Time delayed electroluminescence overshoot in single layer polymer light-emitting diodes due to electrode luminescence quenching

We investigate the transient response of single layer polymer light-emitting diodes (LEDs) based on poly[2-methoxy, 5-(2'ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV). An electroluminescence overshoot is observed between 1 and 2 μs after turning off the voltage pulse, depending on temperature. We attribute the delay between overshoot and voltage turnoff to detrapping of majority charge carriers at the polymer/cathode interface. Due to the luminescence quenching zone of the metal the luminescence is found to decrease rapidly after the voltage is switched off, but then rise again to up to a third of the steady state value. The overshoot is found to be independent of the applied bias, but is strongly influenced by temperature and pulse length, indicating a trapping process in interfacial traps is responsible for the effect. The overshoot area corresponds to the charge located on interfacial traps near the cathode and scales as a power law with the pulse length. Our findings demonstrate the presence of extrinsic traps in MEH-PPV devices and highlight the importance of distinguishing between the effects of extrinsic and intrinsic traps in polymer LEDs. Spatial correlations of electrons and holes due to their mutual Coulombic field also appear to be important.