Sensitivity of Molecular Packing and Photovoltaic Performance to Subtle Fluctuation of Steric Distortions within D–A Copolymer Backbones

How molecular conformation variation due to the existence of steric torsions within the conjugated backbones plays a role in molecular packing and resultant polymer solar cell (PSC) performance was investigated by synthesizing two isomeric alternating D–A copolymers. In these copolymers, poly­{3′,4′-dihexyl-(2,2′:5′,2′′-terthiophene)-5,5′′-diyl-alt-[4′,7′-di-2-(4-(2′-ethylhexyl))­thienyl-(5′,6′-difluorobenzo­[c]­[1′,2′,5′]­thiadiazole)]-5,5-diyl} (PTDTffBT­(C6/EH)) and poly­{3′,4′-di­(2′-ethylhexyl)-(2,2′:5′,2′′-terthiophene)-5,5′′-diyl-alt-[4′,7′-di-2-(4-hexyl) thienyl-(5′,6′-difluorobenzo­[c]­[1′,2′,5′]­thiadiazole)]-5,5-diyl} (PTDTffBT­(EH/C6)), which had comparable molecular weight, the linear hexyl and branched 2-ethylhexyl chains are interchanged between the donor and the acceptor units. Such molecular design could offer two isomeric donors with limited conformational steric distortions by positioning the given alkyl chains with fine steric disparity in the same conjugated backbone. The interchange of the side chains caused a fluctuation of ∼5° of the dihedral angles between the thiophenes within the donor units and between the ending thiophenes from the adjacent donor and acceptor units. The subtle transform on backbone steric distortions of the two copolymers leads to a negligible impact on electronic structures but a distinct one on molecular packing in film. The copolymers both embody polymorph molecular packing with preferential edge-on orientation in neat films. The (100) and (010) distance, corresponding to the lamellar stacking between the alkyl chains and the π–π stacking between the conjugated backbones, are both improved in the PTDTffBT­(C6/EH) film with enhanced crystallinity than that in the PEDTffBT­(EH/C6) film. Similar molecular packing feature remains for the BHJ blends of the two copolymers with the acceptor of PC71BM. Moreover, PTDTffBT­(C6/EH) exhibits the apparant coexistence of face-on orientation with improved crystallinity. The PTDTffBT­(C6/EH): PC71BM PSC devices offer a much improved maximum power conversion efficiency (PCE) of 8.24% over 6.13% of the PTDTffBT­(EH/C6) device, mainly due to more efficient charge generation and balanced charge transport resulted from the optimized film microstructure. The investigation clearly shows the sensitivity of molecular packing and corresponding PSC device performance to subtle steric distortions within conjugated backbones.