Band diagram and band line‐up of the polycrystalline CdS/Cu(In,Ga)Se2 heterojunction and its response to air annealing

Contact potential difference measurements in the dark and under illumination are used to derive the conduction band offset (ΔEC) in the CdS/Cu(In,Ga)Se2 junction before and after air annealing. This junction was formed by chemical bath deposition of CdS on state‐of‐the‐art, solar cell quality, polycrystalline thin films of Cu(In,Ga)Se2. Based on our experimental measurements and estimates made for dipole contributions, we find that the junction in the cell is of type II, i.e. without a spike in the conduction band (ΔEC=80 meV±100 meV). Air annealing improves cell performance and also markedly increases the surface photovoltage spectral response. However, the annealing treatment is shown to have no significant effect on the band line‐up of the heterojunction, which remains type II. This indicates that air annealing of these junctions does not change the built‐in voltage or the band line‐up.