TITLE: Electrical degradation of polymer light emitting diodes AUTHORS (FIRST NAME, LAST NAME): Quan Niu, Paul W. M. Blom, Irina N Craciun INSTITUTIONS (ALL): Max-Planck-Institut für Polymerforschung, Mainz, Germany.
ABSTRACT: In recent years the understanding on the operation mechanism of polymer light-emitting diodes (PLEDs) has reached a level such that its electrical characteristics can be predicted when the electron- and hole transport properties are known.1 Their efficiency is determined by a competition between radiative Langevin recombination and non-radiative trap-assisted recombination. This knowledge can now be used to investigate the mechanisms of electrical degradation of PLEDs. Typically, an increase of driving voltage and a decrease of luminance and efficiency is observed under (accelerated) degradation conditions (i.e. constant high current). Degradation of the PLED can be due to a decrease of charge carrier mobility, change of the charge injection of the electrodes, or the formation of charge traps. In order to disentangle these mechanisms we have performed transient electroluminescence measurement to study the hole transport during degradation. In order to monitor the competition between Langevin and trap-assisted recombination we have studied the dependence of the open-circuit voltage Voc of a PLED as a function of light-intensity. Furthermore, we also investigated the luminance ideality factor of the PLEDs in the diffusion regime as a function of degradation. We observe that the (transient) hole transport is not affected, but that there is an increased contribution of trap-assisted recombination during device degradation as well as an increase of the built-in electric field. These measurements are consistent with the formation of hole traps near the cathode during electrical degradation. 1 M. Kuik, G.A.H. Wetzelaer, H.T. Nicolai, N.I. Craciun, D.M. de Leeuw, P.W.M. Blom, Adv. Mater. 26, 512–531 (2014)