Polarization effects on energy-level alignment at the interfaces of polymer organic semiconductor films

The thickness-dependent evolutions of the Fermi level EF and ionization potential Ip of ultrathin films of regioregular poly(3-hexylthiophene) and poly[2,5-bis(3-tetradecylthiophene-2-yl)thieno[3,2-b]thiophene], deposited as edge-on π-stacked lamellae on gold and on p-doped poly(3,4-ethylenedioxythiophene) electrodes, have been measured by ultraviolet photoemission spectroscopy. The Ip increases by 0.3 eV on going from a monolayer film to a few-nanometer-thick film. Correspondingly, the EF pinning depth increases from 0.2 eV to 0.5 eV. This valence “band bending” which occurs for a constant vacuum level can be quantitatively modeled by microelectrostatic self-consistent polarization field calculations that incorporate both substrate and organic semiconductor film effects. The EF pinning to the first monolayer is relatively shallow.