Ink formulation of phosphorescent emitter with mixed host and its application in inkjet-printed organic light-emitting devices

For organic light-emitting diodes (OLEDs), inkjet printing technology has attached more attention due to the characteristics of low-cost, easy patterning preparation and high registration accuracy. In this work, we report an ink formulation of green phosphorescence with poly-(N-vinyl carbazole) (PVK) and 1,3-di-9-carbazolylbenzene (mCP) mixed host for applications of inkjet-printed phosphorescent OLEDs (PhOLEDs). The ink properties were optimized by changing assistant solvent and the ratio of binary solvent system of butyl benzoate (BB) and 1,2-dichlorobenzene (o-DCB), generating a homogeneous printed film morphology. Moreover, we investigated the characteristics of inks and interactions between the capillary flow and the Marangoni flow. When the volume ratio of BB:o-DCB is 9:1, the inward Marangoni flow offsets the outward capillary flow due to the increased viscosity and surface tension gradient of ink, forming a coffee ring-free film with low roughness. In addition, to fabricate inkjet-printed multi-layer PhOLEDs, we investigated the printing pitch of PEDOT:PSS and emitting layer, and employed inkjet printing technique to ultimately achieve a maximum current efficiency of 29.58 cd/A at 1000 cd/m2 brightness with optimized ink formula, which is around 74% of the efficiency of a spin-cast reference device. And the current efficiency of PhOLEDs with both poly(3,4-ethylenedioxythiophene):poly-(4-styrenesulphonate) (PEDOT:PSS) and emitting layer prepared with inkjet-printed technique is 14.09 cd/A. This work made a significant contribution to the selection of solvents for inkjet printing and could be applied in fabricating low-cost multi-layered OLED displays using inkjet printing technology.