D-A non-equivalent random strategy to achieve donor polymers for stable organic solar cells with efficiency over 17%

High efficiency organic solar cells with great stability are crucial for future commercialization. Herein, a simple electron-donating monomer 2,6-dibromo-4,8-bis(octyloxy)benzo[1,2-b:4,5-b'] dithiophene (BDT-O) was synthesized as the third component to be introduced into the D-A backbone of PM6 by D-A non-equivalent random copolymerization. A series of wide-bandgap D-A-D-D1-type random terpolymer donors PM6-Ox (x = 5, 10, 30) were obtained, in which BDT-O served as the third component D1, fluorinated bis-2-ethylhexyl thiophene substituted benzodithiophene served as the D component and the typical 1,3-bis(4-(2-ethylhexyl) thiophen-2-yl)5,7-bis(2-alkyl) benzo[1,2-c:4,5-c']dithiophene-4,8-dione was used as the A component. Morphological and photoelectronic characterizations are investigated to reveal the effects of BDT-O unit on the photovoltaic (PV) performances of polymers. Small content of BDT-O unit introduced into PM6 can effectively tune the absorption, energy levels and molecular arrangement of polymers for better solar light-harvesting and enhanced charge transport. By fabricating organic solar cells (OSCs) with the common acceptor Y6, the PM6-O5:Y6-based device achieved a much higher PCE of 17.65% than that of the reference PM6:Y6-based device. Besides, PM6-O5:Y6based devices exhibit high thermal- and storage-stability, as well as outstanding photo-stability. This work will provide valuable clues on the molecular design of high-efficiency polymer donors via D-A non-equivalent random copolymerization strategy by incorporating a simple D1 component.