Optimisation of graded-mixed transport organic light emitting diode via mixed-source thermal evaporation technique

Continuously graded-mixed transport organic light emitting diodes (CGM-OLED) have been fabricated using the mixed-source thermal evaporation method. Different CGM composition profiles can be controlled by the heating ramp rate, and validated by the X-ray photoelectron spectroscopy depth-profiling measurements. With external charge injection, the transport sub-layers formed in-situ at both sides of the CGM region, can act to confine the charges in the central emissive region, or recombination zone; adding charge-blocking layers may degrade the OLED performance. An optimised Alq3-based CGM-OLED was operated at low turn-on voltage of 2.8 V, which delivered maximum luminance, luminance efficiency and power efficiency of 21,135 cd/m2, 5.1 cd/A and 3.1 lm/W, respectively, and retained >80% of the maximum luminance efficiency between 100 and 15,000 cd/m2.