Regioregular polymers containing benzodithiophene and thienothiophene segments with different electron donating side chains for high-performance polymer solar cells

This study describes a systematic study on side chain effect in the poly(thieno[3,4-b]thiophene)benzo[1,2-b:4,5-b′]dithiophene (PTB) polymer series based regioregular polymers with D₁-A-D₂-A configuration, denoted as PTT-BDT(R₁)-TT-BDT(R₂). The synthesized three regioregular polymers employ the same conjugated backbone but with different side chains (R₁, R₂), P1; hexyldecyl thiophene (HD, R₁) and ethylhexyl thiophene (EH, R₂), P2; hexyldecyl thiophene (HD, R₁) and ethylhexylthio thiophene (S-EH, R₂), P3; hexyldecylthio thiophene (S-HD, R₁) and ethylhexylthio thiophene (S-EH, R₂). Introduction of different side chains influenced optical, electrochemical, molecular packing properties, and device performance. The P3 polymer with alkylthio thiophene groups as R₁ and R₂ showed much broader light absorption and lower HOMO level than other polymers with relatively less alkylthio thiophene groups. Furthermore, the P3 polymer exhibited favorable polymer orientation and short π-π stacking distance for efficient charge transport in the BHJ PSCs. The P3 based bulk-heterojunction (BHJ) polymer solar cell (PSC) showed an improved PCE of 7.20% while the P1 and P2 based BHJ PSC devices showed PCEs of 5.27% and 6.45%, respectively. This result, within our knowledge, is the highest record among BHJ PSCs based PTB polymer series composed of benzo[1,2-b:4,5-b']dithiophene (BDT) segment and non-fluorinated alkyl ester (COOR) side chain substituted thieno[3,4-b]thiophene (TT) segment.