MoO3 surface passivation of the transparent anode in organic solar cells using ultrathin films

An original surface passivation technique of indium tin oxide (ITO) used as anode in organic solar cells is proposed. We demonstrate that a thin MoO3 film (3.5 +/- 1 nm) at the interface ITO/organic donor allows improving significantly the devices' performances. The devices are based on the multiheterojunction structure copper phthalocyanine (CuPc)/fullerene (C-60)/aluminum tris(8-hydroxyquinoline) (Alq(3)). The deposition of MoO3 onto ITO improves the charge transfer from CuPc to ITO. The enhancement in the hole collection efficiency in the presence of an oxide layer can be explained in terms of the reduction in the effective barrier against hole transfer from CuPc into the ITO anode. The contact ITO/MoO3/CuPc behaves like a metal-insulator-semiconductor (MIS) structure, which allows reducing the energy barrier due to the difference between the work function of ITO and the highest occupied molecular orbital of CuPc. It is shown that the optimum MoO3 thickness corresponds to a compromise between an optimum ITO coverage and a sufficient transparency of the trapezoidal barrier for the tunneling of the charge carriers. The MoO3 thin films are discontinuous, and the passivation effect is improved when the oxide thin film is covered by an ultrathin gold film. Such behavior is discussed in the light of band scheme structures after contact and of geometrical considerations.