Digital Printing of Titanium Dioxide for Dye Sensitized Solar Cells

The overall goal of this experiment is to formulate, characterize and measure the performance of Titanium Dioxide inks in printed layers. The viability of inkjet printing for the production of dye sensitized solar cells, was also investigated. Inkjet printing can be used to deposit a wide range of functional materials from a digital based image.

The main advantage of this technique is the ability to fabricate patterns onto a substrate. The development of inks containing functional materials allows the design of complex electronic devices, such as dye sensitized solar cells to be produced using this technique. Inkjet printing can potentially deposit several layers of material within a single unit operation to produce an entire device.

Visual demonstration of this method is critical, as the ink rheology and printing parameters can strongly influence the quality of the printed structures. Demonstrating the procedure alongside me will be Iulia Salaoru, an expert working in the field of additive manufacturing. To begin the ink formulation process, first add 32 grams of a 0.1 millimolar aqueous solution of hydrochloric acid to eight grams of a compatible solvent, such as dimethylformamide.

Next, add 1.5 grams of a 45%aqueous solution of propylene glycol, Tetramethyl-5-Decyn-4, 7-Diol, and ethoxylated alcohol, to act as a dispersing additive. To prevent the ink drying at the printing nozzles, add 10 grams of ethylene glycol to the mixture to act as a humectant. Continue by adding 0.5 grams of a defoaming agent, a 20%solution of acetylenic diol, and methoxypolyethylene glycol, to prevent air bubbles from developing.

Subsequently, test the ink by handshaking an aliquot in a closed container for 60 seconds. If any foam is observed, add another 0.5 grams of the defoaming agent to the ink. After the shake test, stir the ink solution for eight hours at room temperature with a magnetic stirrer, to ensure homogeneity.

Follow by adding the solution to 1.5 grams of titanium dioxide nanoparticles. Complete the formulation of the ink by sonicating it for 15 minutes, using an ultrasound probe. Prior to initiating printing, soak the glass substrates in a two weight percent solution of cleaning detergent in deionized water for 24 hours.

Remove the substrates, and rinse thoroughly with deionized water before use. Before loading the ink into the print cartridge and head, filter it through a five micrometer polyvinylidene fluoride filter, followed by a 1.2 micrometer filter, to remove any large particulates which can clog the nozzles. Flush the print head with ink through the port located on its side, to displace any air or cleaning solution left within the reservoir, or nozzles.

Insert the print head into the printer, and connect it to the head personality board. Continue by loading the ink into the 150 milliliter syringe, located above the print head, and seal it by attaching an airtight cap to the top of the syringe. Follow this by loading the glass substrate into the printer, according to the manufacturer's instructions.

Turn on the attached vacuum pump and purge the ink through the print nozzles, by pressing the purge button located on the vacuum pump. Using the GIS software on the printer's computer, connect to the print server and set the wave form and printing parameters. Select and load the desired pattern, and then print it onto the glass substrate, according to the manufacturer's instructions.

To complete the process, remove the substrate, and heat the printed film on a hot plate. Using a diamond glass cutter, cut around the printed area to produce a smaller piece, approximately 20 millimeters square. To start manufacture of the dye sensitized solar cell, prepare a dye solution by stirring 20 milliliters of ethanol and two milligrams of ruthenium dye, in a glass beaker for eight hours.

Next, submerge the titanium dioxide coated glass in the dye solution, at room temperature for 24 hours, to allow the dye to absorb onto the surface of the titanium dioxide particles. Remove the glass from the solution, and place the coated side upward on tissue paper. Subsequently, place a pre-cut 60 micrometer thick thermoplastic sealing spacer on top of the conductive glass, around the titanium dioxide coating.

Place a platinum coated counter electrode, with a pre-drilled hole in the center, on top of the thermoplastic sealing spacer, so that both active sides face each other and there is enough overlap so that the electrical contact can be made. Heat the cell on a hot plate at 110 degrees Celsius. And apply light pressure with tweezers over the area of this sealing spacer for 30 seconds to seal the electrodes together.

Complete manufacture of the cell by injecting 50 millimolar iodide triiodide electrolyte in acetonitrile into the pre-drilled hole, to fill the gap between the two electrodes. The average size of the ink's titanium dioxide particles was measured using dynamic light scattering. An overall average size of 80 nanometers was observed, well within the requirement of 100 folds smaller than the print nozzle opening of 40 micrometers.

Current density and voltage curves were obtained for inkjet printed and commercial doctor bladed DSSCs, under identical illumination conditions. The short circuit current density for the inkjet printed cell, is significantly lower than that of the doctor bladed cell, indicating a lower conversion efficiency. The key parameters for inkjet printed and doctor bladed cells, are open circuit voltage and short circuit current.

These two parameters were used to calculate the cell's efficiency. The thickness of the inkjet printed titanium dioxide layer, measured using a surface profiler, was 2.6 micrometers, significantly less than the doctor bladed technique of 18 micrometers. This results in a lower efficiency for the inkjet printed cell of 3.5%The fill factor, for both the inkjet printed, and the doctor bladed cells, was low.

This indicates high internal resistance within the cells and room for improvement in both designs. After watching this video, you should have a good understanding of how to create a nanoparticle ink, use digital printing techniques to deposit a layer of titanium dioxide nanoparticles, and manufacture dye sensitized solar cells. Ongoing work is focused on extending these approaches to improve the performance of printed devices, and evaluate how different materials can be incorporated into additional printed layers.