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

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Presentation on theme: "Nanowire dye-sensitized solar cells Sung Hwan Kim EE 235 Presentation 2."— Presentation transcript:

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

2 UC Berkeley, Sung Hwan Kim2 Outline Dye-Sensitized Cells(DSC) / Motivation Nanowire DSC Fabrication of Nanowires and Solar Cell Results and Analysis Summary and Conclusion

3 UC Berkeley, Sung Hwan Kim3 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 TiO 2 ) 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 TiO 2 film Electrons are supplied to the dye from iodide Oxidized iodide receives electron from cathod

4 UC Berkeley, Sung Hwan Kim4 Nanowire DSCs Limitations of DSC: Electron transport in nanoparticle film(TiO 2 layer) is a trap-limited diffusion process(diffusivity D n ≤10 -4 cm 2 /sec) => small diffusion length Efficiency is limited by L n 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(D n ) of 0.05-0.5cm 2 /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 UC Berkeley, Sung Hwan Kim5 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 UC Berkeley, Sung Hwan Kim6 Results and Analysis Solar cells were constructed for various surface areas(0.25-1.14cm 2 ) and tested under 1 Sun(100mA/cm 2 ) J sc = 5.3–5.85 mA/cm 2 V oc = 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 UC Berkeley, Sung Hwan Kim7 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/cm 2 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 UC Berkeley, Sung Hwan Kim8 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

9 UC Berkeley, Sung Hwan Kim9

10 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

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