Composition-Tolerant Terpolymers for Efficient, Nonhalogenated Solvent-Processed Polymer Solar Cells

The design of terpolymers is a compelling strategy to improve the polymer solar cell (PSC) performance. However, the terpolymer composition at which the power conversion efficiency (PCE) of associated PSCs is typically optimized generally falls within a very narrow range, complicating the reproducible fabrication of optimal PSCs. In this study, a series of D–A1–D–A2 type random terpolymers (DTTz-X, where X = 10–60) is designed with structurally similar A1 and A2 accepting units. The sulfur atom of the benzothiadiazole subunit in the A1 (DTBT) unit is replaced by a nitrogen atom in the A2 (DTTz) unit, enabling the introduction of an alkyl solubilizing group while maintaining the overall structural properties of the terpolymer system. Consequently, the DTTz-X PDs maintain good optoelectronic properties at various A1/A2 ratios, while their processability in nonhalogenated solvents is significantly enhanced. Accordingly, the PSC performance of the terpolymer system shows good composition tolerance; i.e., the terpolymer system affords PSCs with PCEs exceeding 15% (up to 16.4%) over a broad range of DTTz compositions (10–40 mol %). This study establishes a useful design strategy for the development of efficient terpolymer donors for reproducible and eco-friendly fabrication of high-performance PSCs.