Control of impurities in PPV light-emitting devices

The influence of different substrates used for the fabrication of poly(p-phenylene vinylene) (PPV) light-emitting devices on the device characteristics is investigated with different experimental techniques, like current-voltage, brightness-voltage and capacitance-voltage measurements. Using thermally stimulated currents we determine the energetic depth and density of states created by doping of PPV during device fabrication. In devices prepared on indium-tin oxide (ITO) substrates doping with InCl3 leads to states with a depth of about 0.15 eV and an ionized acceptor concentration in excess of 1016 cm−3. These carriers are mobile and form a depletion layer of width 120 nm when a metal with low work function, like Al. is used as cathode. This doping is responsible for the observed Schottky diode behaviour in PPV devices on ITO. With fluorine-doped tin dioxide as transparent hole-injecting contact, trap energies increase slightly to 0.2 eV and the ionized acceptor concentration is lowered by a factor of five. The lower doping concentration leads to an increase of the depletion layer width to about 270 nm and thickness-dependent device characteristics. For PPV convened on gold no doping is detectable with capacitance-voltage measurements and thermally stimulated currents. Photoluminescence measurements show a significant quenching of fluorescence in PPV converted on ITO. We regard this as an important limiting factor for single and heterolayer devices with PPV as emissive material.