Extracting Information about the Electronic Quality of Organic Solar-Cell Absorbers from Fill Factor and Thickness

Arguably the most crucial challenge in developing organic solar cells is to optimize the collection of photogenerated charges, and therefore the cell's $f\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}l$ $f\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}r$, a dimensionless parameter directly proportional to its power-conversion efficiency. The authors provide a complete physical picture of the fill factor, with a detailed discussion of the model's assumptions, easy ways to spot deviations from the model, and a simple method of analysis to quantify a material's electronic quality, without needing further equipment. This analysis can also be applied retrospectively to the deep literature on solar cells made of many semiconducting polymers.