Fullerene C₇₀ as a p-type donor in organic photovoltaic cells

Fullerenes and their derivatives have been widely used as n-type materials in organic transistor and photovoltaic devices. Though it is believed that they shall be ambipolar in nature, there have been few direct experimental proofs for that. In this work, fullerene C70, known as an efficient acceptor, has been employed as a p-type electron donor in conjunction with 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile as an electron acceptor in planar-heterojunction (PHJ) organic photovoltaic (OPV) cells. High fill factors (FFs) of more than 0.70 were reliably achieved with the C70 layer even up to 100 nm thick in PHJ cells, suggesting the superior potential of fullerene C70 as the p-type donor in comparison to other conventional donor materials. The optimal efficiency of these unconventional PHJ cells was 2.83% with a short-circuit current of 5.33 mA/cm2, an open circuit voltage of 0.72 V, and a FF of 0.74. The results in this work unveil the potential of fullerene materials as donors in OPV devices, and provide alternative approaches towards future OPV applications.