The colour-tuning effect of 2,9-dimethyl-4,7-diphenyl-1, 10-phenanthroline in blue-red organic light-emitting devices

This paper reports the fabrication and measurement of organic light-emitting devices comprised of indium-tin-oxide (ITO)/4,4′,4″- tris(N-carbazolyl)-triphenylamine (TCTA, 8 nm)/2-pyrenyl-9-phenyl-9- pyrenylfluorene(2P9PPF, 30 nm)/ bathocuproine (BCP, with different thickness)/Alq₃ : DCJTB (DCJTB ≤ 4-(dicyanomethylene)-2-t-butyl- 6(1,1,7,7- tetramethyljulolidyl-9-enyl)-4H-pyran, Alq₃ ≤ tris(8-hydroxyquinolino) aluminium(III), 2% in mass ratio, 30 nm)/Mg : Ag(250 nm). The dependence of electroluminescence (EL) spectra on the BCP layer thickness and operating voltage has been investigated quantitatively. It is shown that the emission colour of the devices changes from red to blue at 8 V when the BCP layer thickness changes from 1 to 10 nm. The emission colour of the devices also varies with the applying voltage even in a given device. The ratio of blue emission originating from 2P9PPF to the red emission originating from DCJTB increases with the applying voltage. Based on the hole blocking effect of the BCP layer, we deduced the dependence of this ratio on the BCP layer thickness and simulated the experimental result well. It is also proposed that the variation of the EL spectrum with the voltage can be attributed to the varying hole blocking effect under the varying electric field, which resulted in a recombination zone shift, and the exciton dissociation effect in the electric field.