Sodium was incorporating at the copper-phthalocyanine (CuPc)/C-60 interface in CuPc/C-60-based small-molecular solar cells to enhance their power conversion efficiency. C-60 was deposited on slightly sodium-doped CuPc. Post-annealing improved the cell properties. Post-annealing doubled the conversion efficiency of the least sodium-doped devices (75 degrees C, 40 min). The electron/hole mobility ratio gradually approached unity as the annealing time increased, indicating that a reduction in the space charge accumulation was the main cause of the increase of the short-circuit current. The mechanism of enhancement of carrier transport by annealing was investigated by making capacitance-voltage measurements and performing corresponding depth-profile analyses. (C) 2013 Elsevier B. V. All rights reserved.