Abstract
Pretreatment of electrodes surface is generally adopted to improve the charge injection efficiency and device lifetime in organic light-emitting diodes (OLEDs) fabrication. Through the modification of electrode with self-assembled monolayers (SAMs), the size and direction of the interface dipole can be varied to modulate the energy alignment at the interface. In this study, a series of phosphonic acid molecules were used, including n-alkylphophonic acids of different chain length and their trifluoromethyl-terminated analogues; binary mixtures with opposite dipoles of 1-butylphosphonic acid and the trifluoromethyl-terminated analogue, p-methyl- and trifluoromethyl-phenyl phosphonic acid, to modify ITO anode through the adsorbed SAMs and applied in the fabrication of OLED devices. The device performance as function of the tunneling barrier for hole injection, which can be modulated by the chain length of the SAM-forming molecules, provides insight to the charge balance situation in the device. Moreover, the binary mixtures of two molecules with the same chain length but opposite dipole were formed on ITO surfaces to tune the work function of ITO over a range from 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 Cs2CO3 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. Besides, for the common approaches of surface treatment such as oxygen plasma treatment, self-assembled monolayer adsorption and PEDOT:PSS coating, different effects on the device lifetime were observed. In the device lifetime measurement, a three- to ten-fold lifetime enhancement was observed for devices with SAM modification compared to the other two. A correlation of the lifetime and the driving voltage change during device operation was proposed and suggestion was made to the effect of different surface modification and the cause of device failure. The surface analyses by AC2 and XPS provide support to the suggestion. Finally, we try to optimize the adsorption process of phosphonic acid SAM on ITO anode. Through the surface analysis of ref-IR spectrum and work function, the results indicated that only a brief immersion of ITO substrate followed by high-temperature baking is enough to achieve a saturated monolayer adsorption. However, the phosphonic acid SAM tends to desorb from ITO surface as long as it had been in contact with water, during adsorption step or washing step.