Thomas D. Anthopoulos

Highly efficient single-layer dendrimer light-emitting diodes with balanced charge transport

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/A) was measured for a first generation fac-tris(2-phenylpyridine) iridium cored dendrimer when blended with 4,4′-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/m2. 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.

Tuning of emission color for blue dendrimer blend light-emitting diodes

We demonstrate efficient tunable blue electroluminescence from blends of two solution-processible light-emitting dendrimers. These materials can be blended to form optical quality thin films with no phase-separation effects, irrespective of the blend ratio. External quantum efficiencies of 1% have been measured for the blend systems and the emission color can be tuned from deep blue (emission peak 401nm) to blue green (477nm) by blend composition. A power efficiency of 1.5lm∕W (at 200Cd∕m2 and 5.4V) is measured for a single layer, first-generation blue-green fluorene-thiophene dendrimer. These results show that by choice of a dendrimer structure with common branching units and surface groups, dissimilar cores can be blended with excellent miscibility. This provides a simple way of tuning the color of organic light-emitting diodes.

Nondispersive hole transport in a spin-coated dendrimer film measured by the charge-generation-layer time-of-flight method

Measurements of the mobility of a first-generation (G1) bis-fluorene cored dendrimer have been performed on spin-coated samples of 500 nm thickness using the charge-generation-layer time-of-flight (TOF) technique. A 10 nm perylene charge generation layer was excited by the 532 nm line of a Q-switched Nd:YAG laser and the generated carriers swept through the dendrimer film under an applied field. We observe nondispersive hole transport in the dendrimer layer with a room-temperature mobility μ=2.0×10−4u2009cm2/Vu200as at a field of 0.55 MV/cm. There is a weak field dependence of the mobility and it increases from μ=1.6×10−4u2009cm2/Vu200as at 0.2 MV/cm to μ=3.0×10−4u2009cm2/Vu200as at 1.4 MV/cm. These results suggest that the measurement of mobility by TOF in spin-coated samples on thickness scales relevant to organic light-emitting diodes can yield valuable information, and that dendrimers are promising materials for device applications.

Simple color tuning of phosphorescent dendrimer light emitting diodes

A simple way of tuning the emission color in solution processed phosphorescent organic light emitting diodes is demonstrated. For each color a single emissive spin-coated layer consisting of a blend of three materials, a fac-tris(2-phenylpyridyl)iridium (III) cored dendrimer (Ir–G1) as the green emitter, a heteroleptic [bis(2‐phenylpyridyl)‐2‐(2′‐benzo[4,5‐α]thienyl)pyridyl]iridium (III) cored dendrimer [Ir(ppy)2btp] as the red emitter, and 4,4′-bis(N-carbazolyl) biphenyl (CBP) as the host was employed. By adjusting the relative amount of green and red dendrimers in the blends, the color of the light emission was tuned from green to red. High efficiency two layer devices were achieved by evaporating a layer of electron transporting 1,3,5-tris(2-N-phenylbenzimidazolyl)benzene (TPBI) on top of the spin-coated emissive layer. A brightness of 100cd∕m2 was achieved at drive voltages in the range 5.3–7.3 V. The peak external efficiencies at this brightness ranged from 31cd∕A(18lm∕W) to 7cd∕A(4lm∕W).

Highly efficient solution-processible phosphorescent dendrimers for organic light-emitting diodes

Currently, most research into organic light-emitting diodes (OLEDs) has focused on two main classes of materials: small organic molecules and conjugated polymers. An alternative approach is to use conjugated dendrimers. We show that conjugated dendrimers are a promising new class of solution-processible materials for use as the active layer in highly efficient organic LEDs. By optimizing the choice of device structure, host material, and electron transport layer, we can obtain efficiencies of 55 cd/A and power efficiencies of 40 lm/W. This is an excellent result for a spin-coated emissive layer.

L-8: Late-News Paper: Highly Efficient Solution-Processible Dendrimer LEDs

The recent progress in the development of organic light emitting diodes (OLEDs) has opened many avenues for the production of large area, full colour, flexible displays that are cheap to manufacture. Currently most research has focussed on two main classes of materials - small organic molecules and conjugated polymers. An alternative approach is to use conjugated dendrimers. We show that conjugated dendrimers are a promising new class of solution-processible materials for highly efficient organic LEDs.

67.1: Invited Paper: Dendrimers — Efficient Solution‐Processed Phosphorescent OLED Materials

Three aspects of recent dendrimer research are reported. It is shown that highly efficient sky blue phosphorescent devices can be made (external efficiency 10.4%, 11 lm/W at 100 Cd/m), that blending dendrimers provides a simple way of colour tuning, and finally that luminescence quenching by exciton-exciton annihilation in dendrimer films is weak and can be controlled by the dendrimer generation. These results suggest that dendrimers are very attractive solution processible materials for OLEDs.

Conjugated dendrimers: a modular approach to materials for full colour displays

Conjugated dendrimers provide an excellent molecular architecture for tuning material properties for organic light emitting diodes. Here we demonstrate a modular approach allowing highly efficient fluorescent and phosphorescent emissive chromophores to be used to make red, green and blue solution-processed light emitting diodes. The choice of a common dendritic architecture ensures good solubility and film forming properties irrespective of the choice of core unit. In addition, this architecture allows blending of dendrimers with different cores without phase separation. We show that blending provides a simple but powerful way of tuning the color of dendrimer LEDs from deep blue to blue-green, and from green to red with little impact on the device properties.