Charge collection and recombination limiting fill factor in polymer solar cells

A fill factor (FF) in polymer solar cells has been least studied but should be deeply understood in the photovoltaic parameters. An empirical equation for FF shows FF is related to not only the photovoltaic parameters but also device parameters. However, FF in polymer solar cells cannot be explained by the empirical equation with such parameters. Rather, it is significantly dependent on charge carrier dynamics in polymer solar cells. In this study, we first discuss how FF can be explained by using an empirical equation for FF based on the equivalent circuit model. Next, we discuss the limiting factors of FF in terms of charge carrier dynamics, which were evaluated by transient optoelectronic measurements. We propose that for high FF to be achieved in polymer solar cells, it is essential that the mobility of the fast component exceed ~10−3 cm2 V−1