Here, means to enhance power conversion efficiency (PCE or eta) in bulk-heterojunction (BHJ) organic photovoltaic (OPV) cells by optimizing the series resistance (R-s)-also known as the cell internal resistance-are studied. It is shown that current state-of-the-art BHJ OPVs are approaching the limit for which efficiency can be improved via R-s reduction alone. This evaluation addresses OPVs based on a poly(3-hexylthiophene):6,6-phenyl C-61-butyric acid methyl ester (P3HT:PCBM) active layer, as well as future high-efficiency OPVs (eta > 10%). A diode-based modeling approach is used to assess changes in R,. Given that typical published P3HT:PCBM test cells have relatively small areas (similar to 0.1 cm(2)), the analysis is extended to consider efficiency losses for larger area cells and shows that the transparent anode conductivity is then the dominant materials parameter affecting R-s efficiency losses. A model is developed that uses cell sizes and anode conductivities to predict current-voltage response as a function of resistive losses. The results show that the losses due to R-s remain minimal until relatively large cell areas (>0.1 cm(2)) are employed. Finally, R-s effects on a projected high-efficiency OPV scenario are assessed, based on the goal of cell efficiencies >10%. Here, R-s optimization effects remain modest; however, there are now more pronounced losses due to cell size, and it is shown how these losses can be mitigated by using higher conductivity anodes.