Improving the uniformity of the inkjet-printed polymer film in a bank by Marangoni flow and contact line sliding

Abstract

Inkjet printing is considered to be the most promising means of pixelated, large-scale processing of organic light-emitting diode (OLED) panels. However, the uniformity of the films printed in the banks and the device performance are major challenges in printed OLEDs. Herein, we propose controlling the uniformity of the film printed in the banks through Marangoni flow and contact line sliding. To verify our idea, three kinds of poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) ink with different Marangoni flow were used, i.e., toluene (TOL)/cyclohexylbenzene (CHB), TOL/butyl benzoate (BB) and TOL/1-chloronaphthalene (1-CN) inks. The inks were printed into the bank (dyed by rhodamine B). The motion of the contact line and the liquid surface profile changes at different times were revealed by the change in the luminescent intensity of F8BT ink in the bank (recorded by laser scanning confocal microscopy) since the intensity of rhodamine B in the bank is unchanged. For the TOL/CHB ink, the contact line can slide downward along the vertical direction of the bank during the drying process since the surface tension difference between TOL and the co-solvent and the relative value of the Marangoni flow (Δγ/η) reach a suitable value. This led to the surface profile of the liquid film remaining horizontal. Thus, a dried film with higher uniformity is obtained. However, for TOL/BB and TOL/1-CN inks, the contact line is pinned on the bank at some stage during the drying process. Thus, the final profile exhibited mount shape and ring shape, leading to poor uniformity of the dried films. The printed OLED device with TOL/CHB ink has better film uniformity than the other two inks, showing the best performance with a maximum luminance of 11 508.8 cd m⁻², current efficiency of 4.7 cd A⁻¹ and an EQE of 1.36%.