Backbone Conformation Tuning of Carboxylate-Functionalized Wide Band Gap Polymers for Efficient Non-Fullerene Organic Solar Cells

Two carboxylate-functionalized wide
band gap polymers, 2TC-TT-BDTFT
and 2T-TTC-BDTFT, which feature a fluorinated benzodithiophene (BDTFT)-alt-2,5-di­(thiophen-2-yl)­thieno­[3,2-b]­thiophene
(2T-TT) backbone having different carboxylate attaching positions,
were designed and synthesized. By variation of the substitution position
of carboxylate groups on the 2T-TT unit, the backbone conformation
of the designed building blocks 2TC-TT and 2T-TTC and their corresponding
donor–acceptor polymers was fine-tuned as demonstrated by single
crystal study and DFT calculation, thus yielding a large device performance
difference in organic solar cells. As a result of the relatively higher
planarity of the 2T-TTC unit in which the two carboxylate groups were
attached on the inner thieno­[3,2-b]­thiophene moiety,
the 2T-TTC-BDTFT polymer exhibited a red-shifted UV–vis absorption,
stronger aggregation, and improved charge transport property than
its polymer analogue 2TC-TT-BDTFT, in which the two outer thiophene
rings were functionalized with carboxylate groups. Benefiting from
the improved exciton dissociation and charge collection efficiency,
better film morphology, and higher photoresponse, non-fullerene organic
solar cells based on 2T-TTC-BDTFT:m-ITIC achieved a power conversion
efficiency (PCE) of 11.15% with a fill factor (FF) of ∼70%,
while the 2TC-TT-BDTFT:m-ITIC cells showed a relatively lower PCE
of 9.65% and FF of 59.31%. The much higher FF of 2T-TTC-BDTFT-based
solar cells reflects the great merit of the carboxylation on thienothiophene
moiety rather than the outer thiophene counterpart. Therefore, the
modulation of the carboxylate position on polymer backbones is an
efficient strategy to tune the backbone conformation, interchain packing,
film morphology, and the resulting optical, electrical, and photovoltaic
properties. Moreover, both the 2T-TTC-BDTFT:m-ITIC and 2TC-TT-BDTFT:m-ITIC
solar cells showed excellent stability during annealing and long-term
storage. These results demonstrate that carboxylate-functionalized
2T-TTC and 2TC-TT have great potentials as a weak electron-accepting
building block for wide band gap polymers for high-performance non-fullerene
organic solar cells, and the carboxylate position on the polymer backbones
is critical for performance improvement of organic photovoltaic devices.