Synthesis of D-A low-bandgap polymer-based thieno[3,4-b]pyrazine and benzo[1,2-b:4,5-b']dithiophene for polymer solar cells

Three D-A low-bandgap polymer-based benzo[1,2-b:4,5-b']dithiophene (BDT) and thieno[3,4-b]pyrazine (TP) unit with different substituents were synthesized using Stille coupling. The substituent effects were explored. It is found that attaching the phenyl group at the 2,3-positions of thienopyrazine unit is beneficial to good coplanarity and regularity of molecular packing. Meanwhile, attaching the longer alkoxyl side chain at BDT unit or phenyl group at the 2,3-positions of thienopyrazine unit had the relatively deep HOMO (LUMO) energy levels. The three D-A polymers showed good thermal stability and exhibited broad absorption, low bandgap, and order packing structure. As a result, the highest PCE of 1.50 % was achieved for the polymer P2 with the substituents, including hexyldecyloxyl at the BDT unit and phenyl at the TP unit.