Charge collection and recombination limiting fill factor in polymer solar cells

A fill factor (FF) in polymer solar cells has been less studied but should be understood in the photovoltaic parameters. The experimental FF is well explained for typical solar cells by an empirical equation with photovoltaic and device parameters such as series and shunt resistances, while it is not true for polymer solar cells. To investigate this discrepancy, we first analyzed FF by using an empirical equation for FF based on the equivalent circuit model. As a result, FF measured was in good agreement with that obtained by the empirical equation with parameters evaluated under illumination condition, but was not consistent with that in the dark condition. This finding suggests device parameters are dependent on illumination conditions. Thus, we next discussed the limiting factors of FF in terms of charge carrier dynamics in the three polymer solar cells. Specifically, two of the devices exhibited the significant decrease in FF down to 0.4 as the active layer thickness increased. On the other hand, the other device maintained a high FF of 0.6 even with a thicker active layer. To discuss this different dependence, we evaluated the recombination kinetics parameters by transient optoelectronic measurements. Interestingly, we found no significant difference in the recombination reduction factor, which was suppressed by two orders of magnitude compared to Langevin recombination for all the devices. With these kinetic parameters, the J–V characteristics were well reproduced for the high FF devices, but not for the low FF devices. In the latter devices, isolated domains, where charge carriers are not collected at the electrode, need to be considered. We further discuss the competition between charge collection and recombination and 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 s⁻¹.