High CRI and stable spectra white organic light-emitting diodes with double doped blue emission layers and multiple ultrathin phosphorescent emission layers by adjusting the thickness of spacer layer

The color rendering index (CRI) and Commission Internationale de L'Eclairage coordinate (CIEx,y) are significant parameters for the white organic light-emitting diodes (WOLEDs). We fabricate a series of the phosphorescent WOLED devices, in which there are two doped phosphorescent layers (for blue emission) on both sides of the emitting layer (EML) and three ultrathin phosphorescent layers (for red, orange and green emission) separated by the spacer layers (mCP) in EML. One doped blue emission layer near the hole transport layer contributes to CRI enhancement by increasing the doping concentration (x %) of the blue phosphorescent dye because Dexter energy transfer increases from the blue emission to the red emission. When x is 20, CRI reaches to 90 at 5 V in device A3. Based on device A3, CRI can be further improved to 94 in device B1 with a thinner thickness of the spacer layer (y = 1 nm) between the green emission layer and orange emission layer in EML because Dexter energy transfer from the green emission layer to the orange and red emission layer becomes easier. Although CRI declines, the spectra become more stable by increasing the thickness of the spacer layer (y = 3 nm) in device B2 or decreasing the thickness of the green emission layer (z = 0.05 nm) in device C, in which the CIEx,y coordinate shift is only (0.0214, 0.0102) from 5 V to 8 V.