Organic solar cells based on P3HT:PCBM bulk heterojunction with an inverted structure have been fabricated using aluminum doped ZnO (AZO) as their electron transport layer, which was prepared by sol-gel route with 2-methoxyethanol as the solvent. The influences of Al concentration on the surface morphology and electrical properties of AZO layer were investigated by atomic force microscopy, X-ray diffraction and Hall effect measurements. The experimental results indicate that Al concentration has major influence on the grain size and only minor influence on crystallite size, leading to different surface morphology for different doping concentrations. Doping at high concentration produces higher charge carrier density, but not for charge carrier mobility and its electrical conductivity, which may be also influenced by the surface morphology. The effects of Al concentration in AZO layer were also observed in the J-V (current density-voltage) and small AC impedance characteristics of the fabricated solar cells. The resistance increases with Al concentration, but the capacitance decreases. The solar cell using AZO layer with doping concentration of 0.5 wt% shows the smallest impedance and the best J-V characteristics with the highest conversion efficiency. These characteristics seem to be correlated with the properties of AZO layer and its interface with the active layer. (C) 2012 Elsevier B.V. All rights reserved.