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 (Jsc) 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(3H)-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 Eg 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 Eg < 1.24 eV. This study presents a framework for designing low-cost, high-performance acceptors with an ultranarrow bandgap.