Fullerene Adducts Bearing Cyano Moiety for Both High Dielectric Constant and Good Active Layer Morphology of Organic Photovoltaics

Power conversion efficiency (PCE) of organic photovoltaics (OPVs) lags behind of inorganic photovoltaics due to low dielectric constants (epsilon(r)) of organic semiconductors. Although OPVs with high epsilon(r) are attractive in theory, practical demonstration of efficient OPV devices with high-epsilon(r) materials is in its infancy. This is largely due to the contradiction between the requirements of high epsilon(r) and good donor: acceptor blend morphology in the bulk heterojunction. Herein, a series of fullerene acceptors is reported bearing a polar cyano moiety for both high epsilon(r) and good donor: acceptor blend morphology. These cyano-functionalized acceptors (epsilon(r) = 4.9) have higher epsilon(r) than that of the widely used acceptor, [6,6]-phenyl-C-61-butyric acid methyl ester (PC61 BM) (epsilon(r) = 3.9). The high epsilon(r) is realized without decrease of electron mobility and change of the lowest unoccupied molecular orbital/highest occupied molecular orbital (LUMO/HOMO) energy levels. Although the cyano-functionalized acceptors have increased polarity, they still exhibit good compatibility with the typical donor polymer. Polymer solar cells based on the cyano-functionalized acceptors exhibit good active layer morphology and show better device performance (PCE = 5.55%) than that of PC61 BM (PCE = 4.56%).