Halogenation on benzo[1,2-b:4,5-b′]difuran polymers for solvent additive-free non-fullerene polymer solar cells with efficiency exceeding 11%†
In this work, two novel two-dimensional (2D) benzo[1,2-b:4,5-b′]difuran (BDF)-based wide bandgap polymers were designed using a halogenation strategy by incorporating fluorine- and chlorine-substituted conjugated side chains, respectively. With the advantages of low-cost and environment-friendly furan units and halogen atoms, the BDF polymers PFTBDF-FBTA (F10) and PClTBDF-FBTA (F11) possessed lower-lying highest occupied molecular orbital (HOMO) energy levels with a large effect on their optical properties. In addition, the intermolecular interactions induced by F(Cl)⋯H(O) and/or F(Cl)⋯F(Cl) can also promote more preferred polymeric chain stacking behavior. In the application of non-fullerene polymer solar cells (NF-PSCs) with m-ITIC as the electron acceptor in the inverted device structure, the fluorinated F10-based NF-PSC exhibited a power conversion efficiency (PCE) of 10.5% with a higher open circuit voltage (Voc) of 0.908 V, which is much higher than that of the non-halogenated counterpart. In the chlorinated polymer F11:m-ITIC-based device, a further higher Voc of 0.921 V with enhanced current density was achieved, which results in a promising PCE of 11.37%, which is mainly attributed to the lower-lying HOMO and improved intermolecular stacking induced by chlorine. It is also noted that these performances were obtained without using any solvent additive or solvent annealing process, which is attractive to large area printing processing technology. These results demonstrate that halogenation engineering of BDF polymer is a very promising molecular design strategy for BDF polymers to reach the state-of-the-art photovoltaic performance, and these results also show that BDF is an efficient building block to construct highly efficient polymer donors for PSC applications.