Green phosphorescent organic electroluminescent devices with 27.9% external quantum efficiency by employing a terbium complex as a co-dopant

Bright green organic electroluminescent devices with a low operating voltage were realized by utilizing a trivalent terbium complex as a co-dopant within an electron dominant light-emitting layer. To optimize the concentration of the co-dopant, a series of co-doped single and double light-emitting-layer(s) devices were firstly fabricated and examined. Compared with the reference device, the co-doped devices exhibited a relatively higher brightness, a higher efficiency, and an even lower operating voltage, ascribed to the improved carrier balance and the broadening of the recombination zone. Moreover, co-dopant emission was invisible in all co-doped devices because the low-lying energy levels of the utilized terbium complex make it very difficult for the holes to situate on co-dopant molecules. Consequently, the double light-emitting layer device co-doped with a 0.6 wt% terbium complex showed a dominant emission peak at 503 nm and a maximum brightness, an external quantum efficiency, a current efficiency and a power efficiency up to 183 729 cd m(-2), 27.9%, 89.42 cd A(-1) and 87.74 lm W-1, respectively, were achieved. Meanwhile, at 4.0 V, this device showed a practical brightness of 1000 cd m(-2) and retained the external quantum efficiency and the current efficiency as high as 23.2% and 74.23 cd A(-1), respectively.