Efficiency enhancement in organic photovoltaic cells: Consequences of optimizing series resistance

Here, means to enhance power conversion efficiency (PCE or η) in bulkheterojunction (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 C₆₁-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 cm²), 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 cm²) 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.