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 10(16) 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 converted 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. (C) 1997 Elsevier Science S.A.