On the relationship of the effective mobility and photoconductance mobility in organic solar cells

Abstract

The efficiency of organic solar cells has increased significantly in the recent years due to the continued improvement in material properties, including the charge carrier mobilities within the bulk heterojunction. However, common strategies to measure the mobility of electrons and holes, such as the space-charge-limited-current approach, rely on purpose-made single carrier diodes, which are operated in the injection regime. Alternatively, impedance spectroscopy measurements can yield an effective mobility as well as a photoconductance mobility for solar cells under realistic operating conditions. There exist various theoretical interpretations that relate the experimentally determined values of the effective mobility with the mobility of the individual charge carriers (i.e. electrons and holes). Furthermore, the relationship between the effective and photoconductance mobility has not been clarified yet. This study shows how the effective and photoconductance mobilities can be combined in a system of equations to calculate the individual mobilities of the faster and slower carriers. Finally, these considerations are applied to determine individual carrier mobilities in several blend systems, including fullerene-based P3HT:PC₆₀BM solar cells, as well as non-fullerene devices based on PM6:Y11-N4, PM6:Y5, PPDT2FBT:Y6, PM6:Y11, PM6:N4, and PM6:Y6. These results were validated with mobility values obtained via the space-charge-limited-current approach.