Investigation of the Interfacial Chemistry Between Vapor-Deposited Metals and Organic Thin Films by Raman Spectroscopy

Abstract

The use of Raman spectroscopy in ultra high vacuum to assess structure and reactivity at the interface of tris-(8-hydroxyquinoline) aluminum (Alq3) with vapordeposited metals is presented. Understanding the structure of the interface between electron transport layer materials such as Alq3 and low work function metals such as Al, Mg and Ca is vital for engineering organic light emitting diodes with high efficiency and low driving voltage. Reactivity at the interface of Al, Mg and Ca with Alq₃ thin films is examined with Raman spectroscopy along with the non-reactive Ag/Alq₃ interface for comparison. Additionally, the effect of a thin LiF barrier layer on reactivity at the Al/Alq₃ and Mg/Alq₃ interfaces is also examined. Raman spectroscopy of post-deposited Ag on Alq3 films confirms preservation of the Alq₃ structure along with evolution of simple surface enhancement of Alq₃ spectral intensities. Changes in key vibrational modes of Alq₃ upon Ag deposition are consistent with weak interaction of Ag with the conjugated ring of the ligand. In contrast, vapor deposition of Al onto Alq₃ films results in the appearance of new Raman modes linked to the formation of an Al-Alq₃ adduct. Additionally, Raman modes associated with graphitic carbon are also noted for the Al/Alq₃ interface and are attributed to partial degradation of the organic film. The Raman spectral results for deposition of Mg onto Alq3 films also indicate formation of a complex interfacial region composed primarily of Mg-Alq₃ adducts and small-grained amorphous or nanocrystalline graphite. Raman spectroscopy of the Ca/Alq₃ interface is also indicative of formation of a Ca-Alq₃ complex; however, the graphitic carbon in this system is noted to be more disordered, sp³-type carbon compared to that observed for Al/Alq₃ and Mg/Alq₃. Examination of the Al/LiF/Alq₃ and Mg/LiF/Alq₃ interfaces illustrates that 5 Å-thick LiF layers partially block reaction chemistry between the metal and organic, while 10 Å thick LiF films completely eliminates reactivity at these interfaces. Implications of the presence of chemical species observed at these metal/organic interfaces on charge transport in devices are also discussed.