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

Besides high short-circuit current density (J(sc)) and fill factor (FF), high open-circuit voltage (V-oc) is urgently necessary for obtaining high overall efficiencies of polymer solar cells (PSCs). In order to produce high V-oc 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 (E-g(opt)), 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 E-g(opt) (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 V-oc over 0.93 V. Notably, PBDTS-TPTI-based PSC achieves the highest PCE of 5.35% accompanied with the highest V-oc as far as 0.99 V and J(sc) up to 12.60 mA cm(-2). This work indicates side-chain engineering on polymers is an impactful and feasible approach to realize high V-oc PSCs by manipulating electronic levels of D-A copolymers.