Neat C70-Based Bulk-Heterojunction Polymer Solar Cells with Excellent Acceptor Dispersion

The replacement of common fullerene derivatives with neat-C-70 could be an effective approach to restrain the costs of organic photovoltaics and increase their sustainability. In this study, bulk-heterojunction solar cells made of neat-C-70 and low energy-gap conjugated polymers, PTB7 and PCDTBT, are thoroughly investigated and compared. Upon replacing PC70BM with C-70, the mobility of positive carriers in the donor phase is roughly reduced by 1 order of magnitude, while that of electrons is only slightly modified. It is shown that the main loss mechanism of the investigated neat-C-70 solar cells is a low mobility-lifetime product. Nevertheless, PCDTBT:C-70 devices undergo a limited loss of 7.5%, compared to the reference PCDTBT:PC70BM cells, reaching a record efficiency (4.44%) for polymer solar cells with unfunctionalized fullerenes. The moderate efficiency loss of PCDTBT:C-70 devices, due to an unexpected excellent miscibility of PCDTBT:C-70 blends, demonstrates that efficient solar cells made of neat-fullerene are possible. The efficient dispersion of C-70 in the PCDTBT matrix is attributed to an interaction between fullerene and the carbazole unit of the polymer.