Design Strategy to Enhance the Performance of Benzothiadiazole-Based Polymers by Tweaking Their Planarity and Backbone Curvature for Non-Fullerene Organic Solar Cells
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Here, we demonstrate a new synthetic strategy for converting low-energy benzo[c][1,2,5]thiadiazole (BT)-based polymers into efficient polymeric donors for non-fullerene acceptor-based organic solar cells (NFA–OSCs). A highly planar 5,6-difluoro-benzo[c][1,2,5]thiadiazole (ffBT)-based alternating polymer, P1, comprising electron-rich 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene (BDTT) and strong electron-deficient 5,6-difluoro-4,7-bis(4-octylthiophen-2-yl)benzo[c][1,2,5]thiadiazole (DTffBT) units is prepared. Additionally, two ternary polymers, P2 and P3, are prepared by replacing 25% and 50% of the DTffBT unit on the P1 backbone with a weak electron-deficient 2,5-dioctyl-4,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione (DTPPD) unit, which has a twisted but well-controlled wavy backbone. The properties of the resulting polymers, P1–P3, are investigated to understand the effects of changing the planarity and curvature of the backbone of the BT-based polymers. Notably, increasing the concentration of the DTPPD unit on P1 results in a blue-shift in absorption band and relatively deep energy levels. Further, the absorption and X-ray diffraction (XRD) spectra of the polymers confirm that the π–π stacking of the polymers is decreased by increasing the amount of DTPPD units on P1. The NFA–OSCs fabricated using P1–P3 as the electron donor and ITIC as the electron acceptor afford maximum power conversion efficiencies ( PCE ) of 3.22%, 3.99%, and 5.16%. The PCE is further improved to 2.46%, 4.52%, and 7.54%, respectively, using Y6 instead of ITIC. Overall, the photovoltaic performance of the BT-based polymers is significantly improved from 2.46% to 7.54% by lowering their planarity and/or changing their backbone curvature.Keywords:
Planarity testing
Tweaking
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