Carboxylic-substituted polymer design strategy enabled halogen-free solvent processed efficient organic solar cells

Carboxylic ester groups can be rationally utilized to regulate the energy levels and crystallinity in molecular design of conjugated polymer donors for organic solar cells (OSCs). In this work, a new polymer named PB4T was designed and prepared by copolymerization of a carboxylate substituted tetra-thiophene unit with the commonly used BDT unit. Due to the strong electron-withdrawing capability and the symmetric conformation of carboxylic ester substitution, PB4T exhibits appropriate HOMO energy level and stable planar geometry. When matching PB4T with five different NFAs with varied energy levels and bandgaps, including eC9, eC9-2Cl, IT-4F, BTA3, and A4T-3, all the OSCs processed with the halogen-free solvent show good photovoltaic performance. The PB4T:eC9- and PB4T:eC9-2Cl-based OSCs can realize high PCEs of 15.50 % and 15.32 %, respectively. This work demonstrates that PB4T has good universality to work well with newly-emerging NFAs, and carboxylate substituted thiophene units show great potential in constructing high-efficiency polymer donors.