Control of molecular curvature and crystallinity of quinacridone-benzoxadiazole copolymers using different π bridge for polymer solar cells

Abstract

Three quinacridone-benzoxadiazole polymers containing different spacers (PQcOx-T, PQcOx-TT and PQcOx-biT) are obtained via Suzuki coupling reaction to control the energy levels and curvature of the polymer backbones. We find a structural relationship between the polymer backbone and crystallinity, especially regarding packing orientation. PQcOx-T exhibits a zigzag conformation with a dihedral angle over 25° and shows an edge-on structure in pristine film. However, PQcOx-TT exhibits a linear conformation with a dihedral angle under 7° and shows crystallized domains and oriented morphology with shoulder peaks in the UV–vis spectra of both solution and film states. Although PQcOx-biT demonstrates a linear conformation, its electron distribution of both the ground and excited-states are localized on the acceptor unit. Consequently, the intramolecular charge transfer (ICT) effect within PQcOx-biT is poor. Linear polymers prefer regular and crystalline domains in the film state and lead to more efficient organic photovoltaic (OPV) devices. PQcOx-TT possesses a PCE value of up to 3.4%. Further, when we fabricated an inverted device, the PCE increased to 3.6% because of its small surface potential between the active layer and MoO3. The polymer packing direction and crystallinity were determined by the curvature of the polymer, which suggests an enhancement in charge transport and increases Jsc. Moreover, the small difference of surface potential can possibly improve Voc, FF and PCE.