Studies of kinetics of charge carrier recombination in organic light-emitting diodes based on beryllium complexes by transient electroluminescence

The kinetics of the charge-carrier recombination in double-layer organic light-emitting diodes composed of indium-tin-oxide/N,N'-di(naphthalene-1-yl)-N,N' -diphenyl-benzidine/beryllium complexes/LiF/Al were studied by transient electroluminescence (EL). Two blue-emitting materials of bis[2-(2-hydroxyphenyl)-pyridine]beryllium (Bepp2) and bis[2-(2-hydroxytoluenyl)-pyridine]beryllium (Betp2) were used as the emissive layer. It was found that the EL efficiency of the Bepp2-based devices was higher than that of the Betp2-based devices. The improvement of the EL efficiency was well interpreted by the results that the Bepp2 shows higher electron mobility and electron-hole recombination coefficients (γ) than Betp2, which are determined from the onset of the transient EL after a rectangular voltage pulse is applied, and from the long-time component of the temporal decay of the EL intensity after the voltage pulse is turned off. The electron mobility and the recombination coefficients γ of the Bepp2 reached 6 x 10-6 cm2 V-1 s-1 and 7.3 x 10-12 cm3 s-1, respectively, which are approximately twice the magnitude of the Betp2