Detection and role of trace impurities in high-performance organic solar cells

Trace impurities in organic solar cells, such as those from residual catalyst material in conjugated polymers, are often ignored but are known to deleteriously affect device performance. Batch-to-batch variations in the nature and quantity of such impurities leads to widespread issues with irreproducible optoelectronic function, yet to date no technique has emerged that is reliably capable of identifying the character of impurities or their concentration in organic photovoltaic active layer blends. Here we focus on state-of-the-art, high-performance bulk heterojunction blends and show that synchrotron-based X-ray fluorescence can detect and quantify trace concentrations of metal impurities in these systems. Adopting a strategy of artificially introducing known quantities of additional catalyst into polymer/fullerene blends, we identify both the threshold concentration at which performance degrades and the mechanism for the degradation. With the knowledge of a target impurity concentration and a technique in hand to accurately measure their presence, researchers can implement materials preparation processes to achieve consistent, high performance in organic solar cells.