Color tuning in inverted blue light-emitting diodes based on a polyfluorene derivative by adjusting the thickness of the light-emitting layer

Abstract

Inverted blue polymer light-emitting diodes (IPLEDs) were fabricated by using polyethylenimine ethoxylate (PEIE) as the interlayer and a polyfluorene derivative (PF-FSO10) as the emissive layer, with the device structure of ITO/ZnO/PEIE/PF-FSO10/MoO₃/Al. Upon insetting the PEIE interlayer, the maximum luminous efficiency (LE_max) of the IPLEDs increases by about three orders of magnitude, compared with that without. The enhancement of device performance could be attributed to the effective electron-injecting/hole-blocking ability of PEIE, and less exciton quenching at the interface of ITO/ZnO/PEIE. As the thickness of PF-FSO10 is tuned, the color coordinates of the devices can change from sky blue (0.18, 0.26) to deep blue (0.16, 0.07), and the LE_max of the devices can also be improved from 1.77 to 5.7 cd A⁻¹, which should contribute to the micro-cavity effects and the expansible recombination zone, respectively. These results provide an excellent method to improve the efficiency and color purity of the inverted blue-light emitting diodes.