Here, we have tried to make bulk heterojunction photovoltaic cells with enhanced nanoscale morphology by adding an ionomer (partially sulfonated polystyrene (PSP)) into a regioregular P3HT/(6,6)-phenyl C-61-butyric acid methyl ester (PCBM) blend. We found that the polar ionic part of PSP may be miscible With PCBM and that the non-ionic part (polystyrene) is well miscible with P3HT. PSP can reduce the interfacial energy between 133HT and PCBM, resulting in nanoscale morphology. We have studied the relationship between photovoltaic characteristics. and the morphology of the active layer. (C) 2008 Elsevier B.V. All rights reserved.