Negligible effect of processing additive in polymer bulk heterojunction photovoltaic cells with unmodified fullerene

Although processing additives such as 1,8-diiodooctane are known to effectively modify the microscopic structure of polymer bulk heterojunction composite using PCBM-type fullerene derivatives and yield efficient photovoltaic cells, it seems not yet to be clarified which part of the PCBM-type fullerene derivatives, i.e. bucky ball or side group, is essential to the phenomena. Here, the effect of processing additive 1,2-dimethylnaphthalene (DMN), which is reported to be effective for polymer bulk heterojunction composite film with [6,6]-phenyl-C71-butyric acid methyl ester (C70-PCBM) coated from 1,2,4-trimethylbenzene, on the composite film with unmodified C70 is examined. It has been observed that DMN shows negligible effects on the photovoltaic performance including the low-light performance as well as the surface morphology. Therefore, not the bucky ball but the side group in the fullerene derivatives seems to be critical to the positive effect of processing additives such as DMN.