Effect of two yellow delta-emitting layers on device performance of phosphorescent white organic light-emitting devices

Phosphorescent white organic light-emitting devices (WOLEDs) with a structure of ITO/TAPC/δ-EML1/mCP:FIrpic/δ-EML2/Bphen/Mg:Ag were fabricated, wherein two ultrathin and host-free emitting layers (EMLs) were formed by using yellow bis[2-(4-tertbutylphenyl)benzothiazolato-N,C2′] iridium (acetylacetonate) [(tbt)2Ir(acac)] and referred to as delta-EMLs (δ-EML1 and δ-EML2). By adjusting the thicknesses of δ-EMLs, a maximum current efficiency of 27.6 cd/A, an external quantum efficiency (EQE) of 10%, together with low efficiency roll-off at high luminance were achieved. The results showed that δ-EML1 played a dominant role on charge carrier trapping, while δ-EML2 had major impact on yellow light emission, which were highly sensitive to the location of δ-EMLs. Furthermore, by introducing 5-nm Au as anode modifying layer, high device efficiency was maintained along with excellent color stability of warm white emission, displaying color coordinates of (0.38, 0.42) and color temperature of 4348 K at a luminance of 7000 cd/m2. Importantly, explanation and analysis for the influence of both ultrathin δ-EMLs and anode modifying layer on device performance were proposed.

► Phosphorescent WOLEDs with two yellow delta-emitting layers (δ-EMLs).
► The ultrathin δ-EMLs were formed by host-free (tbt)2Ir(acac).
► Effects of the δ-EMLs show a dependence on their location in the devices.
► A layer of 5-nm gold was employed as anode modifying layer.
► High device efficiency together with excellent color stability was achieved.