4,7-Di-2-thienyl-2,1,3-benzothiadiazole with hexylthiophene side chains and a benzodithiophene based copolymer for efficient organic solar cells

In this work, a hexylthiophene substituted thiophene-bridge is used to connect benzodithiophene (BDT) and benzothiadiazole (BT) segments to build a new photovoltaic polymer (PBDT-DTTBT). The highest occupied molecular orbital (HOMO) level of the polymer is decreased by 0.2 eV to −5.47 eV compared to the reference polymer without a hexylthiophene side chain, which may be ascribed to the conformational torsion caused by the steric hindrance from the bulky hexylthiophene side group. The band gap of the polymer shows no obvious change compared to a polymer with a same backbone without a hexylthiophene side chain (around 1.7 eV). Polymer solar cells (PSCs) based on PBDT-DTTBT and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) exhibit a maximum power conversion efficiency (PCE) of 6.19% with an open-circuit voltage (Voc) of 0.80 V, a short-circuit current density (Jsc) of 12.72 mA cm−2 and a fill factor (FF) of 60.97%. The work provides a new method to design new conjugated polymers with a deep HOMO level and a low band gap for high Voc PSCs.