Substantial Reduction of Hole Injection Barrier at the Au/Rubrene Organic Semiconductor Interface Using PFBT Self-Assembled Monolayers

Abstract

Surface functionalization using self-assembled monolayers (SAMs) is an effective and widely adopted strategy for optimizing the charge injection and interfacial energetics in organic electronic devices. In particular, 2,3,4,5,6-pentafluorothiophenol (PFBT) SAMs are widely employed to augment the work function of noble metal electrodes, such as Au, and favorably template organic semiconductor molecular packing. However, comprehensive in situ spectroscopic investigations detailing the thickness-dependent energy-level alignment across the PFBT-modified Au/organic semiconductor interface remain scarce. This study investigates the interfacial electronic structures of 5,6,11,12-tetraphenylnaphthacene (rubrene) deposited on air-exposed Au (Air-Au), ultraviolet-ozone-treated Au (UVO-Au), and UVO-Au-functionalized PFBT (UVO-Au/PFBT) using in situ X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). High-resolution XPS reveals that UV–O3 pre-activation forms an ultrathin gold oxide (AuOx) layer, thereby dramatically enhancing the chemisorption kinetics and packing density of the PFBT SAM. While UV–O3 treatment alone increases the work function of Au from 4.78 to 5.06 eV, subsequent PFBT chemisorption further elevates it to 5.33 eV via a pronounced permanent dipole moment directed toward the substrate. Consequently, the hole injection barrier at the Au/rubrene interface is progressively reduced from 0.65 eV for Air-Au to 0.40 eV for UVO-Au and ultimately down to 0.27 eV for UVO-Au/PFBT. This dramatic reduction reaches the thermodynamic Fermi-level pinning regime, providing direct spectroscopic evidence for establishing near-Ohmic contacts. These findings offer fundamental insights into contact-resistance minimization and interface design for high-performance organic optoelectronic devices.

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