Rheological characterization of liquid electrolytes for drop-on- demand inkjet printing Merve € Ozkan a, * , Katarina Dimic-Misic a , Alp Karakoc a, c , Syed Ghufran Hashmi b , Peter Lund b , Thad Maloney a , Jouni Paltakari a a Department of Forest Products Technology, School of Chemical Technology, Aalto University, Finland b New Energy Technologies Group, Department of Applied Physics, Aalto University, Finland c Department of Civil and Environmental Engineering, University of California Los Angeles, Finland article info Article history: Received 31 May 2016 Received in revised form 2 September 2016 Accepted 3 September 2016 Keywords: Inkjet printing Rheology Dilatancy Electrolyte Dye-sensitized solar cells abstract Physico-chemical properties of inkjet printing liquids significantly affect the quality of print-out, thus being the key parameter in the performance of printed electronic device (PEDs). Complex hydrodynamic interactions that inks are subjected to in an inkjet printing device has an influence on their rheological response, thus final drop formation, jetting, and drying kinetics. This paper provides a systematic comparison of three PED electrolytes based on different solvents i.e. Sulfolane, 3-Methoxypropionitrile and Acetonitrile that gave them different physico-chemical properties. Rheological properties of prin- ted electrolytes were found to strongly influence the quality of print-outs, which is investigated both optically and morphologically. Best printing results were obtained with the sulfolane-based electrolyte that has the most uniform temperature and shear rate dependent rheological behavior as well as the lowest evaporation rate. By carefully controlling the printing temperature window, it is possible to subject PED electrolytes to higher shearing viscosity profiles while avoiding undesirable dilatant behavior which results in clogged printing nozzles and disrupted droplet trajectory. © 2016 Elsevier B.V. All rights reserved. 1. Introduction As the most mature practice of direct writing (DW) technology [1], inkjet printing offers mask-free material deposition with micrometer accuracy and cost saving of the materials [2]. For the last decades drop-on-demand (DoD) inkjet printing has been extensively used in lab-scale research and it has been one of the most promising technique for massive production of printed elec- tronic devices (PEDs), in particular integrated circuits, small an- tennas, solar cells, batteries, thin-film transistors and light-emitting diodes [3e8]. Development of PED sets an on-going demand for improved resolution and printability on different substrates. Such a target can be achieved only by optimal, reliable, and sustainable droplet formation, jetting and substrate-drop interaction which depend on the physical characteristics of the inks such as surface tension and viscosity [9]. Upon applying a sufficient voltage to the piezoelectric actuator in the microchannel of the printer nozzle, the ink flows through the channel depending on its viscosity [10]. Once kinetic energy and surface energy of the ink overcomes the energy required for ejec- tion of a spherical droplet, with the help of contraction within the chamber, a droplet is formed and gets ejected from the nozzle [9,11]. Printing inks are semi-diluted suspensions consisting of the complex mix of particles which are segregated due to the mutual immiscibility and cross-material repulsive forces in the stationary state, such that the system remains de-mixed [12e14]. After strain and shear are applied, ink may first exhibit an induced elasticity as the resistance to mixing reaches a maximum, which induces dilatancy [15,16]. Dilatancy induced in high shear rate jetting con- ditions accompanied with rapid evaporation upon ejection leads to undesirable clogging of nozzles [17,18]. By determining the vis- cosity dependence over a broad shear rate range that affect droplet formation, ink recovery rate and trajectory formation that affect droplet setting, it is possible to correlate the flow parameters with print-out performance [19e21]. In this paper we provide investigation of different rheological and printing performance of three dye-sensitized solar cell inks based on three different solvents (sulfolane, 3- * Corresponding author. E-mail addresses: merve.ozkan@aalto.fi, merve.ozkan85@gmail.com (M. € Ozkan). Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel http://dx.doi.org/10.1016/j.orgel.2016.09.001 1566-1199/© 2016 Elsevier B.V. All rights reserved. Organic Electronics 38 (2016) 307e315