Significantly improving the performance of polymer solar cells by the isomeric ending-group based small molecular acceptors: Insight into the isomerization

Compared to benzene-fused end-capping groups (EGs), thiophene-fused EGs have some unique characteristics due to the non-centrosymmetric structure of the thiophene ring, which make them easy to form different types of isomers. Here, we develop three isomeric brominated thiophene-fused EGs, which are linked to the IDTT core to acquire three novel isomeric small-molecule acceptors (SMAs) named ITC-2Br, ITC-2Br1, and ITC-2Br2. From ITC-2Br to ITC-2Br1, the change of the bromine substituent group on the thiophene ring has only a minor impact on the physicochemical properties and photovoltaic performance. However, from ITC-2Br to ITC-2Br2, the change in the fused sites on the thiophene leads to dramatically modified absorption, energy levels, and photovoltaic performance. Theoretical simulations provide an in-depth understanding of the absorption and electrochemical differences among the three acceptors. Thanks to the favorable properties, the ITC-2Br2-based polymer solar cells (PSCs) yield a significantly higher power conversion efficiency (PCE) (13.1%) than the devices based on ITC-2Br (10.9%) and ITC-2Br1 (11.9%). From the ITC-2Br-, ITC-2Br1-to the ITC-2Br2-based devices, the J_SC and FF exhibit a monotonic increase similar to the trend of PCE, which demonstrates the success of the isomerization strategy, highlighting its future prospects for the development of high-performance SMAs.