Benzo[1,2‐b:4,5‐b′]Difuran‐Based Polymer for Organic Solar Cells with 17.5% Efficiency via Halogenation‐Mediated Aggregation Control

Abstract The realization of high‐efficiency organic solar cells (OSCs) from renewable sources will bring a real green energy technology. Benzo[1,2‐b:4,5‐b′]difuran (BDF) is such a building block for photovoltaic polymers as it can be built from furfural, which is available from trees and vegetables. However, the device performance of BDF‐based polymers is limited by aggregation properties and unfavorable active layer morphology. Herein, two new BDF‐based wide bandgap‐conjugated polymers, PFCT‐2F and PFCT‐2Cl, are developed by copolymerizing the fluorinated or chlorinated BDF units with the 3‐cyanothiophene unit for use as electron donors in OSCs. Benefitting from the more rotatable nature of the side‐chain thiophene rings on the BDF unit, PFCT‐2Cl exhibits more adjustable aggregation, higher π–π stacking ordering, and appropriate miscibility with the electron acceptor. As a result, a high power conversion efficiency (PCE) of 17.2% is offered by PFCT‐2Cl, which is nearly two times higher than that of PFCT‐2F (8.9%). A more remarkable PCE up to 17.5% is further achieved by PFCT‐2Cl in a ternary OSC, which is the new efficiency record for BDF‐based polymers. This success proves the critical role of aggregation control in BDF‐based polymers and the bright future for constructing high‐performance OSCs from bio‐renewable sources.