Simple white organic light emitting diodes with improved color stability and efficiency using phosphorescent and fluorescent emitters

White organic light emitting diodes (WOLEDs) with both phosphorescent and fluorescent emitting layers (EML) usually adopt an interlayer between them to achieve high efficiency by preventing mutual quenching, but insertion of the interlayer causes a higher operating voltage as well as additional fabrication steps. Here, we demonstrate that simple-structure WOLEDs without an interlayer could be achieved using the combination of phosphor-sensitized-fluorescent red and phosphorescent blue EMLs. In addition, the main cause of the color shift with increasing current density was identified, and the color shift of the WOLED was successfully suppressed by properly balancing emission from the red and blue EMLs. Consequently, a maximum external quantum efficiency of 6.2% (a current efficiency of 14.3 cd/A) and very stable color coordinates of (0.32 +/- 0.01,0.42 +/- 0.002) were achieved. However, the elimination of an interlayer for the combination with a fluorescent blue EML causes about 50% decrease in the efficiency and a large change in the color coordinates with the driving current density. (C) 2008 American Institute of Physics.