Non-doped Devices Research Articles

Ultra‐Deep‐Blue Aggregation‐Induced Delayed Fluorescence Emitters: Achieving Nearly 16% EQE in Solution‐Processed Nondoped and Doped OLEDs with CIEy < 0.1

AbstractUltra‐deep‐blue aggregation‐induced delayed fluorescence (AIDF) emitters (TB‐tCz and TB‐tPCz) bearing organoboron‐based cores as acceptors and 3,6‐substituted carbazoles as donors are presented. The thermally activated delayed fluorescence (TADF) properties of the two emitters are confirmed by theoretical calculations and time‐resolved photoluminescence experiments. TB‐tCz and TB‐tPCz exhibit fast reverse intersystem crossing rate constants owing to efficient spin–orbit coupling between the singlet and triplet states. When applied in solution‐processed organic light‐emitting diodes (OLEDs), the TB‐tCz‐ and TB‐tPCz‐based nondoped devices exhibit ultra‐deep‐blue emissions of 416–428 nm and high color purity owing to their narrow bandwidths of 42.2–44.4 nm, corresponding to the Commission International de l´Eclairage color coordinates of (x = 0.16–0.17, y = 0.05–0.06). They show a maximum external quantum efficiency (EQEmax) of 8.21% and 15.8%, respectively, exhibiting an unprecedented high performance in solution‐processed deep‐blue TADF‐OLEDs. Furthermore, both emitters exhibit excellent device performances (EQEmax = 14.1–15.9%) and color purity in solution‐processed doped OLEDs. The current study provides an AIDF emitter design strategy to implement high‐efficiency deep‐blue OLEDs in the future.

3D Triptycene‐Fused Acridine Electron Donor Enables High‐Efficiency Nondoped Thermally Activated Delayed Fluorescent OLEDs

AbstractOrganic light‐emitting diodes (OLEDs) with simple nondoped device structure are extremely attractive for real applications since excitons can be utilized without considering the possible charge trapping and morphology issues in the host–dopant systems. Emitters with suppressed concentration annihilation capability are essential for nondoped devices. Herein, two thermally activated delayed fluorescence (TADF) emitters based on a novel 3D electron donor, namely, TDMAC, are constructed. The TDMAC donor consists of a conventional donor with fused triptycene scaffold. Benefitting from the unique rigid and bulky triptycene moiety, these TADF emitters exhibit excellent thermal, morphological, and photophysical properties. Consequently, highly efficient OLEDs with external quantum efficiencies up to 24.2% and 23% are realized for doped and nondoped devices, respectively.

Effective Energy Transfer for Green, Orange, and Red Phosphorescent Organic Light‐Emitting Diodes Based on a Bipolar Deep‐Blue Emitter with Low Efficiency Roll‐Off at High Brightness

Of paramount significance in phosphorescent organic light‐emitting diodes (PHOLEDs) are the facts that Förster resonance energy transfer (FRET) must be sufficient and back energy transfer should be prohibited. Herein, the FRET efficiencies for high‐performance PHOLEDs based on a simple bipolar luminogen 1‐phenyl‐2‐(5′‐phenyl‐[1,1′:3′,1″‐terphenyl]‐4‐yl)‐1H‐phenanthro[9,10‐d]imidazole (PHT‐PPI) with high triplet energy of 2.44 eV are systematically investigated. As a result, with PO‐01 as dopant, the high‐performance orange PHOLED is achieved with maximum external quantum efficiency (EQEmax) of 26.14% and ultrahigh brightness of 152 000 cd m−2, which are the highest reported efficiencies for orange OLEDs with such high brightness. In addition, the green and red PHOLEDs are also fabricated with EQEmax of 15.38% and 16.12%, respectively. Furthermore, the nondoped device based on PHT‐PPI as a deep‐blue emitter is also obtained with an EQEmax of 5.12% and the Commission Internationale de L’Eclairage (CIE) index of (0.15, 0.08). More importantly, the blue, green, orange, and red devices exhibit low efficiency roll‐off, particularly, of which the orange PHOLED is extremely small (<9%) even at the brightness of 10 000 cd m−2.

Tuning of spin-flip efficiency of blue emitting multicarbazolyl-substituted benzonitriles by exploitation of the different additional electron accepting moieties

Aiming to improve spin-flip efficiency of emitters exhibiting thermally activated delayed fluorescence (TADF), to five derivatives of tetra-tert-butyl-carbazolyl-substituted benzonotriles the different additional electron accepting moieties were attached, i.e. benzonitrile, benzotriazole, 2-benzotrifluoride, methyl 2-benzoate or methyl 3-methylbenzoate (compounds named as CNCN, CN4T, CNCF3, CNCOA and CNCOAM, respectively). The selection of the additional acceptor moieties provided not only different electron accepting abilities but also enabled different non-covalent intramolecular interactions (e.g., due to C–H···F, O, N hydrogen bonds) which affected emission intensity. Despite the similarity of the molecule structures of the compounds, they demonstrated very different TADF efficiencies with reverse intersystem crossing rates in the wide range from 0.047 × 106 to 2.32 × 106 s−1 observed for their oxygen-free toluene solutions. These solutions exhibited photoluminescence spectra peaking in the range from 456 to 506 nm and photoluminescence quantum yields reaching 82%. The effect of the additional acceptor substituents was on electroluminescent properties of OLEDs based on the designed TADF emitters was observed. External quantum efficiencies of non-doped and doped devices based on the newly synthesized emitters decreased in order of CNCN > CNCOAM > CNCOA > CNCF3 > CN4T. The best blue devices exhibiting electroluminescence peaked at 468 and 457 nm with CIE coordinates of (0.15, 0.17) and (0.15, 0.13), respectively, showed external quantum efficiency of 18.3 and 14.1% which was considerably higher than 11.5% observed for the reference device which was fabricated using a previously published TADF emitter and showed electroluminescence intensity maximum at 480 nm.

High-performance non-doped pure-blue electroluminescent device based on bisphenanthroimidazole derivative with twisted donor-acceptor structure

Abstract The development of efficient pure-blue emitters is of great significance for achieving high-quality display and lighting. Herein, a pure-blue emitter TPA-DPPI comprised of bisphenanthroimidazole and triphenylamine units with bipolar carrier transport property is designed and synthesized. For the two phenanthroimidazole units, one acts as the electron acceptor moiety, and the other is introduced to twist the molecule in order to suppress the intermolecular interactions in neat film. Highly twisted molecular configuration endows the compound with a high photoluminescence quantum efficiency yield of 0.56 in neat film. The non-doped device using TPA-DPPI as an emissive layer shows a pure-blue emission with a Commission International de l'Eclairage coordinates of (0.146, 0.097) and a high external quantum efficiency of 5.20%.

Investigation of the imidazole-derived moiety/spiro[fluorene-9,9′-xanthene] hybrid compounds for blue luminescent materials

Three new imidazole-derived moiety/spiro[fluorene-9,9′-xanthene] hybrid compounds, 1-phenyl-2-(4-(spiro[fluorene-9,9′-xanthen]-2-yl)phenyl)-1H-benzo[d]imidazole (SFX-PBI), 1-(4-(tert-butyl)phenyl)-2-(4-(spiro[fluorene-9,9′-xanthen]-2-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (SFX-PPI), and 1-(4-(tert-butyl)phenyl)-4,5-diphenyl-2-(4-(spiro[fluorene-9,9′-xanthen]-2-yl)phenyl)-1H-imidazole (SFX-PIM), were successfully synthesized for highly efficient blue light-emitting devices. The thermal stabilities, photophysical and electrochemical properties of the hybrid compounds were investigated. These hybrid compounds exhibited strong deep blue emission in solutions. Furthermore, the vacuum-processed nondoped devices based on these compounds were fabricated, in which the device based on the compound SFX-PPI exhibited the best electroluminescence performance with a maximum brightness of 4542 cd/m2, a maximum current efficiency of 4.51 cd/A and a maximum external quantum efficiency (EQE) of 5.03%.

Y-shaped donor-π-acceptor based deep-blue electroluminescent material for Non-doped organic light emitting devices

Extensive efforts have been put together to create deep-blue fluorophores with desired efficiency of organic light emitting devices (OLEDs). But, deep-blue fluorophores demonstrating high performance for non-doped OLEDs are yet finite in numbers. Herein, a Y-shaped donor-acceptor based fluorophore, 4′,4″,4'''-(1-phenyl-1H-imidazole-2,4,5-triyl)tris(N,N-diphenylbiphenyl-4-amine) (3TPAI) is constructed by integrating imidazole and triphenyl-amine moieties for OLEDs. Non-doped OLED device is constructed by applying 3TPAI as emissive layer which reveals deep-blue light at 443 nm. An EQE of 4.16% is attained with current efficiency (CE) of 5.1 cd/A and a power efficiency (PE) of 2.87 lm/W. High thermal stability is realized with decomposition temperature of more than 500 °C (Td; 516 °C). This material reveals the bright future of imidazole based donor-π-acceptor fluorophores for highly quality non-doped deep-blue OLEDs.

Rational Molecular Design of Multifunctional Blue-Emitting Materials Based on Phenanthroimidazole Derivatives.

High-performance deep-blue emitters with external quantum efficiencies (EQEs) exceeding 5 % are still scarce in organic light-emitting diodes (OLEDs). In this work, by introducing a [1,2,4]triazolo[1,5-a] pyridine (TP) unit at the N1 position of phenanthroimidazole (PI), two luminescent materials, PTPTPA and PTPTPA, were obtained. Systematic photophysical analysis showed that the TP block is suitable for constructing hybridized local and charge-transfer (HLCT) emitters. Its moderate electron-withdrawing ability and rigid planar structure can enhance the CT component while ensuring color purity. In addition, compared with PTPTPA, the additional phenyl ring of PTPBPTA not only increased the oscillator strength, but also decreased the Stokes shift. TDDFT calculations pointed out facile reverse intersystem crossing processes in PTPTPA from high-lying triplet states to the singlet excited state. A nondoped device based on PTPTPA as emitter showed impressive performance with EQEmax of 7.11 % and CIE coordinates of (0.15, 0.09). At the same time, it was also an efficient host for yellow and red phosphorescent OLEDs. By doping yellow (PPYBA) and red (BTPG) phosphorescent dyes into PTPTPA, a white OLED with a high EQE of 23.85 % was achieved. The successful design of PTPTPA not only provided an optimization choice for OLED emitters, but also demonstrated the empirical rules for the design of multifunctional deep-blue emitters.

Sterically-Locked Donor-Acceptor Conjugated Polymers Showing Efficient Thermally Activated Delayed Fluorescence.

Donor-acceptor (D-A) conjugated polymers often possess a significant frontier molecular orbital overlap because of the conjugation elongation, leading to no thermally activated delayed fluorescence (TADF) caused by a large singlet-triplet energy splitting (▵EST ). Herein a novel steric locking strategy is proposed by incorporating methyl groups into D-A conjugated polymers. Benefitting from the methyl hindrance, the torsion between the donor and acceptor can be well tuned to form a sterically-locked conformation, so that the unwanted relaxation toward planarity and thus conjugation elongation is prevented to boost hole-electron separation. The resultant D-A conjugated polymer achieves an extremely low ΔEST of 0.09 eV to enable efficient TADF. The corresponding doped and non-doped devices are fabricated via a solution process, revealing a record-high external quantum efficiency (EQE) of 24.0 % (79.4 cd A-1 , 75.0 lm W-1 ) and 15.3 % (50.9 cd A-1 , 47.3 lm W-1 ).

Largely Color-Tuning Prompt and Delayed Fluorescence: Dinuclear Cu(I) Halide Complexes with tert-Amines and Phosphines

Luminescent copper(I) halide complexes with bi- and tridentate rigid ligands have gained wide research interests. In this paper, six tetracoordinate dinuclear copper(I) halide complexes, Cu2X2(ppda)2 [ppda = 2-[2-(dimethylamino)phenyl(phenyl)phosphino]-N,N-dimethylaniline, X = I (1), Br (2), Cl (3)] and Cu2X2(pfda)2 [pfda = 2-[2-(dimethylamino)-4-(trifluoromethyl)phenyl(phenyl)phosphino]-N,N-dimethyl-5-trifluoromethylaniline, X = I (4), Br (5), Cl (6)], were successfully prepared and systematically characterized on their structures and photophysical properties. Complexes 1-5 have a centrosymmetric form with a planar Cu2X2 unit, and complex 6 has a mirror symmetry form with a butterfly-shaped Cu2X2. Solid complexes 1, 4, and 5 emit delayed fluorescence at room temperature, intense blue to greenish yellow (λmax = 443-570 nm) light, and their peak wavelengths are located at 443-570 nm with microsecond lifetimes (τ = 0.4-19.2 μs, ΦPL = 0.05-0.48). Complexes 2, 3, and 6 show prompt fluorescence, very weak yellowish green to yellow (λmax = 534-595 nm) emission with peak wavelengths at 534-595 nm, and lifetimes in nanoseconds (τ = 4.4-9.3 ns, ΦPL < 0.0001). (Metal + halide) to ligand and intraligand charge transitions are the main origin of the emission of the complexes. Solution-processed, complex-4-based nondoped and doped devices emit yellow green light with CIE coordinated at (0.41, 0.51), a maximum EQE up to 0.17%, and luminance reaching 75.52 cd/m2.

Highly efficient near-infrared thermally activated delayed fluorescence material based on a spirobifluorene decorated donor

The development of red/near-infrared (NIR) thermally activated delayed fluorescence (TADF) emitters are relatively lagging due to the spin statistics and energy gap law. Herein, we designed and synthesized a new NIR TADF emitter, 3-(4-(9,9′-spirobi[fluorene]-3-yl(phenyl)amino)phenyl)acenaphtho[1,2-b]pyrazine-8,9-dicarboni-trile (SDPA-APDC), by incorporating a spiro-type electron-donating moiety N,N-diphenyl-9,9′-spirobi[fluorene]-2-amine (SDPA) to an electron-withdrawing unit acenaphtho[1,2-b]pyrazine-8,9-dicarbonitrile (APDC). The photophysical, electrochemical and thermal properties of SDPA-APDC have been systematically explored. Consequently, the emitter was found high photoluminescence quantum yield (PLQY), narrow bandgap, small singlet-triplet energy gap (ΔEST) and excellent thermal stability. Furthermore, SDPA-APDC was developed for electroluminescence devices. The doped devices of SDPA-APDC achieved a red emission peak at 696 nm with a maximum external quantum efficiency (EQE) of 10.75%. And the non-doped device exhibited a NIR emission peak at 782 nm with a maximum EQE of 2.55%

Benzo[d]imidazole-functionalized triazatruxenes as the emitting materials for solution-processed non-doped OLEDs

Two benzo[d]imidazole/triazatruxene hybrid compounds, 5,10,15-tributyl-2,7,12-tris(4-(1-phenyl-1H-benzo[d]imidazole-2-yl)phenyl)-10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazole (2,7,12-TBI-TAT) and 5,10,15-tributyl-3,8,13-tris(4-(1-phenyl-1H-benzo[d]imidazole-2-yl)phenyl)-10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazole (3,8,13-TBI-TAT), were synthesized by the introduction of three benzo[d]imidazole (BI) moieties at 2-, 7-, 12-positions and 3-, 8-, 13-positions of the alkylated triazatruxene (TAT) core, respectively. Their thermal stabilities, photophysical and electrochemical properties were systematically investigated. These hybrid compounds exhibited good solubility as well as excellent thermal stability. Besides, the solution-processed doped and non-doped OLEDs based on the compounds were fabricated, in which the non-doped device fabricated from 2,7,12-TBI-TAT exhibited the best electroluminescence performance with a maximum brightness of 10075 cd/m2, a maximum current efficiency of 5.35 cd/A and a maximum external quantum efficiency (EQE) of 2.25%.

Dimers with thermally activated delayed fluorescence (TADF) emission in non-doped device

Two novel TADF molecules, with donor#1–σ–donor#2–π–acceptor (D1–σ–D2–π–A) conformation, were designed and synthesized through-bond charge transfer (TBCT) and through-space charge transfer (TSCT).

Ultrathin non-doped thermally activated delayed fluorescence emitting layer for highly efficient OLEDs.

We report highly efficient, ultrathin non-doped green and bluish-green organic light-emitting diodes (OLEDs) using a thermally activated delayed fluorescence (TADF) emitter. The green OLED with an ultrathin (∼1 nm) EML showed a 2.6-fold higher external quantum efficiency (EQEmax) of 13.5% with a luminance of 17 250 cd m-2 than the conventional (30 nm) non-doped device.

Cyanophenyl spiro[acridine-9,9′-fluorene]s as simple structured hybridized local and charge-transfer-based ultra-deep blue emitters for highly efficient non-doped electroluminescent devices (CIEy ≤ 0.05)

Cyanophenyl substituted spiro[acridine-9,9′-fluorene]s featured with HLCT excited states yielded highly efficient non-doped OLEDs with emissions in high-definition television standard blue color, CIEy ≤ 0.05, and CEmax/EQEmax of 6.54 cd A−1/4.63%.

Purely organic and saturated red emitters for a non-doped electroluminescent device with an EQE of 6.3% and low efficiency roll-off

Using a 2,1,3-naphthodiazole derivative as the fluorescent emitter, a non-doped red OLED has been obtained with a maximum LE of 5.3 cd A−1, a maximum EQE value of 6.3%, good CIE coordinates of (0.64, 0.35) and low efficiency roll-off feature.

High performance non-doped green organic light emitting diodes via delayed fluorescence

Anthracene-based non-doped OLED devices show EQE = 7.2% due to the delayed fluorescence resulting from the triplet-triplet annihilation (TTA). The hyperfluorescence device, in combination with a TADF assistant dopant, provides a high EQE of 17.8%.

Triphenylamine/benzothiadiazole-based compounds for non-doped orange and red fluorescent OLEDs with high efficiencies and low efficiency roll-off

Orange and red light-emitting materials were successfully obtained by linking TPA, BT and AN. Owing to their AIE features, the performances of non-doped devices are superior to those of the doped ones.

High-Efficiency, Non-doped, Pure-Blue Fluorescent Organic Light-Emitting Diodes via Molecular Tuning Regulation of Hot Exciton Excited States.

Tremendous efforts have been made on researching triplet-triplet annihilation (TTA) and thermally activated delayed fluorescence (TADF) materials for realizing high-efficiency blue organic light-emitting diodes (OLEDs) through utilizing triplet exciton conversion to the lowest singlet excited state (S1) from the lowest triplet excited state (T1). However, hot exciton conversion from the upper triplet energy level state (Tn, n > 1) to the lowest singlet excited state (S1) is an increasingly promising method for realizing pure-blue non-doped OLEDs with performances comparable to those of TTA and TADF materials. Herein, two pure-blue fluorescent emitters of donor (D)-π-acceptor (A) type, PIAnCz and PIAnPO, were designed and synthesized. The excited-state characteristics of PIAnCz and PIAnPO, confirmed by theoretical calculations and photophysical experiments, demonstrated these materials' hot exciton properties. Based on PIAnCz and PIAnPO as emission layer materials, the fabricated non-doped devices exhibited pure-blue emission with Commission Internationale de l'Eclairage (CIE) coordinates of (0.16, 0.12) and (0.16, 0.15), maximum luminescences of 10,484 and 15,485 cd m-2, and maximum external quantum efficiencies (EQEs) of 10.9 and 8.3%. Besides, at a luminescence of 1000 cd m-2, the EQEs of PIAnPO-based devices can still be high at 7.7%, and the negligible efficiency roll-off was 6.0%. The device performance of both materials demonstrates their outstanding potential for commercial application.

Rational design of pyridine-containing emissive materials for high performance deep-blue organic light-emitting diodes with CIEy ~ 0.06

Charge balance does matter for emission materials to obtain high-performance organic light-emitting diodes (OLEDs), and it is well-known that the electron-transporting ability is inferior to the hole-transporting for the majority of organic emitting materials, especially for blue-emitting compounds. Hence, systematically investigating the effect of the electron-withdrawing group on fluorophore is of vital importance. In this study, we designed and synthesized two deep-blue phenanthro[9,10-d]imidazole (PI) based materials named DPy-PPI and DmPy-PPI by using pyridine-containing groups as electron acceptor as well as adjusting the conjugation length. The photophysical, theoretical, thermal and electrochemical properties of the compounds were investigated systematically, and the relationship between the conjugation length of substituent groups on phenanthroimidazole and the EL performance was clarified. Both of them exhibited good thermal stability and high photoluminescence quantum yields. Non-doped devices based on DPy-PPI and DmPy-PPI as emitter achieved deep-blue emissions with the Commission Internationale de L'Eclairage (CIE) index of (0.14, 0.06) and (0.15, 0.08) and high external quantum efficiencies (EQEmax) of 4.24% and 3.74%, respectively. Meanwhile, using DPy-PPI and DmPy-PPI as the host materials, yellow-orange phosphorescent organic light-emitting diodes (PHOLEDs) were fabricated with EQEmax, CEmax and PEmax of 20.55%, 63.86 cd/A, 37.08 lm/W and 18.14%, 55.84 cd/A, 32.47 lm/W, respectively. Furthermore, the red PHOLEDs were also constructed using DPy-PPI and DmPy-PPI as the host with EQEmax, CEmax and PEmax of 14.53%, 17.04 cd/A, 18.51 lm/W and 16.62%, 23.58 cd/A, 21.16 lm/W, respectively. And we believe this work can provide some insight suggestions for scientific researchers to design deep-blue emitting materials.