Green-Solvent Engineering for Depositing Qualified Phenyl-C61-butyl Acid Methyl Ester Films for Inverted Flexible Perovskite Solar Cells

Flexible perovskite solar cells (fPSCs) with the inverted structure (p-i-n structure) show a promising commercialization future, owing to their lightweight and high efficiencies. Phenyl-C61-butyric-acid methyl ester (PCBM) is widely used as the n-type material due to its excellent conductivity and solvent processability. However, the commonly used chlorobenzene (CB), as the solvent of PCBM solution, is well recognized as a halogenated contaminant in the environment and is harmful to human health. There is an imperative need to develop nonhalogenated green solvents to replace CB. This work discusses the selection of green solvents based on the Hansen solubility parameters (HSPs). It is found that 2-methylanisole (2-MEA) acts as an excellent alternative to CB, with which high-quality PCBM films could be deposited. The experimental and theoretical studies demonstrate that 2-MEA can suppress the formation of PCBM aggregations during the solvation process compared with CB. The more uniform PCBM film achieved from the 2-MEA solution benefits carrier extraction at the electronic transport layer (ETL)/perovskite interface. As a result, better efficiencies are received among fPSCs based on the 2-MEA-processed PCBM, superior to that of the fPSCs based on the CB-processed PCBM. Moreover, using 1,8-diiodooctane (DIO) as a solvent additive is proven to further increase the solubility of PCBM in the 2-MEA solution, resulting in enhanced efficiencies of the flexible PSCs by more than 5% (from 19.25 to 20.30%). The developed green-solvent strategy is of great importance for the future large-scale production of environmentally sustainable fPSCs.