A Benzobis(thiazole)-Based Copolymer for Highly Efficient Non-Fullerene Polymer Solar Cells

In recent years, non-fullerene polymer solar cells have attracted much attention due to their great potential for achieving high power conversion efficiencies, and in addition to the already existing donor polymers, varieties of excellent acceptors have been developed. To further improve the performance, the main challenge is now to identify perfect donor–acceptor pairs with suitable electronic properties and complementary optical absorption. In this article, we have investigated a donor–acceptor alternating copolymer poly[(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene)-2,6-diyl-alt-(2,6-bis(5-bromo-4-(2-butyloctyl) thiophen-2-yl)benzo[1,2-d:4,5-d′]bis(thiazole))] with benzo[1,2-d:4,5-d′]bis(thiazole) (BBT) as the accepting unit and benzo[1,2-b:4,5-b′]dithiophene as the donor unit. The polymer shows a wide band gap of 2.1 eV with absorption peaks at 515 and 554 nm, matching well with the strongest region of the solar radiation spectrum. A blend of this polymer with the narrow-bandgap acceptor ITIC-F, (3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-6-fluoro-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene)(mixture with 7-fluoro-indanone isomer), as the active layer in combination with a solvent vapor annealing process led to solar cells exhibiting a high efficiency of 13.3% with an open-circuit voltage of 0.91 V, a short current density of 20.9 mA cm–2, and a fill factor of 0.70. The solvent vapor annealing method led to an improvement of the molecular packing, exciton dissociation, and charge transport, thus enhancing the power conversion efficiency. It is noteworthy that external quantum efficiency spectra show excellent photoresponse, especially in the wavelength range from 430 to 570 nm, demonstrating that this donor–acceptor combination efficiently absorbs in the wavelength range where the solar radiation spectrum has its maximum. The results indicate that polymers based on BBT are very promising candidates for high-performance non-fullerene polymer solar cells.