Efficiency Enhancement in Organic Photovoltaic Cells: Consequences of Optimizing Series Resistance

Abstract Here, means to enhance power conversion efficiency (PCE or η ) 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 ( η > 10%). A diode‐based modeling approach is used to assess changes in R s . Given that typical published P3HT:PCBM test cells have relatively small areas (∼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.