Kinetics of charge carrier recombination in organic light-emitting diodes

The temporal behavior of the electroluminescence (EL) from thin-film indium-tin-oxide (ITO)/aromatic diamine (TPD)/Al(III) 8-hydroxyquinoline (Alq3)/Mg/Ag light-emitting diodes (LEDs) upon the application of a rectangular driving voltage is analyzed in terms of the electron-hole recombination on the TPD/Alq3 interface. From the long-time component of the temporal decay of the EL intensity the electron-hole recombination coefficient γ=(1.1±0.5)×10−10 cm3/s is determined in good agreement with the value expected on the basis of the Langevin theory of recombination with the recombination process controlled by the motion of holes in the hole-transporting TPD layer of the LED.