Jonathan P. J. Markham

Green phosphorescent dendrimer for light-emitting diodes

Highly efficient organic LEDs made by solution processing are reported. It is shown that the dendritic architecture (see Figure) can be used to solubilize luminescent chromophores and form uniform films of blends. The simple device structures containing a light-emitting chromophore are amongst the most efficient solution-processed devices reported. Thanks to this technique, the inkjet printing of phosphorescent materials becomes feasible.

High-efficiency green phosphorescence from spin-coated single-layer dendrimer light-emitting diodes

We demonstrate very high-efficiency green phosphorescence from a single-layer dendrimer organic light-emitting diode formed by spin-coating. A first generation fac-tris(2-phenylpyridine) iridium cored dendrimer doped into a wide-gap 4,4′-bis(N-carbazole) biphenyl host displays a peak external quantum efficiency of 8.1% (28 Cd/A) at a brightness of 3450 Cd/m2 and a current density of 13.1 mA/cm2. A peak power efficiency of 6.9 lm/W was measured at 1475 Cd/m2 and 5 mA/cm2. We attribute this exceptionally high quantum efficiency for a single-layer device to the excellent film forming properties and high photoluminescence quantum yield of the dendrimer blend and efficient injection of charge into the emissive layer. These results suggest that dendrimers are an effective method for producing efficient phosphorescent devices by spin-coating.

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.

Charge transport in highly efficient iridium cored electrophosphorescent dendrimers

Electrophosphorescent dendrimers are promising materials for highly efficient light-emitting diodes. They consist of a phosphorescent core onto which dendritic groups are attached. Here, we present an investigation into the optical and electronic properties of highly efficient phosphorescent dendrimers. The effect of dendrimer structure on charge transport and optical properties is studied using temperature-dependent charge-generation-layer time-of-flight measurements and current voltage (I-V) analysis. A model is used to explain trends seen in the I-V characteristics. We demonstrate that fine tuning the mobility by chemical structure is possible in these dendrimers and show that this can lead to highly efficient bilayer dendrimer light-emitting diodes with neat emissive layers. Power efficiencies of 20 lm/W were measured for devices containing a second-generation (G2) Ir(ppy)(3) dendrimer with a 1,3,5-tris(2-N-phenylbenzimidazolyl)benzene electron transport layer

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=2.0×10−4 cm2/V s at a field of 0.55 MV/cm. There is a weak field dependence of the mobility and it increases from μ=1.6×10−4 cm2/V s at 0.2 MV/cm to μ=3.0×10−4 cm2/V s 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.

Solution processable phosphorescent rhenium(I) dendrimers

A family of (1,10-phenanthroline)rhenium(I)(CO)3Cl complex cored first generation dendrimers with one, two or three dendrons have been prepared. The first generation dendrons attached to the core complex are comprised of biphenyl units with 2-ethylhexyloxy surface groups at their distal ends. The number and position of attachment of the dendron to the core was found to have an effect on the properties of the dendrimers. When dendrons were attached to both the 2- and 9-positions of the 1,10-phenanthroline ligand, thus straddling the rhenium(I), the dendrimers became more electrochemically stable and less susceptible to solvatochromism. The dendrimers were generally found to have their emission blue-shifted and a higher photoluminescence quantum yield in the solid state than in the solution. The origin of this rigidochromism effect is discussed. A single layer device with a neat dendrimer film was found to have an external quantum efficiency of 0.4% (0.8 cd A−1) and power efficiency of 0.2 lm W−1 at 100 cd m−2 and 12.8 V.

Relating the physical structure and optical properties of conjugated polymers using neutron reflectivity in combination with photoluminescence spectroscopy

Understanding the effect of physical structure and the role of interfaces is critical for gaining insight into the optoelectronic properties of conjugated polymers and their behavior in semiconductor devices such as organic light-emitting diodes and photovoltaic cells. We have developed an in situ neutron reflection measurement that allows the direct relationship between film photoluminescence and structure to be studied. In addition, we have found that by judicious deuteration of the conjugated polymers, the polymer/indium tin oxide (ITO) interface can be probed. Critically for both poly[2-(2-d17-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] and poly[9,9′-(2-d17-ethylhexyl)-2,6-fluorene] of thickness of order 140–150 nm on ITO, we found that a thermally stable low-density layer of 20 A thickness was present between the polymer film and the ITO. The presence of the low-contact layer means that measurements involving these two families of polymers directly deposited onto ITO may need re-evaluating, and s...

Effect of meta-linkages on the photoluminescence and electroluminescence properties of light-emitting polyfluorene alternating copolymers

Alternating copolymers poly[9,9-dihexylfluorene-alt-(1,3-phenylene)x(1,4-phenylene)1−x] (x = 0, 0.05, 0.1, 0.25, 0.5, 0.75, 1.0) were prepared, and the absorption spectra showed the expected hypsochromic shift with increasing meta linkages. In the solution photoluminescence (PL) emission spectra, all but the all-meta polymer (x = 1) showed a spectrum similar to the all-para polymer, indicating rapid intrachain energy transfer to para-linked segments. Similar spectra were observed from thin films in which inter-chain energy transfer is also available. Annealing of the thin films in air above the Tg of the polymer for several hours led to an increase in long wavelength emission, but the increase was smaller for the polymers containing more meta linkages. IR experiments confirmed the formation of fluorenone defects during the annealing process. For those polymers with high meta-linkage incorporation (i.e. 75% and 100% meta) the long wavelength emission is suppressed. This suggests that when the conjugation length of the polymer is reduced then energy transfer to fluorenone defects is less likely, due to lower exciton delocalisation and mobility. The long wavelength emission was more prominent in the electroluminescence (EL) spectra of devices prepared from the polymer, and, while initially less intense in the high meta linkages polymers, became dominant even for the all-meta linked polymer after a few minutes of operation. In summary, the meta linkages had a beneficial effect in suppressing the undesirable long wavelength emission in both the PL and EL output.

Polarized organic electroluminescence: Ordering from the top

We demonstrate a method for achieving polarized organic electroluminescence for liquid crystalline conjugated polymers that allows the polymer to be deposited directly onto the anode. The technique utilizes a top-down alignment approach whereby the predeposited polymer was aligned from above using a rubbed polyimide master and a smectic liquid crystal transfer layer. The liquid crystal/polyimide master bilayer was sandwiched with the liquid crystalline polymer that had been deposited onto the electrode. The sandwiched layers were then heated to achieve alignment before the removal of the polyimide master and liquid crystal transfer layer. Using this method, poly[2,7-{9,9-di(2-ethylhexyl)}fluorene] (PF2-6) was aligned to give an anisotropic polymer film. Light emitted from single layer light-emitting diodes containing the aligned PF2-6 had integrated dichroic ratios of up to 9.7. At 100 cd/m(2), the single layer devices had external quantum and power efficiencies of 0.08% and 0.05 lm/W, respectively. Bilayer devices containing an electron transport layer between the PF2-6 and the cathode gave emitted light with good dichroic ratios and with the external quantum and power efficiencies at 100 cd/m(2) being increased to 2.2% and 1.1 lm/W