Study on spectral stability of white organic light-emitting diodes with mixed bipolar spacer based on ultrathin non-doped phosphorescent emitting layers

Abstract We have demonstrated color stable white organic light-emitting diodes (WOLEDs) with bipolar mixed spacer based on ultrathin non-doped phosphorescent emitting layers (EMLs). The introduced bipolar mixing spacer which contributes to the uniform distribution of recombined excitons plays an important role in stabilizing the spectrum. Through different emitters orders: blue/green/orange (named B-G-O) and orange/green/blue (named O-G-B), the thickness ratio of the EML at the anode and cathode side is proved to be the core factor on spectral stability. The spectral stability is attributed to two key mechanisms: sequential energy transfer offsets the change of emission intensity caused by exciton recombination zone (RZ) drift under different voltages, and direct carrier trapping effect compensates the emission intensity at low voltage. The resulting three-color WOLED shows an insignificant chromatic coordinate change of (0.001, 0.001) when the luminance increases from 1000 cd/m2 to 10000 cd/m2. This discovery provides a significant method for achieving high-performance WOLED with superior color stability.