Controlling the polarity and viscosity of small molecule ink to suppress the contact line receding and coffee ring effect during inkjet printing

The film quality, especially the film homogeneity and uniformity, plays an important role in organic light emitting devices (OLEDs) efficiency and lifetime made by inkjet printing technology. However, the contact line receding behavior and coffee ring effect always lead to the solute materials deposited in the two edges of linear pixels during the drying process, which influence the film uniformity and OLED device performance. By adding the high viscosity co-solvents, reducing the capillary flux is proposed to suppress the 'coffee ring effect' to form a homogeneous film for organic small molecule material (DR301). Firstly, cyclohexanone (CYC) with polar group was selected as a main solvent to pin up the contact line to suppress the contact line receding. Secondly, cyclohexylbenzene (ChB) and benzyl alcohol (BA) with high boiling point and high viscosity were chosen as the co-solvents to decrease the capillary flux. The low capillary flux decreased the transferred quantity of solvent from the center to the edge to eliminate the coffee ring effect. By optimizing the ratio of the main solvent and co-solvents, a uniform film was obtained at a CYC/ChB/BA = 60/35/5 ratio, resulting in the inkjet-printed device performance with 3.09 cd/A current efficiency and 1.87 lm/W power efficiency.