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Non halogenated ink formulations for inkjet printed photoactive layers

Presentation held at MRS Fall Meeting & Exhibit, November 27 - December 2, 2016, Boston, Massachusetts
: Janietz, Silvia; Lange, Alexander; Gruber, Björn; Pingel, Patrick

Materials Research Society (Fall Meeting) <2016, Boston/Mass.>
Fraunhofer IAP ()
electronic material; inkjet printing
Anfrage beim Institut / Available on request from the institute

For the solution processing of organic photovoltaics on an industrial scale, the exclusion of halogenated solvents is a necessity. Inkjet printing is a mask-less and non-contact deposition technique that is potentially suited for rapid prototyping and/or manufacturing of large-area thin-film polymer organic semiconductor devices. Nevertheless two challenges need to be overcome before flat films with a suitable macromorphology are available with inkjet printing. On the one side is the limited solubility of most semiconducting polymer/fullerene blends in non-halogenated solvents results in ink formulations with low viscosities which poses limitations to the use of roll-to-roll compatible deposition processes. On the other side the fluid migration over the substrate and evaporation-induced so lvent flow within the fluid puddle, can severely distort film shape and profile, e.g., through the "coffee-stain" effect. Here we report the application of an amorphous absorber polymer based on fluorene with different content of dialkyl substituted diphenyl-benzopyrazine or diphenyl-thienopyrazine and triphenylamine units (PFDTBTP). This polymer was synthesized with high molecular weights (Mn ~ 105 g/mol) over the environmentally friendly Suzuki polymerization with very good solubility in non-chlorinated aromatic solvents. The blend with PC70BM leads to moderate efficiencies in inverted OPV cells of about 6% performance. In this case the deposition of the photoactive layer was done by spin coating from dichlorobenzene. Based on these results the blend layers were prepared with inkjet prin ting and a special emphasis was placed on finding a chlorine-free solvent system which could be used to produce devices with high performance. Solar cells with inkjet printed photoactive layers that were deposited from chloro-/trichlorobenzene had efficiencies of 3.5% which compares to 2.7% for layers that were printed from anisole/tetralin in a normal device structure. Printed blend films were analyzed AFM and EF-TEM to better understand the device efficiency. A coarser phase separation was found for films that were printed from anisole/tetralin which could be related to the partial solubility of PFDTBTP and PC61BM in anisole. In general, chlorine-free solvent systems can be used to inkjet printed PFDTBTP:PC61BM without greatly reducing the device efficiency. PFDTBTP and other polymers wi th similar structures are good candidates for industrial scale processing of organic solar cells because they can be printed and hazardous chlorinated solvents are not needed to achieve acceptable performance.