Tetrabenzo[<i>a</i>,<i>c</i>]phenazine Backbone for Highly Efficient Orange–Red Thermally Activated Delayed Fluorescence with Completely Horizontal Molecular Orientation

Wang

et al. 2023

Adv Funct Materials

Multiple-resonance (MR) thermal activated delayed fluorescence (TADF) emitters have attracted increasing attention in organic electroluminescence devices, owing to their superior quantum efficiency and narrowband emission for high color purity. However, MR-TADF materials often suffer from severe aggregation-caused quenching (ACQ) and efficiency roll-off problems due to their rigid planar structures and the lack of sufficient charge-transfer (CT) characters with inefficient reverse intersystem crossing (RISC). Herein, by attaching electron-rich triphenylamine (TPA) with twisted spatial conformation to the MR framework, two efficient narrowband MR-TADF emitters, namely BNCz-pTPA and BNCz-mTPA, are developed. The TPA substituent endows the new emitters with aggregation-induced emission enhancement (AIEE) for ACQ suppression. The unprecedented AIEE-MR-TADF emitters exhibit CT character in high-lying triplet excited states for faster RISC, while the locally-excited (LE) character of the first singlet excited state is retained for narrowband emission with high emission efficiency. An organic light-emitting diode (OLED) based on BNCz-pTPA exhibits a maximum external quantum efficiency of 27.3%with slow efficiency roll-off, demonstrating much higher performances than those of the BNCz-based OLED. This study may provide a simple but effective approach to constructing high-performance emitters for wide-colorgamut OLED displays.

Section: Resultsmentioning

confidence: 99%

Triphenylamine‐Functionalized Multiple‐Resonance TADF Emitters with Accelerated Reverse Intersystem Crossing and Aggregation‐Induced Emission Enhancement for Narrowband OLEDs

Wang

et al. 2023

Adv Funct Materials

“…The distinct difference in transition nature between S 1 and T 1 states is beneficial for the occurrence of RISC processes, rendering increased RISC rate. [ 22,42–44 ] In contrast, the second triplet excited (T 2 ) and third triplet excited (T 3 ) states of both molecules are dominated by the CT characters. Theoretically, up‐conversion of CT‐featured triplet states to CT‐featured singlet states is difficult.…”

Section: Resultsmentioning

confidence: 99%

High‐Performance Orange–Red Organic Light‐Emitting Diodes with External Quantum Efficiencies Reaching 33.5% based on Carbonyl‐Containing Delayed Fluorescence Molecules

Jiang

Líu

et al. 2021

Advanced Science

Developing orange to red purely organic luminescent materials having external quantum efficiencies (ηexts) exceeding 30% is challenging because it generally requires strong intramolecular charge transfer, efficient reverse intersystem crossing (RISC), high photoluminescence quantum yield (ΦPL), and large optical outcoupling efficiency (Φout) simultaneously. Herein, by introducing benzoyl to dibenzo[a,c]phenazine acceptor, a stronger electron acceptor, dibenzo[a,c]phenazin‐11‐yl(phenyl)methanone, is created and employed for constructing orange–red delayed fluorescence molecules with various acridine‐based electron donors. The incorporation of benzoyl leads to red‐shifted photoluminescence with accelerated RISC, reduced delayed lifetimes, and increased ΦPLs, and the adoption of spiro‐structured acridine donors promotes horizontal dipole orientation and thus renders high Φouts. Consequently, the state‐of‐the‐art orange–red organic light‐emitting diodes are achieved, providing record‐high electroluminescence (EL) efficiencies of 33.5%, 95.3 cd A−1, and 93.5 lm W‒1. By referring the control molecule without benzoyl, it is demonstrated that the presence of benzoyl can exert significant positive effect over improving delayed fluorescence and enhancing EL efficiencies, which can be a feasible design for robust organic luminescent materials.

“…Considering its deep red emission at 636 nm, this is one of the high EQE values at high luminance reported in the literature. Figure S9 (Supporting Information) [ 32,35,37,40,64,73,74,77–95 ] summarizes EQE values of all reported pure red TADF emitters (above 630 nm) at a luminance of 100 cd m −2 in which only p CNQ‐TPA [ 74 ] and T‐DA‐2 emitter [ 85 ] exhibited higher EQE than the t BuTPA‐CNQx emitter at 100 cd m −2 . However, the EQE of the T‐DA‐2 emitter was considerably dropped to 7% at 1000 cd m −2 , highlighting the importance of the t BuTPA‐CNQx emitter for low‐efficiency roll‐off in the deep red OLEDs.…”

Section: Resultsmentioning

confidence: 99%

“…Several high EQE orange and red TADF OLEDs were reported by utilizing such types of highly planar and rigid acceptor‐based TADF emitters. [ 30–38 ] However, because of the large dihedral angle between the donor and acceptor units in the red TADF emitters (600–650 nm), their oscillator strength ( f ) value is reduced, resulting in low PLQY and hence low EQE values. [ 39–43 ] Furthermore, more rigid and planar aromatic skeletons of red TADF emitters are favorable to form π–π stacking, but they trigger severe exciton quenching and hence large efficiency roll‐off.…”

Section: Introductionmentioning

confidence: 99%

Solution‐Processed Pure Red TADF Organic Light‐Emitting Diodes With High External Quantum Efficiency and Saturated Red Emission Color

et al. 2023

light-emitting diodes (OLEDs) and are advancing to replace the current stateof-the-art phosphorescent emitters. [1][2][3][4][5][6][7][8][9][10][11][12] Based on the thermal up-conversion of triplet excitons into singlet excitons via reverse intersystem crossing (RISC), TADF emitters can attain 100% internal quantum efficiency like phosphorescent emitters, and they can easily overcome the disadvantages of phosphorescent emitters such as high cost, need for rare materials, and environmental sustainability. [13][14][15][16][17] The emitting material used in TADF OLEDs plays a key role in controlling the important device parameters such as efficiency, lifetime, and color purity of the device. Two contradictory requirements, that is, high photoluminescence quantum yield (PLQY) and a small energy gap (ΔE ST ) between the lowest singlet (S 1 ) and triplet (T 1 ) excited states need to be balanced in the design of highly efficient TADF emitters. Although highly efficient blue and green TADF OLEDs with more than 30% external quantum efficiency (EQE) were reported recently, [18][19][20][21][22][23][24][25][26][27] it is challenging to design high-efficiency pure red emitters because of the inherent limitation of the energy gap law. [28,29] Especially, in the design of pure red TADF emitters with emission wavelengths more than 600 nm, internal conversion (IC) (non-radiative decay) of In spite of recent research progress in red thermally activated delayed fluorescence (TADF) emitters, highly efficient solution-processable pure red TADF emitters are rarely reported. Most of the red TADF emitters reported to date are designed using a rigid acceptor unit which renders them insoluble and unsuitable for solution-processed organic light-emitting diodes (OLEDs). To resolve this issue, a novel TADF emitter, 6,7-bis(4-(bis(4-(tert-butyl)phenyl) amino)phenyl)-2,3-bis(4-(tert-butyl)phenyl)quinoxaline-5,8-dicarbonitrile (tBuTPA-CNQx) is designed and synthesized. The highly twisted donoracceptor architecture and appropriate highest occupied molecular orbital/ lowest unoccupied molecular orbital distribution lead to a very small singlettriplet energy gap of 0.07 eV, high photoluminescence quantum yield of 92%, and short delayed fluorescence lifetime of 52.4 µs. The peripheral t-butyl phenyl decorated quinoxaline acceptor unit and t-butyl protected triphenylamine donor unit are proven to be useful building blocks to improve solubility and minimize the intermolecular interaction. The solution-processed OLED based on tBuTPA-CNQx achieves a high external quantum efficiency (EQE) of 16.7% with a pure red emission peak at 662 nm, which is one of the highest EQE values reported till date in the solution-processed pure red TADF OLEDs. Additionally, vacuum-processable OLED based on tBuTPA-CNQx exhibits a high EQE of 22.2% and negligible efficiency roll-off.