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Comprehensive Summary: Red thermally activated delayed fluorescence (TADF) emitters are essential for the development of organic light‐emitting diodes (OLEDs). To better understand and develop red TADF emitters, herein, the effect of quasi‐degenerate orbital distribution was investigated on their performance. Two red TADF emitters 10,10'‐(11,12‐difluorodibenzo[a,c]phenazine‐3,6‐diyl)bis(10H‐phenoxazine) (DPXZ‐BPF) and 10,10',10"‐(12‐fluorodibenzo[a,c]phenazine‐3,6,11‐triyl)tris(10H‐phenoxazine) (TPXZ‐BPF) were constructed by combining two or three phenoxazine (PXZ) donor units with a rigid coplanar acceptor core. As expected, a couple of PXZ units can induce quasi‐degenerate orbital distribution by virtue of the high steric hindrance between each donor unit and acceptor core, which enables multiple excited states engaged in the reverse intersystem crossing (RISC) process. Consequently, DPXZ‐BPF and TPXZ‐BPF exhibit fast RISC rates (kRISCs) of 1.07 × 106 s–1 and 1.29 × 106 s–1 and high PLQYs of 74.6% and 81.1%, respectively. And the higher kRISC and PLQY of TPXZ‐BPF are mainly attributed to the additional quasi‐degenerate orbitals, which further enhance the triplet RISC process and the singlet radiative process. As a result, TPXZ‐BPF‐based OLEDs achieved improved efficiencies with a red‐shifted emission compared with DPXZ‐BPF‐based OLEDs. These results demonstrated the tremendous potential of introducing quasi‐degenerate orbital distribution in developing red TADF emitters. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

White organic light-emitting diodes (WOLEDs) based on full thermally activated delayed fluorescence (TADF) emitters have attracted enormous attention for their promising applications. However, all-TADF WOLEDs with a single emitting-layer (SEL) still exhibited limited stability till now. Herein, we developed a new sky-blue TADF emitter 2,3,4,5,6-pentakis(3,6-dimethyl-9H-carbazol-9-yl)benzonitrile (5PCzCN) by modifying methyl groups at the 3- and 6-position of carbazole units to construct efficient and stable all-TADF SEL-WOLEDs. In 5PCzCN, peripheral methyl substituents would act as the shield to weaken intermolecular interactions, thus avoiding concentration quenching. By employing 5PCzCN as the emitter, the optimized OLED exhibited a sky-blue emission with a peak at 504 nm, a maximum EQE of 32.1%, and a half-lifetime (LT 50) of 95.5 h at 1000 cd m−2. All-TADF WOLEDs were then constructed by combining blue emitter 5PCzCN and orange emitter 4CzTPN-Ph in a SEL and realized a maximum EQE of 20.2%. Moreover, the white OLED impressively exhibited a long LT 50 of 10009.7 h at 100 cd m−2. These results demonstrated the bright prospect of all-TADF SEL WOLEDs. [Display omitted] • A sky-blue TADF emitter 5PCzCN was developed by introducing methyl groups at the 3- and 6-positions of carbazole units. • The optimized OLED based on 5PCzCN exhibits sky-blue emission with a forward-viewing maximum EQE of 32.1% and a LT 50 of 95.5 h at 1000 cd m−2. • All TADF WOLED was constructed by employing a single-emitting-layer structure with a long LT 50 of 10009.7 h at 100 cd m−2. [ABSTRACT FROM AUTHOR]

High efficiency, high color rendering index (CRI) and excellent color-stability are important requirements for high-performance white organic light emitting diodes (WOLEDs). To realize these issues for the WOLEDs based on the full thermally activated delayed fluorescence (TADF) system, we constructed WOLED devices by employing CDBP:PO-T2T exciplex as the host, while 2CzPN and AnbTPA as blue and red dopants, respectively. We carefully optimize the device to realize balanced carrier transporting property of each emitting layer and good exciton confinement in the device. As a result, the WOLED achieves stable white emission with a high CRI of 82, a CIE coordinate of (0.33, 0.38) at a luminance of 1000 cd m −2 , and a small CIE variation value of (0.00, 0.02) in the luminance range of 100–3000 cd m −2 . It also demonstrates high maximum forward-viewing external quantum efficiency of 19.2%, current efficiency of 36.7 cd A −1 and power efficiency of 46.2 lm W −1 . Our work presents a novel and useful approach to develop highly efficient, color-stable and high-CRI WOLEDs through the full TADF mechanism. [ABSTRACT FROM AUTHOR]

Thermally activated delayed fluorescence (TADF) emitters have only realized limited performance in solution-processed organic light-emitting diodes (OLEDs) comparing to in evaporation-processed OLEDs. To address this issue, a novel D-D′-A structure, where A is the electron-accepting group, D is the primary electron-donating group and D′ is the secondary electron-donating group, was proposed to develop efficient solution-processable TADF emitters in this work. As the intermediate D′ spacer weakens the direct intramolecular interaction between D and A groups, D-D′-A structure molecules simultaneously possess intramolecular and intermolecular charge-transfer transition channels, suppressing the aggregation-caused quenching induced by solution process. Accordingly, a novel TADF emitter 2-(3,6-bis(9,9-dimethylacridin-10(9H)-yl)-9H-carbazol-9-yl)thianthrene 5,5,10,10-tetraoxide (DMAC-Cz-TTR) was designed and synthesized. In the optimized evaporation- and solution-processed OLEDs, DMAC-Cz-TTR successfully realized similar maximum external quantum efficiencies (EQEs) of 21.1% and 20.6%, respectively. To the best of our knowledge, this is the first TADF emitter realizing nearly equal performance in both evaporation- and solution-processed OLEDs with over 20% EQEs. The outstanding performance of DMAC-Cz-TTR successfully demonstrates the feasibility of the D-D′-A structure to develop efficient solution-processable TADF emitters. [Display omitted] • A novel D-D′-A structure was proposed to develop efficient solution-processed TADF emitters. • A new TADF emitter DMAC-Cz-TTR was developed accordingly with intra- and inter-CT transition channels simultaneously. • In the evaporation- and solution-processed OLEDs, DMAC-Cz-TTR realized maximum EQEs of 21.1% and 20.6%, respectively. [ABSTRACT FROM AUTHOR]

[Display omitted] • Three TADF emitters were designed and synthesized with red to deep-red emission. • Quasi-degenerate molecular orbitals and intramolecular H-bonds are integrated. • k f and k RISC of TTPA-DCPPm are simultaneously improved. • The TTPA-DCPPm-based deep-red OLED showed an EQE of 26.5% with 640 nm emission. Triphenylamine (TPA) has been commonly used as the electron donor to construct red and near-infrared thermally activated delayed fluorescence (TADF) emitters as the strong intramolecular charge transfer with electron acceptors to induce rapid radiative decay to compete with the small energy gap. For TPA-derivates, even stronger conjugation, accelerated up-conversion and enhanced rigidity are highly desired to further overcome the serious non-radiative decay and promote exciton utilization. Targeting at optimizing TPA-based emitters, herein, three new molecules were developed. Among them, 4,4′,4′'-(pyrazino[2′,3′:5,6]pyrazino[2,3-f][1,10]phenanthroline-3,6,11-triyl)tris(N,N-diphenylaniline) (TTPA-DCPPm) were designed by incorporating intramolecular hydrogen bonding and quasi-degenerate molecular orbitals. The rigidity is enhanced and radiative decay as well as reverse intersystem crossing (RISC) are promoted. Accordingly, TTPA-DCPPm demonstrates an order of magnitude smaller non-radiative decay rate, a 2.6 times larger fluorescence decay rate and a 1.8 times faster RISC rate compared with the control group, which supports its OLED with a maximum external efficiency of 26.5% with an emission peak at 640 nm. This work paves the way for developing efficient TPA-based deep-red emitters. [ABSTRACT FROM AUTHOR]

[Display omitted] • Three TADF emitters were synthesized by finely modifying the acceptor units. • The donor–acceptor intramolecular hydrogen bond is formed in TPA-APm. • TPA-APm exhibits both twofold higher k F and an order of magnitude lower k nr. • TPA-APm-based red OLEDs achieved multiply increasing efficiency >20%. As one of the three primary colors, red organic light-emitting diodes (OLEDs) are indispensable in practical applications. However, red emitters are generally subject to severe non-radiative exciton loss due to their narrow energy gap. In this work, three new thermally activated delayed fluorescence (TADF) emitters were developed, namely 4-(acenaphtho[1,2-b]quinoxalin-9-yl)-N,N-diphenylaniline (TPA-AP), 4-(acenaphtho[1,2-b]pyrido[2,3-e]pyrazin-10-yl)-N,N-diphenylaniline (TPA-APy), and 4-(acenaphtho[1,2-b]pyrazino[2,3-e]pyrazin-9-yl)-N,N-diphenylaniline (TPA-APm), employing a series of finely modified acenaphtho[1,2-b]quinoxaline (AP) derivatives as acceptor units. Among three TADF emitters, an intramolecular hydrogen bond is formed between the donor (D) and acceptor (A) units in TPA-APm. Consequently, the overlap of the frontier molecular orbitals (FMOs) of TPA-APm can increase appropriately, and the fluorescence radiative rate (k F) of TPA-APm is nearly twofold than that of TPA-AP and TPA-APy. Furthermore, the non-radiative decay rate (k nr) of TPA-APm is less than that of TPA-AP and TPA-APy by an order of magnitude, which is attributed to the improved molecular rigidity caused by intramolecular hydrogen bond. As a result, TPA-APm-based OLEDs achieved a multiplied external quantum efficiency (EQE) of 21.1% with the electroluminescence peak at 590 nm, comparing to only 7.0% and 11.5% for the TPA-AP-based and TPA-APy-based devices, respectively. These results demonstrate appropriate intramolecular hydrogen bond can suppress the influence of non-radiative decay by simultaneously enhancing molecular rigidity and facilitating the fluorescence process, and have great potential in the design of efficient red TADF emitters. [ABSTRACT FROM AUTHOR]