Two types of electroluminescence (EL), injection-controlled (IC) with tau(rec)>tau(T), and volume-controlled (VC) with tau(rec)<tau(T), in organic light-emitting diodes (LEDs) are discussed using the notions of recombination (tau(rec)) and transit (tau(T)) times of charge carriers. The results of experimental and theoretical studies presented show that the EL intensity (Phi(EL)) is, in general, a nonlinear function of the current density (j) in an EL diode. Often it can be approximated by a power-type function Phi(EL)alpha j(n), with the power, n, dependent on LED structure, the nature of light-emitting material, injection and transport mechanisms of the charge carriers. In the LEDs with carrier mobilities independent of electric field 1 less than or equal to n less than or equal to 2 for both ICEL and VCEL operation modes. In the ICEL, n=1+omega(1)/omega(2), where omega(1) and omega(2) are quantities characterizing various charge injection mechanisms into the EL sample. Field-dependent mobility and charge carrier trapping lead Phi(EL) to be a sublinear or supralinear function of j. Selected examples of experimental Phi(EL)(j) relationships on single-layer ITO/Alq(3)/Mg/Ag (8-hydroxyquinoline aluminium complex, Alq(3)), double-layer ITO/TPD/Alq(3)/Mg/Ag (aromatic diamine, TPD), ITO/QAC/Alq(3)/Mg/Ag (quinacridone, QAC), and triple-layer ITO/QAC/Alq(3)/PBP/Mg/Ag (perylene bisimide pigment, PBP) are presented briefly to illustrate various types of operation modes of organic LEDs.
