[Effect of spacer on red and green phosphorescent organic light-emitting devices]

We have investigated the performances of organic light-emitting diodes (OLED) with different spacer, the structure was fabricated as ITO/MoO3 (40 nm)/NPB(40 nm)/TCTA(10 nm)/CBP:GIr114:R-4B2%(30 nm)/spacer (3 nm)/CBP: GIr114%:R-4B2%(30 nm)/BCP(10 nm)/Alq3 (40 nm)/LiF(1 nm)/Al(100 nm), the spacers were CBP, TCTA, TPBI and BCP separately, GIr1 and R-4B were green and red phosphorescent dye respectively. The results showed that compared to the reference device utilized CBP as the spacer layer, TCTA, TPBI and BCP had higher current efficiency in excess of 59%, 79% and 93%, the maximum current efficiency of 16.91 cd x A(-1) was achieved with BCP as the spacer at voltage of 5 V, TPBI and BCP as the spacer layer obtained the higher current density and lower efficiency roll-off. We attributed to these results to the follow reasons, the first was that carriers and excitons were limited to a narrow recombination region because of TCTA with higher LUMO energy level and triplet energy, which improved the probability of carriers recombination, in addition, more serious quenching at higher current density. The second reason was that TPBI and BCP had the higher HOMO energy level and electron mobility, which broadened excitons recombination zone. In addition, the spacer layer caused the accumulation of electrons or holes and the formation of high space electric field, leading to carrier injection and transport more effectively. In particular, we obtained a better stability of phosphorescent organic light-emitting diodes since the way for the red and green co-doped with host material.