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

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.