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Sensitivity of Molecular Packing and Photovoltaic Performance to Subtle Fluctuation of Steric Distortions within D–A Copolymer Backbones
journal contribution
posted on 2018-07-30, 00:00 authored by Jianhong Gao, Wei Wang, Shubin Liu, Chun Zhan, Shengqiang Xiao, Xinhui Lu, Wei YouHow 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.