Tetramerized Small-Molecule Acceptor for Organic Solar Cells with Enhanced Efficiency, Stability, and Mechanical Robustness: Impact of Chain Length and Dispersity Effects

Discrete dimer or multimer acceptors have enhanced the stability of organic solar cells (OSCs) due to their slow diffusion kinetics resulting from their large molecular sizes. However, development of multimer acceptors with chain length longer than trimers has been challenging, which often require multistep reactions with low synthetic yield. In this study, a new discrete tetramer acceptor (TetA) using one-pot reaction and subsequent purification processes is developed. During the purification, dimer (DA) and trimer acceptor (TA) are also obtained. The OSCs for TetA demonstrate a higher power conversion efficiency (PCE) of 16.14% than those for the discrete acceptors with shorter chain lengths, such as monomer acceptor (MA, 12.85%), DA (14.31%), and TA (15.10%). Additionally, despite having a similar number-average molecular weight, TetA-based OSCs exhibit a significantly higher PCE (16.14%) compared to OSCs based on a mixture of the acceptors (MixA) with dispersity (10.72%). Furthermore, the TetA-based OSCs have the highest photostability and mechanical robustness among the series. For example, TetA-based OSCs demonstrate superior photostability (t(70%) lifetime = 2180 h under 1-sun illumination) and mechanical robustness (crack-onset strain (COS) = 8%) compared to those based on MA (t(70%) = 220 h, and COS = 2%), or MixA (t(70%) = 745 h, and COS = 6%).