Electroluminescent Properties Research Articles

Axially chiral TADF-active materials with π-extended acceptors for highly efficient circularly polarized electroluminescence

In this work, a π-extended acceptor strategy for designing chiral thermally activated delayed fluorescence (TADF)-active materials with highly efficient circularly polarized electroluminescence (CPEL) properties has been proposed. Consequently, three pairs of TADF-active enantiomers based on axially chiral biphenyl skeleton, namely R/S-Ax-CN, R/S-Ax-H, R/S-Ax-OCH3, were designed and constructed. Various para-substituted push–pull electron groups were introduced into the axially chiral skeleton to comprehend the structure–property relationship. Compared with R/S-Ax-H and R/S-Ax-OCH3, R/S-Ax-CN exhibited a higher PLQY of 88.6% and a smaller ΔEST of 0.13 eV, ascribing to the pull electron effect of CN group to make a more delocalized distribution of LUMO orbital and a lesser overlapping of frontier molecular orbitals. Consequently, the CP-OLEDs of R/S-Ax-CN showed a remarkable external quantum efficiency up to 21.0% and 19.7% and intense CPEL signals with the electroluminescent dissymmetry factors (gEL) up to +4.2 × 10−3/−4.4 × 10−3, respectively. This work can provide an efficient and facile way to construct highly efficient CP-OLEDs based on chiral TADF-active emitters by simple molecular engineering of introducing π-extended acceptor.

Organic Light-Emitting Diodes with Electrospun Electrodes for Double-Side Emissions.

Transparent conductive electrodes (TCE) obtained by the electrospinning method and gold covered were used as cathodes in the organic light-emitting diodes (OLEDs) to create double side-emission. The electro-active nanofibers of poly(methyl methacrylate) (PMMA) with diameters in the range of several hundreds of nanometers, were prepared through the electrospinning method. The nanofibers were coated with gold by sputtering deposition, maintaining optimal transparency and conductivity to increase the electroluminescence on both electrodes. Optical, structural, and electrical measurements of the as-prepared transparent electrodes have shown good transparency and higher electrical conductivity. In this study, two types of OLEDs consisting of indium tin oxide (ITO)/ poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT-PSS)/ Ir(III) complex (8-hydroxyquinolinat bis(2-phenylpyridyl) iridium-IrQ(ppy)2 20 wt% embedded in N, N'-Dicarbazolyl-4,4'-biphenyl (CBP) sandwich structure and either gold-covered PMMA electrospun nanoweb (OLED with electrospun cathode) were fabricated together with a similar structure containing thin film gold cathodes (OLED with thin film cathode). The luminance-current-voltage characteristics, the capacitance-voltage, and the electroluminescence properties of these OLEDs were investigated.

Molecular Modulation by σ Conjugated Spacer Enables Efficient Ultraviolet/Deep-Blue Emissive Organic Light-Emitting Diodes

Phenanthroimidazole (PI) has great proficiency to design efficient near UV deep blue emitting materials due to its bipolarity behavior. The tuning of N1 and C1 centers of the PI further reveals the improvement of the optical and electronical properties. Thorough investigation is also executed to explore the influence of the alkyl chain on the optical and electroluminescence (EL) properties of these emissive materials. The incorporation of the alkyl chain in the materials can tune the optical properties, leading to better solid state emission and enhancement of the photoluminescence quantum yield (PLQY) of thin film (∼74%) as compared to solution (∼65%). The EL spectra were also exhibited between 395 and 420 nm (in the near-UV spectral region). Furthermore, for boosting of the device efficiency and color purity, the OLED device is fabricated by doping CBP as host matrix. Selectively, CBP is used as a host because of the similar energy level (HOMO and LUMO) of the synthesized emitters which assist efficient charge trapping. Predominantly, the emitters in OLED device blended with 0.5 wt % concentration achieved brilliant device efficiency and luminance properties. Among all the synthesized fluorophores, PIPP (0.5 wt %) doped OLED device showed 4.4% maximum external quantum efficiency (EQEmax), 1202 cd/m2 highest luminance, 2.2 lm/W power efficiency (PE), 2.7 cd/A current efficiency (CE) with Commission International de L’Eclairage (CIE) coordinates x = 0.17, y = 0.10 (nearer the NTSC standard value).

Electroluminescent property of new chrysene derivatives having bulky side group of aromatic amine moiety

Two blue emitting materials, 4,4'-(chrysene-6,12-diyl)bis(N,N-diphenylaniline) (TPA-C-TPA) and 12-(4-(diphenylamino)phenyl)-N,N- Diphenylchrysen-6-amine (TPA-C-DPA) were prepared with the composition of chrysene core moiety and dipenylamine (DPA) and triphenylamine (TPA) moieties as the side group. Two compounds were used to emitting layer as the dopants (EMLs) in doped device. The doped device using TPA-C-TPA showed current efficiency of 2.49 cd/A and external quantum efficiency of 4.26% at a current density of 10 mA/cm2. The doped device using TPA-C-DPA exhibited CE of 2.28 cd/A and EQE of 3.52% at a current density of 10 mA/cm2. TPA-C-TPA and TPA-C-DPA show the Commission Internationale de L’Eclairage (CIE) coordinates of (0.15, 0.06) and (0.14, 0.06) corresponding to the electroluminescence peak at 439 and 444 nm.

Photo- and electro-luminescent properties of V-shaped bis-coumarin derivatives containing aromatic π-conjugated imidazole moieties

Two new V-shaped bis-coumarin derivatives containing aromatic π‒conjugated imidazole moieties, 3-(4-(1-(4-(tert‑butyl)phenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)-10-(diethylamino)-6H,7H-chromeno[3,4-c]chromene-6,7‑dione (VBC-PI) and 3-(4-(9-(4-(tert‑butyl)phenyl)-3a1,5a1-dihydro-9H-pyreno[4,5-d]imidazol-10-yl)phenyl)-10-(diethylamino)-6H,7H-chromeno[3,4-c]chromene-6,7‑dione(VBC-PyI), were successfully synthesized and characterized by NMR (1H and 13C) and high resolution mass spectra and elemental analyses. The photophysical and electrochemical properties and thermal stabilities of VBC-PI and VBC-PyI were investigated systematically. The effect of π-conjugation of aromatic imidazole moieties on the properties of V-shaped bis-coumarin derivatives were revealed by combining theoretical study with experimental data. The solution-processed doped devices using the blend of CBP:VBC derivative as the emitting layers exhibited green emission with maximum external quantum efficiency (EQEmax) of 1.97% for VBC-PI and 1.87% for VBC-PyI, respectively.

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.

Inside Back Cover: Aggregation‐induced emission luminogens for organic light‐emitting diodes with a single‐component emitting layer

Aggregation-induced emission luminogens (AIEgens) have received considerable attention from researchers owing to their high photoluminescence quantum yield (PLQY) in the aggregated state. In article number (e199), Dong Hoon Choi and co-workers have reviewed the recent molecular design strategies as well as photophysical and electroluminescent properties of AIEgens used in non-doped organic light-emitting diodes.

Highly transparent and stretchable organic light-emitting diodes with ultrathin metal films as double electrodes

Organic light-emitting diodes (OLEDs) with both high transparency and stretchability are of importance for next-generation wearable and deformable displays, which have not been studied in depth. In this work, highly flexible and transparent OLEDs (TrOLEDs) are achieved by using ultrathin metal films as both anode and cathode to fabricate stretchable TrOLEDs (S-TrOLEDs) based on a wrinkling strategy. The TrOLEDs show a transmittance value of 73% at 550 nm and a negligible micro-cavity effect with near Lambertian emission characteristics at a wide angle of view. Furthermore, almost symmetric electroluminescent properties are obtained from the anode and cathode, which is important for dual-side display. Two-dimensional S-TrOLEDs with random wrinkles show 50% stretchability and retain 80% of their initial luminance after 200 cycles of stretching. The effect of random wrinkles on the transmittance of S-TrOLEDs is researched and the transparency losses of the device are analyzed deeply. We hope that this work is meaningful for promoting the development of S-TrOLEDs in the display industries.

Mechanochromic Materials Based on Tetraphenylethylene-Substituted Phenothiazines: Substituent-Dependent Hypsochromic and Bathochromic Switching of Emission

Phenothiazine- and tetraphenylethylene (TPE)-based mechanochromic materials (PTZTPE-1CHO, PTZTPE-4CHO, and PTZTPE-4CN) with different electron-withdrawing substituents (−CHO and −CN) were designed and synthesized by the Suzuki cross-coupling reaction of bromo-TPEs with the corresponding boronate esters of phenothiazine. The photophysical, aggregation-induced emission, mechanochromic, thermal, and electroluminescence properties of the synthesized compounds were investigated. The cyano group-bearing derivative PTZTPE-4CN exhibited reversible mechanochromism, and its emission maximum shifted bathochromically when mechanically ground, whereas the formyl group-containing derivatives PTZTPE-1CHO and PTZTPE-4CHO exhibited hypsochromically shifted mechanochromism. The solution of PTZTPE-4CN was weakly emissive, and PTZTPE-1CHO and PTZTPE-4CHO exhibit strong fluorescence in the solution state due to the intramolecular charge transfer transitions. The electroluminescence properties of the synthesized compounds as non-annealed and annealed non-doped prompt fluorescent emitters were studied in organic light-emitting diodes, which showed a maximum external quantum efficiency of 1.2% and a luminance of 8762 cd·m–2.

Tweaking the Optoelectronic Properties of S-Doped Polycyclic Aromatic Hydrocarbons by Chemical Oxidation.

Peri-thiaxanthenothiaxanthene, an S-doped analog of peri-xanthenoxanthene, is used as a polycyclic aromatic hydrocarbon (PAH) scaffold to tune the molecular semiconductor properties by editing the oxidation state of the S-atoms. Chemical oxidation of peri-thiaxanthenothiaxanthene with H2 O2 led to the relevant sulfoxide and sulfone congeners, whereas electrooxidation gave access to sulfonium-type derivatives forming crystalline mixed valence (MV) complexes. These complexes depicted peculiar molecular and solid-state arrangements with face-to-face π-π stacking organization. Photophysical studies showed a widening of the optical bandgap upon progressive oxidation of the S-atoms, with the bis-sulfone derivative displaying the largest value (E00 =2.99 eV). While peri-thiaxanthenothiaxanthene showed reversible oxidation properties, the sulfoxide and sulfone derivatives mainly showed reductive events, corroborating their n-type properties. Electric measurements of single crystals of the MV complexes exhibited a semiconducting behavior with a remarkably high conductivity at room temperature (10-1 -10-2 S cm-1 and 10-2 -10-3 S cm-1 for the O and S derivatives, respectively), one of the highest reported so far. Finally, the electroluminescence properties of the complexes were tested in light-emitting electrochemical cells (LECs), obtaining the first S-doped mid-emitting PAH-based LECs.

Synthesis, photoluminescence and electroluminescence properties of a new blue emitter with aggregation-induced emission and thermally activated delayed fluorescence characteristics

The emitters with aggregation-induced emission (AIE) and thermally activated delayed fluorescence (TADF) characteristics are in high demand in organic light-emitting diodes (OLEDs) owing to their strong fluorescence and high exciton utilization under electrical excitation. Herein, a blue emitter, 10-(3-((3,5-di(9H-carbazol-9-yl)phenyl)sulfonyl)phenyl)-9,9-dimethyl-9,10-dihydroacridine (m-CZ-DPS-DMAC), was synthesized by incorporating carbazole as skeleton, acridine as electron donor, and diphenyl sulfone as electron acceptor. m-CZ-DPS-DMAC emits weak fluorescence in good solvent, while it is obviously enhanced in the aggregate state, which is typical of AIE molecules. Meanwhile, the energy levels of the singlet and triplet states (ΔEST) of the molecule is relatively small, and it also exhibits obvious temperature dependence and oxygen sensitivity, which directly proves its TADF properties. In view of the above properties, a series of non-doped and doped OLEDs were prepared using m-CZ-DPS-DMAC as light-emitting layers. Among them, non-doped OLED (device A) displays blue emission (488 nm) with the turn-on voltage (Von), the maximum luminance (Lmax), the maximum current efficiency (CEmax), the maximum power efficiency (PEmax) and the maximum external quantum efficiency (EQEmax) of 2.6 V, 3460 cd m−2, 26.09 cd A–1, 29.26 lm W−1 and 10.05%, respectively. Doped OLED (device C) constructed based on m-CZ-DPS-DMAC doped 30% in DPEPO shows the satisfactory performance with the maximum emission peak of 486 nm, the Von of 2.8 V, the Lmax of 4571 cd m−2, the CEmax of 21.37 cd A–1, the PEmax of 22.37 lm W−1, and the EQEmax of 9.44%, respectively. The outstanding performance of m-CZ-DPS-DMAC proves that it is a potential material for designing blue OLEDs with AIE-TADF properties.

Improved electroluminescence efficiency derived from functionalized decoration of 1,3,4-oxadiazole (OXD)-based Ir(III) complexes.

The performance of organic light emitting devices (OLEDs) fabricated using Ir(III) complexes bearing 1,3,4-oxadiazole (OXD)-based cyclometallic ligands still needs to be improved. In this work, Ir3+ was coordinated with a 2-(9,9-diethyl-9H-fluoren-2-yl)-1,3,4-oxadiazole (F-OXD) fragment, which was modified with various functionalized substituents, including fluorenyl, OXD and carbazolyl groups. Three complexes, named Ir-Flu, Ir-OXD and Ir-Cz, were synthesized successfully and their photophysical, electrochemical and electroluminescence properties were investigated in detail. All these complexes exhibited yellow-orange emission in solution and a distinct aggregation-induced phosphorescent emission (AIPE) phenomenon was observed. Monochrome OLEDs were fabricated using these phosphorescent dopants, and the turn-on voltage (V), luminance (L) and current efficiency (CE) showed significant improvement compared to analogous OXD-based Ir(III) complexes reported before. In particular, the device with Ir-OXD as the dopant achieved the highest maximum brightness of 25 014 cd m-2 and the lowest efficiency roll-off (42.6%) at the maximum luminance among all the devices. These results provided a proven strategy of functionalized decoration of OXD-based complexes to achieve superior luminous efficiency devices.

Излучательная рекомбинация в разъединенном гетеропереходе II типа InAs/InSb с квантовыми точками на интерфейсе

The electroluminescent properties of narrow-gap type II InAs/InSb/InAs heterostructures containing a single layer of InSb quantum dots placed at the interface of the p-n junction in InAs were studied. The features of the electroluminescence spectra depending on the surface density of nanoobjects at a broken-gap type II heterointerface were investigated both at forward and reverse bias. When applying a reverse bias to the heterostructures under study, the suppression of negative interband luminescence and the dominance of interface recombination transitions at the InSb/InAs type II heterojunction were observed at room temperature. The radiation, which corresponded to recombination transitions involving localized states of InSb quantum dots, was recorded at low temperature.

Efficient thermally activated delayed fluorescence emitters with regioisomeric effects for red/near-infrared organic light-emitting diodes

The regioisomeric effect of cyano substituents on the photoluminescence and electroluminescence properties of thermally activated delayed fluorescence molecules is studied.

Molecular engineering with triptycene groups endows homoleptic Ir(iii) complexes with enhanced electroluminescence properties

Complex Ir2 exhibited a high PLQY of 0.87, a Td of 445 °C and a peak EQE of 27.5%.

Multifunctional guanidinium thiocyanate additive to enhance the performance of quasi-two-dimensional perovskite blue-emitting devices

Phase distribution, defect density, and charge transport of quasi-two-dimensional perovskites are the important factors determining the performance of the related light-emitting devices. Herein, the effects of the guanidinium thiocyanate (GaSCN) addition on the photo-physical, crystallographical, morphological, charge transport, and electroluminescent properties of the quasi-two-dimensional perovskites were studied. SCN − adjusted the phase distribution by increasing the n = 2 phase content and decreasing the n = 3 and n = 4 phase contents, which facilitated the charge funneling. Ga + mainly contributed to reducing the defect density. In addition, GaSCN increased the crystallinity, promoting the charge transport. Consequently, the quasi-two-dimensional perovskite blue-emitting device with GaSCN showed the maximum external quantum efficiency of 8.39%, which was 4.26 times that of the control device without GaSCN, together with the improved spectral and operational stabilities. This work highlights that the incorporation of a pseudohalide additive to simultaneously adjust the phase distribution, passivate defects, and promote the charge transport is promising for enhancing the performance of quasi-two-dimensional perovskite blue-emitting devices.

Effect of dendrimer generation and surface groups on the optoelectronic properties of green emitting bis-tridentate iridium(iii) complexes designed for OLEDs

The role of surface groups (t-butyl versus 2-ethylhexyloxy) and dendron generation (first versus second) on the photoluminescent and electroluminescent properties of solution-processable bis-tridentate iridium(iii) complexes has been investigated.

Design, Synthesis and Electroluminescent Properties of Multiresonant Thermally Activated Delayed Fluorescence Materials Based on Tetrahydroquinoline

Design, Synthesis and Electroluminescent Properties of Multiresonant Thermally Activated Delayed Fluorescence Materials Based on Tetrahydroquinoline

Current injection and luminescence properties of wurtzite InP nanowires with crystal phase transition

We characterized the current injection and electroluminescence (EL) properties of wurtzite (WZ) InP nanowire (NW) light-emitting diodes (LEDs) with axial junctions. The EL spectra of two samples with the same LED junction structure exhibited two different behaviors. One showed a single EL peak originating from the zinc-blende (ZB)-InP bandgap. The other showed two EL peaks originating from the ZB and WZ phases. This difference in EL behavior is attributed to the difference in the contact position and to depletion layer spreading. Clarification of the origin of the different EL peaks is important for optimizing the NW-LED structure.

Progresses and Perspectives of Near-Infrared Emission Materials with "Heavy Metal-Free" Organic Compounds for Electroluminescence.

Organic/polymer light-emitting diodes (OLEDs/PLEDs) have attracted a rising number of investigations due to their promising applications for high-resolution fullcolor displays and energy-saving solid-state lightings. Near-infrared (NIR) emitting dyes have gained increasing attention for their potential applications in electroluminescence and optical imaging in optical tele-communication platforms, sensing and medical diagnosis in recent decades. And a growing number of people focus on the "heavy metal-free" NIR electroluminescent materials to gain more design freedom with cost advantage. This review presents recent progresses in conjugated polymers and organic molecules for OLEDs/PLEDs according to their different luminous mechanism and constructing systems. The relationships between the organic fluorophores structures and electroluminescence properties are the main focus of this review. Finally, the approaches to enhance the performance of NIR OLEDs/PLEDs are described briefly. We hope that this review could provide a new perspective for NIR materials and inspire breakthroughs in fundamental research and applications.