Transient electroluminescence from polymer-based LED devices

Abstract

We report on the observation of switching or 'off-phase' electroluminescence (EL) spikes in thick devices (300 nm to 600 nm) based on poly-2-methoxy,5-(2'-ethylhexoxy)-1,4- phenylenevinylene (MEH-PPV) conjugated polymer sandwiched between Aluminum and Indium-Tin-Oxide (ITO) electrodes. We also report on impedance spectroscopy studies on the same samples. Transient EL measurements were carried out using voltage pulses with pulse width in the range between several microseconds and 10 nanoseconds and voltage pulse height up to 1000 V. The short pulses are obtained using a home-built voltage pulse source consisting of inorganic photo-switch governed by light pulses from Nd:YaG laser. The electrical pulse rise-time is determined by the laser pulse rise-time, which is around 3 nanoseconds. The off-phase EL is manifested by the appearance of two EL spikes at the turn-on and the turn-off edges of the voltage pulse; for very short voltage pulses the two spikes merge. We demonstrate that the EL emission spectrum associated with the off-phase EL is similar, although not identical, to the regular injection EL spectrum of MEH-PPV, suggesting that the off-phase EL is also due to singlet exciton recombination. We show, for the first time, transient EL as a function of the pulse height up to 200 V using pulse width as narrow as 15 nanoseconds. For such strong and short pulses the EL emission is dominated by the off-phase EL mechanism. We provide evidence from impedance spectroscopy that one of the relaxation processes has a characteristic time of the order of magnitude of the time scales observed in the off-phase EL. This may shed light on the relaxation processes in the switching EL.