Efficient solution-processed dendrimer OLEDs

Materials, Devices, and Systems for Display and Lighting, 2002

Light-emitting dendrimers are a new distinct class of material for OLEDs. Dendrimers consist of a light-emitting core, dendrons and surface groups. Dendrimers are designed for solution coating and have a number of advantages over conjugated polymers. We report our recent results for solution processed green dendrimer OLEDs. The OLEDs were fabricated by spin-coating a blend of first generation dendrimer/host material followed by the evaporation of a hole blocking layer and a LiF/Al cathode. Power efficiencies of 50 lm/W at practical brightness levels were achieved for these structures.

Advanced Functional Materials, 2005

npj Flexible Electronics

Controlling the orientation of the emissive dipole has led to a renaissance of organic light-emitting diode (OLED) research, with external quantum efficiencies (EQEs) of >30% being reported for phosphorescent emitters. These highly efficient OLEDs are generally manufactured using evaporative methods and are comprised of small-molecule heteroleptic phosphorescent iridium(III) complexes blended with a host and additional layers to balance charge injection and transport. Large area OLEDs for lighting and display applications would benefit from low-cost solution processing, provided that high EQEs could be achieved. Here, we show that poly(dendrimer)s consisting of a non-conjugated polymer backbone with iridium(III) complexes forming the cores of firstgeneration dendrimer side chains can be co-deposited with a host by solution processing to give highly efficient devices. Simple bilayer devices comprising the emissive layer and an electron transport layer gave an EQE of >20% at luminances of up to ≈300 cd/ m 2 , showing that polymer engineering can enable alignment of the emissive dipole of solution-processed phosphorescent materials.

Organic Electronics, 2003

Iridium-based phosphorescent dendrimers have shown much promise as highly efficient light emitting materials for organic light emitting diodes (OLEDs). Here we report the effects of modifying the chemical structure on the emissive and charge transport properties of Ir(ppy) 3 based electrophosphorescent dendrimers. We investigate a novel para linked first generation (G1) iridium dendrimer. This material is compared to G1 and G2 meta linked dendrimers. We show that by blending these dendrimers into a CBP host, high external quantum efficiencies of over 10% and luminous efficiencies of 27 lm/W can be achieved.

Advanced Materials, 2007

We are grateful to CDT Oxford Ltd, the Engineering and Physical Sciences Research Council and the Scottish Funding Council for financial support. Dendrimers are now an important class of light-emitting material for use in organic light-emitting diodes (OLEDs). Dendrimers are branched macromolecules that consist of a core, one or more dendrons, and surface groups. The different parts of the macromolecule can be selected to give the desired optoelectronic and processing properties. The first light-emitting dendrimers were fluorescent but more recently highly efficient phosphorescent dendrimers have been developed. OLEDs containing light-emitting dendrimers have been reported to have external quantum efficiencies of up to 16 %. The solubility of the dendrimers opens the way for simple processing and a new class of flat-panel displays. In this Review we show how the structure of the light-emitting dendrimers controls key features such as intermolecular interactions and charge transport, which are important for all OLED materials. The advantages of the dendrimer architecture for phosphorescent emitters and the way the structure can be varied to enhance materials performance and device design are illustrated. REVIEW P. L. Burn et al./Light-Emitting Dendrimers for Displays 1676 www.advmat.de

SPIE Proceedings, 2008

Organic light-emitting diodes (OLEDs) have great potential for displays and lighting applications. For large area displays the ideal materials would be both phosphorescent and solution processible. These requirements mean that the materials need to be able to be patterned and the most advanced method for forming pixelated displays is inkjet printing. Light-emitting phosphorescent dendrimers have given high efficiency monochrome displays with the emitting layer deposited by spin-coating. However, the viscosity of the dendrimer solutions is insufficient for inkjet printing. We report the development of a new class of light-emitting materials, namely poly(dendrimers) in which a green emissive phosphorescent dendrimer is attached to a poly(styrene) backbone. Free radical polymerization of a dendrimer-styrene monomer gave a poly(dendrimer) with a weight average molecular weight of 24000 and a polydispersity of 3.6. A dilute solution of the dendrimer had a viscosity 15% higher than the neat solvent. Comparison of the photophysical studies of the poly(dendrimer) versus a model monomer dendrimer showed that the PL spectrum was broader and red-shifted, and the PL quantum yield around 50% lower. This was attributed to intermolecular interactions of the emissive dendrimers, which are held closely together on the polymer backbone.

Applied Physics Letters, 2003

High-efficiency single-layer-solution-processed green light-emitting diodes based on a phosphorescent dendrimer are demonstrated. A peak external quantum efficiency of 10.4% ͑35 cd/A͒ was measured for a first generation f ac-tris(2-phenylpyridine) iridium cored dendrimer when blended with 4,4Ј-bis(N-carbazolyl)biphenyl and electron transporting 1,3,5-tris(2-N-phenylbenzimidazolyl)benzene at 8.1 V. A maximum power efficiency of 12.8 lm/W was measured also at 8.1 V and 550 cd/m 2 . These results indicate that, by simple blending of bipolar and electron-transporting molecules, highly efficient light-emitting diodes can be made employing a very simple device structure.

Optics Express, 2012

We show that it is possible to produce an efficient solutionprocessable phosphorescent poly(dendrimer) OLED with a 32 lm/W power efficiency at 100 cd/m 2 without using a charge transporting host or any improvements in light extraction. This is achieved by using the dendrimer architecture to control inter-chromophore interactions. The effects of using 4,4,4-tris(N-carbazolyl)triphenylamine (TCTA) as a charge transporting host and using a double dendron structure to further reduce interchromophore interactions are also reported.

2010

a b s t r a c t 24 A uniform dispersion of highly soluble phosphorescent dendrimer emitters is achieved by 25 blending with a polymer host poly(9-vinylcarbazole) (PVK) containing N,N 0 -diphenyl-N, 26 N 0 -(bis(3-methylphenyl)-[1,1-biphenyl]-4,4 0 -diamine (TPD) and 2-(4-biphen-4 0 -yl)-5-(4-27 tert-butylphenyl)-1,3,4-oxadiazole (PBD). No visible aggregation or self-quenching was 28 observed for guest-to-host weight ratios of up to 33:67. The dendrimers contain a fac-29 tris(2-phenylpyridyl)iridium(III) [Ir(ppy) 3 ] core, first generation biphenyl-based dendrons, 30 and 2-ethylhexyloxy surface groups. The guest-host blend is used for all solution pro-31 cessed organic light-emitting diodes. A maximum external and current efficiency of 32 10.2% and 38 cd/A (at 5 V and a brightness of 50 cd/m 2 ), and a maximum brightness of 33 27,000 cd/m 2 (at 14.5 V), were obtained when a CsF/Al cathode was used. Blade coating 34 was used to fabricate a multi-layer structure that also contained an electron-transport 35 layer. The device that had a LiF/Al cathode had a maximal efficiency of 40 cd/A correspond-36 ing to an external quantum efficiency of 10.8% (at 5 V and a brightness of 19 cd/m 2 ). The 37 maximum brightness of the second device was 17,840 cd/m 2 at 14 V.

Advanced Materials, 2022

demonstration in high-efficiency organic light-emitting diodes (OLEDs). [2] A number of benchmark red, [3] green, [4] and blue [5] emitters have already been developed exhibiting more than 20% maximum external quantum efficiency, EQE max , highlighting the significant and rapid advances in TADF materials' design and demonstrating their viability as replacement materials for state-of-the-art phosphorescent (for red and green) and fluorescent (blue) emitters in commercial OLEDs. However, the aforementioned high efficiency of the OLEDs relies not only on the intrinsic photophysical properties of the emitter but also on the rather complicated multilayered device architecture typically used in vacuum-deposited devices, which increases the cost of device fabrication. An alternative strategy would be to fabricate the OLED using lower-cost solution-processing techniques such as ink-jet printing. [6] TADF dendrimers are perfect candidates for solution-processed host-free OLEDs for the following reasons: 1) the singlet-triplet gap, ΔE ST , can be easily adjusted within the modular molecular design using dendronized donors or acceptors; 2) intermolecular quenching can be largely avoided by careful design of the dendron motif; and 3) intersystem crossing (ISC) or reverse intersystem crossing The development of high-performance solution-processed organic light-emitting diodes (OLEDs) remains a challenge. An effective solution, highlighted in this work, is to use highly efficient thermally activated delayed fluorescence (TADF) dendrimers as emitters. Here, the design, synthesis, density functional theory (DFT) modeling, and photophysics of three triazine-based dendrimers, tBuCz2pTRZ, tBuCz2mTRZ, and tBuCz2m2pTRZ, is reported, which resolve the conflicting requirements of achieving simultaneously a small ΔE ST and a large oscillator strength by incorporating both meta-and para-connected donor dendrons about a central triazine acceptor. The solution-processed OLED containing a host-free emitting layer exhibits an excellent maximum external quantum efficiency (EQE max) of 28.7%, a current efficiency of 98.8 cd A −1 , and a power efficiency of 91.3 lm W −1. The device emits with an electroluminescence maximum, λ EL , of 540 nm and Commission International de l'Éclairage (CIE) color coordinates of (0.37, 0.57). This represents the most efficient host-free solution-processed OLED reported to date. Further optimization directed at improving the charge balance within the device results in an emissive layer containing 30 wt% OXD-7, which leads to an OLED with the similar EQE max of 28.4% but showing a significantly improved efficiency rolloff where the EQE remains high at 22.7% at a luminance of 500 cd m −2 .