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While monochrome organic light‐emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) emitters have achieved over 30% external quantum efficiencies (EQEs), all‐TADF white OLEDs (WOLEDs) are still lagging behind. Herein, a simple system based on two color‐complementary TADF emitters is exploited to realize high‐performance WOLEDs. By doping a high‐performance orange–red TADF fluorophor (BPPZ‐DPXZ) into a blue TADF host (DBFCz‐Trz), energy transfer, and triplet‐to‐singlet conversion in the host‐dopant system can be optimized to simultaneously achieve full exciton utilization and color balance. With this design, all‐TADF single‐emitting‐layer WOLEDs with a maximum EQE up to 32.8% are demonstrated. This high efficiency surpasses EQEs of reported WOLEDs based on both TADF as well as phosphorescence. It is expected that this finding can provide new insight for designing highly efficient all‐TADF WOLEDs. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Thermally activated delayed fluorescence (TADF) emitters generally require good separation of highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) overlaps to minimize the singlet–triplet energy offset. Observations of excellent TADF performance in several planar emitters with large HOMO–LUMO overlaps are counter-intuitive and not fully explained. To understand this, we prepared two isomeric TADF molecules, 7-methyl-2-(10H-phenoxazin-10-yl)-5H-pyrano[4,3-b]pyridin-5-one (PXZ-PPO) and 2-methyl-7-(10H-phenoxazin-10-yl)-4H-benzo[d][1,3]oxazin-4-one (PXZ-BOO). PXZ-BOO has a stable highly-twisted configuration with a small HOMO–LUMO overlap leading to good TADF performance as expected. PXZ-PPO has a stable nearly-planar form and a large HOMO–LUMO overlap and is not expected to give TADF. While the solid crystal of PXZ-PPO only shows conventional fluorescence at 420 nm, its diluted solution is dominated by TADF at 610 nm. With a combined experimental and theoretical approach, PXZ-PPO is shown to have a stable nearly-planar form and a metastable highly-twisted form in the ground state. While the highly-twisted form only has a small population (<1%) in the ground state, its excited state has lower energy than the nearly-planar form. As a result, when the high population nearly-planar form is excited, its excited state energy can decay by transforming to the highly-twisted form which shows TADF characteristics. This work points out the importance of low population but energetically stable excited states and explains for the first time the origin of TADF in such nearly-planar emitters with large HOMO–LUMO overlaps. [ABSTRACT FROM AUTHOR]

Developing red thermally activated delayed fluorescence (TADF) emitters, attainable for both high‐efficient red organic light‐emitting diodes (OLEDs) and non‐doped deep red/near‐infrared (NIR) OLEDs, is challenging. Now, two red emitters, BPPZ‐PXZ and mDPBPZ‐PXZ, with twisted donor–acceptor structures were designed and synthesized to study molecular design strategies of high‐efficiency red TADF emitters. BPPZ‐PXZ employs the strictest molecular restrictions to suppress energy loss and realizes red emission with a photoluminescence quantum yield (ΦPL) of 100±0.8 % and external quantum efficiency (EQE) of 25.2 % in a doped OLED. Its non‐doped OLED has an EQE of 2.5 % owing to unavoidable intermolecular π–π interactions. mDPBPZ‐PXZ releases two pyridine substituents from its fused acceptor moiety. Although mDPBPZ‐PXZ realizes a lower EQE of 21.7 % in the doped OLED, its non‐doped device shows a superior EQE of 5.2 % with a deep red/NIR emission at peak of 680 nm. [ABSTRACT FROM AUTHOR]

Developing red thermally activated delayed fluorescence (TADF) emitters, attainable for both high‐efficient red organic light‐emitting diodes (OLEDs) and non‐doped deep red/near‐infrared (NIR) OLEDs, is challenging. Now, two red emitters, BPPZ‐PXZ and mDPBPZ‐PXZ, with twisted donor–acceptor structures were designed and synthesized to study molecular design strategies of high‐efficiency red TADF emitters. BPPZ‐PXZ employs the strictest molecular restrictions to suppress energy loss and realizes red emission with a photoluminescence quantum yield (ΦPL) of 100±0.8 % and external quantum efficiency (EQE) of 25.2 % in a doped OLED. Its non‐doped OLED has an EQE of 2.5 % owing to unavoidable intermolecular π–π interactions. mDPBPZ‐PXZ releases two pyridine substituents from its fused acceptor moiety. Although mDPBPZ‐PXZ realizes a lower EQE of 21.7 % in the doped OLED, its non‐doped device shows a superior EQE of 5.2 % with a deep red/NIR emission at peak of 680 nm. [ABSTRACT FROM AUTHOR]

Energy differences (ΔESTs) between the lowest singlet and triplet excited states as well as the oscillator strengths (fs) are two contradictory factors controlling performance of thermally activated delayed fluorescence (TADF) emitters. To figure out their optimum balance, we developed two orange–red isomeric TADF emitters 12-(13,13-dimethyl-5-phenyl-5,13-dihydro-11H-indolo[2,3-b]acridin-11-yl)dibenzo[a,c]dipyrido[3,2-h:2′,3′-j]phenazine (IDAC–BPPZ) and 12-(13,13-dimethyl-11-phenyl-11,13-dihydro-5H-indolo[2,3-b]acridin-5-yl)dibenzo[a,c]dipyrido[3,2-h:2′,3′-j]phenazine (ACID–BPPZ), which are constructed with the same electron-donor (D) and the same electron-acceptor (A) cores via different connecting sites. ACID–BPPZ has a weaker D–A conjugation and realizes an extremely small ΔEST of 0.02 eV, but its f is also very small (0.001). These factors are better compromised in IDAC–BPPZ which has a more conjugated D–A connections. This successfully realizes a much higher f of 0.068 and a reasonably small ΔEST of 0.07 eV. As a result, the orange–red OLEDs using IDAC–BPPZ as emitter achieve a maximum external quantum efficiency 18.3% comparing to that of 14.7% in the ACID–BPPZ-based device. The results suggest that a balance between ΔEST and f is important for developing high performance TADF emitters. [ABSTRACT FROM AUTHOR]

Sind gründlich studiert worden, allerdings verlief die Entwicklung von roten/nahinfraroten TADF-Emittern viel langsamer als die von grünen und blauen Emittern (TADF: thermisch aktivierte verzögerte Fluoreszenz). Sind gründlich studiert worden, allerdings verlief die Entwicklung von roten/nahinfraroten TADF-Emittern viel langsamer als die von grünen und blauen Emittern (TADF: thermisch aktivierte verzögerte Fluoreszenz). Zhang et al. präsentieren in ihrem Forschungsartikel auf S. 14808 zwei neuartige rote TADF-Emitter. [Extracted from the article]