We investigated the effect of processing additives on the morphology, charge transport and device performance of poly[N-9-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl C61 butyric acid methyl ester (PC60BM) based bulk heterojunction (BHJ) photovoltaic cells. To carry out this investigation, dimethylsulfoxide (DMSO) and dimethylformamide (DMF) were separately added as processing additives in ortho dichlorobenzene (o-DCB) solution of PCDTBT. A progressive aggregation of PCDTBT in both solutions and films were resulted through the addition of both DMSO and DMF additives. The optimization of the additives resulted in fine tuning the morphology of PCDTBT/PC60BM based solar cells, leading to improved domain structure and hole mobility on the active layer, and significantly enhanced photovoltaic performance.