Blue electroluminescence with ladder-type poly(para-phenylene) and para-hexaphenyl

Conjugated polymers and oligomers are very interesting materials, with a number of possible electronic, optoelectronic and photonic applications. New synthesis techniques allow the preparation of extremely pure soluble ladder-type poly(para-phenylenes) (LPPPs), showing a steepness at the band edge comparable to conventional semiconductors. One consequence of this high purity is an improved photoluminescence quantum yield, around 30% for thin films. Oligomers of poly(poly-phenylene), like para-hexaphenyl (PHP) can be produced with extremely high purity, either as homogeneous disordered layers or well-ordered thin films, which show a high photoluminescence quantum yield in excess of 30%. All these materials are applied as the active layer in light-emitting devices (LEDs), giving bright blue light electroluminescence (EL) emission, and we report on their performance and efficiencies. For the LEd device with LPPP as the active layer, we observe the maximum of the EL emission around 460 nm, whereas for the PHP-based EL devices the emission maximum is located between 400 and 450 nm. The light emission of these EL devices is very high and exceeds a luminance of 2000 cd/m2 for a typical PHP multi-heterolayer EL device. For the overall EL quantum efficiency we obtain values up to 4% in homolayer EL devices. Ordering effects in thin films of conjugated materials, polymers and oligomers, strongly influence their optical properties. This ordering occurs in the active layers of EL devices and determines the threshold field for EL mission. Finally, we demonstrate that the emission colour of our EL devices can be controlled by the driving electric field.