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 (V_oc) of OSCs, compared to those of other types of solar cells (i.e., 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 V_oc. The dimerized SMA (DYBO) connected by a benzodithiophene (BDT) conjugated linker affords OSCs with excellent PCEs (> 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 V_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 (T_g) of DYBO of 179 °C (the T_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.