Injection-controlled and volume-controlled electroluminescence in organic light-emitting diodes

Two types of electroluminescence (EL), injection-controlled (IC) with τrec>τ, and volume-controlled (VC) with τrec < τT, in organic light-emitting diodes (LEDs) are discussed using the notions of recombination (τrec) and transit (τT) times of charge carriers. The results of experimental and theoretical studies presented show that the EL intensity (ΦEL) 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 ΦELαjn, 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 ≤ n≤ 2 for both ICEL and VCEL operation modes. In the ICEL, n= 1 + ω1/ω2, where ω1 and ω2 are quantities characterizing various charge injection mechanisms into the EL sample. Field-dependent mobility and charge carrier trapping lead ΦEL to be a sublinear or supralinear function of j. Selected examples of experimental ΦEL(j) relationships on single-layer ITO/Alq3/Mg/Ag (8-hydroxyquinoline aluminium complex, Alq3), double-layer ITO/TPD/Alq3/Mg/Ag (aromatic diamine, TPD), ITO/QAC/Alq3/Mg/Ag (quinacridone, QAC), and triple-layer ITO/QAC/Alq3/PBP/Mg/Ag (perylene bisimide pigment, PBP) are presented briefly to illustrate various types of operation modes of organic LEDs.