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- 1. Testing with theOpen-Cell Method Layered TiO2 and ITO Structures in Dye-Sensitized Solar Cells Kirsten M. Runyan, Nicholas O. Chisholm, Hans H. Funke, and John L. Falconer Department of Chemical and Biological Engineering, University of Colorado at Boulder Research Objective • Thicker layers of TiO2 mean greater dye loading which would mean greater light harvesting and higher current if it weren’t for electron recombination • Electrons have an electron diffusion length (modeled by random walk) that is characterized by the point where recombination with the oxidized dye molecules or electrolyte becomes likely • The electron diffusion length is observed to be approximately 15-20µm (see Figure 1) Adding a Conductive Layer to Effectively Increase the Electron Diffusion Length Conclusions and Future Work Preliminary Work on ITO Coated Glass 1. The work function of porous ITO on glass is lower than the work function of dense ITO on glass due to the voltage drop 2. The electron diffusion length is about 10µm 3. Cell type 4 is about 2 times the current of type 2 4. Cell type 5 having a lower current than type 4 suggests there may be some electron recombination sites on nanoparticle ITO Recent Work with FTO Coated Glass 1. The work function of FTO on glass is greater than the work function of porous ITO creating an electron barrier, thereby, inhibiting electron flow 2. Film cracking may reduce electron mobility resulting in lower photocurrent Future Work 1. Further development of paste recipe that results in crack-free, mechanically stable films 2. Other deposition methods for ITO and TiO2 nanoparticles (sputtering, spin coating) 3. Replication of preliminary work on ITO coated glass using in-house prepared ITO nanoparticles 4. Investigation of other ITO and TiO2 layered structures 5. Investigate film cracking on photocurrent and efficiency Background How Do DSSCs Work? • Dye Sensitized Solar Cells (DSSCs) are electrochemical multi-junction photovoltaics with an anode and a cathode, first created by Gratzel • The anode is comprised of a mesoporous semiconductor layer of TiO2 nanoparticles dyed with a molecular sensitizer (a ruthenium dye) all layered on a transparent conducting oxide (TCO) film References 1. Man Gu Kang, Kwang Sun Ryu, Soon Ho Chang, Nam Gyu Park, Jin Sup Hong†, and Kang-Jin Kim, Dependence of TiO2 Film Thickness on Photocurrent-Voltage Characteristics of Dye-Sensitized Solar Cells, 2003. Results Preliminary Studies on Indium Tin Oxide Coated Glass • Preliminary studies by Dr. Miao Yu suggest that the addition of a porous conductive layer of ITO betweenTiO2 layers on ITO coated glass (porous and dense) increases the photocurrent of the cell Recent Studies on Fluorine Tin Oxide Coated Glass • Results from adding porous ITO layers between TiO2 layers on FTO coated glass (dense) indicate the photocurrent of the cell decreases with the addition of the conductive layer Acknowledgements Many thanks for contributions and guidance from Miao Yu along with the support and direction of Rich D. Noble and Prashant Nagpal. Thank you to the National Science Foundation and Undergraduate Research Opportunities Program (UROP) at the University of Colorado at Boulder for funding. Preparation of TiO2 films • The cathode of the cell is conductive glass coated in platinum • The anode and the cathode are joined by an iodide/triiode electrolyte • A photon creates an electron-hole pair in the dye molecule which is immediately split with the electron percolating to the TiO2 and the electrolyte acting as the hole carrier • Injection of the electron to the TiO2 results in oxidation of the dye molecules, the electrolyte replenishes electrons in the dye • Electrons diffuse through the TiO2 to the conductive layer on the glass where they then pass through the external circuit to the counter electrode then replenish electrons in the electrolyte • Ideally, after the electrons leave the dye molecules; they will pass through theTiO2 film without recombining with any oxidized dye molecules or electrolyte molecules Contact Information Kirsten Runyan Nicholas Chisholm kirsten.runyan@colorado.edu nicholas.chisholm@colorado.edu 720-317-5229 Figure 1 : Plots from literature demonstrating the decline in efficiency and current as the thickness of the TiO2 film increases. 1 • The addition of a layer of transparent conductive oxide (TCO) in this case Indium Tin Oxide (ITO) will provide a closer path by which the electrons may to travel to the external circuit • TiO2 wet films are heated slowly to 300 0C to volatilize off organics from the colloidal paste and then slowly heated to 500 0C to sinter the nanoparticles to anatase • A colloidal nanoparticle suspension of P25 TiO2 nanoparticles is created with water and ethanol as the solvent, an organic binder, a surfactant, and acetic acid • Acetic acid is used to increase dye loading by exposing more particle surface • Films are deposited by the doctor blade method • The figures to the right illustrates the full doctor blade method for producing films • Films were characterized by a profilometer to check for uniform thickness and roughness • Cells are tested without fully sealing the solar cells • A spacer is placed between the anode and the cathode, alligator clips are attached and the electrolyte is injected with a micropipette • A 5mm x 5mm area is masked • This method is easier than the closed cell method and prevents bubbles from occurring in the electrolyte solution • Fewer shunt and series losses observed in IV curves • Seems to contradict preliminary results • This result is possibly due to observed cracking in films that prevents electron mobility through the TiO2 and the FTO layer • Dense FTO could also have a much greater work function than porous ITO, acting as an electron barrier and lowering current 100 µm