Simultaneously improving photovoltaic parameters of organic solar cells via isomerization of benzo[b]benzo[4,5]thieno[2,3-d]thiophene-based octacyclic non-fullerene acceptors

Fused-ring electron acceptors (FREAs) have attracted immense interest owing to their ability for facile structural modification and good thermal and optical characteristics. Among these acceptors, isomerized building blocks originate from multiple reaction sites affect the electronic structures, morphological properties and resulting photovoltaic performance, but have rarely been studied. Herein, three isomeric FREAs, Z1-aa, Z1-ab, and Z1-bb, were synthesized using different reaction sites of benzo[b]benzo[4,5]thieno[2,3-d]thiophene (BTBT)-based fused-ring cores and were used in organic solar cells (OSCs). The relatively high-lying lowest unoccupied molecular orbital of these isomeric FREAs provided a high open-circuit voltage (Voc). As compared to Z1-aa and Z1-ab, Z1-bb exhibited red-shifted absorption and a higher maximum molar extinction coefficient. When blended with PM6, Z1-bb-based OSCs exhibited more balanced charge transport compared to those with the PM6:Z1-aa and PM6:Z1-ab blend films, which favored higher short-circuit current density (Jsc) and fill factor (FF). As a result, the OSC devices based on Z1-bb exhibited a power conversion efficiency (PCE) of 12.66% with Voc = 0.98 V, Jsc = 18.52 mA cm-2, and FF = 70.05%, respectively, which are significantly higher than the values recorded for Z1-ab-based (PCE of 9.60%) and Z1-aa-based (PCE of 4.56%) devices. These results indicate that the isomerization of a fused-ring core originating from a special reaction site could be a promising approach to achieve high-performance OSCs with high Jsc, Voc, and FF.