ABSTRACT
Pretreatment of the indium tin oxide (ITO) surface is generally adopted to improve the charge injection and device performance in the fabrication of organic light-emitting diodes (OLEDs). For the common approaches of surface treatment, such as oxygen plasma treatment, self-assembled monolayer (SAM) adsorption, and the PEDOT:PSS coating, different effects on the device lifetime were observed. A distinctly different driving voltage change with device operation time was obtained and was correlated with the device lifetime. The fast increase in driving voltage for devices based on oxygen-plasma-treated ITO is attributed to the work function change as a result of the change in the composition of the interface with device operation, whereas a rather stable work function for SAM-modified ITO is suggested due to the permanent dipoles associated with the monolayer and the protecting effect of the covalently bound monolayer on the surface composition.
ABSTRACT
Self-assembled monolayers (SAMs) of binary mixtures of 1-butylphosphonic acid and the trifluoromethyl-terminated analogue (4,4,4-trifluoro-1-butylphosphonic acid) were formed on ITO surfaces to tune the work function of ITO over a range of 5.0 to 5.75 eV by varying the mixing ratio of the two adsorbents. The mixed SAM-modified ITO surfaces were used as the anode in the fabrication of OLED devices with a configuration of ITO/SAM/HTL/Alq3/MX/Al, where HTL was the NPB or BPAPF hole-transporting layer and MX was the LiF or Cs(2)CO(3) injection layer. It was shown that, depending on the HTL or MX used, the maximum device current and the maximum luminance efficiency occurred with anodes of different modifications because of a shift in the point of hole/electron carrier balance. This provides information on the charge balance in the device and points to the direction to improve the performance.
ABSTRACT
Self-assembled monolayers (SAMs) of binary mixtures of n-decanethiol and the fluorinated analogue (3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluoro-1-decanethiol) were formed on silver surface. The film structure was characterized by reflection absorption IR and XPS to be a homogeneous mixture of the two components. The mixed monolayers serve to tune the work function of silver over a wide range by varying the surface composition of the mixed monolayer from 4.1 to 5.8 eV. The mixed SAM-modified Ag surfaces were used as the anode in the fabrication of hole-only devices with the device structure Ag/SAM/HTL/Ag, where HTL represents a hole-transporting layer. It is shown that depending on the HTL used and thus the HOMO level involved, the maximum current injection into the device occurred with differently modified Ag. Top-emitting organic light-emitting diodes fabricated with differently modified silver electrodes showed that the maximum current and maximum luminance efficiency occur at anodes of different modifications due to a change in the hole-electron charge balance.
