Analysis of pinning effect at the Alq3/metal interface in organic light emitting diodes

In order to study the improvement of electrons injection in organic light emitting diodes (OLEDs), we have realized ITO/PEDOT/PF6/Alq3/cathode OLEDs with four different metallic electrodes: Ca, Mg, Al, and Ag. The cathode barrier increment/decrement has been estimated between the four kinds of devices by using the thermionic injection theory. These results have allowed to see a very low dependence of the cathode barrier on the metal work function, explained in terms of pinning at the cathode interface. The authors propose that this pinning effect is due to the presence of a distribution of states at the Alq3/metal interface; the application of the Cowley-Sze model has returned a density of states of about 6 x 10(14) eV(-1) cm(-2), high value satisfying the observed independence of the cathode barrier on the employed metallic electrodes. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4731719]