Improvement of the surface roughness and power conversion efficiency (PCE) of bulk hetero-junction (BHJ) solar cells was made by the addition of organic additives for the cells based on a low energy-gap polymer, poly [2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4-b'] dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT), as an electron donor and [6,6]-phenyl C61 butyric acid methyl ester (PC61BM) as the electron acceptor. The PCPDTBT: PC61BM active layers were prepared by spin-coating process from four different organic solvents: pure chlorobenzene, chlorobenzene with 2.5 vol% 1,2-ethanedithiol (EDT) additive, chlorobenzene with 2.5 vol% 1,8-octanedithiol (ODT) additive, and chlorobenzene with a combination of 2.5 vol% EDT and 2.5 vol% ODT additives. The smoothest surface of the active layer, which was observed by AFM, was obtained in the case of PCPDTBT:PC61BM prepared with a combination of EDT and ODT additives in chlorobenzene, and a maximum PCE of 3.5% was achieved.