We discovered that fringe structures of ZnO film in solar cells of inverted structures penetrate through the active layer made of low bandgap polymers. The ZnO penetration leads to leakage current of the solar cells. To prevent the ZnO penetration, the thin film morphology was modified. By changing the solvent and inserting TiO2 nanorods between ZnO film and the active layer, penetration can be avoided, so Rsh is improved from 110.3 Omega cm(2) to 374.7 Omega cm(2) and V-oc from 0.36 V to 0.66 V. This result provides a promising route for employing low bandgap polymers to air-stable inverted structures with solution process. It is possible that inverted low bandgap polymer solar cells can achieve the goals of high performance and air stability. (C) 2012 Elsevier B.V. All rights reserved.