Margreet M. de Kok

Improving color purity and stability in a blue emitting polyfluorene by monomer purification

Monomer purification has been tailored to ensure reduced levels of fluorenone defects in the corresponding polyfluorene, which results in greater resistance to fluorescence degradation when exposed to high temperatures and demonstrates pure blue, more stable electroluminescence.

Ink-jet printing of polymer light-emitting devices

Spin coating is a suitable technique for the fabrication of monochrome light-emitting polymer devices. For color displays, however, it is not the optimal solution when different polymers are applied. In principle, there are several technologies available for patterning light-emitting polymers. In this paper we discuss the advantages of drop-on-demand ink-jet printing over other printing methods. Special attention is given to some fundamental aspects of the printing process, such as drop formation and pixel filling. Examples of both monochrome and full color ink-jet printed passive matrix displays will be discussed.

Hole transport in blue and white emitting polymers

Hole transport in a blue emitting polyspirobifluorene polymer and in a white emitting polymer consisting of a polyspirobifluorene backbone and two dyes (green and red) was studied. The hole mobility was measured using the time-of-flight method as a function of the electric field and temperature in the range 105−106 V/cm and 285−335 K, respectively. The observed temperature and electric field dependence of the hole mobility was analyzed in the framework of the Bassler disorder model. Also, steady-state current-voltage characteristics were measured over a wide range of electric fields and temperatures and the hole mobility was determined. Our measurements have shown that the hole mobility in the white emitting polymer is the same as in the blue emitting polymer. The performed disorder model analysis gives the same values for the effective energetic disorder (115 meV) and for the positional disorder (1.85) for both polymers. Therefore, we have concluded that the added green and red dyes do not act as hole tr...

High efficiency polymer LEDs: triplets and novel devices

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.

Technology and materials for full-color polymer light-emitting displays

The status of the development of full-color polymer light emitting diodes will be presented. The focus of current materials research is on state-of-the-art red, green, and blue light-emitting polymers (LEP) with high efficiency, optimum color points, low driving voltages and long lifetimes in devices. A general overview of the progress of red, green and blue LEP lifetimes and efficiencies will be given and compared to requirements for both full-color passive and active matrix-displays for mobile display applications. Further, the status of ink-jet printing of LEPs for the industrialization of full-color displays will be discussed, and a comparison of the performance of spin coated and inkjet printed devices will be presented. In addition, two material-related topics studied recently will be discussed; namely, the lifetime of blue light-emitting devices correlated to processing, anodes, cathodes and the blue polymers themselves, and second, the consequences of pulsed-driving schemes on efficiency and lifetime.

1H and13C NMR Full Assignment of a Series of Precursor Polymers Derived from the Sulfinyl Route Towards Poly(p-phenylene vinylene)

A series of precursor polymers towards poly(p-phenylene) vinylene) is analyzed by different complementary NMR techniques. Resonance assignments were achieved by the use of one- and two-dimensional NMR techniques, such as 1 H, 13 C, APT, 2D-HETCOR, FLOCK, COSY and INADEQUATE. The series consists of the sulfinyl precursor poly[p-phenylene(1-butylsulfinyl)ethylene] and the derived sulfanyl (poly[p-phenylene(1-butylsulfanyl)ethyelen] and sulfonyl derivative (poly[p-phenylene(1-butylfulsonyl)ethylene]). Chiral induction during polymerization of sulfinyl monomers was quantified by high resolution 13 C NMR.

Complete assignment of 1H and 13C NMR spectra of a set of sulphinyl and sulphonyl monomers toward poly(p-phenylenevinylene) precursors

The complete assignment of the 1H and 13C NMR spectra of sulphinyl and sulphonyl monomers toward poly(p‐phenylenevinylene) precursors is presented. For all compounds the same protocol could be used. Resonance assignments were achieved by the use of one‐ and two dimensional NMR techniques such as 1H, 13C APT and 2D HETCOR (J = 8 Hz, J = 140 Hz), INADEQUATE and COSY. Copyright

Status of red, green and blue light emitting polymers for passive matrix displays

An overview of the requirements for full color passive matrix displays and their implications for the light emitting materials will be presented. Using the performance of light emitting polymers tested in Philips devices the status of the light emitting polymers is reviewed. It will be shown that the performance of light emitting polymers is at the edge of being acceptable for practical applications. Red and green light emitting polymers can already be used for certain monochrome applications. However, for the high-resolution displays used in mobile telecom applications the efficacy for red and the lifetime for green are still somewhat low. Optimization routes for further improvement in terms of efficacies and lifetimes for red and green are identified. The peformance of blue light emitting polymers has rapidly improved over the last year, but the lifetime is still too short for full color applications. Improvement routes for the blue light emitting polymers and its device structure are outlined.