Benzannulation of Furan: A Strategy for Stable and High-Performance Furan-containing Giant Electron Acceptor with Efficiency Exceeding 20%

Abstract

The use of furan as a building block for electron acceptors in organic solar cells has been limited by its instability, particularly its susceptibility to Diels-Alder cycloaddition with singlet oxygen. In this study, we address the degradation challenges historically associated with furan through a benzannulation strategy, in which one of furan's double bonds is incorporated into an aromatic sextet, rendering the benzenoid aromaticity energetically unfavorable to disrupt via Diels-Alder cycloaddition. We synthesized a benzotrifuran-centered giant electron acceptor (BQx-O) with three Y6 arms, alongside benzotrithiophene (BQx-S) and benzotrisselenophene (BQx-Se) analogues for comparison. Quantum chemical calculations indicate that the BQx-O maintains a near-planar structure, promoting enhanced π-conjugation and molecular packing compared to the more twisted thiophene and selenophene counterparts. As a result, BQx-O achieved an outstanding power conversion efficiency (PCE) of 18.6% in binary OSCs, outperforming BQx-S (14.9%) and BQx-Se (12.6%), and setting a new benchmark for all furan-containing and all giant electron acceptors. In optimized ternary OSCs, BQx-O further reached a remarkable PCE of 20.11%. Moreover, the planar conformation of BQx-O and its reduced diffusion coefficient contribute to superior morphological and operational stability. This study demonstrates benzannulation as a straightforward yet powerful strategy for designing high-performance, photostable furan-containing acceptors, expanding possibilities for innovative electron acceptor designs.