Capacitance-voltage characterization of polymer light-emitting diodes

The capacitance-voltage (C-V) characterization of polymer light-emitting diodes (PLEDs) employing poly[5-(2(')-ethylhexyloxy)-2-methoxy-1,4-phenylene vinylene], as the light-emitting layer are reported. Several metals, such as calcium (Ca), aluminum (Al), and gold (Cu) were used as the cathode in order to investigate the influence of the charge injection on the C-V characteristics. Under forward bias, the capacitance increases with majority charge carrier injection into the polymer layer, and, afterwards, decreases upon minority charge carrier injection which results in recombination of electrons and holes in the active polymer layer. The increase in the value of capacitance follows the same dependence as the increase in the value of current density through the device, which suggests that the capacitance depends not only on the amount of charge trapped in the polymer near the interface, but also, and mainly, on the amount of charge injection into the polymer layer from the electrodes. The C-V behavior of PLEDs with added amount of ionic defects was also studied. The capacitance values are higher for devices with higher amount of added impurities in form of ionic electrolytes, and, the increase in the capacitance under forward bias also depends on the ionic defect concentration. (C) 2005 American Institute of Physics.