pubs.acs.org/Langmuir Continuous Modulation of Electrode Work Function with Mixed Self-Assembled Monolayers and Its Effect in Charge Injection Kun-Yang Wu, Szu-Yen Yu, and Yu-Tai Tao* Institute of Chemistry, Academia Sinica Taipei Taiwan, Republic of China 115 Received January 6, 2009. Revised Manuscript Received March 5, 2009 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. Introduction Charge injection at the interface between a metal electrode and an organic semiconductor material is involved in a variety of organic electronic devices, including organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), organic photovoltaics, etc. A Schottky barrier is present at the metal/ organic interface due to different energy level alignments of the metal work function and the highest occupied molecular orbital (HOMO) or the lowest unoccupied molecular orbital (LUMO) of the organic molecule, depending on the type of charges to be injected. Reducing the barrier between the electrode and organic layer is in general desirable for efficient charge injection. 1 To reduce the barrier for charge injection from the metal electrode to the organics, one can use an organic material with proper HOMO (or LUMO) level by judicious choice of the organic materials or by structural modification of the material that has been demonstrated to be promising. These approaches would nevertheless introduce other variables such as different charge mobility for the new material or change in another inter- face involved in a multilayered device structure. Alternatively, one can use metal electrodes of different work function. This ap- proach would be limited by the adequacy of the particular metal in terms of conductivity, stability, or transparency/reflectivity of the metal involved. Furthermore, one can modulate the work function of a metal by surface treatment/modification. 2 The size and direction of the interface dipole introduced by the modifica- tion are suggested to effect the modulation. 3 Among various modifications, the use of a self-assembled monolayer (SAM) grafted on a metal surface has been shown to have great potential for systematic modulation of the work function through struc- tural change of the molecule used. Thus, by utilizing mercaptan- based SAMs, the energy barrier for the hole injection from the Ag (or Au) anode to an organic layer could be varied. 4 Besides introducing an oriented dipole, the SAM also imposes a tunneling barrier through which the charges have to pass to reach the organic layer. The current response as a function of the tunneling barrier, which can be modulated by the chain length of the SAM-forming molecules, provides insight to the charge balance situation in the device. We recently reported the use of SAMs of various organothiols on Ag for the fabrication of efficient top- emitting OLEDs. 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Published on Web 4/6/2009 © 2009 American Chemical Society DOI: 10.1021/la900046b Langmuir 2009, 25(11), 6232–6238 6232