Improving CRI of white phosphorescence organic light-emitting diodes by controlling exciton energy transfer in the planar heterojunction

Abstract

Abstract In four-color phosphorescent white organic light-emitting diodes (WOLEDs), the thickness ratio of the planar heterojunction spacer (NPB/mCP or NPB/TPBi) between green ultrathin emitting layer (UEML) and yellow UEML can change the capability and direction of exciton energy transfer in this heterojunction. When the thickness (y) of TPBi is 1.5 nm, the value of color rending index (CRI) increases to 93 in device B22 with NPB/TPBi spacer because that the position of recombination zone (RZ) and direction of exciton energy transfer in device B22 are different from that in device A without the planar heterojunction spacer. With little decline for CRI from 93 to 91, the luminance can be improved by reducing the FIrpic doping concentration (z %) in the blue EML (close to electron transport layer) of device C2 based on the structure of device B22. As z is 3.3, the maximum luminance is 36230 cd/m2, the shift of CIEx,y is (0.0440, 0.0029) from 5.5V to 8.5V in device C2.