Difluoroterthiophene as promising block to build highly planar conjugated polymer for polymer photovoltaic cells

Abstract

Developing highly planar semiconducting polymer is essential for achieving high mobility and improving the photovoltaic performance of bulk-heterojunction polymer solar cells. In this contribution, two novel low-bandgap donor-acceptor copolymers, P1-3T and P2-3T2F, consisting of electron-accepting benzothiadiazole acceptor segment and electron-donating terthiophene donor segment with and without fluorine atoms were designed and synthesized. The density functional theory calculations and ultraviolet–visible absorption studies demonstrate that the fluorinated P2-3T2F polymer has more planar backbone conformation, deeper HOMO level, broader absorption spectrum with a vibronic shoulder peak, and higher absorption coefficient compared with the non-fluorinated analogue polymer, P1-3T. Furthermore, P2-3T2F exhibits good crystallinity and high hole mobility, presumably due to the well-ordered lamellar packing and the π-π stacking interaction between the backbones of the conjugated polymers. The optimized polymer solar cell based on P2-3T2F:PC61BM exhibits a short-circuit current density (Jsc) of 12.77 mA cm−2, a fill factor (FF) of 68.99%, an open-circuit voltage (Voc) of 0.80 V, and a maximum power conversion efficiency (PCE) of 7.14%, which is approximately 46% higher than that of the P1-3T:PC61BM device. The enhanced PCE is primarily due to increased light-harvesting ability and interconnected morphology with finely dispersed polymer-rich and PC61BM-rich domains, which improves the efficiency of exciton dissociation and rises the mobility of charge carriers, thus boosting the short-circuit current density and the FF value. Moreover, the relatively deep HOMO of P2-3T2F effectively increases the Voc.