An efficient star-shaped fused-ring electron acceptor with C3h-symmetric core via thieno[3,2-b]thiophene extending conjugation strategy

Abstract

The thieno[3,2-b]thiophene (TT) unit is a classic building block to construct high-performance fused-ring electron acceptors (FREAs) for organic solar cells (OSCs). In this work, a star-shaped FREA BTTCTT-ICF based on a new C_3h-symmetric fused-ring core BTTCTT was designed and synthesized by introducing the TT unit to extend the π-conjugation. Due to its strong electron-donating ability and efficient π-electron delocalization, BTTCTT-ICF exhibited a narrow optical bandgap of 1.54 eV and a high absorption extinction coefficient of 3.6 × 10⁵ M⁻¹ cm⁻¹. The blend film of BTTCTT-ICF and the wide bandgap polymer PM6 produced a complementary absorption and a broad EQE spectral range covering 300 to 850 nm. When treated via thermal annealing, the PM6:BTTCTT-ICF blend film exhibited more ordered molecular packing and a suitable phase morphology, which improved its charge mobility and device efficiency. Consequently, the PM6:BTTCTT-ICF-based inverted device achieved the best power conversion efficiency (PCE) of 10.39% with a high short-circuit current density (J_sc) of 16.77 mA cm⁻², open-circuit voltage (V_oc) of 0.91 V and fill factor (FF) of 68.08%. Our results demonstrate that the extension of the star-shaped fused-ring core by TT units is a rational and effective strategy for achieving highly efficient star-shaped FREAs.