Manipulating electronic energy levels of wide-bandgap D–A copolymers via side-chain engineering to realize high open-circuit voltage polymer solar cells

Abstract Besides high short-circuit current density (Jsc) and fill factor (FF), high open-circuit voltage (Voc) is urgently necessary for obtaining high overall efficiencies of polymer solar cells (PSCs). In order to produce high Voc PSCs, herein, we developed three wide-bandgap donor–acceptor (D-A) alternate copolymers (PBDTO-TPTI, PBDTT-TPTI, and PBDTS-TPTI ) of benzodithiophene (BDT) and thienopyridothieno-isoquinoline-5,11(4H,10H)-dione (TPTI) moieties. These copolymers possess a uniform BDT-TPTI framework, but various side chains (alkoxyl, alkylthienyl, alkylthiothienyl) on the BDT unit. The resultant data convincingly reveal that the spectral absorption, optical bandgap (Egopt), aggregation characteristic, energy levels, charge transport properties and active layer morphology of the D–A copolymers can be effectively manipulated via side-chain engineering on the BDT segment. The gradually increased Egopt (1.92–1.95 and then to 1.97 eV) and gradually decreased HOMO/LUMO levels (−5.43/−3.47 to −5.54/−3.53 and then to −5.56/−3.76 eV) are found while the side group on the BDT unit is varied from alkoxyl (PBDTO-TPTI) to alkylthieyl (PBDTT-TPTI) and then to alkylthiothienyl (PBDTS-TPTI). Importantly, the geometric and optoelectronic properties of these polymers are supported by theoretical predictions. PSCs based on all the three copolymers with a fullerene-based acceptor (PC71BM) exhibit power conversion efficiencies (PCEs) exceeding 5% and a Voc over 0.93 V. Notably, PBDTS-TPTI-based PSC achieves the highest PCE of 5.35% accompanied with the highest Voc as far as 0.99 V and Jsc up to 12.60 mA cm−2. This work indicates side-chain engineering on polymers is an impactful and feasible approach to realize high Voc PSCs by manipulating electronic levels of D–A copolymers.