DOI: 10.1002/ente.201500096 Inkjet-Printed TiO 2 Nanoparticles from Aqueous Solutions for Dye-Sensitized Solar Cells (DSSCs) Ruth Cherrington,* [a] Darren J. Hughes, [a] S. Senthilarasu, [b] and Vannessa Goodship [a] Introduction Dye-sensitized solar cells (DSSCs) provide an interesting ap- plication of nanotechnology that offers a potentially low-cost alternative to conventional commercial solar cells. Typically for DSSC production, mesoporous titanium dioxide (TiO 2 ) layers are deposited by screen-printing or doctor blading. Screen-printing inks usually require large amounts of binders and thickeners to produce the high viscosities of between 1000 and 10 000 mPa s required for reproducible and reliable film production. [1] Viscosities can be increased with the addi- tion of polymeric additives such as glycerol, ethylene glycol, or ethyl cellulose. [2] However, high-temperature curing in the range of 450–500 8C is required to completely remove these large amounts of material from the printed layers. [3] Screen- printed pastes can sometimes result in significant cracking and peeling of layers from the substrate. These are some of the major limitations of the screen-printing method and yet it is still one of the most reported methods of producing the nanostructured metal-oxide layer within DSSCs. [4, 5] Several unconventional methods to produce porous TiO 2 photoano- des have been investigated in the literature including UV-sin- tering methods, which dramatically speed up the sintering time from hours to seconds, [6] novel sol-preparation methods to produce coated nanoparticles, [7] production of complex porous structures which improve electrical power conversion efficiencies, [8] emulsion templating to produce hollow spheres with improved photocatalytic activity, [9] and doctor-blade deposition techniques. [10, 11] Inkjet printing allows for the direct deposition of a liquid onto a substrate to produce a pre-determined pattern or shape, for which layer-by-layer printing can be used to in- crease the thickness. By controlling the amount and the loca- tion of the ink deposition, the production of undesirable waste materials during the manufacturing process is avoided. These advantages result in an environmentally friendly, low- cost production method that can be adapted for mass pro- duction. Successful inkjet printing requires careful optimiza- tion of the ink and substrate properties as well as the printer parameters. Inkjet printing inks require low-viscosity fluids (between 2 and 20 mPa s) to allow droplets to be ejected from the nozzle and achieve appropriate spreading of the droplets on the substrate. [12] Viscosity and surface tension are the two key factors that enable reliable flow through the inkjet nozzle to produce a good quality film. The ideal sur- face tension of ink must be high enough to be held in the nozzle and avoid premature droplet formation, but it must also be low enough to allow the droplet to spread over the substrate surface to result in the formation of a continuous film. A variety of solvents and additives are often used to op- timize formulations to provide successful jetting, drop forma- tion, wetting, and drying behavior alongside functional per- formance. The role of a solvent within inkjet ink is to act as a carrier to deliver the functional material to the substrate. The solvent is then removed by a drying mechanism. The carrier solvent is usually the largest constituent within inkjet ink and therefore has a large impact on the overall proper- ties of the ink. The choice of solvent is also important to de- termine which ingredients can be added. One of the most im- portant factors to consider when choosing a solvent is the evaporation rate. Slow-drying solvents minimize drying in This work reports on the formulation of suitable ink for inkjet printing of TiO 2 by investigating the critical parame- ters of particle size, pH, viscosity, and stability. Aqueous sus- pensions of TiO 2 nanoparticles (Degussa, P25) were pre- pared with the addition of 25 wt % polyethylene glycol 400 as a humectant to minimize drying at the printer nozzles and reduce the likelihood of nozzle blockage. The inkjet-printed TiO 2 layers were assembled into dye-sensitized solar cells. The current–voltage (I–V) characteristics were measured under one sun (air mass 1.5, 100 mW cm À2 ) using a source meter (Model 2400, Keithley Instrument, Inc.), and the active area of the cell was 0.25 cm 2 . The inkjet-printed TiO 2 photoanode produced a device with a short-circuit current (I sc ) of 9.42 mA cm À2 , an open-circuit voltage (V oc ) of 0.76 V, and a fill factor (FF) of 0.49, resulting in a power conversion efficiency (PCE) of 3.50 %. [a] R. Cherrington, Dr. D. J. Hughes, Dr. V. Goodship WMG University of Warwick Coventry, CV47AL (United Kingdom) E-mail: R.Cherrington@warwick.ac.uk [b] Dr. S. Senthilarasu Environment and Sustainability Institute University of Exeter Penryn, Cornwall TR10 9FE (United Kingdom) Energy Technol. 2000, 00, 1 – 6  2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim &1& These are not the final page numbers! ÞÞ