Charge Carrier Tunneling in the Light-Emitting Diodes of Poly (p-phenylene) Thin Films

We have studied the temperature dependence of the current-voltage (I-V) and the electroluminescence-voltage (EL-V) characteristics in the blue light-emitting diodes of vacuum-deposited poly (p-phenylene) (PPP) thin films in the temperature range between 14 and 290 K. The onset of the EL occurs at an electric field of about 7×107 V/m, independent of the thickness of the PPP layer. The I-V and EL-V dependences show very weak temperature dependences and fit very well with the Fowler-Nordheim tunneling formula. The results suggest that charge carrier injection is a tunneling process through an energy barrier of about 0.6∼0.8 eV in indium tin oxide (ITO)/PPP/Al devices. I