Space-charge limited current and electroluminescence in conjugated polymer-based LEDs

We have studied the current versus voltage (I-V) and the luminescence versus voltage (EL-V) characteristics in PPV-based LED devices for different film thickness, d. For voltages below the EL threshold voltage, the I-V curves exhibit a linear regime (I similar to V) at low voltages and quadratic (I similar to V-2) regime for higher voltages. The later is attributed to the space charge limited current mechanism assuming a single shallow trap level. The fit of the model to the experimental results is very good with a single fitting parameter, namely, the effective mobility. The extracted mobilities are compared to those measured independently by time-of-flight (TOF) photoconductivity. The SCLC model gives a reasonable explanation for the voltage threshold in EL.