Fluorination of conjugated polymers has been a widely adopted approach to tune optoelectronic properties of conjugated polymers, aiming for enhancing the power conversion efficiency (PCE) of their solar cells. Most fluorinated donor-acceptor (D-A) polymers have the fluorine atoms placed on the acceptor units; for example, in the well-studied PBnDT-FTAZ, two fluorine atoms are substituted onto the benzotriazole (TAZ) unit. Compared with the non-fluorinated counterpart (PBnDT-HTAZ), PBnDT-FTAZ shows one order of magnitude higher hole mobility in its bulk heterojunction (BHJ) blend with phenyl-C61-butyric acid methyl ester (PC61BM), which directly translates into a remarkable improvement in fill factor (FF) of its BHJ device, leading to the observed efficiency of 7% (vs. 4% for PBnDT-HTAZ). In this study, we intend to direct our attention to the fluorination of the two connecting thiophene units between the TAZ unit and the BnDT unit. Four structurally related polymers based on PBnDT-HTAZ were then designed and successfully synthesized, featuring thiophene units fluorinated at different positions and/or fluorinated TAZ units. It was found that the fluorination positions on thiophene units (3 vs 4 position) have a strong impact on the reactivity of monomers, the conformation and twisting angles between adjacent units, the energy levels of the new polymers and the aggregation of polymer chains. More interestingly, the fluorination of thiophene units can efficiently deepen the HOMO level and increase hole mobility of these new polymers. When these newly designed polymers were tested for their photovoltaic performance, remarkably high Voc (as high as 0.99 V) and high fill factor were achieved owing to the deep HOMO levels and high hole mobility, respectively. The synthetic chemistry to control the fluorination positions at thiophene units and the strategy of incorporation of fluorinated thiophene units to improve the efficiency of PSCs developed in this study will provide the community useful material design guidance.

Fluorinated thiophenes in PBnDT-TAZ for bulk heterojunction solar cells: The impact of fluorination

253rd National Meeting (2017)

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