We investigated the effect of additives on the morphology of a 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):(6,6)-phenyl C 71-butyric acid methyl ester (PC 71BM) blended onto a surface of poly (3, 4-ethylendioxythiophene): poly(styrensulfonate)(PEDOT:PSS) to form photoactive films. Films of PCPDTBT: PC 71-BM bulk heterojunctions were prepared by spin-coating from a solution in chlorobenzene (CB) and were processed with and without the addition of 2%, 4%, and 6 vol% 1-chloro naphthalene (CN) or 1, 8-octanedithiol (ODT) as additives. For all samples, the PCPDTBT:PC 71BM molar ratio was 1:2 (wt%), and the additives in 1 ml were prepared with a concentration of 30 mg of PCPDTBT:PC 71BM. Optical absorption spectroscopy measurements of the films indicated shifts in the absorption peaks in the range from 500-800 nm which was attributed to PCPDTBT. XRay diffraction (XRD) was used to investigate the nature of the molecular stacking in the polymer thin films. Topographic images which were obtained by using an atomic force microscope, of the PCPDTBT:PC 71BM layers with 2 vol% ODT additive, were found to have the highest surface roughness. The best performing device shows a power conversion efficiency of 2. 15% for a 2-vol% ODT additive. © 2012 The Korean Physical Society.