Non-fullerene polymer solar cells with VOC > 1 V based on fluorinated quinoxaline unit conjugated polymers

To achieve efficient non-fullerene polymer solar cells (NF-PSCs), it is important to design and synthesize donor materials. To investigate the substituent effects of electron-withdrawing fluorine atoms and different alkyl chains (-C8C12 or -C10C14) on the thiophene pi bridge, four novel conjugated polymers composed of dicyanodistyrylbenzene (DCB)-based and quinoxaline-based units were synthesized and applied in NF-PSC photovoltaic devices. It was found that the different alkyl side groups had a minimal influence on the molecular energy level but a moderate effect on the absorption coefficient, whereas the highest occupied molecular orbital (HOMO) of the resulting copolymers could be effectively lowered by introducing highly electronegative fluorine atoms into the quinoxaline moiety. As a result, the NF-PSCs based on the fluorinated quinoxaline-based copolymer exhibited high open-circuit voltages (V-OC) of up to 1.043 V, which is the highest value to date for devices based on quinoxaline moiety copolymers. Moreover, fluorination also improved the copolymer carrier mobility and absorption coefficient, leading to enhanced J(SC) and FF, thus giving rise to higher overall efficiencies. NF-PSCs based on PDCB-DFQ812: ITIC exhibited the best performance, with a power conversion efficiency (PCE) of 8.37%. Our comparative research indicates that fluorinated quinoxaline-based conjugated polymers are promising donor materials for NF-PSCs.