Non-doped Devices Research Articles

Deep red donor-acceptor type thermally activated delayed fluorescence emitter with o-carborane as an acceptor

o-Carborane is a strong acceptor having a three-dimensional structure of 10 borons and 2 carbons. Thus, this study explores the potential of using o-carborane as a new acceptor in designing donor-acceptor (D-A) type thermally activated delayed fluorescence (TADF) materials. Accordingly, a D-A type TADF emitter was designed using o-carborane as an acceptor and 9,9-dimethyl-9,10-dihydroacridine as a donor. The TADF emitter exhibited deep red emission with a peak wavelength of 642 nm as proven by delayed fluorescence in the transient photoluminescence analysis. Deep red emission with a peak wavelength of 651 nm was also observed in both doped and non-doped devices, as demonstrated using electroluminescence devices. Finally, the maximum external quantum efficiency of DMAC-oCB devices was found to be 5.2% in the doped device and 2.1% in the non-doped device.

D-O-A based organic phosphors for both aggregation-induced electrophosphorescence and host-free sensitization

Pure organic phosphors capable of room-temperature phosphorescence show a great potential in organic light-emitting diodes, while it is limited by the big challenge to realize efficient electroluminescence under electric excitation. Herein, we develop a class of organic phosphors based on acridine as the electron donor, triazine as the electron acceptor and oxygen as the bridge between them. Benefitting from the characteristic donor-oxygen-acceptor geometry, these compounds are found to behave an exciting aggregation-induced organic room-temperature electrophosphorescence, and achieve a record-high external quantum efficiency of 15.8% for non-doped devices. Furthermore, they can sensitize multi-resonant emitters in the absence of any additional wide bandgap host, leading to an effective narrowband emission with a peak external quantum efficiency of 26.4% and a small full-width at half maximum of 26 nm. The results clearly indicate that donor-oxygen-acceptor geometry is a promising strategy to design organic phosphors suitable for organic light-emitting diodes.

An efficient and stable blue-emission OLED based on the A-π-A configuration by “hot exciton” strategy

The “hot exciton” channel, which effectively converts triplet excitons to singlet excitons from the high triplet excited state (Tn, n > 1) to the singlet excited state (Sm, m ≥ 1), can theoretically achieve 100% utilization efficiency (ηs) of the singlet excitons. In this work, two deep-blue “hot exciton”-based fluorescent emitters, IAM and TAM, with the asymmetric structure of acceptor(A)-π-acceptor(A) configuration, were designed and synthesized. Both compounds have excellent thermal stability with thermal decomposition temperatures of up to 505 and 486 °C, respectively. The outstanding thermal stability makes them easy to form amorphous films, during device vaporization, which is beneficial for luminescence. The optimized non-doped device based on IAM achieves a deep-blue emission centered at 452 nm with International Commission on Electron Emission (CIE) coordinates of (0.148, 0.145), and reaches a maximum external quantum efficiency (EQEmax) of 6.09% and a maximum luminance of 9197 cd m−2. The EQE maintains 5.1% at 1500 cd m−2, with a low efficiency roll-off, by virtue of the excellent carrier balance derived from the rational molecular regulation of IAM. The excellent deep-blue electroluminescent performance of IAM indicates that the “hot exciton” channel has unique advantages in achieving superior deep-blue emission. Meanwhile, the A-π-A design strategy is an effective way to realize the “hot exciton”, which sheds a light on the research on the “hot exciton” materials.

Effective electroluminescence performance improvement of bis-tridentate iridium complexes by triptycene group modification

To inhibit concentration quenching, three new bis-tridentate iridium complexes modified with triptycene groups were successfully synthesized. The rigid sterically demanding triptycene endows these new complexes with high photoluminescence quantum yields (PLQYs) and low concentration quenching. In air, their PLQYs exceed 0.68 in poly(methyl methacrylate) (PMMA) films and 0.17 in neat powder form, which are significantly higher than that of the reference complex (0.57 and 0.03, respectively). At the same time, the thermal stability of the complex Ir1 is improved and the luminescence wavelength of the complex Ir1 is less affected by the modification of triptycene group. The doped OLED device based on complex Ir1 exhibits excellent electroluminescent efficiencies of 80.1 cd A−1 and 24.0%, which are improved by more than 50% compared with that of reference device. More significantly, the maximum efficiencies of its non-doped device are 17.6 cd A−1, 7.4%, which are improved by more than 300% compared with that of reference device. These results indicate that the triptycene group is very effective functional group to inhibit the concentration quenching of bis-tridentate iridium complexes.

Deep blue fluorescent emitters based imidazole and carbazole for non-doped organic light emitting diodes with CIEy≤0.05

Two fluorescent emitters based imidazole and carbazole were synthesized for deep blue emission organic light-emitting diodes (OLED) with non-doped structure. The photophysical, thermal and electrochemical properties are investigated with high decomposition temperature up to 391 and 380 °C, and strong deep blue emission. Single carrier devices were also fabricated to show that two compounds have good bipolar carrier transport characteristic. The non-doped fluorescent OLED devices using the two compounds as emitting layers are designed among which the devices achieved the maximum external quantum efficiency of 1.66% and 1.56%, respectively. Besides, the deep blue emission peak of 412 nm with Commission Internationale de l’Eclairage (CIE) coordinate of (0.16, 0.04), and 420 nm with CIE coordinate of (0.16, 0.05) were also realized. The small CIEy value of 0.04 and 0.05 obtained in this work are very significant for high color purity full-color display, which is also among the best CIEy value results ever reported as deep blue bipolar emitters.

Substituent effect on anthracene-benzophenone triad system: Photophysical properties and application to OLEDs with “hot-exciton” characteristics

‘‘Hot-exciton’’ materials exhibiting high-lying reverse intersystem crossing (hRISC) are promising candidates for organic light-emitting diode (OLED) materials owing to their improved exciton utilization efficiency and efficiency roll-off. To investigate the excited-state structure–property relationship in anthracene/benzophenone-based materials, we designed and prepared three anthracene-benzophenone derivatives by varying substituents at peripheral benzophenone units, namely, AnBP-F, AnBP-H, and AnBP-OMe. Systematic investigation of the excited-state properties of these compounds and DFT calculations showed that they have hybridized local and charge-transfer (HLCT) characteristics in their emissive transitions. All prepared compounds were adapted in a non-doped OLED device, which exhibited an external quantum efficiency (EQE) of up to 2.88%. To further study the exciton dynamics in the device, magnetoelectroluminescence was measured, which exhibits the combined characteristics of triplet–triplet annihilation (TTA) and HLCT mechanisms.

Harvesting Triplet Excitons in High Mobility Emissive Organic Semiconductor for Efficiency Enhancement of Light-Emitting Transistors.

Organic light-emitting transistors (OLETs), a kind of highly integrated and minimized optoelectronic device, demonstrate great potential applications in various fields. The construction of high-performance OLETs requires the integration of high charge carrier mobility, strong emission, and high triplet exciton utilization efficiency in the active layer. However, it remains a significant long-term challenge, especially for single component active layer OLETs. Herein, the successful harvesting of triplet excitons in a high mobility emissive molecule, 2,6-diphenylanthracene (DPA), through the triplet-triplet annihilation process is demonstrated. By incorporating a highly emissive guest into the DPA host system, an obvious increase in photoluminescence efficiency along with exciton utilization efficiency results in an obvious enhancement of external quantum efficiency of 7.2times for OLETs compared to the non-doped devices. Moreover, well-tunable multi-color electroluminescence, especially white emission with Commission Internationale del'Eclairage of (0.31, 0.35), from OLETs is also achieved by modulating the doping concentration with a controlled energy transfer process. This work opens a new avenue for integrating strong emission and efficient exciton utilization in high-mobility organic semiconductors for high-performance OLETs and advancing their related functional device applications.

Cyanopyridone-cored fluorophores with triphenylamine peripheries: From molecular design to OLED fabrication studies

• Seven novel cyanopyridone-based D-A-D type molecules with a fixed triphenylamine donor, were synthesized. • Theoretical, optical, solvatochromic, thermal, electrochemical, electrical, and EL studies were performed to evaluate structure-property relationships. • Doped OLEDs using novel fluorophores as dopants outperform non-doped devices. • OLEDs emit yellow light with maximum efficiencies of 10.72 cd/A, 7.87 lm/W, and 5.32%. Amongst many types of fluorescent organic materials, compounds with π-conjugation between an electron donor (D) and electron acceptor (A) have been intensively investigated as effective organic electroluminescence (EL) device components. Herein, we report the synthesis and structural characterization of a new series of D-A-D configured small molecule-based fluorophores ( TPA-CyP1-7 ) having triphenylamine ( TPA ) as a donor unit and 3-cyanopyridine-2-one ( CyP ) as an acceptor core with different architects for use as efficient yellow light emitters in fluorescent organic light-emitting diodes (OLED). The detailed photophysical, solvatochromic, thermal, electrochemical, and EL properties, including quantum chemical calculations, were systematically investigated to study their relation between structure and properties. All the fluorophores show high fluorescent quantum yields in the solid film state and display high thermal stability with decomposition temperatures above 350 o C. The study reveals that they possess appropriate HOMO and LUMO energies level for effective charge injection. Finally, these yellow luminogens were employed to fabricate new OLED devices as sole emitters and dopants with CBP host materials. Interestingly, the host-guest devices doped with CBP host emitters show a remarkable improvement in the overall device performance. Among them, the TPA-CyP3 -based doped device has achieved a maximum current efficiency ( η c ), high power efficiency ( η p ), and good external quantum efficiency ( η EQE ) of 10.72 cd/A, 7.87 lm/W, and 5.32%, respectively.

Hyperbranched Conjugated Polymer with Multiple Charge Transfer Enables High-Efficiency Nondoped Red Electroluminescence with Low Driving Voltage

Conjugated polymers featuring thermally activated delayed fluorescence (TADF) attract tremendous attention in both academic and industry communities due to their easy solution processing for fabricating large-area and low-cost high-performance polymer light-emitting diodes (PLEDs). However, current nondoped solution-processed PLEDs frequently encounter significant efficiency roll-offs and unreasonably high operating voltages at high brightness, especially for red-emitting polymers. Herein, we design hyperbranched conjugated polymers (HCPs) with D-A-D type TADF characteristics for high-performance red-emitting PLEDs. Multiple intramolecular charge transfer (ICT) channels induced by quasi-equivalent donors of the TADF core strongly boost the reverse intersystem crossing (RISC) process and singlet excitons radiative transition. Coupling with the efficient energy transfer process generated by structure advantages of HCPs, the strongly electron-withdrawing oxygen atoms located on the TADF cores further accelerate hole transportation from the host chains to the TADF cores. Under a rational regulation of the TADF core ratio, the related nondoped red-emitting device performs an outstanding performance with an EQEmax of 8.39% and exhibits no roll-off while the luminance is less than 100 cd/m2 and only 3.3% decrease at 500 cd/m2. Simultaneously, the EQE can maintain 7.4% under 1000 cd/m2. Furthermore, the corresponding nondoped device exhibits a low turn-on voltage of around 2.5 V and achieves a luminance of 500 cd/m2 at 3.5 V and even 1000 cd/m2 at 3.9 V. To our knowledge, this is the best performance among all nondoped red PLEDs with high brightness obtained at low operating voltage.

A new multifunctional fluorescent molecule for highly efficient non-doped deep-blue electro-fluorescence with high color-purity and efficient phosphorescent OLEDs

A multifunctional molecule DTPCZTZ was synthesized, which can not only realize high efficiency nondoped deep blue device, but also achieve high efficiency phosphorescent devices.

Novel electro-fluorescent materials with hybridized local and charge-transfer (HLCT) excited state for highly efficient non-doped pure blue OLEDs

Two molecules with HLCT characteristics were synthesized through fine-tuning the substituent on PPI based on the PPI–TAZ skeleton, and the non-doped device based on PPITZCN achieved higher efficiency with an EQEmax of 7.5% and CIE of (0.16, 0.10).

Highly efficient deep-blue electrofluorescence with optimized excited state composition and "hot-exciton" channel

The properties of excited states of emitters play an important role in OLEDs. How to obtain balanced hybrid localized charge transfer (HLCT) state by reasonably designing molecules is still a challenge. Herein, three HLCT states D-π-A type blue fluorophores with different LE/CT hybrid degree were designed and synthesized, namely 3,6-di-tert-butyl-9-(4-(6-(4-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)phenyl)pyren-1-yl)phenyl)-9H carbazole (DTPCZPYTZ), 3,6-di-tert-butyl-9-(4-(4-(4-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)phenyl)naphthalen-1-yl)phenyl)-9H-carbazole (DTPCZNATZ) and 3,6-di-tert-butyl-9-(4''-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)-[1,1':4′,1″-terphenyl]-4-yl)-9H-carbazole (DTPCZPHTZ). DTPCZPHTZ achieved quasi-equivalent hybrid local and charge-transfer (HLCT) excited state and efficient “hot-exciton” channel, the non-doped deep-blue device based on DTPCZPHTZ has an electroluminescence emission peak (λEL) of 427 nm with Commission International de L'Eclairage (CIE) of (0.157, 0.037), the maximum external quantum efficiency (EQE) is up to 5.7% and the efficiency roll-off is only 5% (under 1000 cd m−2), which is outstanding in the reported deep-blue emitters. This study proved that the quasi-equivalent hybrid HLCT state is beneficial to improving device performance.

Novel Ir(III) Complexes with NHC-Based Ancillary Ligands for Efficient Nondoped OLEDs

Nondoped organic light-emitting diodes (OLEDs) are of paramount importance for display and lighting applications owing to their advantages of facile fabrication and outstanding stability. However, nondoped OLEDs achieving extraordinary electroluminescence (EL) performance and low turn-on voltage (Von) remain sparse. Here, three Ir(III) complexes featuring N-heterocyclic carbene (NHC) auxiliary ligands functionalized with electron-deficient aromatic sulfonyl or phosphine oxide groups are reported as promising emitters for nondoped OLEDs. All Ir(III) complexes exhibit green emission with relatively high neat film efficiency. Although the photoluminescence spectra of three complexes reveal similarities, there are distinct differences in the nondoped EL performance. The nondoped device N3 based on tBu-Ir-ISO displays the most eminent EL performances and presents a low Von of 2.1 V, a power efficiency of 30.7 lm W-1, and a maximum current efficiency of 27.0 cd A-1, which can be attributed to steric hindrance and balanced carrier-transporting ability induced by electron-deficient substituents. Moreover, doped devices D1-D3 also realize excellent EL performance. It is believed that the strategy reported herein is a simple and efficient way of constructing excellent Ir(III) complexes for nondoped phosphorescent OLEDs.

Tuning hybridized local and charge transfer emission in pyrene derivatives by incorporating phenanthroimidazole to realize high exciton harvest in blue organic light-emitting diodes

Pyrene derivatives capable of harvesting triplet excitons and displaying blue electroluminescence are limited. A new series of pyrene-based blue-emitting hybridized local and charge transfer materials is designed by asymmetrically substituting at the 1,3-positions of pyrene with phenanthroimidazole and N-phenylcarbazole. Electronic excitations due to pyrene localized and the charge transfer from phenanthroimidazole to pyrene segments dominate the absorption of the dyes. Incorporation of carbazole enhanced the charge transfer characteristics in the compounds and lead to the hybridization of local and charge transfer states. Theoretical studies revealed the possibility of exciton harvesting via the high lying triplet states, which have matching energy with the first excited singlet state. Lippert-Mataga plots for the solvatochromic data of the dyes exhibited bilinear relationship attributable to the formation of local state in the non-polar solvents and charge transfer state in the polar solvents. All the derivatives served as emitting layers in doped as well as non-doped multilayered organic light-emitting devices and exhibited deep-blue electroluminescence. The doped devices displayed much higher EL performance on comparing with that of the non-doped devices due to the proper alignment of energy levels leading to efficient exciton harvesting. The device containing carbazole derivative as dopant with tris(4-carbazoyl-9-ylphenyl)amine host displayed the highest external quantum efficiency of 8.4% with 50% exciton harvest.

The effect of electron donor and acceptor conformations on thermally activated delayed fluorescence

A novel organic molecule, 2-(9,9-dimethyl-9 H -acridin-10-yl)-anthraquinone (DMAC-AQ), is designed and synthesized by introducing a dual-conformation electron donor 9,9-dimethyl-9,10-dihydroacridine to a typical electron acceptor anthraquinone. DMAC-AQ exhibits decent thermally activated delayed fluorescence (TADF) property. However, using DMAC-AQ as an emitter, the doped and non-doped electroluminescence devices show maximum external quantum efficiency values of 5.63% and 0.32%, respectively, which are much lower than expected. Crystallographic analysis reveals that DMAC-AQ has two nearly orthogonal conformers (C1 and C2), in which planar and non-planar DMAC moieties are both linked to the AQ group with high twisting angles. Interestingly, the non-planar conformations of donor (D) and acceptor (A) units contribute to the generation of luminescent enantiomers in C2. C1 exhibits obvious TADF while C2 shows no TADF. The results suggest that nonplanar conformation of the donor in the emitter may inhibit TADF and significantly reduce device efficiency. The present work may provide new insight into understanding the effect of D and A conformations in TADF emitters. The non-planar conformations of electron donor and acceptor groups in thermally activated delayed fluorescence (TADF) emitters may inhibit TADF and significantly reduce device efficiency. • A TADF emitter with dual conformations was designed and synthesized. • The effect of donor and acceptor conformations on TADF was investigated. • Nonplanar conformation of the donor in the emitter may inhibit TADF.

A sensitization strategy for highly efficient blue fluorescent organic light-emitting diodes

Highly efficient blue fluorescent materials have recently attracted great interest for organic light-emitting diode (OLED) application. Here, two new pyrene based organic molecules consisting of a highly rigid skeleton, namely SPy and DPy, are developed. These two blue light emitters exhibit excellent thermal stability. The experiment reveals that the full-width at half-maximum (FWHM) of the emission spectrum can be tuned by introducing different amounts of 9,9-diphenyl-N-phenyl-9H-fluoren-2-amine on pyrene units. The FWHM of the emission spectrum is only 37 nm in diluted toluene solution for DPy. Furthermore, highly efficient blue OLEDs are obtained by thermally activated delayed fluorescence (TADF) sensitization strategy. The blue fluorescent OLEDs utilizing DPy as emitters achieve a maximum external quantum efficiency (EQE) of 10.4% with the electroluminescence (EL) peak/FWHM of 480 nm/49 nm. Particularly, the EQE of DPy-based device is boosted from 2.6% in non-doped device to 10.4% in DMAc-DPS TADF sensitized fluorescence (TSF) device, which is a 400% enhancement. Therefore, this work demonstrates that the TSF strategy is promising for highly efficient fluorescent OLEDs application in wide-color-gamut display field.Graphical

Approaching 800 nm near-infrared thermally activated delay fluorescence materials via simple malononitrile condensation strategy

The development of organic materials with near-infrared (NIR) thermally activated delayed fluorescence (TADF) has drawn growing attention in recent years, but most of the reported NIR TADF emitters have emission peaks less than 780 nm with considerable visible emission remnants. To achieve pure NIR emission, we developed a strong electron acceptor fluoren-9-ylidene malononitrile ( FM ) by malononitrile condensation with fluorenone. Exploiting FM as acceptor, two novel molecules 3TPAFM and 36DTPAFM with pure NIR TADF were facilely synthesized and displayed high thermal stability. Compared with the orange emissive non-TADF fluorenone precursors, 3TPAFM and 36DTPAFM demonstrated significantly red-shifted emission of up to 217 nm and largely reduced energy gaps between the lowest singlet and triplet states, leading to pure NIR TADF with emission peak at 802 nm in neat films. The photophysical study revealed that their nonradiative decay rates are comparable with fluorenone counterparts even with such significantly reduced optical energy gap, manifesting largely attenuated limitation from the energy gap law. The non-doped OLEDs based on 3TPAFM and 36DTPAFM achieved pure NIR emission with λ EL at 792 and 788 nm, and external quantum efficiencies of 0.21% and 0.32% at 1 mA cm −2 , respectively. This work provides a simple and powerful strategy to achieve NIR TADF by malononitrile condensation with ketone group and presents FM group as a desirable acceptor candidate to construct pure and efficient NIR emitting materials. • Two novel NIR TADF emitters were constructed through simple malononitrile condensation strategy. • They achieved significant redshifts of 217 nm while maintaining similar nonradiative decay rates with fluorenone precursors. • Their non-doped OLED devices realized pure and efficient NIR emission with peaks around 790 nm.

A novel asymmetric fused chromophore for non-doped organic light-emitting diode with small efficiency roll-off

We have designed and synthesized a novel asymmetric amine‒carbonyl fused emitter, BACQ, having a hybridized local and charge-transfer emissive state. From the single-crystal analysis, the BACQ exhibited non-planar configuration with multiple C–H‧‧‧O and C–H‧‧‧π interactions, which could rigidify the molecule and provide good thermal stability. Non-doped device based on BACQ present sky-blue emission with maximum luminance of 11180 cd m−2, and external quantum efficiency of 2.48% with small roll-off.

A new amine‒carbonyl fused emitter with hybridized excited state for green OLED with high luminance and low efficiency roll-off

In this work, a novel green emitter, namely 16,16-dimethylbenzo[a]quinolino[3,2,1-de]acridin-10(16H)-one (GACQ), with amine‒carbonyl fused structure has been designed and synthesized. The single-crystal structure analysis confirmed that the fused structure shows non-planar conformation with dihedral angle of 48.97° from the front-view direction. The photophysical and theoretical results confirmed that GACQ exhibited hybridized local and charge-transfer (HLCT) feature with strong green emissions of 513 and 535 nm in solution and film, respectively. The non-doped device based on GACQ achieved high luminance (15810 cd m−2) and good efficiency stability with negligible external quantum efficiency (EQE) roll-off. Furthermore, the doped device exhibited high maximum EQE of 4.8% with CIE coordinate of (0.38, 0.58).

Highly efficient and concentration-insensitive OLEDs based on alkyl sterically modified red homoleptic phenylphthalazine iridium complexes

Four red homoleptic iridium (III) complexes (Ir1–Ir4) with steric phenylphthalazine ligands have been synthesized. Single X-ray structural analysis shows that the steric hindrance groups can prevent molecular aggregation. All complexes exhibited red emission with peaks between 614 and 629 nm in CH2Cl2 and photoluminescence quantum yields (QYs) of 44%–86% in doped films. They were concentration-insensitive, their QYs in neat powder and air atmosphere can reach a maximum of 15%. These complexes exhibited excellent thermal stability with Td higher than 416 °C. The OLEDs based on these complexes showed good performances at 10 wt% high doping concentration. Especially, the device based on Ir1 gave a peak current efficiency and external quantum efficiency (EQE) of 25.0 cd A−1 and 24.2% with mild efficiency roll-off, which is the most efficient OLED based on phthalazine iridium complexes. The maximum EQEs of non-doped devices are between 4.0% and 5.6%, which are considerably high, compared with the reported efficiency of iridium complexes based red-emitting non-doped devices. These studies show that increasing the density of the rigid steric groups of iridium complexes is very effective to achieve high efficiency and low concentration sensitive red phosphorescent devices.