Rong-Huei Yi

Publication date: December 2022

Source: Dyes and Pigments, Volume 208

Author(s): Jia-Xiong Chen, Hui Wang, Lu Zhou, Kai Wang, Jia Yu, Xiao-Hong Zhang

Organic light-emitting devices (OLEDs) using inexpensive, aluminum-foil-based electrode substrates with an extremely high surface roughness are fabricated. A low leakage current is realized by incorporating extraordinary thick buffer layers of phosphotungstic acid exhibiting the negative differential resistance property. The OLEDs with a light-emission area of 64 cm² emit steady luminescence even in crumpled and torn states.

Abstract

Electrical leakage is one of the fatal faults associated with thin-film devices (TFDs) including organic light-emitting devices (OLEDs). To prevent electrical leakage, highly smooth electrodes are required to avoid the concentration of the electric field at local points. However, this requires the use of expensive electrode substrates. Here, OLEDs using inexpensive, aluminum-foil-based electrode substrates with an extremely high surface roughness are fabricated. A low leakage current is realized by incorporating extraordinary thick buffer layers of phosphotungstic acid (PWA) exhibiting the negative differential resistance (NDR) property. In its pristine state, PWA exhibits a high electrical resistance. However, upon applying electric current, the resistance is significantly decreased, resulting in increased conductivity and expression of NDR property, owing to the formation of conductive filaments and charge-storage effects. The OLEDs produced with PWA achieve a low-driving voltage and high external quantum efficiency simultaneously. Aluminum-foil-based OLEDs with a light-emission area of 64 cm² are fabricated. They emit steady luminescence even in crumpled and torn states. To the best of authors’ knowledge, this is the first report of a large-area OLED exhibiting steady luminescence in such deformed states.

Exciton–polaron quenching induced by spontaneous orientation polarization (SOP) is generally quantified and modeled in organic light-emitting devices (OLEDs). Quenching is probed using photoluminescence and engineered by varying electron transport layer SOP through materials selection and dilution with a nonpolar material. This work underscores the significance of SOP-induced quenching in limiting OLED efficiency and provides a means to tune its severity.

Abstract

Many electron transport layer (ETL) materials employed in organic light-emitting devices (OLEDs) show a preferred orientation of the molecular permanent dipole moments. This phenomenon is known as spontaneous orientation polarization (SOP) and results in the formation of bound polarization charge. In an OLED, this leads to the accumulation of polarons (typically holes) at the ETL/emissive layer interface to balance this charge. Previous work on phosphorescent OLEDs has found that exciton–polaron quenching due to SOP-induced hole accumulation can reduce peak efficiency by ≈20%. In this work, the generality of this phenomenon is systematically established by probing polaron accumulation and quenching in phosphorescent OLEDs with varying degrees of SOP. Exciton quenching is quantified by optically probing the photoluminescence of the device emissive layer during operation. It is found that the degree of SOP-induced luminescence quenching and reduction in device efficiency scale directly with ETL SOP. It is further demonstrated that the degree of polarization and amount of quenching can be tuned by mixing the polar ETL with a nonpolar host (dipolar doping). This work establishes a ubiquitous role for SOP in determining OLED efficiency and demonstrates dipolar doping as a means to tune the underlying exciton–polaron quenching.

Twisted interlocked acceptor core-based thermally activated delayed fluorescence (TADF) molecules are designed and synthesized for solution-processed deep-blue organic light emitting diodes (OLEDs). A KCTBC-based doped device shows 9.0% maximum external quantum efficiency (EQE_max), with CIE (0.17, 0.13). 4CzFCN as an assistant dopant boosts the performance of the KCTBC-based hyperfluorescent deep-blue device with an EQE_max of 13.9%. A high-efficiency warm white OLED using the TADF hybrid method achieves 9.0% EQE_max.

Abstract

Solution processed deep-blue organic light emitting diodes (OLEDs) with high external quantum efficiency (EQE) and a long operational lifetime are still constrained. In this context, two thermally activated delayed fluorescence (TADF) emitters are synthesized utilizing a new design strategy of twisted interlocked acceptor core integrated with carbazole (KCCz) and tert-butylcarbazole (KCTBC) as donors, respectively, for solution processed deep-blue TADF OLEDs. Twisting of the acceptor core by two methyl groups results in complete separation of highest occupied molecular orbital and lowest unoccupied molecular orbital, along with cyanide group facilitating the generation of low-lying triplet excited states as suggested by theoretical simulation. The combined effect of both results in tuning of emission in ultradeep blue region through the efficient population of triplet excitons and concurrently reverse intersystem crossing to produce highly efficient devices. A doped device based on KCTBC shows EQE_max of 9.0% along with low efficiency roll-off with long operational device half lifetime of 72 min at initial brightness of 1000 cd m⁻², and Commission Internationale de L'Eclairage (CIE) coordinates of (0.17, 0.13). In addition, with 12.5 wt% of 4CzFCN as assistant dopant/cohost the performance of the KCTBC-based device is enhanced to an EQE_max of 13.9% and CIE coordinates of (0.18, 0.13). Further, a high-efficiency warm white OLED adopting the TADF hybrid approach is realized with EQE_max of 9.0%.

Organoboron-based thermally activated delayed fluorescence (TADF) materials that enable high-efficiency and high-color-purity electroluminescence are actively researched in recent years. This review focuses on recent advances in narrowband emissive TADF materials with spectral full width at half maxima narrower than 50 nm, from the perspective of molecular design strategies, photophysical properties, and electroluminescence performance in organic light-emitting diodes.

Abstract

Organic thermally activated delayed fluorescence (TADF) materials have attracted significant research interest in the field of organic electronics because of their inherent advantage of 100% exciton utilization capability in organic light-emitting diodes (OLEDs) without the use of noble metals. However, despite their high internal electroluminescence quantum efficiencies approaching unity, broad emission spectra with sizable full width at half maxima (FWHM; 60–100 nm) present a critical issue that must be solved for their application in ultrahigh-definition OLED displays. Recently, a new paradigm of TADF materials featuring the multiple resonance (MR) effect based on heteroatom-doped polycyclic aromatic frameworks, referred to as MR-TADF materials, has emerged and garnered considerable research interest owing to their remarkable features of efficient narrowband emissions with extremely small FWHMs (≤30 nm). Currently, MR-TADF materials occupy a prominent position in the cutting-edge research on organic light-emitting materials from both chemical and physical perspectives. This review article focuses on recent progress in narrowband emissive MR-TADF systems from the perspective of molecular design, photophysical properties, and electroluminescence performance in OLEDs. The current status and future prospects of this advanced material technology are discussed comprehensively.

By using assistant dopant f-CF₃ and f-PhCF₃ to convey its energy to terminal emitter t-DABNA and 2TCzBN, the fabricated hyper-organic light-emitting diode devices gave external quantum efficiency (EQE) of 23.8%, full-width at half-maximum (FWHM) of 30 nm, and CIE _{x , y} of (0.13, 0.14), and EQE of 24.0%, FWHM of 28 nm, and CIE _{x , y} of (0.11, 0.36), respectively.

Abstract

Homoleptic fac-substituted Ir(III) carbene complexes exhibit higher emission energy (in purple region) in comparison to their mer-counterparts, prohibiting them to be employed in fabrication of blue emissive organic light-emitting diode (OLED) devices. Now, the design of two distinctive CF₃-functionalized purin-8-ylidene Ir(III) complexes, namely, m- and f-CF₃ and m- and f-PhCF₃ , from new carbene motifs, 9-(3-(tert-butyl)phenyl)-7-isopropyl-2-(trifluoromethyl)-7,9-dihydro-8H-purin-8-ylidene (A4) and 9-(3-(tert-butyl)phenyl)-7-methyl-6-phenyl-2-(trifluoromethyl)-7,9-dihydro-8H-purin-8-ylidene (B7), having notably stabilized lowest unoccupied molecular orbital energy levels is reported. Hence, the corresponding f-isomers f-CF₃ and f-PhCF₃ exhibit electroluminescence with peak max. at 478 and 495 nm, max. external quantum efficiencies (EQEs) of 10.4% and 12.8%, respectively. By using f-CF₃ as assistant dopant to convey its energy to terminal emitter t-DABNA and from f-PhCF₃ donor to 2TCzBN acceptor, two hyper-OLED devices are successfully fabricated, giving high max. EQE of 23.8%, full-width at half-maximum (FWHM) of 30 nm, and CIE _{x , y} coordinate of (0.13, 0.14) for the acceptor t-DABNA, and max. EQE of 24.0%, FWHM of 28 nm, and CIE _{x , y} of (0.11, 0.36) for the acceptor 2TCzBN, confirming the advantages of these purin-8-ylidene Ir(III) complexes.

Three green Ir(III) complexes with newly created cycloalkyl fused dibenzofuran ligands demonstrate that the cycloalkyl fused dibenzofuran ligand can modify the intermolecular interactions and alter the horizontal emitting dipole orientation ratios of dopants.

Abstract

In this study, the photophysical characteristics and electroluminescence of three green Ir(III) complexes with newly created cycloalkyl fused dibenzofuran ligands are examined. The results demonstrate that the cycloalkyl fused dibenzofuran ligand can modify the intermolecular interactions and alter the horizontal emitting dipole orientation ratios of dopants in phosphorescent organic light-emitting diodes (PhOLEDs). One of the three phosphors, Ir(TBF)₂(mppy), shows high photoluminescence quantum yield of 0.96 and a high horizontal emitting dipole orientation ratio of 0.80. Therefore, an optimized PhOLED using Ir(TBF)₂(mppy) exhibits a peak external quantum efficiency (EQE) of 27.1% and a very small efficiency roll-off with a high EQE of 25.1% at 10 000 cd m⁻². It is significant that the high EQE device maintains the CIE chromaticity coordinates over a wide doping concentration range (3–10 wt%), thus creating new possibilities for the dopant design strategy for the practical use of PhOLEDs.

Two novel tetradentate Pt(II) complexes are prepared and applied as blue phosphorescent emitters in organic light-emitting diode devices. These displayed low turn-on voltages (<3.1 V) and outstanding efficiencies (EQEs of >26%) and potential device stabilities, with CIEy values of <0.31.

Abstract

Although the remarkable growth of the organic light-emitting diode (OLED) industry has occurred via continuous, extensive efforts toward the utilization of various organometallic luminophores as phosphorescent emitters, the development of blue phosphorescent emitters with improved efficiencies and high electrochemical stabilities is essential. To this end, herein the preparation of two novel tetradentate Pt(II) complexes, Pt1 and Pt2, and their application as blue phosphorescent emitters in OLED devices is described. Both complexes exhibit intense bluish emission in the solution and solid states. In addition, these complexes display very high phosphorescent quantum efficiencies (>89%) with host materials and thermal stabilities (>390 °C). Multilayer phosphorescent OLEDs containing Pt1 or Pt2 as emitters with mCBP/CNmCBP-CN mixed-host systems are fabricated. The devices exhibit outstanding performances, including high current, power, external efficiencies, and potential device lifetime in addition to sky-blue (Pt1) or blue (Pt2) electroluminescence. These results clearly suggest that these tetradentate Pt(II) dopants are promising candidates as highly efficient, stable blue phosphorescent emitters in OLEDs.

(Sub)micron-sized hybrid supraparticles that bear a fire-proof identification (ID) fingerprint and thus enable postmortem identification of a marked product after an event of fire are presented herein. Due to their thermostable magnetic components, these markers can be magnetically retrieved from fire residues and purified afterwards. Finally, their ratiometric luminescence signal, derived from the luminescent components, can be detected.

Abstract

Counterfeit electronic products not only cause financial losses but also come with safety risks. The worst-case failure scenario certainly is a fire event. Since manufacturers are liable for damages and suffer image loss, fire-proof postmortem taggants are needed, enabling differentiation between originals and counterfeits even after a fire incident. This work presents such taggants: optomagnetic supraparticles (SPs), i.e., complex microscale particles composed of luminescent and magnetic nanoparticles (NPs) are generated. Their hybrid nature is pivotal, as magnetic separation can effectively remove the tags from light-absorbing fire debris, and a fire-proof identification (ID) fingerprint is based on ratiometric luminescence signals. To achieve thermally stable magnets, iron oxide (IO) NPs are wet-chemically coated with a SiO₂-3-aminopropyltriethoxysilane (APTES) shell. Subsequently, these magnetic NPs are assembled with luminescent nanophosphors (lanthanide-doped calcium phosphate NPs with a SiO₂-core) and SiO₂-APTES spacer NPs by spray-drying to form hybrid SPs. The careful choice of type and ratios of the NPs and process parameters make it possible to achieve and precisely tune the desired functionality of the resulting fire-proof taggants. A proof-of-concept is demonstrated, in which the taggants are incorporated as additives into coatings and subjected to real fire simulations.

High energy density and long lifespan sodium metal batteries. In this review, recent progress in high energy density and long lifespan SMBs is summarized from the aspects of modification of the sodium metal anode, electrolyte exploration, and cathode design. In addition, some insights toward their practical applications are introduced.

Abstract

Sodium metal batteries (SMBs) have been widely studied owing to their relatively high energy density and abundant resources. However, they still need systematic improvement to fulfill the harsh operating conditions for their commercialization. In this review, we summarize the recent progress in SMBs in terms of sodium anode modification, electrolyte exploration, and cathode design. Firstly, we give an overview of the current challenges facing Na metal anodes and the corresponding solutions. Then, the traditional liquid electrolytes and the prospective solid electrolytes for SMBs are summarized. In addition, insertion- and conversion-type cathode materials are introduced. Finally, an outlook for the future of practical SMBs is provided.

Nature Photonics, Published online: 10 October 2022; doi:10.1038/s41566-022-01079-8

Nature Photonics, Published online: 13 October 2022; doi:10.1038/s41566-022-01083-y

Publication date: 15 January 2023

Source: Chemical Engineering Journal, Volume 452, Part 4

Author(s): Jing-Wen Tai, Yukun Tang, Kai Zhang, Chen-Zong Yang, Ze-Hui Pan, Yu-Ching Lin, Yu-Wei Shih, Chia-Hsun Chen, Tien-Lung Chiu, Jiun-Haw Lee, Chuan-Kui Wang, Chung-Chih Wu, Jian Fan

Accompanying effects of restricted intramolecular rotations and intermolecular interactions in D−A−D’ typed molecules led to successively blue-shifted wavelength and narrowed bandwidth from solution to amorphous film then to crystals, realizing deep blue TADF (λ=424 nm, CIE coordinate of (0.15, 0.08), ΔE _ST=0.07 eV) with FWHM of 64 nm in crystals of t BuO-SOmAD.

Abstract

Narrowband deep blue thermally activated delayed fluorescent (TADF) materials have attracted significant attention. Herein, four asymmetrical structured TADF emitters based on diphenylsulfone (DPS) acceptor and 9,9-dimethyl-9,10-dihydroacridine (DMAC) donor with progressive performances were developed. The tert-butyloxy auxiliary electron-donor was adopted to restrict the intramolecular rotations and provide efficient steric hindrance. Regioisomerization by altering the substitution position of DMAC on DPS unit further enhanced the intra- and inter-molecular interactions. The accompanying effects yielded increased energy level, minimized reorganization energy, and inhibited non-radiative transitions in the crystals of t BuO-SOmAD, which achieved narrowband deep-blue emission peaking at 424 nm (FWHM=64 nm, Φ _F=33.6 %) through aggregation-induced, blue-shifted emission (AIBSE). In addition, deep-blue organic light emitting diodes (OLEDs) based on t BuO-SOmAD realized the electroluminescence (EL) spectrum peaking located at 435 nm and CIE coordination of (0.12, 0.09).

A green phosphorescent OLED with a high external quantum efficiency of 22 % and a long lifetime (LT₅₀) of 89,000 h at 1000 cd m⁻² has been successfully developed by a spirobifluorene-based multifunctional hole transporter referred to as TDBFSBF1.

Abstract

Using a tailored high triplet energy hole transport layer (HTL) is a suitable way to improve the efficiency and extend the lifetime of organic light-emitting devices (OLEDs), which can use all molecular excitons of singlets and triplets. In this study, dibenzofuran (DBF)-end-capped and spirobifluorene (SBF) core-based HTLs referred as TDBFSBF1 and TDBFSBF2 were effectively developed. TDBFSBF1 exhibited a high glass transition temperature of 178 °C and triplet energy of 2.5 eV. Moreover, a high external quantum efficiency of 22.0 %, long operational lifetime at 50 % of the initial luminance of 89,000 h, and low driving voltage at 1000 cd m⁻² of 2.95 V were achieved in green phosphorescent OLEDs using TDBFSBF1. Further, a high-hole mobility μ _h value of 1.9×10⁻³ cm² V⁻¹ s⁻¹ was recorded in TDBFSBF2. A multiscale simulation successfully reproduced the experimental μ _h values and indicated that the reorganization energy was the primary factor in determining the mobility differences among these SBF core based HTLs.

Multi-resonant thermally activated delayed fluorescence (MR-TADF) compounds, DiKTa and Mes₃DiKTa, are shown to be excellent photocatalysts in a range of different reactions, benchmarked against the widely used donor-acceptor TADF photocatalyst, 4CzIPN. Advantages of using these MR-TADF photocatalysts include robust and inexpensive photocatalyst synthesis, lower required photocatalyst loadings and faster reaction rates, while achieving comparable or improved product yields.

Abstract

Donor-acceptor (D−A) thermally activated delayed fluorescent (TADF) compounds, such as 4CzIPN, have become a widely used sub-class of organic photocatalysts for a plethora of photocatalytic reactions. Multi-resonant TADF (MR-TADF) compounds, a subclass of TADF emitters that are rigid nanographene derivatives, such as DiKTa and Mes₃DiKTa, have to date not been explored as photocatalysts. In this study both DiKTa and Mes₃DiKTa were found to give comparable or better product yield than 4CzIPN in a range of photocatalytic processes that rely upon reductive quenching, oxidative quenching, energy transfer and dual photocatalytic processes. In a model oxidative quench process, DiKTa and Mes₃DiKTa gave increased reaction rates in comparison to 4CzIPN, with DiKTa being of particular interest due to the lower material cost (£0.94/mmol) compared to that of 4CzIPN (£3.26/mmol). These results suggest that DiKTa and Mes₃DiKTa would be excellent additions to any chemist's collection of photocatalysts.

Publication date: 15 January 2023

Source: Chemical Engineering Journal, Volume 452, Part 1

Author(s): Jinnan Huo, Shu Xiao, Yuanyuan Wu, Mengxing Li, Hongbo Tong, Heping Shi, Dongge Ma, Ben Zhong Tang

By combining carbazole and acridone units through pyridyl bridge, a bipolar host with highly twisted and rigid structure is developed, which guarantees the green phosphorescence and thermally activated delayed fluorescence OLEDs to exhibit remarkably low efficiency roll-offs.

Abstract

By linking the carbazole unit to the nitrogen atom of acridone through phenyl or pyridyl, two compounds, named 10-(4-(9H-carbazol-9-yl)phenyl)acridin-9(10H)-one (AC-Ph-Cz) and 10-(5-(9H-carbazol-9-yl)pyridin-2-yl)acridin-9(10H)-one (AC-Py-Cz) were designed and synthesized. These two materials, characterized with highly twisted and rigid structure, good thermal stability, and balanced carrier-transporting properties, were employed as host materials for green phosphorescent and thermally activated delayed fluorescent organic light-emitting diodes (OLEDs). The carbazole group, despite its small contribution to the highest occupied molecular orbitals (HOMOs) of these two materials, plays an essential role as an intramolecular host in energy delivering and improving the hole transporting ability of these two hosts. The incorporation of the electron-deficient pyridyl group as a linking group slightly improves the electron transporting capability of AC-Py-Cz. The green phosphorescent OLED (PhOLED) based on AC-Py-Cz exhibited excellent device performance with a turn-on voltage of 2.5 V, a maximum power efficiency and an external quantum efficiency (η _ext) of 89.8 lm W⁻¹ and 25.2 %, respectively, benefitting from the better charge-balancing ability of AC-Py-Cz host due to the presence of the pyridyl bridge. More importantly, all the devices based on these two hosts showed low efficiency roll-off at high brightness due to the suppressed non-radiative transition in the emitting layer. In particular, the AC-Py-Cz-hosted green PhOLED exhibited an efficiency roll-off of 1.6 % from the maximum next at a high brightness of 1000 cd m⁻² and a roll-off of 15.9 % at an extremely high brightness of 10000 cd m⁻². This study manifests that acridone-based host materials have great potential in fabricating OLEDs with low efficiency roll-off.