Dipole Layer Formation by Surface Segregation of Regioregular Poly(3-alkylthiophene) with Alternating Alkyl/Semifluoroalkyl Side Chains

A regioregular poly(3-alkylthiophene) with alternating alkyl/semifluoroalkyl side chains (P3DDFT) spontaneously segregates on the surface of poly(3-dodecylthiophene) (P3DDT) during a spin-coating process and forms a thin fluoroalkyl layer with a thickness of less than 3 nm. This surface layer is as dense as fluoroalkyl-based self-assembled monolayers (SAMs) on metals, as demonstrated by angle-dependent X-ray photoelectron spectroscopy and water contact angle measurements. The semifluoroalkyl chains aligned at the air/solid interface form a large molecular dipole moment that continuously shifts the ionization energy by as much as +1.8 eV depending on the surface density of P3DDFT. This can be a novel approach to controlling the energy-level alignment at organic/(in)organic interfaces, analogous to the work function shifts of metals induced by the formation of SAMs.