Efficient Polymer Solar Cells Enabled by A-DA′D-A Type Acceptors with Alkoxypheny-Substituted Quinoxaline as the Fused-Ring Core

Polymer solar cells (PSCs) have made tremendous advances over the past three years due to the advantages offered by A-DA′D-A type small molecule electron-acceptors (SMEAs). To this day, the power conversion efficiencies (PCEs) have exceeded 19% in the PSCs incorporated by appropriate wide-bandgap polymer electron-donors and structure fine-tuned A-DA′D-A SMEAs. Despite high fill factor (FF) and short-circuit current (J_SC) that were successfully achieved in wide-bandgap polymers and these type SMEA-based PSCs, the open-circuit voltage (V_OC) was comparatively small, which hinders the performance further enhancement of PSCs. To boost the V_OC of A-DA′D-A type SMEA-based PSCs, two small molecules (BQ-4F and BQ-4Cl) were designed and synthesized, with the alkoxyphenyl-substituted quinoxaline-containing fused core as the A′ unit and difluorinated and dichlorinated end groups as the A segments. Benefiting from the introduced alkoxyphenyl-substituted quinoxaline-containing fused core, the two SMEAs all have elevated lowest unoccupied molecular orbital energy levels. PSCs based on PM6:BQ-4F achieved a high V_OC of 0.916 V and a promising PCE of 12.45%. Despite the device of PM6:BQ-4Cl realizing a slightly decreased V_OC of 0.906 V, it accomplished high J_SC (22.47 mA cm^-2) and FF (66.2%), thus giving rise to a high efficiency of 13.48%. Besides, after adding 20% of these two small molecules in PM6:Y6-based devices, the fabricated BQ-4F- and BQ-4Cl-based ternary devices achieved high PCEs of 16.75% and 16.77%, respectively, which were superior to the 15.81% value of the PM6:Y6 binary PSC. The findings from this study should assist in the design and construction of new quinoxaline-based A-DA′D-A SMEAs that would be used for the manufacture of binary and ternary PSCs with high V_OC and efficiencies.