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

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 pi-pi 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.