting diode (OLED) exhibits a record‐setting external quantum efficiency (EQE) of 39.1% at a doping ratio of 10 wt.%, peaking at 620 nm. Moreover, its nondoped NIR OLED affords a champion EQE of 14.3% at 711 nm and retains outstanding EQEs of 5.40% and 2.35% at current densities of 10 and 100 mA cm−2, respectively, which are the highest values among all NIR‐TADF OLEDs at similar density levels. This work validates the feasibility of such mild‐twist A‐D‐A‐type molecular design for precisely controlling molecular aggregation while maintaining high efficiency, thus providing a promising pathway for high‐performance red/NIR TADF OLEDs.
State‐of‐the‐art red/near‐infrared organic light‐emitting diodes are realized by modulating molecular aggregation based on an intrinsic yellow acceptor‐donor‐acceptor‐type molecule with mild twists. Its doped red device delivers a record‐setting external quantum efficiency (EQE) of 39.1% at 620 nm. Furthermore, its nondoped near‐infrared device also offers a topmost EQE of 14.3% at 711 nm and retains decent EQEs at high current densities.
In this work, we propose a novel concept to develop two fluorophores 2-(10H-phenothiazin-10-yl)thianthrene 5,5,10,10-tetraoxide (PTZ-TTR) and 2-(4-(10H-phenothiazin-10-yl)phenyl)thianthrene ...5,5,10,10-tetraoxide (PTZ-Ph-TTR) showing dual conformations for highly efficient single-emitter white organic light-emitting diodes (WOLEDs). Both molecules exist in two stable conformations. Their nearly orthogonal forms own lower energy levels and show thermally activated delayed fluorescence (TADF) characteristics, whereas their nearly planar conformers possess higher energy levels and show only prompt fluorescence. These dual conformers were exploited for fabricating WOLEDs with complementary emission colors contributed by the two conformations. Moreover, the originally wasted triplet energy on the nearly planar conformation can be transferred to the nearly orthogonal one and then harvested via the TADF channel, realizing full exciton utilization. A PTZ-TTR-based single-emitter device exhibits standard white emission with a CIE coordinate of (0.33, 0.33) and a high color rendering index value of 92. On the other hand, the PTZ-Ph-TTR-based single-emitter device realizes an emission approaching warm white light and a high maximum external quantum efficiency of 16.34%. These results demonstrate an alternative approach for designing high-performance WOLEDs based on single TADF emitters.
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