Energy Level Alignment at Tpd/Etal Interfaces Studied by Kelvin Method

Abstract

ABSTRACTThe vacuum level shift A was determined by Kelvin probe method for TPD/metal systems prepared in ultrahigh vacuum (TPD: N, N'-bis(3-methylphenyl)-N, N'-diphenyl-[1, 1'-biphenyl]-4,4'-diamine). The energy of the vacuum level sharply changed at the initial stage of depositing TPD on the metal substrates, and further bending was not observed up to 100 nm thickness of TPD. This result is consistent with our previous studies of ultraviolet photoelectron spectroscopy. These results indicate the invalidity of traditional model with a common vacuum level at organic/metal interface. We discussed the dependence of the vacuum level shift on the work function Φm of the metal substrate (Au, Cu, Ag, Mg, and Ca). A liner correlation between Δ and Φm was observed only in the region of 3.8 eV < Φm < 4.5 eV, indicating that Fermi level alignment is not achieved at least for some interfaces. We found that Δ for TPD film on the air-exposed metal substrate is smaller than that of TPD on clean metals and was observed a liner relation between Φm and Δ except for the Cu substrate.