Solution-processed Organic Light-emitting Diodes Research Articles

Carbazole-dendronized thermally activated delayed fluorescent molecules with small singlet-triplet gaps for solution-processed organic light-emitting diodes

Based on a multi-carbazole encapsulation strategy, two novel solution-processable thermally activated delayed fluorescence (TADF) molecules, named 2,8-bis(2,7-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)-9,9-dimethylacridin-10(9H)-yl)dibenzo[b,d]thiophene 5,5-dioxide (1CzAcDBTO) and 2,8-bis(9,9-dimethyl-2,7-bis(3,3″,6,6″-tetra-tert-butyl-9′H-[9,3':6′,9″-tercarbazol]-9′-yl)acridin-10(9H)-yl)dibenzo[b,d]thiophene 5,5-dioxide (2CzAcDBTO), were synthesized. Both compounds exhibit excellent thermal stability and a small energy gap between the lowest singlet and triplet state, i.e., 0.02 eV for 1CzAcDBTO and 0.04 eV for 2CzAcDBTO, respectively. Attributed to the more effective reverse intersystem crossing process, the molecule 2CzAcDBTO as the neat emitter realized better electroluminescent performances. The simplified devices with the neat dendrimers as the emissive layers exhibit significant improvements of the external quantum efficiencies (EQEs) by factors of 19.5 and 22.5, respectively compared with the device incorporating the small molecular core AcDBTO. By blending an additional TADF sensitizer with the dendrimers in a common host, enormous enhancements of efficiency and luminance were realized, e.g., a maximum EQE of 7.9% and a maximum luminance of 16380 cd/m2. Remarkably, the EQE remains as high as 7.4% at the high luminance of 1000 cd/m2, revealing a negligible efficiency roll-off.

Highly Efficient, Solution-Processed Organic Light-Emitting Diodes Based on Thermally Activated Delayed-Fluorescence Emitter with a Mixed Polymer Interlayer

The efficiency of solution-processed organic light-emitting diodes based on pure organic thermally activated delayed-fluorescence (TADF) emitters is limited by the hole-injection/severe exciton quenching of poly(styrenesulfonate)-doped poly(3,4-ethylenedioxythiophene) (PEDOT:PSS) and the exction quenching of TADF emitters due to the self-aggregation. In order to effectively enhance the hole-injection from PEDOT:PSS and to alleviate the exciton quenching of the PEDOT:PSS, the mixed interlayer of poly(9-vinylcarbazole) (PVK) and poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-s-butylphenyl)diphenylamine)) (TFB) was inserted between emitting layer (EML) and the PEDOT:PSS layer. Additionally, the cohost of 3′,5′-di(carbazol-9-yl)-[1,1′-biphenyl]-3,5-dicarbonitrile (DCzDCN) and bis(3,5-di(9H-carbazol-9-yl)phenyl)diphenylsilane (SimCP2) was used to suppress the self-aggregation of the TADF emitter of 2-(9-phenyl-9H-carbazol-3-yl)-10,10-dioxide-9 thioxanthone (TXO-PhCz) and tune the charge carrier transport....

Fabrication of solution-processed pure blue fluorescent OLED using exciplex host

Exciplex host in organic light-emitting diodes (OLEDs) can use radiative energy of upconverted singlet excitons by reverse intersystem crossing and thus transfer the energy to guest emission to effectively enhance the device performance. However, investigation of OLED with pure blue emission using exciplex host has rarely been reported. In this contribution, solution-processed OLEDs using exciplex host with pure blue fluorescence-type guest emission have been achieved. Through optimization of guest concentration, improved efficiency and blue-shifted electroluminescence spectrum of the OLED device was obtained. This study provides a method to fabricate efficient OLEDs with deeper blue emission for application in full-color display and white lighting source with high color rendering index.

Highly Efficient Soluble Blue Delayed Fluorescent and Hyperfluorescent Organic Light-Emitting Diodes by Host Engineering.

Solution-processed high-efficiency fluorescent organic light-emitting diodes with an external quantum efficiency over 18% were developed by engineering a host material and device structure designed for solution process. A high triplet energy host material designed for the solution process, (oxybis(3-(tert-butyl)-6,1-phenylene))bis(diphenylphosphine oxide) (DPOBBPE), worked efficiently as the host of blue fluorescent devices because of good solubility, high photoluminescence quantum yield, and good film properties. The DPOBBPE host enabled a high external quantum efficiency of 18.8% in the fluorescent organic light-emitting diodes by the solution process. Moreover, 25.8% external quantum efficiency in the soluble blue thermally activated delayed fluorescent devices was also realized. The 25.8% external quantum efficiency of the DPOBBPE delayed fluorescent device and 18.8% external quantum efficiency of the fluorescent device are the highest efficiency values achieved in the solution-processed blue fluorescent organic light-emitting diodes. Moreover, the solution-processed fluorescent device showed an improved blue color coordinate of (0.14, 0.20) compared to (0.17, 0.31) of the delayed fluorescent device.

Preparation of Cyano-Substituted Tetraphenylethylene Derivatives and Their Applications in Solution-Processable OLEDs.

Creation of organic luminescent materials with high solid-state efficiency is of vital importance for their applications in optoelectronic fields. Here, a series of AIE luminogens (AIE gens), (Z)-2,3-bis(4-(9,9-bis(6-(9H-carbazol-9-yl)hexyl)-9H-fluoren-2-yl)phenyl)-3-phenylacrylonitrile (SFC), and 2,3-bis(4-(9,9-bis(6-(9H-carbazol-9-yl)hexyl)-9H-fluoren-2-yl)phenyl)fumaronitrile (DFC), utilizing 2,3,3-triphenylacrylonitrile and 2,3-diphenylfumaronitrile as respective centers, are designed and synthesized by Suzuki coupling reactions with high yields. The cis- and trans-isomers of DFC are also successfully obtained. All of them are thermally stable and show good solubility in common organic solvents. They all emit weakly in solution, but become strong emitters when fabricated into solid films. It is found introduction of one additional cyano group in DFC induced a big red-shift in solid-state emission, owing to its high electron-withdrawing ability. The cis- and trans-DFC show similar photophysical and Cyclic voltammogram (CV) behaviors. Non-doped solution-processed organic light-emitting diodes (OLEDs) using the three compounds as light-emitting layers are fabricated. SFC gives the best device performance with a maximum luminance of 5201 cd m−2, a maximum current efficiency of 3.67 cd A−1 and a maximum external quantum efficiencies (EQE) of 1.37%. Red-shifted EL spectra are observed for cis- and trans-DFC-based device, and the OLED using trans-DFC as active layer exhibits better performance, which might derive from their different conformation in film state.

Solution processable small molecule based organic light-emitting devices prepared by dip-coating method

Abstract Solution-processed organic light-emitting diodes (OLEDs) are currently attracting a lot of attention for their simple fabrication process. However, most laboratory-scale solution-processed devices are fabricated by spin-coating method which is not the optimum method for fabricating devices with a larger area as much of the material is wasted. Herein, a dip-coating method is introduced for its advantages for producing large-sized devices. The method is a more reliable and material saving fabrication process. In the present study, both the hole-transporting layer (HTL) and emitting layer (EML) were prepared by dip coating, and solution-processed green thermally activated delayed fluorescence (TADF) based OLEDs with high efficiencies were fabricated.

Highly efficient deep-red emitting methyl substituted thiophenylquinoline based Ir(III) complexes for solution-processed organic light-emitting diodes

ABSTRACTHerein, we synthesized two new methyl substituted thiophenyl-quinoline based heteroleptic Ir(III) complexes as guest molecules for deep-red emitting solution-processed phosphorescent organic light-emitting diodes (PhOLEDs). Their thermal stabilities, photophysical properties and electroluminescence (EL) properties are systematically investigated. (MTPQ)2Ir(pic) and (MTPQ)2Ir(acac) showed photoluminescence emission of 614 and 629 nm and a photoluminescence quantum yield of 21% and 15%, respectively. Solution-processed deep-red emitting PhOLEDs were fabricated using standard structure with PEDOT:PSS and TPBi as hole and electron transport layers, respectively and, a mixed host of TCTA:TPBi (1:1) doped with the guest Ir(III) molecules. The emission electroluminescence wavelength was slightly red shifted by 11 nm for both (MTPQ)2Ir(pic) (625 nm) and (MTPQ)2Ir(acac) (640 nm). A maximum external quantum efficiency of 8.46% and maximum current efficiency of 7.37 cd/A was achieved for (MTPQ)2Ir(pic) with a deep-red CIE coordinates of (0.679, 0.318).

Highly Efficient TADF Polymer Electroluminescence with Reduced Efficiency Roll-off via Interfacial Exciplex Host Strategy.

Solution-processed organic light-emitting diodes (s-OLED) consisting of TAPC/TmPyPB interfacial exciplex host and polymer PAPTC TADF emitter are prepared, simultaneously displaying ultralow voltages (2.50/2.91/3.51/4.91 V at luminance of 1/100/1000/1000 cd m-2), high efficiencies (14.9%, 50.1 lm W-1), and extremely low roll-off rates (J50 of 63.16 mA cm-2, L50 of ca. 15000 cd m-2). Such performance is distinctly higher than that of pure-PAPTC s-OLED. Compared to pure-PAPTC, the advanced emissive layer structure of TAPC:PAPTC/TmPyPB is unique in much higher PL quantum yield (79.5 vs 36.3%) and nearly 4-fold enhancement in kRISC of the PAPTC emitter to 1.48 × 107 s-1.

Efficient solution-processed orange-red organic light-emitting diodes based on a novel thermally activated delayed fluorescence emitter

A novel thermally activated delayed fluorescence (TADF) emitter was designed and synthesized for solution-processed red organic light-emitting diodes (OLEDs).

A Cu-NHC based phosphorescent binuclear iridium(iii)/copper(i) complex with an unpredictable near-linear two-coordination mode.

A novel Cu-NHC based phosphorescent binuclear iridium(iii)/copper(i) complex has been prepared. The copper(i) center adopts an unpredictable near-linear two-coordination mode with short Cu-N and Cu-C bond lengths and two obvious CH-π interactions, which support its stability in the solid state. It also shows better optical properties (higher luminescence efficiency and shorter emission lifetime) in both solution and solid states relative to the model binuclear copper(i)/copper(i) complex, and was successfully applied in solution-processed organic light-emitting diodes.

All solution-processed red organic light-emitting diode based on a new thermally cross-linked heteroleptic Ir(iii) complex

A novel cross-linkable red iridium(iii) complex and electron transport material were designed and synthesized. The newly cross-linkable red Ir(iii) complex was successfully thermally cross-linked with a cross-linkable host in the emitting layer (EML). After cross-linking, the EML was not damaged by organic solvents.

Power-efficient and solution-processed red phosphorescent organic light-emitting diodes by choosing combinations of small molecular materials to form a well-dispersed exciplex co-host

Intrinsic compatibility of exciplex couple determines the EL performance of the resultant solution-processed phosphorescent OLEDs, particularly driving voltage behaviours.

The application of a high boiling point dissolution solvent on a poly(N-vinylcarbazole) host toward improving the performance of blue electrophosphorescent devices via a solution process

ODCB triggers the formation of a p-PVK conformation, a low content PVK electromer, enhancing the performance of blue phosphorescent s-PhOLEDs.

A new cross-linkable 9,10-diphenylanthracene derivative as a wide bandgap host for solution-processed organic light-emitting diodes

We present a cross-linkable wide bandgap host based on 9-(4-(10-phenylanthracene-9-yl)phenyl)-9H-carbazole.

Efficient solution-processed yellow/orange phosphorescent OLEDs based on heteroleptic Ir(Ⅲ) complexes with 2-(9,9-diethylfluorene-2-yl)pyridine main ligand and various ancillary ligands

Abstract Efficient solution-processible yellow/orange emissive phosphors are highly required in solution-processed phosphorescent organic light-emitting diodes (s-PhOLEDs). With respect to phenyl pyridine (ppy)-type Ir(III) complexes, 2-(9,9-diethylfluoren-2-yl)pyridine (Flpy)-type Ir(III) complexes have huge potential in yellow/orange s-PhOLEDs due to their extended conjugation and excellent solubility in common organic solvents. Herein, a new series of phosphorescent Ir(III) complexes, i.e. Ir(Flpy)2pic-N O, Ir(Flpy-CF3)2pic-N O, Ir(Flpy-CF3)2dbm and Ir(Flpy-CF3)2acac, based on Flpy main ligand and picolinic N-oxide (pic-N O), acetylacetone (acac), and dibenzoyl methane (dbm) ancillary ligands are synthesized. The photophysical, electrochemical, and electroluminescent (EL) properties of these phosphorescent Ir(III) complexes are investigated in details. By introducing the CF3 group in Flpy ligand and varying the ancillary ligands, the emission spectra of these phosphorescent complexes are tuned in a large range with EL emission peaks from 544 nm to 588 nm. The s-PhOLEDs employing these phosphors show promising EL performance with maximum external quantum efficiency (EQE) from 15.4% to 23.7% and high power efficiency from 61.9 l m W−1 to 80.4 l m W−1. These highly efficient phosphorescent Ir(III) complexes may serve as ideal chromaticity components in solution-processed full-color displays and lighting devices.

Improved light outcoupling efficiency in organic light-emitting diodes with nanoparticle-embedded charge transport layers

Abstract We demonstrate high performance internal light outcoupling structures using nanoparicle-embedded hole transport layers (NE-HTLs) for solution-processed organic light-emitting diodes (OLEDs). The NE-HTLs show smoothly formed corrugated surfaces. The OLEDs with NE-HTLs show enhanced power efficiencies by a factor of 1.44 for polystyrene (PS) nanoparticles and 1.78 for SiO2 nanoparticles, respectively. These improvements are mainly attributed to the reduced total internal reflection at the interfaces of organic layers and transparent electrodes in devices by introducing NE-HTLs. Because the internal light outcoupling structures are prepared on top of the transparent electrode, a damage of organic layers and deterioration of the electrical and optical properties of transparent electrodes are not induced. In addition, the OLEDs with NE-HTLs show the angle-independent light outcoupling enhancement. It is expected that the NE-HTLs fabricated by the simple solution process can contribute to the development of efficient internal outcoupling structures for OLEDs.

Anchoring a Sulfonate Group to an Electron‐Transporting Molecule by an Alkyl Chain and Its Use as the Counter Anion in a Phosphorescent Cationic Iridium Complex

An electron-transporting anion prepared by anchoring a sulfonate group onto an electron-transporting molecule through a flexible alkyl chain and its use as the counter anion in a cationic iridium complex is reported. The flexible alkyl chain improves the solubility of the bulky anion in water or in polar organic solvents. The anion exhibits similar photophysical and electrochemical properties to the parent electron-transporting molecule. Within the complex, the anion does not disturb the phosphorescence of the cation in the solid film. Solution-processed small-molecule organic light-emitting diodes (OLEDs) using the complex as the dopant show superior performances over the reference device using the conventional complex with a PF6 - counter anion. It is revealed that anchoring anionic groups to optoelectronically active molecules with flexible alkyl chains is a feasible approach to develop optoelectronically active anions for assembling ion pairs with advanced optoelectronic properties.

Sulfone-Based Deep Blue Thermally Activated Delayed Fluorescence Emitters: Solution-Processed Organic Light-Emitting Diodes with High Efficiency and Brightness

Low efficiencies of soluble blue emitter materials remain a major issue in the development of printed organic light-emitting devices. n-Alkylated carbazoles or sterically demanding ortho-substituted diphenylamines were employed as donor elements to increase solubility and to preserve blue emission of thermally activated delayed fluorescence (TADF) donor–acceptor–donor emitters employing a p-bis(phenylsulfonyl)benzene acceptor described in the literature. The soluble molecules exhibited increased steric hindrance of the amine donors and a small ΔEST as low as 0.32 eV. Thermally activated delayed fluorescence occurs, and photoluminescence quantum yields of ≤82% were achieved. Application of these TADF molecules in solution-processed organic light-emitting diodes resulted in high brightnesses of ≤10000 cd/m2, current efficiencies of ≤9.5 cd/A, and external quantum efficiencies of ≤8.5%, while retaining deep blue emission ranging from 466 to 436 nm with color coordinates low as CIEy = 0.08.

Highly Efficient Solution-Processed Deep-Red Organic Light-Emitting Diodes Based on an Exciplex Host Composed of a Hole Transporter and a Bipolar Host.

With the aim to achieve highly efficient deep-red emission, we introduced an exciplex forming cohost, 4,4',4″-tris(3-methylphenylphenylamino)triphenylamine (m-MTDATA): 2,5-bis(2-(9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole (o-CzOXD) (1:1). Due to the efficient triplet up-conversion processes upon the exciplex forming cohost, excellent performances of the devices were achieved with deep-red emission. Using the heteroleptic iridium complexes as the guest dopants, the solution-processed deep-red phosphorescent organic light-emitting diodes (PhOLEDs) with the iridium(III) bis(6-(4-(tert-butyl)phenyl)phenanthridine)acetylacetonate [(TP-BQ)2Ir(acac)]-based phosphorescent emitter exhibited an electroluminescent peak at 656 nm and a maximum external quantum efficiency (EQE) of 11.9%, which is 6.6 times that of the device based on the guest emitter doped in the polymer-based cohost. The unique exciplex with a typical hole transporter and a bipolar material is ideal and universal for hosting the red PhOLEDs and tremendously improves the device performances.

Near-infrared emission of dinuclear iridium complexes with hole/electron transporting bridging and their monomer in solution-processed organic light-emitting diodes

To obtain efficient near-infrared (NIR) emitting materials, a novel iridium (III) complex based on a triphenylamine functionalized pyridylpyrene ligand, namely, Mono-Ir, as well as its two corresonding dinuclear iridium (III) complexes possessing π-conjugated bridging linkages of hole-transporting carbazole (Caz) unit or electron-transporting 2,5-diphenyl-1,3,4-oxadiazole (OXD) unit, namely D-Ir-Caz and D-Ir-OXD, were successfully synthesized and characterized. Under photo-excitation, similar phosphorescence spectrum were obtained for the three complexes with intense NIR emission peak at approximately 698 nm and emission wavelength ranging from 650 to 900 nm. Also, in their single-emissive-layer OLEDs with a device structure of ITO/PEDOT:PSS/TFB/CBP:PBD:complexes/TmPyPB/Liq/Al, similar NIR electroluminescent (EL) emission peaked at 698 nm with a shoulder at 762 nm were obtained. The Mono-Ir-based device showed an external quantum efficiency (EQE) of 1.29% at low current density of 3.5 mA cm−2, while a relatively lower efficiency of 0.27% for the D-Ir-Caz-based device and 0.41% for the D-Ir-OXD-based device, accompanied with negligible efficiencies roll-off at high current densities were achieved, respectively. The better device performance should be achieved when coupled with electron transporting bridging, especially at high currents.