Frisch, J.; Vollmer, A.; Koch, N.: Energy level pinning of an n-type semiconducting polymer on conductive polymer electrodes: Effects of work function and annealing. Journal of Applied Physics 112 (2012), p. 033712/1-5
10.1063/1.4745017
Abstract:
Using photoelectron spectroscopy, we investigated the energy level alignment at interfaces between the organic n-type semiconductor poly{[N,N0-bis(2-octyldodecyl)naphthalene-1,4,5,8- bis(dicarboximide)-2,6-diyl]-alt-5,50-(2,20-bithiophene)} [P(NDI2OD-T2] and poly(ethylenedioxy thiophene):poly(styrenesulfonate) (PEDT:PSS) electrodes with different work function (U). 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 U5.30 eV, whereas the valence band of P(NDI2OD-T2) becomes Fermi level pinned for higher U values. In situ annealing of un-pinned P(NDI2OD-T2) films on electrodes with U 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 U below the semiconductor polymer due to annealing. Pinning the P(NDI2OD-2T) energy levels at the conduction band with a low U electrode allowed estimating the charge transport gap of this polymer to be1.7 eV.