Dimerized Small-Molecule Acceptors Enable High-Performance Organic Solar Cells with High Open-Circuit Voltage and Prolonged Life-Time

The power conversion efficiencies (PCEs) of small molecule acceptor (SMA)-based organic solar cells (OSCs) have remarkably increased in recent years, but their thermal and long-term stability are insufficient for commercialization. In addition, the low open-circuit voltage (<i>V</i>_oc) of OSCs, compared to those of other types of solar cells (<i>i.e.</i>, perovskite solar cells), should be addressed to further improve their PCEs. Here, we demonstrate that the dimerization of an SMA resolves the performance limitations of SMA-based OSCs, in terms of stability and <i>V</i>_oc. The dimerized SMA (DYBO) connected by a benzodithiophene (BDT) conjugated linker affords OSCs with excellent PCEs (&gt; 18%), which outperform OSCs based on its monomer counterpart (MYBO, PCE ~ 17.1%). The electron-donating BDT linker in DYBO effectively upshifts the lowest unoccupied molecular orbital energy level and reduces the voltage loss, synergistically increasing the <i>V</i>_oc of DYBO-based OSCs. Importantly, DYBO-based OSCs exhibit excellent thermal and photo stability. For example, DYBO-based OSCs retain more than 80% of their initial PCE even after 6000 hr of thermal exposure at 100 °C, whereas the PCE of MYBO-based OSCs sharply degrade to ~80% of their initial value in only 36 hr. The improved stability of DYBO-based OSCs is attributed to (1) the high glass transition temperature (<i>T</i>_g) of DYBO of 179 °C (the<i> T</i>_g of MYBO is 80 °C) due to its extended chain, which stabilizes the blend morphology under thermal stress, and (2) the improved miscibility of DYBO with the BDT-based polymer donor. Thus, we highlight the significance of the molecular design of dimerized SMAs in realizing OSCs with excellent PCEs and stabilities.