Effect of alkylthiolated hetero-aromatic rings on the photovoltaic performance of benzodithiophene-based polymer/fullerene solar cells

Three new donor–acceptor (D–A) copolymers based on benzodithiophene (BDT) with alkylthiolated aromatic rings furan, thiophene, and benzene and 5,6-difluoro-4,7-bis(4-(2-octyldodecyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole (DTBT) units, namely PBDTFS-DTBT, PBDTTS-DTBT, and PBDTPS-DTBT, respectively, were synthesized and applied in solution-processed fullerene type organic solar cells (OSCs). The target polymers were systematically investigated by thermogravimetric analysis (TGA), ultraviolet-visible (UV–vis) absorption spectra, cyclic voltammetry (CV), density functional theory (DFT) and photovoltaic measurements. The experimental results imply that the electron push-pull effect and torsion angles between the conjugated side hetero-aromatic rings and BDT core play a key role in tuning the optical absorption bands, molecular energy levels, hole mobilities, and morphology of the corresponding polymers. The OSCs based on PBDTTS-DTBT and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) show the highest power conversion efficiency (PCE) of 7.80% among the three polymeric analogues, with an open-circuit voltage (V_oc) of 0.92 V, a short-circuit current density (J_sc) of 11.95 mA cm⁻² and fill factor (FF) of 70.76%. Hence, the results in this work indicate that the photovoltaic properties of the BDT-BT-based polymers can be effectively modulated by introducing conjugated alkylthiolated hetero-aromatic rings with the varied electron donating ability and steric hindrance.