Highly flattened donor-acceptor polymers based on fluoride-substituent acceptors for efficient heterojunction solar cells

Abstract

For high performance polymer solar cells, the effects of planarity and alkyl chain steric hindrance are systematically examined in the polymer poly[1-(4-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-6-methylbenzo[1,2-b:4,5-b′]dithiophen-2-yl)-6-methylthieno[3,4-b]thiophen-2-yl)-2-ethylhexan-1-one] (P4) constructed with a benzo[1,2-b:4,5-b′] dithiophene (BDT) plane with symmetrical thiophene side chains, poly[2-ethyl-1-(6-methyl-4-(6-methyl-4,4-dioctyl-4H-silolo[3,2-b:4,5-b′]dithiophen-2-yl)thieno[3,4-b]thiophen-2-yl)hexan-1-one] (P5), composed of silaindacenodithiophene, poly[2-ethyl-1- (6-methyl-4-(6-methyl-4-octyl-4H-dithieno[3,2-b:2′,3′-d]pyrrol-2-yl)thieno[3,4-b]thiophen-2-yl)hexan-1-one] (P6), based on pyrrole moieties and the corresponding F-containing polymers, poly[1-(4-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-6-methylbenzo[1,2-b:4,5-b′]dithiophen-2-yl)-3-fluoro-6-methylthieno[3,4-b]thiophen-2-yl)-2-ethylhexan-1-one] (P1), Poly[2-ethyl-1-(3-fluoro-6-methyl-4-(6-methyl-4,4-dioctyl-4H-silolo[3,2-b:4,5-b′]dithiophen-2-yl)thieno[3,4-b]thiophen-2-yl)hexan-1-one] (P2), and Poly[2-ethyl-1-(3-fluoro-6-methyl-4-(6-methyl-4-octyl-4H-dithieno[3,2-b:2′,3′-d]pyrrol-2-yl)thieno[3,4-b]thiophen-2-yl)hexan-1-one] (P3). Except for P1 and P4, the alkyl chain is directly linked to the main chain in the polymers. P1 has a high degree of flatness because flanking thienyls can decrease the torsion angle between conjugated units. In addition, the introduction of fluorine (F) on the thienothiophene acceptors of P4 reduces the dihedral angle between the BDT and thienothiophene, resulting in a more planar P1. Moreover, the introduction of short alkyl and thiophene side chains minimizes the intermolecular steric hindrance of P1 and ensures strong inter-chain aggregation and π-π stacking. However, the introduction of F in P2 and P3 increases the dihedral angle relative to non-F-containing P5 and P6. Finally, an efficiency of 7.1% is obtained for P1, leading to a 50% increase in power conversion efficiency (PCE) compared with P4. This work is a step towards enhanced polymer planarity and minimal intermolecular steric hindrance for the development of efficient polymer heterojunction solar cells.