Benzodithiophene with multiple side-chains for efficient wide-bandgap D–A copolymers

Abstract

The limited selections of wide-bandgap polymer donors have prevented the further increase in the power conversion efficiency (PCE) of organic solar cells. In this work, we report two novel wide-bandgap alternating copolymers (PL4 and PL5), which were synthesized by copolymerizing a new benzodithiophene-derived electron-rich unit (D) with two electron-deficient units (A) of 5,6-difluoro-2-(2-hexyldecyl)-4,7-di(thiophen-2-yl)-2H-benzo[d][1,2,3]triazole and 1,3-bis(2-ethylhexyl)-5,7-di(thiophen-2-yl)-4H,8H-benzo[1,2-c:4,5-c′]dithiophene-4,8-dione, respectively. PL5 shows a broader and red-shifted absorption spectrum compared to PL4 due to the more electron-withdrawing feature of its A moiety. Moreover, the energy levels of PL5 are relatively down-shifted compared to those of PL4. When blended with a low-bandgap non-fullerene acceptor (Y6), the best-performing PL5:Y6-based device exhibits a significantly higher PCE of 15.77% with an enhanced V_OC of 0.818 V, an excellent J_SC of 26.60 mA cm⁻², and a higher fill factor (FF) of 72.47% in comparison with the PL4:Y6-based PSC which shows a PCE of 7.53% with a V_OC of 0.718 V, a J_SC of 21.60 mA cm⁻², and a FF of 48.58%. The improved PCE of the PL5:Y6-based device can be ascribed to its more efficient charge transfer, dramatically increased charge carrier mobilities and reduced charge recombination.