The anisotropic conductivity of thin films of poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is correlated to the film morphology as obtained from scanning tunneling and atomic force microscopy images. The material was found to consist of layers of flattened PEDOT‐rich particles that are separated by quasi‐continuous PSS lamella (see figure).

A series of novel carbazole compounds was synthesized and tested for their suitability as host for triplet emitters in an organic-light emitting diode (OLED). In these compounds, a carbazole unit is either connected to other carbazole units to form carbazole dimers and trimers or to fluorene and oxadiazole derivatives to form mixed compounds. The HOMO level of carbazole compounds can be tuned by substitution at the 3, 6, and/or 9 positions. Making oligomers by connecting carbazole molecules via their 3 (3') positions shifts the HOMO level to higher energy, while replacing alkyl groups at the 9 (9') positions by aryl groups shifts the HOMO level to lower energy. Furthermore, it has been found that the triplet energy of these compounds is determined by the presence of poly(p-phenyl) chains in the molecular structure. By identifying the longest poly(p-phenyl) chain, one can predict whether a compound will be a suitable host for a high-energy triplet emitter. An overview of HOMO levels, singlet and triplet levels, and exchange energies is given for all carbazole compounds synthesized. Finally, OLEDs employing two selected carbazole compounds as host and fac-tris(2-phenylpyridine)-iridium (Ir(ppy)(3)) as guest were constructed and characterized electrically.

A carbazole homopolymer and carbazole copolymers based on 9,9'-dialkyl-[3,3']-bicarbazolyl, 2,5-diphenyl-[1,3,4]-oxadiazole and 9,9-bis(4-[3,7-dimethyloctyloxy]phenyl)fluorene were synthesized and their electrical and photophysical properties were characterized with respect to their application as host in phosphorescent polymer light-emitting diodes. It is shown that the triplet energy of a polymer depends on the specific connections between its building blocks. Without changing the composition of the polymer, its triplet energy can be increased from 2.3 to 2.6 eV by changing the way in which the different building blocks are coupled together. For poly(9-vinylcarbazole) (PVK), a carbazole polymer often used as host for high-energy triplet emitters in polymer light-emitting diodes, a large hole-injection barrier of about 1 eV exists due to the low-lying HOMO level of PVK. For all carbazole polymers presented here, the HOMO levels are much closer to the Fermi level of a commonly used anode such as ITO and/or a commonly used hole-injection layer such as PEDOT:PSS. This makes high current densities and consequently high luminance levels possible at moderate applied voltages in polymer light-emitting diodes. A double-layer polymer light-emitting diode is constructed comprising a PEDOT:PSS layer as hole-injection layer and a carbazole-oxadiazole copolymer doped with a green triplet emitter as emissive layer that shows an efficacy of 23 cd/A independent of current density and light output.

We present results and a discussion of highly efficient polymer Light-Emitting Diodes (polymer LEDs, PLEDs). The external quantum efficiency in current standard devices reaches up to 2-4% only. We have explored two routes to enhance this value. In the first route, PEDOT/PSS is replaced with a novel anode or hole injection layer. The efficiency with some Light Emitting Polymers (LEP) is improved significantly, resulting in an efficacy of 35 cd/A for a yellow emitting poly-(para-phenylene-vinylene) and 20 cd/A for a blue emitting poly-(spirobifluorene). We attribute the major improvement compared to standard devices, where about 10 and 5 cd/A are obtained, respectively, to a combination of improved exciton formation efficiency and light out-coupling efficiency, and to less quenching of the radiative decay under actual device operating conditions. In the second route, we developed a new host polymer with high triplet energy such that transition metal-based green-emitting phosphorescent dyes can be used without significant back transfer of triplet excitons to the polymer host. First results using this system showed about 25 cd/A using a soluble green Ir-based emitter. Importantly, all data are obtained in a standard two-layer device of a hole transport/injection layer and the LEP.

In this paper, we present the results of our ongoing work towards achieving high-efficiency high-energy (i.e. green and blue) phosphorescent polymer light-emitting diodes (pLEDs). Green phosphorescent pLEDs have been prepared with efficiencies that are comparable to those of state-of-the art OLEDs based on small molecules (smOLEDs), and the first steps towards extending these concepts to blue phosphorescent pLEDs are shown.