Solution-processed Organic Light-emitting Diodes Research Articles

Synergetic interface and morphology modification to achieve highly efficient solution-processed sky-blue organic light-emitting diodes

Abstract Solution-processed organic light-emitting diodes (OLEDs), especially blue OLEDs, generally suffer from the low efficiency. Herein, we report an efficient approach to achieve high efficiency by synergetic interface and morphology modification with a polymer, Poly(9-vinylcarbazole) (PVK). Sky-blue thermally activated delayed fluorescent (TADF) material, 10-(4-(4,6-diphenyl-1,3,5- triazin-2-yl)phenyl)-10H- spiro[acridine-9,9′-fluorene] (SpiroAC-TRZ), is used as the emitter. The incorporation of PVK thin layer at the hole injection layer/emission layer (EML) interface and a small amount of PVK inner the EML modify the carrier behaviors at the interface and improve the EML morphology. As a result, balanced carrier distribution and reduced carrier recombination are realized at the interface and inner the EML. Through these strategies, the maximum external quantum efficiency and current efficiency of the optimal OLED achieve 25.1% and 53.5 cd/A. To the best of our knowledge, the efficiencies are the highest values ever achieved by the solution-processed sky-blue TADF OLEDs.

Study of All Solution Processed Exciplex Organic Light-Emitting Diode

Organic light-emitting diodes (OLEDs) are widely used in full-color display panels, fluorescent tubes and solid-state lighting devices. In recent years, due to the material utilization yield of the vacuum deposition method is very low and maintaining a high vacuum further increases the fabrication cost and complexity, solution processing method has become a research hotspot. However, the development of solution-processed OLEDs has a great challenge in multilayer structure formation and nearly all prior reports on solution-processed OLEDs were utilizing a vacuum deposited electron-transport layer (ETL). Therefore, it is of great significance to further research on all solution-processed OLEDs whose ETL were spin-coated. In this paper, spin-coated ETL based OLED with high current efficiency that comparable with vacuum deposited ETL based OLED were successfully achieved.

Improved film morphology and reduced defects in solution-processed red phosphorescent emission layer of the organic light-emitting diodes

The fabrication of highly efficient organic light-emitting diodes (OLEDs) by solution processes has become a hot research point due to the low production cost, high material utilization ratio and simple fabrication process. Herein, highly efficient, solution-processed red phosphorescent organic light-emitting diodes (PhOLEDs) using chlorobenzene (CB) and isopropanol (IPA) as mixed solvents are proposed. It is found that both the roughness and the defect states of emission layer (EML) prepared with mixed solvents are reduced. Thereby, the electroluminescence performance of PhOLEDs are enhanced compared with that of the ones with pure CB as the solvent. With CB:IPA ratio of 5:1, the current efficiency and the external quantum efficiency of device reach 27.1 cd/A and 17.2 %, respectively. These results prove that the use of IPA can improve the device performance even if it is not able to dissolve the organic materials, which breaks the limitation of the use of alcohol solvent in solution-processed OLEDs.

Effect of methyl-substitution on carbazole/oxadiazole donor-acceptor (D-A) type host materials for efficient solution-processed green organic light-emitting diodes

Two new bipolar hosts methyl-substituted carbazole/oxadiazole donor-acceptor (D-A) type compounds of 2,5-bis(2-(3,6-dimethyl-9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole (3,6-MeCzOXD) and 2,5-bis(2-(2,7-dimethyl-9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole (2,7-MeCzOXD) are reported. The thermostability and solubility were greatly improved by attaching two methyl groups on carbazole in contrast to that of parent o-CzOXD without any substituents. The different positions of termination between the methyl substituents and the carbazole unite have almost no impact on thermal stability, whereas methyl-substituted carbazole at 3,6-positions shows more effective π-conjugation and significantly better electrochemical stability than 2,7-positions. The triplet energies of 3,6-MeCzOXD and 2,7-MeCzOXD were measured to be 2.56 and 2.64 eV. Solution-processed green PhOLEDs based on the two new o-CzOXD derivatives as hosts were achieved with CEmax of 32 cd A−1 and PEmax of 26 lm W−1. Besides, green TADF devices employing 3,6-MeCzOXD as host material maintain the values with CEmax of 32.4 cd A−1 and PEmax of 17.2 lm W−1.

Mixed-Host Systems with a Simple Device Structure for Efficient Solution-Processed Organic Light-Emitting Diodes of a Red-Orange TADF Emitter.

Charge balance, concentration quenching, and exciton confinement are the most important factors for realizing the use of thermally activated delayed fluorescence (TADF) emitters for organic light-emitting diodes. Red-orange organic light-emitting diodes of a TADF emitter 2-[4 (diphenylamino)phenyl]-10,10-dioxide-9H-thioxanthen-9-one (TXO-TPA) have been reported by doping in a mixed p-type host system of poly(N-vinylcarbazole) (PVK) and 1,3-bis(N-carbazolyl)benzene (mCP) via solution-processed. We have demonstrated the peak external quantum efficiency of 9.75%, maximum current efficiency of 19.36 cd/A, and power efficiency of 12.17 lm/W along with a CIE coordinate of (0.45, 0.51). The devices were compared with different doping concentrations of TXO-TPA, and a comparative investigation on the effect of the thickness electron transport layer was studied. The results clearly indicated that this solution-processed TXO-TPA device structure is a promising strategy to develop highly efficient but simple OLED structures.

Bipolar fluorophores based on intramolecular charge-transfer moieties of sulfone for nondoped deep blue solution-processed organic light-emitting diodes

Dipolar emitters exhibited excellent performance in organic light-emitting diode (OLED). However, these molecules had intramolecular charge-transfer (ICT) properties, which posed challenge to obtain deep blue emission. In this study, three fluorophores were designed by introducing carbazole and diphenylamine as electron donors and sulfone as electron acceptor due to their mild charge-accepting properties and twisted angles. These materials appeared almost in vertical angles of the dihedral configuration, and exhibited high thermal and electrochemical stability, suitable for solution-processed OLED. The solution-processed non-doped devices based on these three emitters were realized, where two emissions within the standard deep blue emission range were achieved with the Commission International e de l’Eclairage (CIE) coordinates of (0.16, 0.12) and (0.16, 0.15).

Fluorene based amorphous hole transporting materials for solution processed organic light-emitting diodes

Abstract For an organic light-emitting diode (OLED), enhancing hole injection into the emissive layer for charge balance is a priority to achieve efficient device performance, while solution processing enables organic devices to be fabricated cost-effectively with a large area-size via continuous roll-to-roll manufacturing. However, a limited number of small molecules-based hole-transporting materials (HTMs) were reported with the solution process feasibility and high performance. Here, we demonstrate a series of low cost, efficient and solution processable small molecule HTMs, namely, 2,7-Di(4-fluorophenyl)-9,9-diethylfluorene (Fl-PyEyF), 2,7-Di(3,5-difluorophenyl)-9,9diethylfluorene (di-Fl-PyEyF) and 2,7-Di(2,4,6-trifluorophenyl)-9,9-diethylfluorene (tri-FlPyEyF), designed by using two fluorophenyl, difluorophenyl or trifluorophenyl fragments as common end capping groups with 9,9-Diethylfluorenes cores, respectively, for highly efficient OLEDs. The glass transition temperatures of the molecules were estimated to be higher than 80 °C, which can provide morphologically stable amorphous thin films. All the molecules possess good solubility in common organic solvents. Moreover, all the synthesized molecules have not only appropriate highest occupied molecular orbital energy levels for good hole injection ability, but also sufficient lowest unoccupied molecular orbital for electron blocking capability, adequate ionization potentials (6.15 eV) and suitable triplet energies, which make them suitable hole transporting materials. The current efficiency of a conventional iridium(III) bis(4phenylthieno[3,2-c]pyridinato-N,C-2՜) acetylacetonate based phosphorescent yellow OLED device increases from 40.2 to 61.0 cd/A, an increment of 51.7% by substituting the conventional HTM, N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB), with the tri-Fl-PyEyF counterpart. These findings suggest that the reported materials can serve as potential molecular HTMs in solution-processed OLED.

Rational molecular design of novel host material combing intra- and intermolecular charge transfers for efficient solution-processed organic light-emitting diodes

Two novel D-σ-A molecules bearing 2,4,6-diphenyl-1,3,5-triazine (TRZ) units as electron-accepting segment and 1,3-di (9H-carbazol-9-yl)benzene (mCP) or 9'-(4-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenoxy)phenyl)-9′H-9,3':6′,9″-tercarbazole (TCz) as electron-donating moiety, named as 9,9'-(5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl) phenoxy) −1,3- phenylene) bis(9H-carbazole) (mCP-o-TRZ) and 9'-(4-(4-(4,6-diphenyl-1,3,5-triazin-2-yl) phenoxy)) phenyl)- 9′H-9,3':6′,9″-tercarbazole (TCz-o-TRZ), are designed and synthesized. The oxygen atom is applied as a bridge between the various donor and acceptor parts to realize the conjugation-forbidden linkage of the molecular backbone. The photophysical properties, solvatochromism, theoretical calculations, thermal properties and device performance are systematically investigated. As anticipated, the mCP-o-TRZ molecule was endowed with intramolecular charge transfer (intra-CT) and intermolecular charge transfer (inter-CT), while only intra-CT can be obtained in the TCz-o-TRZ material. The solution-processed OLEDs employed mCP-o-TRZ as host for the phosphorescence guest (tris(2-methylphenylpyridine) iridium (III)) (Ir (mppy)3) realized a maximum EQE of 9.0%, which is superior to that of TCz-o-TRZ. The results demonstrate that the development of new D-σ-A molecule equipped with dual CTs is a promising strategy for fabricating organic light-emitting diodes.

Highly efficient solution-processed red phosphorescent organic light-emitting diodes employing an interface exciplex host

To cut cost and improve efficiency, an interface exciplex system of NPB/TPBi was employed as a host in solution-processed red phosphorescent organic light-emitting diodes (PhOLEDs).

Room temperature perylene based columnar liquid crystals as solid-state fluorescent emitters in solution-processable organic light-emitting diodes

Perylene tetraesters based columnar liquid crystal employed as solid-state fluorescent emitter in solution-processed OLEDs and enables to achieve maximum external quantum efficiency of 6.5%.

Blue-green-emitting cationic iridium complexes with oxadiazole-type counter-anions and their use for highly efficient solution-processed organic light-emitting diodes

Cationic iridium complexes with 2,5-diphenyl-1,3,4-oxadiazole type counter-anions afford highly-efficient solution-processed blue-green organic light-emitting diodes.

Efficiency improvement in solution-processed multilayered phosphorescent white organic light emitting diodes by silica coated silver nanocubes

Localized surface plasmon resonance (LSPR) effect of metal nanoparticles (MNs) has been widely applied in organic light-emitting diodes (OLEDs) to improve the radiation of excitons. The LSPR wavelength and intensity of MNs and the coupling between MNs and excitons greatly affect the LSPR effect on exciton radiation. In this work, silica coated silver nanocubes (Ag@SiO<sub>2</sub> NCs) were doped in the electron transport layer (ETL) of a solution-processed multilayered white OLED (WOLED). Due to the sharp edges and corners, Ag NCs have strong LSPR effect and can effectively enhance the radiation of nearby excitons. With an appropriate concentration of Ag@SiO<sub>2</sub> NCs, the WOLED achieved two fold improvement in the current efficiency comparing with the control device without Ag@SiO<sub>2</sub> NCs incorporated. The working mechanism of the Ag@SiO<sub>2</sub> NCs based WOLED was investigated in detail. For the solution-processed OLED, excitons usually form and recombine near the interface of emission layer and electron transport layer (EML/ETL) because the commonly used host material (such as polyvinylcarbazole, PVK) has the unipolar hole transport property. So the Ag@SiO<sub>2</sub> NCs in ETL greatly enhanced the radiation of the excitons located near the EML/ETL interface, which mostly contributed to the performance enhancement of the Ag@SiO<sub>2</sub> NCs based WOLED. Study on a group of devices with Ag@SiO<sub>2</sub> NCs doped in different locations indicated that Ag@SiO<sub>2</sub> NCs in ETL showed more effective LSPR effect than those in hole injection layer. Electroluminescence and photoluminescence spectra of the WOLEDs declared that the Ag@SiO<sub>2</sub> NCs simultaneously improved the radiation intensities of the blue and yellow excitons and helped the WOLED maintain the good chromaticity stability, which was mainly attributed to the wide LSPR wavelength range (450–650 nm) of the Ag@SiO<sub>2</sub> NCs. The SiO<sub>2</sub> coating layer of the Ag@SiO<sub>2</sub> NCs played the important role in the LSPR enhanced emission. On the one hand, it formed an appropriated distance between the Ag NCs and the extions, helping to generate the strong coupling between them. On the other hand, it suppressed the effect of Ag NCs on charge trapping, keeping the stability of the carrier transport in the device. Our research demonstrate MNs can effectively improve the performance of OLEDs by carefully designing the device structure.

Homoleptic mer-Ir(iii) complexes for highly efficient solution-processable green phosphorescent organic light-emitting diodes with high current efficiency

Highly efficient green-emitting phosphorescent Ir(iii) complexes with meridional configurations are introduced for solution-processed PHOLEDs that achieve high CE with high EQE. The unexpected high PLQYs leads to high luminous efficiency.

Two novel neutral and ionic Ir(iii) complexes based on the same bipolar main ligand: a comparative study of their photophysical properties and applications in solution-processed red organic light-emitting diodes

A bipolar ligand design for solution-processed red Ir(iii) complex OLED.

Sky-blue-emitting cationic iridium complexes with carbazole-type counter-anions and their use for efficient solution-processed organic light-emitting diodes.

The control of counter-anion is a facile approach to tune the overall properties of cationic iridium complexes for optoelectronic applications. Here, we report for the first time the use of electron-rich carbazole-type counter-anions in cationic iridium complexes and the use of such complexes as dopants in solution-processed organic light-emitting diodes (OLEDs). PF6-, 4-(9H-carbazol-9-yl)benzenesulfonate (CAZ-SO3-), and 4-(9'H-[9,3':6',9''-tercarbazol]-9'-yl)benzenesulfonate (TCAZ-SO3-) have been employed as the counter-anions of sky-blue-emitting complexes R, 1 and 2, respectively. The carbazole-type counter-anions do not largely disturb the phosphorescence of the cations and the complexes show similar emission properties in solution and films. The close proximity of the carbazole-type anions to the phosphorescent cations allows efficient energy-transfer from the former to the latter in films. When used as dopants in solution-processed OLEDs, complexes 1 and 2 show higher performances than complex R because the hole-trapping effects of the carbazole-type counter-anions largely improve the carrier-recombination balance in the emissive layers. In particular, the double-layer sky-blue device based on complex 2 with strong hole-trapping by TCAZ-SO3- affords a high peak current efficiency of 27.1 cd A-1. The work reveals that electron-rich carbazole-type anions are promising counter-anions for cationic iridium complexes toward optoelectronic applications.

An 850 nm pure near-infrared emitting iridium complex for solution-processed organic light-emitting diodes

The first example of an iridium(iii) emitter in pure near-infrared OLEDs with an emission peak at 850 nm and full emission over 700 nm.

Highly Photoluminescent Blue Ionic Platinum-Based Emitters.

New cycloplatinated N-heterocyclic carbene (NHC) compounds with chelate diphosphines (P^P) as ancillary ligands: [Pt(R-C^C*)(P^P)]PF6 (R = H, P^P = dppm (1A), dppe (2A), dppbz (3A); R = CN, P^P = dppm (1B), dppe (2B), dppbz (3B)) have been prepared from the corresponding starting material [{Pt(R-C^C*)(μ-Cl)}2] (R = H, A, R = CN, B) and fully characterized. The new compound A has been prepared by a stepwise protocol. The photophysical properties of 1A-3A and 1B-3B have been widely studied and supported by the time-dependent-density functional theory. These compounds show an efficient blue (dppe, dppbz) or cyan (dppm) emission in PMMA films (5 wt %), with photoluminescence quantum yield (PLQY) ranging from 30% to 87% under an argon atmosphere. This emission has been assigned mainly to transitions from 3ILCT [π(NHC) → π*(NHC)] excited states with some 3LL'CT [π(NHC) → π*(P^P)] character. The electroluminescence of these materials in proof-of-concept solution-processed organic light-emitting diodes containing 3A and 3B as dopants was investigated. The CIE coordinates for devices based on 3A (0.22, 0.41) and 3B (0.24, 0.44) fit within the sky blue region.

Synthesis and Electroluminescent Properties of Through-Space Charge Transfer Polymers Containing Acridan Donor and Triarylboron Acceptors.

We report the design, synthesis and electroluminescent properties of three kinds of through-space charge transfer (TSCT) polymers consisting of non-conjugated polystyrene backbone, acridan donor and triarylboron acceptors having different substituents such as hydrogen (H), fluorine (F), and trifluoromethyl (CF3). Owing to the weak electron interaction between acridan donor and triarylboron acceptor through non-conjugated connection, blue emission with peaks in range of 429–483 nm can be achieved for the polymers in solid-state film, accompanied with photoluminescence quantum yields of 26–53%. The resulting TSCT polymers exhibit small ΔEST values below 0.1 eV owing to the separated HOMO and LUMO distributions, showing thermally activated delayed fluorescence with lifetimes in range of 0.19–0.98 μs. Meanwhile, the polymers show aggregation-induced emission (AIE) effect with the emission intensity increased by up to ~33 folds from solution to aggregation state. Solution-processed organic light-emitting diodes based on the polymers containing trifluoromethyl substituent exhibit promising electroluminescent performance with maximum luminous efficiency of 20.1 cd A−1 and maximum external quantum efficiency of 7.0%, indicating that they are good candidates for development of luminescent polymers.

AB- Versus AA+BB-Suzuki Polycondensation: A Palladium/Tris(tert-butyl)phosphine Catalyst Can Outperform Conventional Catalysts.

A Pd/Pt-Bu3 catalyst having bulky, electron-rich ligands significantly outperforms conventional "step-growth catalysts" Pd(PPh3 )4 and Pd(Po-Tol3 )3 in the Suzuki polycondensation of the AB-type arylene-based monomers, such as some of the substituted fluorenes, carbazoles, and phenylenes. In the AA+BB polycondensation, Pd/Pt-Bu3 also performs better under homogeneous reaction conditions, in combination with the organic base Et4 NOH. The superior performance of Pd/Pt-Bu3 is discussed in terms of its higher reactivity in the oxidative addition step and inherent advantages of the intramolecular catalyst transfer, which is a key step joining catalytic cycles of the AB-polycondensation. These findings are applied to the synthesis of a carbazole-based copolymer designed for the use as a hole conductor in solution-processed organic light-emitting diodes.

Lead-Halide Perovskite as the Host Material for Solution-Processed Phosphorescent Organic Light-Emitting Diodes

Phosphorescent organic light-emitting diodes (PhOLEDs) are a kind of highly efficient and solution-processable devices for displays and light sources, which employ phosphorescent material as the gu...