Solution-processed Emissive Layer Research Articles

Origins of the improved device performance in solution-processed TADF organic light-emitting devices with a polymer underneath layer

In solution-processed organic light-emitting devices (OLEDs), a polymer interface layer underneath the emission layer is essential to improving the device's performance. The typical polymer is Poly (9-vinylcarbazole) (PVK), which can dramatically increase the device efficiency whereas the mechanisms are not well understood. In this work, we explored the related mechanisms by smart experimental design and measurements. The results reveal that in addition to reducing the hole current density and balancing the carrier distribution, the PVK underneath layer also improves the film morphology and enhances the horizontal orientation of the transition dipole moment of the thermally activated delayed fluorescence (TADF) emitters. Consequently, the device's efficiency and stability are notably increased by about three times. This work provides insights into the origins of improved device performance in solution-processed TADF OLEDs with a polymer underneath layer and also highlights the effect of the underneath layer on the film formation of the solution-processed emission layer.

Enhanced performance in solution-processed blue emission layer of organic light-emitting diodes with an alcohol soluble amphiphilic polymer as the hole modify layer

Solution-processed emission layer of organic light-emitting diodes (OLEDs), especially blue OLEDs, are generally inefficient. In this work, we use an alcohol-soluble amphiphilic polymer, Poly[9, 9- bis[3- (ethyldimethylammoni o) propyl] 9′, 9′- dioctyl[2, 2′- bi- 9 H- fluorene] - 7, 7′- diyl bromide (1:2)] (PFN-Br), to modify the hole transport layer, which is deposited on the surface of Poly(9-vinyl carbazole) (PVK) layer for interface modification to achieve high efficient blue OLEDs based on the thermally activated delayed fluorescent (TADF) material 10-[4-(4,6-Diphenyl-1,3,5-triazin-2-yl)− 2-methylphenyl]-spiro[acridine-9(10 H),9’-[9 H]fluorene] (TTSA) as the emitter. It is shown that adding PFN-Br layer can reduce the defect density at the interface between PVK and the emission layer, then the exciton quenching of TTSA is reduced. After optimizing the thickness of PFN-Br layer, the maximum external quantum efficiency and current efficiency of the optimal OLED achieve 22% and 42 cd/A, respectively. To the best of our knowledge, these values are superior to the highest values reported for this material in the literature.

A red-orange carbazole-based iridium(III) complex: Synthesis, thermal, optical and electrochemical properties and OLED application

A novel heteroleptic iridium(III) acetylacetonate (acac) complex, (L-5-CHO)2Ir(acac) (3b), was synthesised from 2-(9’-hexylcarbazole-3’-yl)-5-formylpyridine (L-5-CHO) (1b). The complex 3b was determined to be thermally and electrochemically stable. The photoluminescence properties of the compound were studied, with a dichloromethane solution of 3b giving structureless emission at 662 nm, showing that the formyl group red-shifted the emission by 151 nm compared to the parent complex. Complex 3b was also shown to possess a moderate photoluminescence quantum yield (67%) and a short emission lifetime (τ = 280 ns). Organic light-emitting diodes (OLEDs) were fabricated with a solution-processed emissive layer (EML) consisting of poly(N-vinylcarbazole) (PVK), 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (PBD) and iridium complex (3b). The OLEDs containing complex 3b showed red-orange electroluminescence (EL) at 624 nm. The influence of the host materials was studied and the best performance was achieved with both PVK and PBD in the emissive layer, with the resulting OLEDs exhibiting a current efficiency of 0.84 cd/A, a power efficiency of 0.20 lm/W, and an external quantum efficiency (EQE) of 0.66% at a brightness of 2548 cd/m2.

Managed carrier density and distribution in solution-processed emission layer to achieve highly efficient and bright blue organic light-emitting devices

In this paper, solution-processed blue organic light-emitting devices (OLEDs) with high efficiency and high luminance are realized. A blue thermally activated delayed fluorescence (TADF) material, 4CzFCN, is used as the emitting material. A host material with high hole mobility, mCP, and an electron transport material with high electron mobility and lower LUMO level, B4PyMPM, are employed to increase the electron density and balance the carrier distribution inside the emission layer. As a result, a luminance of 7625.0 cd/m 2 , a current efficiency of 49.3 cd/A, and an external quantum efficiency of up to 24% are obtained. To the best of our knowledge, these values are superior to the highest values reported for this material in the literatures. • By managing the carrier density and the carrier balance inside the EML, the solution-processed blue OLEDs performance was improved greatly. • The maximum current efficiency of the champion device is 49.3 cd/A, which is the highest efficiency reported from OLEDs with this material. • The high-efficiency device also achieved a high luminance of 7625 cd/m 2 , which is the highest luminance reported from OLEDs with this material.

Highly efficient and bright blue organic light-emitting devices based on solvent engineered, solution-processed thermally activated delayed fluorescent emission layer

Abstract Inefficient blue emission is one of the key bottlenecks limiting the development of solution processable displays and white light sources, which is essentially governed by the film quality of the solution-processed emission layer (EML). Here, we use an efficient blue thermally activated delayed fluorescent (TADF) emitter, bis [4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]solfone (DMAC-DPS), as EML and propose a solvent engineering strategy to achieve high efficiency and brightness. The strategy employs synergetic solvents to improve the film morphology of the solution-processed DMAC-DPS EML. The improved film quality enhances the carrier injection while reduces the surface trap charges, as revealed by the hole-/electron-only device performance and the transient electroluminescence measurements. As expected, the blue organic light-emitting devices (OLEDs) employing synergetic solvent strategy presents an increment of 22.8% in efficiency and 19.4% in luminescence, which reaches the highest efficiency of 15.76 cd/A and an efficiency of 9.63 cd/A at a luminance of 1000 cd/m2. These values are generally higher than the reported values from the blue OLEDs based on solution-processed DMAC-DPS EML.

Achieving high efficiency by high temperature annealing of hole transporting polymer layer in solution-processed organic light-emitting devices

We developed highly efficient solution-processed phosphorescent organic light-emitting device by annealing of hole transporting polymer layer at about glass transition temperature. Poly(N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine) (poly-TPD) was used as a hole transporting polymer layer and tris(2-phenylpyridine)iridium(III) [Ir(ppy)3] doped N,N'-dicarbazolyl-3,5-benzene (mCP) was used as a solution-processed emission layer. The annealing temperature was critical to the well-defined interface between the poly-TPD and mCP:Ir(ppy)3 layers. In addition, the carrier recombination was significantly enhanced by high temperature annealing of poly-TPD layer. A current efficiency of 61.5cd/A and an external quantum efficiency of 17.5% were achieved by annealing of poly-TPD layer at about glass transition temperature.

Substituent position engineering of diphenylquinoline-based Ir(iii) complexes for efficient orange and white PhOLEDs with high color stability/low efficiency roll-off using a solution-processed emission layer

The location of –CF3 group on 2,4-diphenylquinoline based Ir(iii) complexes is key for attaining high performance in solution-processed PhOLEDs.

Highly efficient green organic light emitting diode with a novel solution processable iridium complex emitter

We demonstrate a high-efficiency green organic light-emitting diode (OLED) with a solution-processed emissive layer composed of a novel green light emitting iridium complex, bis [5-methyl-8-trifluoromethyl-5H-benzo(c) (1,5)naphthyridin-6-one]iridium(pyrazinecarboxylate). By coupling with a proper host, the green device shows at 1000 cd m−2 an external quantum efficiency of 23.8%, current efficiency of 95.6 cd A−1, and efficacy of 60.8 lm W−1, the highest among all reported OLEDs with a solution-processed emissive layer. The high efficiency may be attributed to the host possessing a zero electron injection barrier, resulting in a more balanced carrier-injection.

Using light-emitting dyes as a co-host to markedly improve efficiency roll-off in phosphorescent yellow organic light emitting diodes

We discovered in this study the feasibility of using regular light-emitting dyes as an effective co-host, rather than a sensitizer, to markedly improve the efficiency of phosphorescent organic light emitting diodes. At 10 000 cd m−2, for example, the efficacy of a yellow emitter containing device was increased from 11.7 lm W−1 to 15.4 lm W−1, an increment of 32%, as a sky-blue phosphorescent dye, bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium(III) (FIrpic), was blended into a host of 4,4′,4′′-tri(N-carbazolyl)triphenylamine (TCTA). The efficacy at 1000 cd m−2 was 26.7 lm W−1, the highest among all reported yellow OLEDs with a solution-processed emissive layer. The marked efficiency improvement may be attributed to the co-host having an electron trapping character, enabling excitons to generate on itself instead of on the guest, creating an additional efficiency-effective energy transfer route, and having a very efficient co-host to guest energy transfer. The most effective co-host may vary with the variation of the host employed, depending on the energy level pairing of the co-host and host.

High-efficiency low color temperature organic light emitting diodes with solution-processed emissive layer

Low color temperature (CT) lighting provides a warm and comfortable atmosphere and shows mild effect on melatonin suppression. A high-efficiency low CT organic light emitting diode can be easily fabricated by spin coating a single white emission layer. The resultant white device shows an external quantum efficiency (EQE) of 22.8% (34.9lm/W) with CT 2860K at 100cd/m2, while is shown 18.8% (24.5lm/W) at 1000cd/m2. The high efficiency may be attributed to the use of electroluminescence efficient materials and the ambipolar-transport host. Besides, proper device architecture design enables excitons to form on the host and allows effective energy transfer from host to guest or from high triplet guest to low counterparts. By decreasing the doping concentration of blue dye in the white emission layer, the device exhibited an orange emission with a CT of 2280K. An EQE improvement was observed for the device, whose EQE was 27.4% (38.8lm/W) at 100cd/m2 and 20.4% (24.6lm/W) at 1000cd/m2.

Polymers with Carbazole-Oxadiazole Side Chains as Ambipolar Hosts for Phosphorescent Light-Emitting Diodes

Polymethacrylates, polystyrenes, and polynorbornenes bearing 2-(3-(carbazol-9-yl)phenyl)-5-phenyl-1,3,4-oxadiazole, 2-(4-(carbazol-9-yl)phenyl)-5-phenyl-1,3,4-oxadiazole, 2-(3,5-di(carbazol-9-yl)phenyl)-5-phenyl-1,3,4-oxadiazole) groups linked to the polymer backbone through the 3-position of their terminal phenyl groups have been synthesized for use as solution-processable ambipolar hosts for phosphorescent light-emitting diodes. The polymers exhibit good thermal stabilities, with no weight loss below 350 °C, and have glass-transition temperatures in the range 118–209 °C. Spectroscopic, electrochemical, and quantum-chemical data for small-molecule model compounds suggest that the side chain groups are suitable hosts for a range of phosphors, including the green-emitter fac-tris(2-phenylpyridinato-N,C2′)iridium (Ir(ppy)3). Light-emitting diodes were fabricated, each with a solution-processed photo-cross-linked hole-transport layer, a solution-processed emissive layer composed of a carbazole-oxadiazole-fun...

Combined thermal evaporated and solution processed organic light emitting diodes

Highly efficient, partly solution processed phosphorescent red, green and white organic light emitting diodes with small molecular weight host materials are prepared from commercially available starting compounds. Starting from an evaporated reference device, layers are stepwise replaced by solution processed layers. Replacing the evaporated hole transport layer by a solution processed polymer interlayer does not affect the performance and allows spincoating of the emissive layer after annealing of the polymer. Devices with, next to the spincoated hole injection and transport layer, a solution processed emission layer show similar characteristics and efficiencies as the reference device. Additionally, white light emitting devices are demonstrated in which the first emissive layer is solution processed, for which lifetimes in excess of 1000 h are achieved. Thus, we show that, in order to reduce production costs, solution processing can be used as an alternative to – and in conjunction with – Thermal evaporation.

Small Molecular Reddish-White Light Emitting Electrophosphorescent Devices with Solution Processed Emission Layer

We compared the electrical characteristics of polymer, poly(N-vinylcarbazole) (PVK) and small molecule, 4,4′-N,N′-dicarbazole-biphenyl (CBP), N,N′-dicarbazolyl-3,5-benzene (mCP), as host material in white organic light-emitting diodes (OLEDs) fabricated by solution process. In order to obtain the white emission, iridium(III) bis(4,6-difluorophenylpyridinato)-picolinate) (Firpic) and iridium(III) bis(2-phenylquinoline)(acetylaceton) [Ir(pq)2acac] were used as blue and red emitting dopant, respectively. As a result, devices with PVK, CBP, and mCP have the max. luminous efficiencies of 10.5 cd/A, 9.8 cd/A, and 4.2 cd/A, respectively.

White Light Emitting Electrophosphorescent Devices with Solution Processed Emission Layer

White light emitting diodes with high efficiency and brightness have been fabricated using phosphorescent materials by spin coating an emission layer. The emission layer consisted of sky blue and orange light emitting materials based on iridium complexes, which were dissolved in toluene solvent with 4,4'-bis(carbazoyl-9-yl)biphenyl (CBP) as a host and polystyrene as a binder. The introduction of polystyrene binder resulted in not only good film forming properties but also better device performance than that of poly(vinyl carbazole) conventionally used as a host material. The device structure was ITO/PEDT:PSS/emission layer/BAlq/LiF/Al. The color of the devices varied according to the composition of light emitting materials and the device having the closest to white emission showed CIE coordinates of (0.33,0.39) with an external quantum efficiency and a current efficiency of 6.1% and 13 cd/A, respectively, at 100 cd/m2.