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.