Vinylene π-bridge: A simple building block for ultra-narrow bandgap nonfullerene acceptors enable 14.2% efficiency in binary organic solar cells

Ultra-narrow bandgap (ultra-NBG) nonfullerene acceptors (NFAs) are essential for advancing the development of organic solar cells (OSCs). In this paper, a synergistic strategy of π-bridge and asymmetric engineering is applied to state-of-the-art Y-series NFAs, and it achieves ultra-NBG NFAs for efficient OSCs. Introduction of vinylene π bridges into the Y-series NFA in a symmetric pattern, resulting in a molecule, BTP-2V-2F, which exhibits moderate power conversion efficiency (PCE) of 11%, despite extending absorption near-infrared wavelength range. Through tailoring the number of vinylene π-bridges in the asymmetric backbone, another molecule, BTP-1V-2F, results which not only shows near-infrared absorption improvement, but also low-lying frontier molecular orbital levels. Benefiting from the maintained photocurrent along with the higher open-circuit voltage and the fill factor, a superior efficiency of 14.24% is achieved, which is one of the highest values for binary single-junction OSCs based on ultra-NBG NFA with a bandgap below 1.29 eV. The results demonstrate that the effectiveness of the vinylene π-bridge and asymmetric engineering on designing ultra-NBG NFAs, which aids the avenue toward developing more efficient ultra-NBG-based OSCs.