Nanowire dye-sensitized solar cells

1. Nanowire dye-sensitized solar cells Sung Hwan Kim EE 235 Presentation 2

2. Outline • Dye-Sensitized Cells(DSC) / Motivation • Nanowire DSC • Fabrication of Nanowires and Solar Cell • Results and Analysis • Summary and Conclusion

3. Dye-Sensitized Cells(DSC) • A type of photochemical cell that consists of an electrolyte sandwiched between a cathod and transparent anode • Anode is a thick film of nanoparticles (~10μm TiO2) coated with a photosensitive dye(ruthenium-polypyridine) • Electrolyte(iodide solution) consists of redox couples • Giving up electrons(accepting holes) oxidizes • Accepting electrons changes from oxidized to reduced state • When sunlight enters through anode, photons strike the dye, injecting electrons into the conduction band of TiO2 film • Electrons are supplied to the dye from iodide • Oxidized iodide receives electron from cathod

4. Nanowire DSCs • Limitations of DSC: • Electron transport in nanoparticle film(TiO2 layer)is a trap-limited diffusion process(diffusivity Dn≤10-4cm2/sec) => small diffusion length • Efficiency is limited by Ln in the film, surface area of the electrodes, and low absorbance near 400-800nm where much of the solar spectrum is incident • Nanowire DSCs • For a single nanowire(ZnO), measured electron diffusivity(Dn) of 0.05-0.5cm2/sec is several hundred times larger than the highest reported diffusivity for TiO2 => provides faster carrier extraction • Provides large surface area for dye loadings • Overall increase in carrier collection efficiency

5. Fabrication of Nanowires and Solar Cell • 3-4nm in diameter ZnO quantum dots deposited in FTO substrate and nanowires grown submerged in a complex solution • Thermally platinized FTO counter electrodes were used to sandwich nanowires separated by 40μm thick spacers • Internal space of the cell was filled with iodide electrolyte by capillary action

6. Results and Analysis • Solar cells were constructed for various surface areas(0.25-1.14cm2) and tested under 1 Sun(100mA/cm2) • Jsc = 5.3–5.85 mA/cm2 • Voc = 0.61–0.71V • FF = 0.36–0.38 • η = 1.2–1.5% • FF for nanowire cells is relatively insensitive to device area => Nanowire cells are less affected by series resistance

7. Fill factor falls off with increasing light intensity owing to the development of a large photo-shunt => efficiency is fairly constant above light intensity of 5mW/cm2 For nanoparticle cells, there is a rapid saturation and decline of the current with increasing roughness factor => transport efficiency falls off above certain film thickness Results and Analysis

8. Summary and Conclusion • Some thoughts: • Lifetime of DSC solar cells • Semiconductor-electrolyte operation => susceptibility of semiconductor to photoenhanced corrosion? • Depends too heavily on dye loadings • Nanowire electrodes increase the rate of electron transport • Dye-sensitized solar cells are promising devices for inexpensive, large-scale solar energy conversion • Further work is required to accommodate the red region of the spectrum and to achieve higher dye loadings

10. Notes • Slide 3: electrolyte – semiconductor or liquid // DSC low cost • Slide 3: , since drift transport is prevented by the ions in the electrolyte • Slide 4: At the electrodes, since drift is not possible, carriers diffuse(percolate) to the contacts with transit times in miliseconds => small diffusion length • Roughness factor = surface area x TiO2 weight