Suppressing Efficiency Roll-Off of TADF Based OLEDs by Constructing Emitting Layer With Dual Delayed Fluorescence

Enhanced performance of thermally activated delayed fluorescent light emitting diodes by optimized host polarity

The interaction between the intrinsic polarity of the host material and the TADF guest material affects charge injection and transport, exciton formation, charge recombination, and emission mechanisms. Therefore, understanding and controlling the interaction between the intrinsic polarity of the host material and the TADF guest material is very important to realize efficient TADF-OLED devices. This study investigated the molecular interaction between different polar host materials and a thermally activated delayed fluorescence material (DMAc-PPM). It has been found that interaction between the host and guest (π-π stacking interaction, multiple CH/π contacts) greatly influence the molecular transition dipole moment orientation of the guest. And the OLED devices based on the strong polar host (DPEPO) exhibited the highest EQEmax and lowest luminescence intensity, while devices using the weaker polar hosts mCP and CBP achieved higher luminance and lower EQEmax. Then, the strong polar host DPEPO was mixed with the weaker polar hosts CBP and mCP, respectively. The devices prepared based on the mixed-host DPEPO: mCP showed a 2.2 times improvement in EQEmax from 6.3% to 20.1% compared to the single-host mCP. The devices prepared based on the mixed-host DPEPO: CBP showed a 3.1 times improvement in luminance intensity from 1023 cd/m2 to 4236 cd/m2 compared to the single host of DPEPO. This suggests that optimizing the polarity of host materials has the potential to enhance the performance of solution prepared OLED devices.

Multifunctional derivatives of pyrimidine-5-carbonitrile and differently substituted carbazoles for doping-free sky-blue OLEDs and luminescent sensors of oxygen

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Pyrimidine-5-carbonitrile-based compounds with efficient TADF exceeding reverse intersystem crossing rates of 10⁶ s⁻¹.

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AIEE properties for the designed compounds allowing to reach PLQYs up to 50% in solid state.

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Bipolar charge-transporting properties showing hole mobility of 1.6 × 10^-4 cm²/V·s and electron mobility of 1.37 × 10^-5 cm²/V·s.

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Non-doped sky-blue OLED with external quantum efficiency of 12.8%.

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Oxygen probes with fast response, high sensitivity and good stability.

Introduction

Evolution of organic light-emitting diodes (OLEDs) reached the point, which allows to obtain maximum internal quantum efficiency of 100% partly using heavy-metal-free emitters exhibiting thermally activated delayed fluorescence (TADF). Such emitters are also predictively perfect candidates for new generation of optical sensors since triplet harvesting can be sensitive to different analytes (at least to oxygen). Although many organic TADF emitters have been reported so far as OLED emitters, the investigation of materials suitable for both OLEDs and optical sensors remains extremely rare.

Objectives

Aiming to achieve high photoluminescence quantum yields in solid-state and triplet harvesting abilities of organic semiconductors with efficient bipolar charge transport required for application in both blue OLEDs and optical sensors, symmetrical donor–acceptor-donor organic emitters containing pyrimidine-5-carbonitrile electron-withdrawing scaffold and carbazole, tert-butylcarbazole and methoxy carbazole donor moieties were designed, synthesized and investigated as the main objectives of this study.

Methods

New compounds were tested by many experimental methods including optical and photoelectron spectroscopy, time of flight technique, electrochemistry and thermal analyses.

Results

Demonstrating advantages of the molecular design, the synthesized emitters exhibited sky-blue efficient TADF with reverse intersystem crossing rates exceeding 10⁶ s⁻¹, aggregation-induced emission enhancement with photoluminescence quantum yields in solid state exceeding 50%, hole and electron transporting properties with charge mobilities exceeding 10^-4 cm²/V·s, glass-forming properties with glass transition temperatures reaching 177 °C. Sky-blue OLEDs with non-doped light-emitting layers of the synthesized emitter showed maximum external efficiency of 12.8% while the doped device with the same emitter exhibited maximum external efficiency of 14%. The synthesized emitters were also used as oxygen probes for optical sensors with oxygen sensitivity estimated by the Stern-Volmer constant of 3.24·10^-5 ppm⁻¹.

Conclusion

The developed bipolar TADF emitters with pyrimidine-5-carbonitrile and carbazole moieties showed effective applicability in both blue OLEDs and optical sensors.

Dual emission in purely organic materials for optoelectronic applications

Purely organic molecules, which emit light by dual emissive (DE) pathways, have received increased attention in the last decade. These materials are now being utilized in practical optoelectronic, sensing and biomedical applications. In order to further extend the application of the DE emitters, it is crucial to gain a fundamental understanding of the links between the molecular structure and the underlying photophysical processes. This review categorizes the types of DE according to the spin multiplicity and time range of the emission, with emphasis on recent experimental advances. The design rules towards novel DE molecular candidates, the most perspective types of DE and possible future applications are outlined. These exciting developments highlight the opportunities for new materials synthesis and pave the way for accelerated future innovation and developments in this area.

High-Efficiency Diphenylpyrimidine Derivatives Blue Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes

Organic light-emitting diodes with thermally activated delayed fluorescence emitter have been developed with highly twisted donor-acceptor configurations and color-pure blue emitters. Synthesized 4-(4-(4,6-diphenylpyrimidin-2-yl)phenyl)-10H-spiro[acridine-9,9'-fluorene] (4,6-PhPMAF) doped device with spiroacridine as a donor unit and diphenylpyrimidine as acceptor exhibits the device characteristics such as the luminescence, external quantum efficiencies, current efficiencies, and power efficiencies corresponding to 213 cd/m², 2.95%, 3.27 cd/A, and 2.94 lm/W with Commission International de l'Eclairage (CIE) coordinates of (0.15, 0.11) in 4,6-PhPMAF-doped DPEPO emitter. The reported 10-(4-(2,6-diphenylpyrimidin-4-yl)phenyl)-10H-spiro[acridine-9,9'-fluorene] (2,6-PhPMAF) doped device exhibit high device performance with 1,445 cd/m², 12.38%, 19.6 cd/A, and 15.4 lm/W, which might be originated from increased internal quantum efficiency by up-converted triplet excitons to the singlet state with relatively smaller ΔE _ST of 0.17 eV and higher reverse intersystem crossing rate (k _RISC) of 1.0 ×10⁸/s in 2,6-PhPMAF than 0.27 eV and 3.9 ×10⁷/s in 4,6-PhPMAF. Despite low performance of 4,6-PhPMAF doped device, synthesized 4,6-PhPMAF has better color purity as a deep-blue emission with y axis (0.11) than reported 2,6-PhPMAF with y axis (0.19) in CIE coordinate. The synthesized 4,6-PhPMAF has higher thermal stability of any transition up to 300°C and decomposition temperature with only 5% weight loss in 400°C than reported 2,6-PhPMAF. The maximum photoluminescence emission of 4,6-PhPMAF in various solvents appeared at 438 nm, which has blue shift about 20 nm than that of 2,6-PhPMAF, which contributes deep-blue emission in synthesized 4,6-PhPMAF.

Asymmetric Thermally Activated Delayed Fluorescence Materials With Aggregation-Induced Emission for High-Efficiency Organic Light-Emitting Diodes

The exploitation of thermally activated delayed fluorescence (TADF) emitters with aggregation-induced emission is highly prerequisite for the construction of highly efficient electroluminescent devices in materials science. Herein, two asymmetric TADF emitters of SFCOCz and SFCODPAC with charming aggregation-induced emission are expediently designed and prepared based on highly twisted strong electron-withdrawing acceptor (A) of sulfurafluorene (SF)-modified ketone (CO) and arylamine donor (D) in D1-A-D2 architecture by simple synthetic procedure in high yields. High photoluminescence quantum yields up to 73% and small singlet-triplet splitting of 0.03 eV; short exciton lifetimes are obtained in the resultant molecules. Strikingly, efficient non-doped and doped TADF organic light-emitting diodes (OLEDs) facilitated by these emitters show high luminance of 5,598 and 11,595 cd m-2, current efficiencies (CEs) of 16.8 and 35.6 cd/A, power efficiencies (PEs) of 9.1 and 29.8 lm/W, and external quantum efficiencies (EQEs) of 7.5 and 15.9%, respectively. This work furnishes a concrete instance in exploring efficient TADF emitter, which is highly conducive and encouraging in stimulating the development of TADF OLEDs with high brightness and excellent efficiencies simultaneously.