Small molecule donor third component incorporating thieno[3,2-b]thiophene unit enables 19.18% efficiency ternary organic solar cells with improved operational stability

The rational design of the third component to enhance the efficiency and stability of organic solar cell (OSC) is a critical and challenge. Herein, a thieno[3,2-b]thiophene (TT) substituted wide-bandgap small molecule donor BTTC, was designed, synthesized and incorporated into the state-of-the-art PM6:L8-BO binary to construct ternary OSC. The rigid side chain and planar molecule skeleton enables strong crystallinity and complementary absorption with binary system. The introduction of BTTC enhanced the molecular packing, fibril-like film and preferred vertical phase separation, yielding balanced and higher charge transport, more efficient exciton dissociation and suppressed charge recombination. Besides, lower energy loss was attained and thus leading to higher open-circuit voltage (VOC). Consequently, the introduction of BTTC achieved the highest efficiency of 18.8% along with extended operational stability. Furthermore, replacing L8-BO to BTP-eC9 further boosted the ternary PCE to 19.18%, standing one of the best charge management in ternary OSC. This work presents efficient rigid side chain substitution strategy to construct the third component of small molecule donor for high performance ternary OSCs with comprehensively improved photovoltaic parameters. This strategy broadens the selection of guest materials toward realizing the actual commercial application of OSCs.