Efficient multi-light-emitting layers warm and pure white phosphorescent organic light-emitting diodes with excellent color stability

The warm white organic light-emitting diodes (WOLEDs) consisting of four emissive layers (EMLs) as well as pure WOLEDs consisting of three EMLs have been fabricated by using three primary color phosphors as emitters. Experimental results demonstrated that energy transfer between dopants, direct carriers' trapping and main recombination zone are all important factors in determining the performances of obtained devices. By precisely managing dopants' concentrations and emitting layers' thicknesses, we realized the optimized warm WOLED with a Commission Internationale de l’Eclairage coordinates (CIEx,y) of (0.388, 0.383), giving the highest current efficiency (ηc) of 69.00 cd/A, power efficiency (ηp) of 60.19 lm/W and external quantum efficiency (EQE) of 20.63%. Additionally, by increasing the proportion of blue emission, we also realized the pure WOLED (CIEx,y value is (0.331, 0.385)) giving 60.48 cd/A (EQE = 16.90%, CRI = 72.03) and 51.32 lm/W in highest ηc and ηp, respectively. Especially, the optimal warm and pure WOLEDs exhibited the superior color stability with small CIE discrepancies of (0.020, 0.003) and (0.022, 0.004) in the range of 1–10 mA/cm2, respectively.