Chlorinated Benzo[1,2-b:4,5-c ']dithiophene-4,8-dione Polymer Donor: A Small Atom Makes a Big Difference

The position of a chlorine atom in a charge carrier of polymer solar cells (PSCs) is important to boost their photovoltaic performance. Herein, two chlorinated D-A conjugated polymers PBBD-Cl-alpha and PBBD-Cl-beta are synthesized based on two new building blocks (TTO-Cl-alpha and TTO-Cl-beta) respectively by introducing the chlorine atom into alpha or beta position of the upper thiophene of the highly electron-deficient benzo[1,2-b:4,5-c ']dithiophene-4,8-dione moiety. Single-crystal analysis demonstrates that the chlorine-free TTO shows a pi-pi stacking distance (d(pi-pi)) of 3.55 angstrom. When H atom at the alpha position of thiophene of TTO is replaced by Cl, both pi-pi stacking distance (d(pi-pi) = 3.48 angstrom) and Cl center dot center dot center dot S distance (d(Cl-S) = 4.4 angstrom) are simultaneously reduced for TTO-Cl-alpha compared with TTO. TTO-Cl-beta then showed the Cl center dot center dot center dot S non-covalent interaction can further shorten the intermolecular pi-pi stacking separation to 3.23 angstrom, much smaller than that of TTO-Cl-alpha and TTO. After blending with BTP-eC9, PBBD-Cl-beta:BTP-eC9-based PSCs achieved an outstanding power conversion efficiency (PCE) of 16.20%, much higher than PBBD:BTP-eC9 (10.06%) and PBBD-Cl-alpha:BTP-eC9 (13.35%) based devices. These results provide an effective strategy for design and synthesis of highly efficient donor polymers by precise positioning of the chlorine substitution.