Small-molecule organic solar cells (OSCs) with an optimized structure of indium tin oxide/poly (3,4-ethylenedioxythioxythiophene): poly(styrenesulfonate)/copper phthalocyanine (CuPc) (10 nm)/CuPc: fullerene (C60) mixed(20 nm)/C-60 (20 nm)/4,7-diphenyl-1,10-phenanthroline (BPhen) (5 nm)/Ag were fabricated. In this study, the cesium carbonate-doped BPhen (Cs2CO3: BPhen) was adopted as the buffer layer to enhance the efficiency of the OSCs. The photovoltaic parameters of the OSCs, such as the short-circuit current density and fill factor, depend on the doping concentration of Cs2CO3 in the BPhen layer. The cell with a Cs2CO3: BPhen (1: 4) cathode buffer layer exhibits a power conversion efficiency (PCE) of 3.51%, compared to 3.37% for the device with the pristine BPhen layer. The enhancement of PCE was attributed to the energy-level alignment between the C-60 layer and the Cs2CO3: BPhen layer. In addition, the characterization measured using atomic force microscopy shows that the Cs2CO3: BPhen layers have smoother surfaces. (C) 2013 Elsevier B.V. All rights reserved.