Influence of plasma treatment of ITO surface on the growth and properties of hole transport layer and the device performance of OLEDs

Surface energy of indium tin oxide (ITO) surfaces treated by different plasmas, including argon (Ar–P), hydrogen (H₂–P), carbon tetrafluoride (CF₄–P), and oxygen (O₂–P), was measured and analyzed. The initial growth mode of hole transport layers (HTLs) was investigated by atomic force microscope observation of thermally deposited 2 nm thick N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) on the plasma treated ITO surfaces. The results show that different plasma treatments of ITO influence the growth of HTLs in significantly different ways through the modification of surface energy, especially the polar component. The O₂–P and CF₄–P were found to be most effective in enhancing surface polarity through decontamination and increased dipoles, leading to more uniform and denser nucleation of NPB on the treated ITO surfaces. It was further found that increased density of nucleation sites resulted in a decreased driving voltage of OLEDs. Under the same fabricating conditions, a lowest driving voltage of 4.1 V was measured at a luminance of 200 cd/m² for the samples treated in CF₄–P, followed by the samples treated in O₂–P (5.6 V), Ar–P (6.4 V), as-clean (7.0 V) and H₂–P (7.2 V) plasma, respectively. The mechanisms behind the improved performance were proposed and discussed.