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, eta(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 eta(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. (C) 2012 Elsevier B.V. All rights reserved.