Inkjet Printing of Mixed-Host Emitting Layer for Electrophosphorescent Organic Light-Emitting Diodes.

We have investigated the impact of the ink formulation on the properties of an inkjet-printed small molecular mixed host in a phosphorescent organic light-emitting diode (PhOLED). Host solubility, film roughness, and device efficiency improved by blending tris(4-carbazoyl-9-ylphenyl)amine (TCTA) with pyrido[3',2':4,5]furo[2,3- b]pyridine (3CzPFP). At a host ratio of 60:40 (TCTA/3CzPFP), the brightness increased by 33%, the efficiency roll-off at 1000 cd/m2 dropped to well below 10%, and the luminance half-lifetime (LT50) improved by 80% in comparison to the device with a single host (100% TCTA). When the optimized ink was deposited by inkjet printing, a maximum external quantum efficiency of 8.9% and a current efficiency of 28.8 cd/A were achieved at 1000 cd/m2 brightness. This amounted to around 84% of the efficiency of a spin-cast reference device. The obtained results provide a blueprint for designing enhanced PhOLEDs with inkjet-printed mixed hosts.