Design of Ethanol/Water Soluble Polymers/Fullerenes for Aqueous Processed Organic Solar Cells and Importance of Water Contents for Enhancement of Processability and Device Performance

Most of the efficient organic electronic devices have been fabricated using toxic halogenated and/or aromatic organic solvents, which are not desirable in the industrial-scale solution process. Herein, we present a new series of ethanol/water soluble fullerene derivatives with different forms of oligoethylene glycol (OEG) side chains to be utilized in fabricating eco-friendly polymer solar cells (eco-PSCs). Intriguingly, the addition of a typical anti-solvent, water, to ethanol is found to markedly enhance the solubility of the non-ionic OEG side chain-based conjugated polymer (PPDT2FBT-A) and the newly designed fullerene mono-adducts (PC₆₁BO12, PC₆₁BO15, and PC₆₁BO27). A water-ethanol co-solvent with a 1:1 molar ratio provided an increased solubility of PPDT2FBT-A from 2.3 to 42.9 mg mL^-1 and that of PC₆₁BO12 from 0.3 to 40.5 mg mL^-1. Owing to the improved processability, efficient eco-PSCs with a power conversion efficiency of 2.05% were successfully fabricated. To date, this value is the highest among the devices based on the active layer of water/ethanol-soluble conjugated materials. Our results not only provide important guidelines for the design of electroactive materials, but also signifies the importance of water addition in fabricating environmentally benign eco-PSCs.