<title>Transient electroluminescence under short and strong voltage pulses</title>

Abstract

ABSTRACT We present high voltage pulsed electroluminescence (EL) measurements on light-emitting diodes (LED) based on thin filmsof poly(p-phenylenevinylene) (PPV) sandwiched between Indium-Tin-Oxide (ITO) and Aluminum electrodes. We observetwo regimes in the LED operation depending on the driving pulsed current density. At low current densities, below 50A/cm2,the pulsed EL follows its DC characteristics with yellow-green emission. Above some threshold current density we observeadditional UV-violet emission (centered at 390 nm, 3.l7 eV); the amplitude of the pulsed UV EL increases exponentiallywith the applied voltage. When the amplitude of the voltage pulses is around 300 V, the current signal exhibits a sharpcurrent peak followed by a dramatic increase in UV EL intensity but only moderate increase of the green emission. Wepropose a possible explanation for the appearance ofthe UV emission upon application of strong electrical pulses. It is due,we believe, to hot carriers in strong fields which partially inhibit the formation of singlet excitons and enhance theprobability for direct inter-band radiative transitions. We show that our very simple device can be operated at current densityas high as 140 A/cm2 and achieve a peak brightness of iO cd/m2 without appreciable degradation.Keywords: electroluminescence, conjugated polymers, high-voltage pulses, PPV, UV emission, light-emitting diode,exciton