Designing narrow bandgap small-molecule acceptors with unilateral vinylene π-bridge for high-performance optoelectronic devices

Abstract

Narrow bandgap (NBG) small-molecule acceptors (SMAs) are crucial for enhancing the performance of organic solar cells (OSCs) and near-infrared organic photodetectors (NIR OPDs). Two novel SMAs, namely, BTP-1V-4F and BTP-1V-4Cl, were developed via a synergistic strategy combining unilateral vinylene π-bridge introduction and terminal halogen atom modulation. The unilateral vinylene π-bridge not only effectively extended the intramolecular conjugated electron cloud distribution, promoting a narrow bandgap and a redshifted absorption, but also synergistically suppressed molecular disorder in conjunction with the terminal halogen atoms, thereby optimizing the orderly packing. Results revealed that although BTP-1V-4Cl possessed a narrower bandgap and higher crystallinity, BTP-1V-4F exhibited better compatibility with the donor material and achieved a more ordered and well-proportioned crystalline morphology in the blends. Consequently, BTP-1V-4F devices delivered superior performance. The NBG OSCs achieved a power conversion efficiency (PCE) of 14.2% with an open-circuit voltage of 0.8 V. Meanwhile, the NIR OPDs exhibited a remarkable specific detectivity (D*) of 3.85 × 10¹³ jones with an ultralow dark current density (J_D) of 5.22 × 10⁻¹⁰ A cm⁻² and a responsivity of 0.497 A W⁻¹ at 870 nm and −1 V bias. This work demonstrates the promise of the BTP-1V-4X series for applications in semi-transparent cells, tandem cells, and photodetectors.