Phosphorescent white organic light-emitting devices (WOLEDs) with a structure of ITO/TAPC/delta-EML1/mCP:FIrpic/delta-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,C-2'] iridium (acetylacetonate) [(tbt)(2)Ir(acac)] and referred to as delta-EMLs (delta-EML1 and delta-EML2). By adjusting the thicknesses of delta-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 delta-EML1 played a dominant role on charge carrier trapping, while delta-EML2 had major impact on yellow light emission, which were highly sensitive to the location of delta-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/m(2). Importantly, explanation and analysis for the influence of both ultrathin delta-EMLs and anode modifying layer on device performance were proposed. (C) 2013 Elsevier Ltd. All rights reserved.