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.