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

Interfacial chemistry of thiophene-based polymers and oligomers in contact with low work function metals is of interest for organic electronic devices. Herein, interfacial reactions of the model thiophene-based oligomer, α-sexithiophene (α-6T), in ultrathin (5 ML) films with vapor-deposited Ag, Al, Mg, and Ca are investigated using surface Raman spectroscopy and X-ray photoelectron spectroscopy under ultrahigh vacuum conditions. Although typically considered an electron donor, results indicate that α-6T is reduced by Al and Ca. The reduction product with Al is the tetrahydrothiophene, whereas complete degradation of the thiophene cores occurs with Ca with the formation of calcium sulfide. In contrast, for the higher work function Ag, contact doping from α-6T to Ag nanoparticles is observed, inducing the formation of polaron states at the interface. Inter-ring torsion by the C-C bond rotation is also induced by electron sharing between α-6T and Ag. Mg, which is expected to undergo electron transfer in a manner similar to Ca based on the work function, instead diffuses through these 5 ML films and deposits on or alloys with the Ag substrate beneath; however, electron transfer from Mg to α-6T is observed in thicker α-6T films suggesting that nucleation into structures with electronic characteristics that resemble those of bulk Mg metal occurs. Overall, the evolution of α-6T interfaces with these low work function metals alters the interfacial energetics through the formation of "gap" states, which ultimately impact the device performance.