Injection, transport and recombination of charge carriers in organic light-emitting diodes

Depending on the magnitude of the energy barrier for charge carrier injection from the electrode(s) the current flowing through a light-emitting diode (LED) can be either space charge-limited or injection-limited The first alternative, reflected in j(E) characteristics obeying Child's law, requires the electrode to be ohmic. Various injection models are discussed, favoring a hopping description for the escape of a charge carrier from the image charge potential in an energetically disordered system. Information concerning the mobility of charge carriers can be inferred from time of flight studies, from space charge limited currents, as well as from transient absorption due to the momentary space charge stored inside the sample. A simple model, building on the proven validity of Langevin-type electron hole recombination in organic solids, is able to explain the recombination yield in single-layer LEDs as a function of the cell current. A more elaborate analytic theory is described for bilayer systems in which internal energy barriers give rise to charge accumulation and to a concomitant redistribution of the electric field. (C) 1998 John Wiley & Sons, Ltd.