Fluorene Side-Chained Benzodithiophene Polymers for Low Energy Loss Solar Cells

Abstract

Here we design and synthesize one novel fluorene side-chained benzodithiophene (BDT) monomer for polymer solar cells (PSCs) donor. By copolymerizing this monomer with 4,7-di(thiophen-2-yl)-2,1,3-benzothiadiazole (DTBT) or 4,7-di(4-(2-ethylhexyl)-2-thienyl)-5,6-difluoro-2,1,3-benzothiadiazole (DTffBT), two donor–acceptor (D–A) conjugated polymers PFBDT–DTBT and PFBDT–DTffBT are prepared. PSCs are prepared with these polymers as donor and PC71BM as acceptor. The maximum power conversion efficiency (PCE) of the two polymers PFBDT–DTBT and PFBDT–DTffBT based PSCs is 7.13% (VOC = 0.90 V, JSC = 13.26 mA cm–2, and FF = 0.598) and 7.33% (VOC = 0.96 V, JSC = 13.24 mA cm–2, and FF = 0.577). The UV–vis absorption and electrochemical cyclic voltammetry test results show that F atoms in DTffBT unit present an obvious influence on intermolecular effect and molecular energy levels of polymers. Furthermore, the energy loss of two PSCs devices in this work is confirmed to be 0.78 and 0.71 eV, lower than most results based on BDT PSCs devices, which is critical to obtain high PCE PSCs devices with a decent trade-off between JSC and VOC.