A universal halogen-free solvent system for highly efficient polymer solar cells†
Power conversion efficiencies (PCEs) of state-of-the-art polymer solar cells (PSCs) have been promoted to over 9%. However, halogenated solvents like chlorobenzene (CB), o-dichlorobenzene (DCB), 1,8-diiodooctane (DIO) or their mixtures are still predominately used in the fabrication of these high performance PSCs. With the rapid progress in PCEs, removing the halogenated solvents from the fabrication processes of PSCs becomes an urgent task for the practical utilization of PSC technology. In this study, a halogen-free solvent system consisting of o-xylene (XY) and N-methylpyrrolidone (NMP) was successfully applied in the fabrication of the PSCs based on a variety of highly efficient polymers including PBDT-TS1 and other eight types of photovoltaic polymers. Notably, utilizing an XY–2% NMP mixture as a processing solvent, the PBDT-TS1/PC71BM-based PSC realized a PCE of 9.47%, which has been the highest value in halogen-free solvent processed PSCs until now. The photovoltaic properties and nanoscale morphology clearly indicated that the halogen-free solvent system featuring the XY–NMP mixture can replace the role of the widely utilized halogenated solvents in fabricating environmentally friendly PSCs with high efficiency.