Linear Fused-Ring Donor Polymer with Suppressed Aggregation and Deep HOMO Level for Efficient Organic Solar Cells

Abstract

Developing wide-bandgap (WBG) polymer donors with well-balanced optoelectronic and morphological properties remain crucial for advancing the performance and scalability of organic solar cells (OSCs). Herein, we report two D-A type WBG polymers P-BTST and P-DT-BTST incorporating a novel six-fused-ring acceptor unit, BTST, and a fluorinated FBDT donor core.The incorporation of the BTST unit enhances the planarity of the polymer backbone and weakens intramolecular charge transfer (ICT), resulting in deeper HOMO energy levels and blue-shifted absorption. Compared to the reference polymer P-DT-BT, both BTST-based polymers exhibit enlarged bandgaps, improved spectral complementarity with the low-bandgap acceptor Y6, and optimized energy level alignment. Notably, P-BTST demonstrates superior film morphology with tighter π-π stacking and reduced aggregation in solution, as evidenced by temperature-dependent UV-vis, AFM, and GIWAXS analyses. These combined effects yield a remarkable power conversion efficiency (PCE) of 15.04%, with a high short-circuit current density (J SC ) of 26.53 mA cm⁻² and open-circuit voltage (V OC ) of 0.83 V. This work highlights the potential of fused BTST-based acceptor units as effective building blocks for the design of high-performance WBG donor polymers in OSCs.