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: Nanowire DSCs
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 nm where much of the solar spectrum is incident
Nanowire DSCs
For a single nanowire(ZnO), measured electron diffusivity(Dn) of cm2/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( cm2) 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. Results and Analysis
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

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