Tuning solvent quality to push performance of donor-acceptor films in OPVs*

Organic photovoltaics (OPVs) have the potential for high specific power, flexibility, and solution processability. Yet the factors governing performance of OPVs are complex due to the interplay between chemistry, processing, and morphology. Tuning the solvent quality prior to casting the active layer can further improve the power conversion efficiency (PCE). Poly[(2,6-(4,8-bis(5-(2-ethylhexyl-3-fluoro)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophene))-alt-(5,5-(1’,3’-di-2-thienyl-5’,7’-bis(2-ethylhexyl)benzo[1’,2’-c:4’,5’-c’]dithiophene-4,8-dione)] (PM6) and 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2",3’':4’,5']thieno[2',3':4,5]pyrrolo[3,2-g]thieno[2',3':4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (Y6) were used to fabricate OPV’s in conjunction with a variety of solvent additives using chloroform as a host solvent. The addition of small amounts of acetone leads to a specific regime of solvent quality where Y6 is well solvated but PM6 has marginal solubility. This leads to perturbations to the active layer bulk heterojunction morphology and improved short circuit current, open circuit voltage, fill factor, and PCE in resulting devices.