Near-Infrared Acceptors Based on Cyano-Thiazole Terminals Enable Efficient Organic Solar Cells with Photoresponse beyond 1000 nm

The effective utilization of near-infrared (NIR) photons is pivotal for promoting the short-circuit current density ( J sc ) and further enhancing the photovoltaic performance of organic solar cells (OSCs). Here, we designed a strong electron-withdrawing terminal group, 2-(4-(4-cyanophenyl)thiazol-2(3 H )-ylidene)malononitrile (TCN), and introduced it into the Y-series acceptors to construct two asymmetric acceptor molecules, Y6-IF-CN and Y6-ICl-CN. The pronounced enhancement of intramolecular charge transfer (ICT) results in both acceptors with an ultranarrow E g of 1.23 eV, enabling the absorption of NIR photons. Interestingly, the developed TCN could be obtained via a two-step synthetic route with high yields, exhibiting distinctly low-cost characteristics. With the enhanced ICT and intermolecular interactions, Y6-ICl-CN shows a bathochromic shift in absorption and more ordered π-π stacking. Moreover, improved miscibility is also observed between D18 and Y6-ICl-CN. Consequently, OSC based on Y6-ICl-CN exhibits more efficient exciton dissociation, enhanced charge transfer, and suppressed carrier recombination, thus demonstrating a significantly improved efficiency of 9.72% compared to the Y6-IF-CN-based device (8.00%). Importantly, the figure of merit (FOM) value for D18:Y6-ICl-CN is 0.1433, representing the highest value reported for NIR-NFAs with an E g < 1.24 eV. This study presents a framework for designing low-cost, high-performance acceptors with an ultranarrow bandgap.