Direct determination of the electronic structure of the poly(3-hexylthiophene):phenyl-[6,6]-C61 butyric acid methyl ester blend

This article focuses on the electronic structure of the poly(3-hexylthiophene):phenyl-[6,6]-C61 butyric acid methyl ester (P3HT:PCBM) blend, widely used in bulk heterojunction (BHJ) solar cells. Given the fact that the surface of the blend film is a nearly pure P3HT wetting layer, we use a lift-off method to access the originally buried surface, which is rich in both P3HT and PCBM and thus representative of the BHJ. The combination of direct and inverse photoemission spectroscopy on this surface leads to a determination of the energy gap between the lowest unoccupied molecular orbital (LUMO) of the acceptor and the highest occupied molecular orbital (HOMO) of the donor. The gap is ∼1.4 eV, which implies a 0.5–0.6 eV interface dipole barrier between the two materials. The energy gap is found to be stable versus in situ annealing up to 100 °C.