Linear fused-ring donor polymer with suppressed aggregation and a deep HOMO level for efficient organic solar cells

Abstract

Developing wide-bandgap (WBG) polymer donors with well-balanced optoelectronic and morphological properties remains 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 wide 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.