Nanoscale thickness and roughness control of gravure printed organic light emitting layer with poly(N-vinyl carbazole) and Ir(ppy)3.

A nanoscale organic light emitting layer of PVK/Ir(ppy)3 was formed by the gravure printing process for the fabrication of organic light emitting diodes (OLEDs) in this work. The control of the nanoscale thickness and roughness by gravure printing is known to be challenging. The thickness and roughness characteristics were investigated for various solvents and printing speeds in this work. The OLED device fabricated in this work has a layer structure of glass/ITO/PEDOT: PSS/PVK + Ir(ppy)3-active layer/LiF/Al. PVK (poly(N-vinyl carbazole)) was used as the host and Ir(ppy)3 as the green-emitting dopant. The thickness of the active layer was controlled by the multiple-printing technique. The effects of the solvent on the surface roughness and OLED brightness were investigated with three different widely used solvents, viz. chloroform (CF), chlorobenzene (CB), and 1,2-dichlorobenzene (DCB). DCB showed the smoothest surface and the best brightness. The low surface roughness obtained with DCB is attributed to its relatively low evaporation rate. We also investigated the effect of the printing speed. In the case of chloroform, the thickness was inversely proportional to the printing speed, because of its high evaporation rate. In the case of DCB, the thickness was proportional to the printing speed. When the evaporation rate is low, a higher rolling speed was preferred for the formation of a thicker layer. The evaporation rate is believed to affect the surface morphology of the active polymer material and the control of the evaporation rate is considered to be an important factor in the gravure printing of nanoscale organic layers.