Modulation of Dielectric Constant and Photovoltaic Properties of 2,1,3-benzothiadiazole-based Alternating Copolymers by Adding Fluorine Atoms to the Backbone of Polymers

In organic solar cells (OSCs), fluorine (F) substitution of conjugated polymers has a variety of effects on molecular properties and device performance. In this work, three conjugated polymers, namely PBDT-4F-BTs, PBDT-8F-FBTs, and PBDT-10F-FBTs, with 4, 8 and 10 fluorine atoms in the backbone were prepared and characterized. The conjugated polymers PBDT-4F-BTs, PBDT-8F-FBTs and PBDT-10F-FBTs showed the same highest occupied molecular orbital (HOMO) energy levels of -5.58 eV, which did not decrease with the increasing number of F atoms in each repeat unit of polymers. As the number of fluorine atoms in each repeat unit of polymers in the backbone increases from 4 to 10, the dielectric constant (epsilon(r)) increases from 4.5 to 6.2 at 1 kHz. The bimolecular compounding was suppressed by introducing F into the polymer backbone and influenced the device compounding kinetics of OSCs from the blend films of polymers paired with the phenyl-C-71-butyric-acid-methyl ester (PC71BM). Thereby the fill factor (FF) and power conversion efficiency (PCE) of the OSCs devices have been improved, the fluorine atoms in each repeat unit of polymers backbone increased from 4 to 10, FF increased from 48.7 % to 70.0 %, and the corresponding PCE increased from 3.43 % to 6.46 %.