ABSTRACT. Electrically stimulated switching of a charge injection barrier at the interface between an organic semiconductor and an electrode modified with a disordered monolayer (DM) is studied by using various benzenethiol derivatives as DM molecules. The switching behavior is induced by a structural change in the DM molecules, and is manifested as a reversible inversion of the polarity of DM-modified Au electrode/rubrene/DM-modified Au electrode diodes. The switching direction is found to be dominantly determined by the push-back effect of the thiol bonding group, while the terminal group modulates the switching strength. A device with 1,2-benzenedithiol DMs exhibited the highest switching ratios of 20, 10 2 , and 10 3 for the switching voltages of 3, 5, and 7 V, respectively. A variation in the tilt angle of benzenethiol DMs owing to the application of 7 V is estimated to be smaller than 23.6° by model calculations. This study 2 offers an understanding for obtaining highly stable operations of organic electronic devices, especially with molecular modification layers.KEYWORDS. molecular switch, charge injection barrier, push-back, electric dipole, selfassembled monolayer.3 Organic or molecular semiconductors are regarded as promising materials for realizing low-cost, large-area fabrication of electronic devices. 1,2 In most organic electronics and optoelectronics applications, intrinsic semiconductors are used as active layers. In this case, the type of charge carriers (electrons or holes) exploited in the electronic and optoelectronic devices is determined by a charge injection barrier at the electrode-semiconductor interface. If the barrier for electron injection is lower than that for hole, the device is classified as n-type, where electrons are the charge carriers. Therefore, it is critical to control the height of the charge injection barrier, ΦB. In a first approximation, the value of ΦB is determined by the relative energy difference between the Fermi level, EF, of the metallic electrode and the molecular orbital of the semiconducting molecule. Once the preferred molecule for the semiconducting layer is determined, the work function of the electrode, Φm, must be tuned for controlling ΦB.Among various methodologies for tuning of Φm, modification of electrode surfaces with wellordered self-assembled molecular monolayers (SAMs) is widely employed in the research of organic electronics. This is partly because organic semiconductor layers can be formed under rather mild conditions by formation techniques such as solution-based coating, 3,4 vacuum deposition with a low sublimation temperature, 5 or simple lamination of single crystals. [6][7][8] In contrast, a significant damage can be easily inflicted on a molecular monolayer by the deposition of inorganic materials on it. 9 In addition, the constituent molecules of SAMs can be flexibly designed by using organic synthetic techniques to obtain terminal group(s) possessing permanent electric dipoles. The Φm values are partly determined by the electric doub...