Efficient polymer solar cell employing an oxidized Ni capped Al:ZnO anode without the need of additional hole-transporting-layer

We show that an effective transparent electrode for bulk-heterostructure organic solar cells (OSCs) can be produced by uniformly depositing a few nm of Ni on a film of aluminum-doped zinc oxide (AZO). After deposition, the Ni capping layer is O-2 plasma treated to form a bilayer of Ni/NiO, as it is evidenced by x-ray photoelectron spectroscopy analysis. The oxidized Ni capped AZO electrode can act as anode and hole-transporting-layer in OSCs, providing an enhancement in transparency, environmental stability, and injection/collection of charges. The S-shaped feature of the IV curve for the OSC using AZO electrodes in conjunction with NiO transporting layer is not present in the case of the proposed electrode structure, clearly indicating the significant role of the Ni metallic interlayer in reducing the energy barrier. The relevant role played by the Ni was further confirmed when a NiO layer was deposited on top of the AZO/Ni bilayer. In that case, the S-shape was not present while a 90% photo-conversion efficiency relative to the ITO/NiO cell was achieved. (C) 2012 American Institute of Physics. [doi:10.1063/1.3673843]