Electroluminescence overshoot effect in single layer pulsed organic light emitting diodes

After switching off the applied voltage pulse to a single layer organic light emitting diode (OLED) such as ITO/MEH-PPV/Al, initially the electroluminescence (EL) decays rapidly corresponding to the discharging of the parallel plate capacitor of the device, however, after a delay of 1–2 μs, the EL intensity rises again, attains a peak value, and then decays slowly without the application of an electric field. The EL intensity and the position of the peak in this overshoot depend on temperature. The intensity of EL overshoot increases with increasing pulse duration but the position of spike is independent of the pulse duration. A phenomenological theory is explored for the EL overshoot produced during the turn off of single layer OLEDs. The EL transient after the turn off of the external bias can be divided into the following three regions: (i) fast decay region, (ii) delayed peak region, and (iii) slow decay region. The initial fast decay is attributed to the recombination of injected charge carriers; the rise and exponential decay just after the peak in the delayed EL peak are due to the recombination luminescence owing to the movement of detrapped holes with noncorrelated electrons; and the slowly decaying EL following power law t−z (z lying between 1.5 and 2) is due to geminate recombination of the correlated electron–hole pairs. The release time τr of holes trapped at Al2O3/polymer interface, the activation energy Ea for the detrapping of holes from the interface and the time–constant of the OLED circuit, can be determined from the measurement of the time dependence of EL overshoot. A good agreement is found between the theoretical and experimental results.