Over 18.7% efficiency for bulk heterojunction and pseudo-planar heterojunction organic solar cells achieved by regulating intermolecular compatibility

The regulation of intermolecular compatibility is important for achieving a suitable morphology for organic solar cells (OSCs). The small molecule acceptor BTP-eC9 is selected as the third component of binary bulk heterojunction (BHJ) or pseudo-planar heterojunction (PPHJ) OSCs with the polymer PM6 as the donor and the small molecule L8-BO as the acceptor to regulate intermolecular compatibility. Good compatibility between L8-BO and BTP-eC9 may promote the formation of a well-mixed acceptor phase. Inferior compatibility between PM6 and BTP-eC9 compared to that between PM6 and L8-BO may enhance the phase separation degree between the donor and acceptor. Different intermolecular compatibility can lead to changes in intermolecular forces, thereby affecting the orderliness of molecular arrangement. The addition of BTP-eC9 can effectively optimize the horizontal and vertical structural phase separation degree, as well as the order of molecular arrangement by regulating the intermolecular compatibility. The power conversion efficiencies (PCEs) of BHJ and PPHJ OSCs are improved to 18.92% from 17.85% and 18.75% from 17.43% after adding BTP-eC9. This work indicates that regulating intermolecular compatibility is a crucial and universal approach to optimize the morphology of the BHJ and PPHJ active layers by selecting the third component based on the rational compatibility between guest molecules and binary host system materials.