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13.9%-Efficiency and Eco-Friendly Nonfullerene Polymer Solar Cells Obtained by Balancing Molecular Weight and Solubility in Chlorinated Thiophene-Based Polymer Backbones.

To industrialize nonfullerene polymer solar cells (NFPSCs), the molecular design of the donor polymers must feature low-cost materials and a high overall yield. Two chlorinated thiophene-based polymers, P(F-Cl) and P(Cl-Cl), are synthesized by introducing halogen effects like fluorine (F) and chlorine (Cl) to the previously reported P(Cl), which exhibits low complexity. However, the molecular weights of these polymers are insufficient owing to their low solubility, which in turn is caused by introducing rigid halogen atoms during the polymerization. Thus, they show relatively low power conversion efficiencies (PCEs) of 11.8% and 10.3%, respectively. To overcome these shortcomings, two new terpolymers are designed and synthesized by introducing a small amount of 1,3-bis(5-bromothiophen-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c']dithiophene-4,8-dione (BDD) unit into each backbone, namely, P(F-Cl)(BDD = 0.2) and P(Cl-Cl)(BDD = 0.2). As a result, both polymers remain inexpensive and show a better molecular weight-solubility balance, achieving high PCEs of 12.7% and 13.9%, respectively, in NFPSCs processed using eco-friendly solvents.