Halogen-free solvent processing for sustainable development of high efficiency organic solar cells

Eliminating processing with halogenated solvents is desirable to achieve sustainable large-scale fabrication of organic solar cells. This work demonstrates a device processing approach completely free of halogenated solvents to yield high-performance (power conversion efficiency, η_P > 6%) polymer:fullerene bulk-heterojunction solar cells comprising a conjugated polymer PIDT-phanQ and [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM). Introducing 2% 1-methylnaphthalene (Me-naph) as a processing additive to toluene alleviates PC₇₁BM solubility problems, reduces phase domain size by two orders of magnitude, and boosts efficiency from η_P = 0.02% to 6.10%. Both AFM and TEM imaging show that the Me-naph additive promotes a more finely phase-separated morphology in spin-coated films, while photoluminescence quenching and photoinduced absorption spectroscopy confirm that this finer morphology results in both better exciton quenching and more efficient charge separation.