Instability of prevailing small molecule acceptors in organic solar cells toward water/nucleophiles

Small molecule acceptors (SMAs) with A-D-A structures have become the key constituents in organic solar cells (OSCs); however, many of them have demonstrated instability toward external factors (including light, heat, water, and oxygen). This work explores the chemical reactivity of the double bond linking the donor (D) and acceptor (A) units in these molecules. Using a model compound, T4CN, constructed by a thiophene (T) and a strong acceptor moiety {2,2'-[1H-indene-1,3(2H)-diylidene]dimalononitrile, 4CN}, we show that water can break the double bond. The rate of this reaction depends on the electronic properties and steric hindrance. We found that common SMAs, including ITIC and Y6, react with nucleophiles such as amines and hydroxide, creating unintended products. These reactions can affect device performance and long-term stability. Among the tested materials, Y6, with its β-position side chains, showed the best resistance to water-induced degradation. Our findings highlight key factors affecting SMA stability and offer insights into designing more robust materials for future development of OSCs.