Penetration and Reaction Depths of Vapor Deposited Ag, Mg, Al, and Ca on Oligothiophene Thin Films

Abstract

Physical vapor deposition of metal contacts onto thin film organic functional materials is widely used in organic electronic technologies. Using the oligothiophenes (OTs) α-sexithiophene (α-6T) and 2,2′:5′2″-terthiophene (3T) as models for thiophene-containing functional organic films, the metal/organic interface chemistry, metal penetration and reaction depths, and surface metallization processes are investigated and compared for Ag, Al, Mg, and Ca contacts using Raman and X-ray photoelectron spectroscopies. Ag shows only modest penetration, with some surface metallization but no reaction chemistry on OTs. Al and Ca induce some OTs reaction but exhibit the least propensity to penetrate these films with the greatest ability to metallize on the surface. In contrast, Mg exhibits complete penetration of 5 ML OT films with no reaction chemistry or surface metallization. On 50 ML OT films, however, Mg still exhibits the greatest depth of penetration with no surface metallization but does induce a small amount of reaction chemistry initiated by electron transfer from Mg to the OT film. These results reveal significant differences in the depth to which the deposited metal chemically impacts OT thin films during physical vapor deposition and reinforce the critical role of interface chemistry on the performance of organic electronic devices formed with such materials.

Chemistry at the Interface of α-Sexithiophene and Vapor-Deposited Ag, Al, Mg, and Ca: A Molecular View

“…This postulate led to a more extensive study of the penetration depth of these various metals with interesting and significant results that are detailed in a separate report. 106 Indeed, the report contains convincing evidence that Mg does, in fact, reduce oligothiophene films in the same way as Ca as expected but only for α-6T films thicker than ∼10−15 ML, a thickness that apparently exceeds the ability of Mg to penetrate. Further evidence that electron transfer does not occur in these 5 ML films due to putative Mg penetration comes from the complete lack of dependence of the XPS S 2p intensity on Mg coverage, as shown in Figure 7.…”

“…The preponderance of Mg diffusion in these films may be driven by a combination of factors including a strong propensity to alloy with the underlying Ag substrate as well as the weaker interaction of Mg with S compared to Ag, Al, and Ca. These factors alter the cluster formation of the metal atoms, which dictates interfacial reaction chemistry and the thickness of the interface. , Further evidence for this, elaborated in our forthcoming publication, indicates that for thicker oligothiophene films, Mg penetration is significantly impeded, and products of the expected electron transfer reactions are, in fact, observed.…”

“…The surprising absence of any evidence for α-6T reduction by Mg led us to hypothesize that Mg atoms diffuse into and completely penetrate these 5 ML α-6T films, depositing onto the Ag substrate beneath, possibly driven by the strong alloying tendency of Mg with Ag. This postulate led to a more extensive study of the penetration depth of these various metals with interesting and significant results that are detailed in a separate report . Indeed, the report contains convincing evidence that Mg does, in fact, reduce oligothiophene films in the same way as Ca as expected but only for α-6T films thicker than ∼10–15 ML, a thickness that apparently exceeds the ability of Mg to penetrate.…”

“…These factors alter the cluster formation of the metal atoms, which dictates interfacial reaction chemistry and the thickness of the interface. 42,43 Further evidence for this, elaborated in our forthcoming publication, 106 indicates that for thicker oligothiophene films, Mg penetration is significantly impeded, and products of the expected electron transfer reactions are, in fact, observed.…”

Water permeation pathways in laminated organic single-crystal devices

“…This is because the top contact formation is known to form an ill-defined interface as a result of impingement of energetic metallic clusters to the soft organic layer during vacuum deposition. 35,36 In the bottom-contact configuration the pre-defined electrodes protrude from the substrate surface. If the SC lamination is not sufficiently conformal, a space should be formed around the electrode edges.…”