Energy level pinning of an n-type semiconducting polymer on conductive polymer electrodes: Effects of work function and annealing

Using photoelectron spectroscopy, we investigated the energy level alignment at interfaces between the organic n-type semiconductor poly{[N,N′-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} [P(NDI2OD-T2] and poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDT:PSS) electrodes with different work function (Φ). The P(NDI2OD-T2) film thickness was varied between monolayer and multilayer (up to 12 nm) coverage. Vacuum level alignment was found for polymer electrode Φ ≤ 5.30 eV, whereas the valence band of P(NDI2OD-T2) becomes Fermi level pinned for higher Φ values. In situ annealing of un-pinned P(NDI2OD-T2) films on electrodes with Φ below 5.3 eV resulted in a transition to the Fermi level pinning regime. This transition is due to an increase of the effective polymer electrode Φ below the semiconductor polymer due to annealing. Pinning the P(NDI2OD-2T) energy levels at the conduction band with a low Φ electrode allowed estimating the charge transport gap of this polymer to be ≥ 1.7 eV.