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Abstract:

The engineering of new electronic and photonic devices requires materials with unique properties. To find these, scientists increasingly harness the versatility of organic chemistry. Three key advantages of organic semiconductors over their inorganic counterparts are a direct result of the chemical diversity of the molecular world: Processability from solution, chemical bandgap tuning, and the ability to self-organise. The first two advantages are now widely exploited, but the third has yet to receive more attention. One area where the use of self-organising materials for organic optoelectronics is comparatively advanced is the field of polarized electroluminescence (EL). A polarized lightsource would lend itself well to use as a backlight for liquid crystal (LC) displays. In this contribution, we report on the development of polarized lightsources based on conjugated LC polymers. This approach is now fairly advanced, and polarization ratios in excess of 20 have been demonstrated. As an alternative approach, we have recently started to extend the versatile reactive mesogen concept to organic semiconductors. We report on first results on this new class of materials.

Abstract:

We have studied the electrical fields in single-layer polymer light-emitting diodes (LEDs) using electroabsorption (EA) spectroscopy. The materials under investigation were PPV derivatives, namely poly(2,5-dialkoxy-para-phenylene vinylene) and a precursor route poly(2-methoxy-5-ethyl(2′-hexyloxy)-para-phenylene vinylene). The applied bias generates an opposing electric field that decays over time. The magnitude of the reversed field increases as the applied DC bias is increased and can be as high as one-third of the applied external field. The development of the internal field is less pronounced in vacuum than in air and it vanishes for devices prepared and tested under inert conditions.